Structural Characterization of Crystalline Ternary Inclusion Compounds at the Air-Water Interface

Article abstract of DOI:10.1021/ja0371404

Crystalline ternary inclusion monolayers consisting of a two-dimensional hydrogen-bonded host network of guanidinium (G) ions and organosulfonate (S) amphiphiles, and biphenylalkane guests, can be generated at the air-water interface through synergistic structural enforcement by hydrogen bonding and host-guest packing. Surface pressure-area isotherms of the 4′ -hexadecylbiphenyl-4-sulfonate (C16BPS) amphiphile in the presence of G, with or without guest, are characterized by lift-off molecular areas expected for the GS sheet based on single-crystal X-ray structures of homologous bulk crystals. Intercalation of biphenylalkane guests (4-C_nH _2n+1-C₆H₄-C₆H₅, n = 1, 4, 6, 10, 16; denoted CnBP) between organosulfonate hydrophobes, which define pocketlike cavities in the GS monolayer host, afford ternary inclusion monolayers with a 1:1 host-guest stoichiometry. These inclusion monolayers are less compressible than the guest-free host, consistent with dense packing of the biphenylalkane moieties of the host and the biphenylalkane guests. The inclusion monolayers are distinguished from the amorphous guest-free host and from selected guanidinium-free mixed monolayers by structural characterization with grazing-angle incidence X-ray diffraction (GIXD). The GIXD data for the ternary (G)C16BPS:C16BP and (G)C16BPS:C6BP inclusion monolayers obtained upon compression are consistent with a rectangular unit cell. The dimensions of these unit cells and refinement of the GIXD data suggest a "rotated shifted ribbon" GS hydrogen-bonding motif similar to that observed in some bulk GS crystals, including (G) (ethylbiphenylsulfonate). GIXD reveals that (G)C16BPS:C16BP and (G)C16BPS:C6BP are more crystalline than the corresponding guanidinium-free mixed monolayers. The (G)C16BPS:C6BP inclusion monolayer is stable upon compression, even though the alkyl-alkyl host-guest interactions are reduced due to the shorter hexyl substituents of the guest, demonstrating an important reinforcing role for the hydrogen-bonded GS sheet. The structure of a C16BPS: tetracosane (C24) mixed monolayer is independent of G; the unit cell symmetry and dimensions suggest a structure governed by alkyl-alkane interactions that prohibit formation of a GS network. These results illustrate that the existence of ternary inclusion monolayers with an intact GS network requires guest molecules that are structurally homologous with the hydrophobes of the host, in this case biphenylalkanes. The observation of these inclusion compounds suggests an approach for introducing functional nonamphiphilic molecules to an air-water interface through inclusion in a well-defined host.

Full text of DOI:10.1021/ja0371404

Published on Web 12/02/2003
Structural Characterization of Crystalline Ternary Inclusion
Compounds at the Air-Water Interface
David J. Plaut,† Stephen M. Martin,† Kristian Kjaer,‡ Markus J. Weygand,‡
Meir Lahav,§ Leslie Leiserowitz,§ Isabelle Weissbuch,*,§ and Michael D. Ward*,†
Contribution from the Department of Chemical Engineering and Materials Science, UniVersity of
Minnesota, Minneapolis, Minnesota 55455, Department of Materials and Interfaces,
The Weizmann Institute of Science, 76100 RehoVot, Israel, and Materials Research Department,
Risø National Laboratory, DK 4000 Roskilde, Denmark
Received July 8, 2003; E-mail: isabelle.weissbuch@weizmann.ac.il; wardx004@umn.edu
Abstract: Crystalline ternary inclusion monolayers consisting of a two-dimensional hydrogen-bonded host
network of guanidinium (G) ions and organosulfonate (S) amphiphiles, and biphenylalkane guests, can be
generated at the air-water interface through synergistic structural enforcement by hydrogen bonding and
host-guest packing. Surface pressure-area isotherms of the 4′-hexadecylbiphenyl-4-sulfonate (C16BPS)
amphiphile in the presence of G, with or without guest, are characterized by lift-off molecular areas expected
for the GS sheet based on single-crystal X-ray structures of homologous bulk crystals. Intercalation of
biphenylalkane guests (4-CnH2n+1-C6H4-C6H5, n ) 1, 4, 6, 10, 16; denoted CnBP) between organosulfonate
hydrophobes, which define pocketlike cavities in the GS monolayer host, afford ternary inclusion monolayers
with a 1:1 host-guest stoichiometry. These inclusion monolayers are less compressible than the guest-
free host, consistent with dense packing of the biphenylalkane moieties of the host and the biphenylalkane
guests. The inclusion monolayers are distinguished from the amorphous guest-free host and from selected
guanidinium-free mixed monolayers by structural characterization with grazing-angle incidence X-ray
diffraction (GIXD). The GIXD data for the ternary (G)C16BPS:C16BP and (G)C16BPS:C6BP inclusion
monolayers obtained upon compression are consistent with a rectangular unit cell. The dimensions of these
unit cells and refinement of the GIXD data suggest a “rotated shifted ribbon” GS hydrogen-bonding motif
similar to that observed in some bulk GS crystals, including (G)(ethylbiphenylsulfonate). GIXD reveals that
(G)C16BPS:C16BP and (G)C16BPS:C6BP are more crystalline than the corresponding guanidinium-free
mixed monolayers. The (G)C16BPS:C6BP inclusion monolayer is stable upon compression, even though
the alkyl-alkyl host-guest interactions are reduced due to the shorter hexyl substituents of the guest,
demonstrating an important reinforcing role for the hydrogen-bonded GS sheet. The structure of a C16BPS:
tetracosane (C24) mixed monolayer is independent of G; the unit cell symmetry and dimensions suggest
a structure governed by alkyl-alkane interactions that prohibit formation of a GS network. These results
illustrate that the existence of ternary inclusion monolayers with an intact GS network requires guest
molecules that are structurally homologous with the hydrophobes of the host, in this case biphenylalkanes.
The observation of these inclusion compounds suggests an approach for introducing functional nonam-
phiphilic molecules to an air-water interface through inclusion in a well-defined host.
Introduction
two-dimensional molecular assemblies at the air-water interface
are more limited in number than their 3-D counterparts, yet they
Crystalline three-dimensional (3-D) organic frameworks have
been examined extensively, owing to their ability to serve as
hosts for inclusion of complementary guest molecules within
well-defined cavities and potential applications in separations,
catalysis, chemical and biological sensors, and electronic
devices.1 Host-guest complexes and inclusion compounds in
span a range of architectures (Figure 1). Mixed monolayers,
typically binary, formed at the air-water interface through
specific interactions and stabilized by hydrophobe-hydrophobe
contacts,2,3 can be viewed as 2-D equivalents of bulk cocrystals
(Figure 1a).4 Amphiphiles that simply bind water-soluble guest
molecules at or beneath the air-water interface through specific
interactions, analogous to “coordinatoclathrate” host-guest
complexes in 3-D crystals,5 sometimes are characterized as
† University of Minnesota.
‡ Risø National Laboratory.
§ The Weizmann Institute of Science.
(1) (a) MacNicol, D. D.; McKendrick, J. J.; Wilson, D. R. In Inclusion
Compounds; Atwood, J. L., Davies, J. E., MacNicol, D. D., Eds.; Academic
Press: London, 1984; Vol. 2. (b) Simard, M.; Su, D.; Wuest, J. D. J. Am.
Chem. Soc. 1991, 113, 4696. (c) Ermer, O.; Lindenberg, L. HelV. Chim.
Acta 1991, 74, 825. (d) Endo, K.; Ezuhara, T.; Koyonagi, M.; Hideki, M.;
Aoyama, Y.; J. Am. Chem. Soc. 1997, 119, 499.
(2) Kuzmenko, K.; Kjaer, K.; Als-Nielsen, J.; Lahav, M.; Leiserowitz, L. J.
Am. Chem. Soc. 1999, 121, 2657.
(3) Gidalevitz, D.; Weissbuch, I.; Kjaer, K.; Als-Nielsen, J.; Leiserowitz, L.
J. Am. Chem. Soc. 1994, 116, 3271.
(4) Burgi, H.-B.; Hulliger, J.; Langley, P. J. Curr. Opin. Sol. State Mater. Sci.
1998, 3, 425.
9
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J. AM. CHEM. SOC. 2003, 125, 15922-15934
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Crystalline Ternary Inclusion Compounds
A R T I C L E S
a key feature that is reminiscent of 3-D crystalline inclusion
compoundssinherent cavities that permit inclusion of a structur-
ally complementary component. These last two examples
suggest that monolayers equipped with spacer molecules at the
air-water interface can have inherent inclusion cavities between
“preexpanded” hydrophobe arrays, affording hosts for nonam-
phiphilic guest molecules and generating two-dimensional
inclusion compounds at the air-water interface (Figure 1h). In
principle, this manner of guest intercalation could permit the
introduction of functionality (i.e., the guest) at the air-water
interface that otherwise may not be achievable.
One of our laboratories has reported numerous 3-D crystalline
materials based on a highly persistent two-dimensional hydrogen-
bonded network of guanidinium (G) ions, C(NH2)3+, and
sulfonate moieties (S) of organomonosulfonate and organodisulfo-
nates.17-20 The GS network, which typically adopts either a
“quasihexagonal” or “shifted ribbon” motif, has been observed
in more than 300 crystals derived from various organomono-
sulfonates and organodisulfonates, usually affording layered
architectures. In the presence of suitable guest molecules,
inclusion compounds can be generated with either (G)(orga-
nomonosulfonate) or (G)2(organodisulfonate) hosts, wherein the
organic substituents of the host support inclusion cavities
between adjacent GS sheets.21,22 The unusual persistence of the
GS network has been attributed to the hydrogen-bonding
complementarity of the G and S ions, the large number of
hydrogen bonds within the network (up to six for each G and
S ion), and an inherent structural flexibility of the GS sheet,
which permits the host frameworks to achieve optimum packing
with the guests, often resulting in packing that mimics that of
the organic components alone.23,24 The metrics of the GS sheet,
in which the S sites are separated by 7.2-7.5 Å because of the
intervening G ions, proscribes voids that permit interdigitation
of organic groups from an opposing GS sheet, as observed in
guest-free (G)(organomonosulfonates) that crystallize in the
discrete bilayer architecture. This can be illustrated by the single-
crystal X-ray structures of (G)(4′-methylbiphenyl-4-sulfonate)
and (G)(4′-ethylbiphenyl-4-sulfonate). Even though these two
compounds display different hydrogen-bonded motifs
(quasihexagonal and shifted ribbon, respectively), both exhibit
the bilayer architecture with interdigitation of organic groups
(Figure 2).25,26
Figure 1. Schematic representations of various Langmuir monolayer
architectures: (a) a mixed monolayer of amphiphiles assembled through
specific interactions at the air-water interface; (b) a host monolayer of
amphiphiles bound to solute guests below the air-water interface through
specific interactions; (c) a host monolayer of amphiphiles bound to water-
soluble “guest” molecules at the air-water interface through specific
interactions (these guest molecules promote loose packing of the hydro-
phobes and amorphous character); (d) a crystalline host monolayer of
amphiphiles, equipped with bulky terminal substituents, assembled through
specific interactions with water-soluble guest molecules at the air-water
interface; (e) a monolayer of cup-shaped amphiphiles with intercalated guest
molecules above the air-water interface; (f) a crystalline mixed monolayer
of amphiphiles assembled through nonspecific interactions, here displayed
with bulky substituents on one of the amphiphiles; (g) a bilayer of self-
interdigitated monolayers, each sufficiently spaced though specific binding
with water-soluble spacer guest molecules; (h) a ternary monolayer
composed of amphiphiles bound to solute molecules at the air-water
interface through specific interactions, with nonamphiphilic guest molecules
intercalating above the air-water interface.
host-guest complexes (Figure 1b-d).6-11 Some of these have
been proven to be crystalline by grazing incidence X-ray
diffraction (GIXD).12,13
Conversely, host-guest assemblies that rely on nonspecific
intercalation into cavities formed by amphiphile hydrophobes
aboVe the air-water interface are analogous to clathrates or
coordination-assisted clathrates in 3-D crystals.14 Fatty acid or
alcohol amphiphiles can exhibit hostlike behavior, expanding
to allow nonspecific intercalation to form crystalline mixed
monolayers (Figure 1f).15 Formation of a self-interdigitated
crystalline bilayer has been achieved through the introduction
of hydrophilic “spacer” molecules to the air-water interface,
thereby separating the hydrophobes of the amphiphiles in one
layer so that intercalation of the other is possible (Figure 1g).16
Although such bilayers do not contain distinct host and guest
components, the architecture of the bottom monolayer displays
These features suggested that GS monolayers based on 4′-
alkylbiphenyl-4-sulfonates could serve as two-dimensional hosts
at the air-water interface, with inclusion cavities for intercalat-
(5) Weber, E. In Topics in Current Chemistry; Springer-Verlag: Berlin, 1987;
Vol. 140.
(6) Bohanon, T. M.; Denziger, S.; Fink, R.; Paulus, W.; Ringsdorf, H.; Weck,
M. Angew. Chem., Int. Ed. Engl. 1995, 34, 58.
(7) (a) Ariga, K.; Kunitake, T. Acc. Chem. Res. 1998, 31, 371. (b) Cha, X.;
Ariga, K.; Kunitake, T. J. Am. Chem. Soc. 1996, 118, 9545. (c) Sasaki, D.
Y.; Kurihara, K.; Kunitake, T. J. Am. Chem. Soc. 1992, 114, 10994.
(8) Siegel, S.; Kindermann, M.; Regenbrecht, M.;. Vollhardt, D.; von Kied-
rowski, G. Prog. Colloid Polym. Sci. 2000, 115, 233.
(9) Plaut, D. J.; Lund, K. M.; Ward, M. D. Chem. Commun. 2000, 9, 769.
(10) Tyson, J. C.; Moore, J. L.; Hughes, K. D.; Collard, D. M. Langmuir 1997,
13, 2068.
(17) Russell, V. A.; Etter, M. C.; Ward, M. D. J. Am. Chem. Soc. 1994, 116,
1941.
(18) Swift, J. A.; Pivovar, A. M.; Reynolds, A. M. J. Am. Chem. Soc. 1998, 24,
5887.
(19) Evans, C. C.; Sukarto, L.; Ward, M. D. J. Am. Chem. Soc. 1999, 121, 320.
(20) Holman, K. T.; Pivovar, A. M.; Ward, M. D. Science 2001, 294, 1907
(21) Russell, V. A.; Evans, C. C.; Li, W.; Ward, M. D. Science 1997, 276, 575.
(22) Holman, K. T.; Pivovar, A. M.; Swift, J. A.; Ward, M. D. Acc. Chem. Res.
2001, 34, 107.
(11) Kuzmenko, I.; Kindermann, M.; Kjaer, K.; Howes, P. B.; Als-Nielsen, J.;
Granek, R.; von Kiedrowski, G.; Leiserowitz, L.; Lahav, M. J. Am. Chem.
Soc. 2001, 123, 3771.
(23) Holman, K. T.; Ward, M. D. Angew. Chem., Int. Ed. 2000, 39, 1653.
(24) Holman, K. T.; Martin, S. M.; Parker, D. P.; Ward, M. D. J. Am. Chem.
Soc. 2002, 123, 4421.
(12) Kuzmenko, I.; Rapaport, H.; Kjaer, K.; Als-Nielsen, J.; Weissbuch, I.;
Lahav, M.; Leiserowitz, L. Chem. ReV. 2001, 101, 1659.
(13) Alonso, C.; Eliash, R.; Jensen, T. R.; Kjaer, K.; Lahav, M.; Leiserowitz,
L. J. Am. Chem. Soc. 2001, 123, 10105.
(25) Single-crystal X-ray structure data for (G)(4′-methylbiphenyl-4-sulfonate)
(1): formula C14H17N3O3S; formula weight 307.37; crystal system mono-
clinic; space group P21/n; a ) 11.902(3) Å; b ) 7.561(7) Å; c ) 34.130-
(7) Å; â ) 94.679(5)°; V ) 3061.4(1) Å3, temperature 173(2) K; Z ) 8;
R1(I > 2σ(I)) ) 0.0580; wR2(I > 2σ(I)) ) 0.1899; GOF ) 1.096.
(26) Single-crystal X-ray structure data for (G)(4′-ethylbiphenyl-4-sulfonate)
(2): formula C15H19N3O3S; formula weight 321.40; crystal system mono-
clinic; space group P21; a ) 12.180(4) Å; b ) 7.250(2) Å; c ) 17.807(5)
Å; â ) 100.204(6)°; V ) 1547.5(8) Å3, temperature 173(2) K; Z ) 4;
R1(I > 2σ(I)) ) 0.0594; wR2(I > 2σ(I)) ) 0.1420; GOF ) 1.032.
(14) Ashwell, G. J.; Dyer, A. N.; Green, A.; Sato, N.; Sakuma, T. J. Mater.
Chem. 2000, 10, 2473.
(15) Alonso, C.; Kuzmenko, I.; Jensen, T. R.; Kjaer, K.; Lahav, M.; Leiserowitz,
L. J. Phys. Chem. B 2001, 105, 8563.
(16) Kuzmenko, I.; Buller, R.; Bouman, W. G.; Kjaer, K.; Als-Nielsen, J.; Lahav,
M.; Leiserovitz, L. Science 1996, 274, 2046.
9
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A R T I C L E S
Plaut et al.
Chart 2
Figure 2. Interdigitated bilayer structures adopted in bulk 3-D crystals of
(a) (G)(4′-methylbiphenyl-4-sulfonate) and (b) (G)(4′-ethylbiphenyl-4-
sulfonate), as determined by single-crystal X-ray diffraction. Half of the
bilayer is depicted in red to aid visualization of the interdigitation. Top
view of the (c) quasi-hexagonal and (d) shifted ribbon GS hydrogen-bonded
network observed in crystalline GS compounds, including the structures in
(a) and (b), respectively. The gray circles represent the regions that may be
occupied by guest molecules in inclusion monolayers. The areas per formula
unit are 48.8 Å2 (box in (c)) and 44.6 Å2 (d).
these inclusion compounds suggests an approach for introducing
functional nonamphiphilic molecules to an air-water interface
through inclusion in a well-defined host.
Chart 1
Results and Discussion
Guest-Free Monolayers. The surface pressure-area (π-A)
isotherm of a C16BPS monolayer on pure water revealed the
formation of a condensed phase with an extrapolated molecular
area of Amol ) 28 Å2/sulfonate (from the rising portion of the
isotherm in Figure 3a). This value is larger than that expected
for close-packed alkyl chains (ca. 19-21 Å2)27 or biphenyl
molecules (ca. 23 Å2),28 revealing the influence of the sulfonate
headgroup. Isotherms of the same amphiphile on an aqueous
subphase of G2CO3, however, exhibited features distinct from
isotherms over pure water. Specifically, when G was present
in the subphase, the surface pressure exhibited a discernible
increase at larger molecular areas (this area is referred to herein
as Alift-off) and the isotherm rose less steeply. The lift-off area
increased logarithmically with increasing G concentration in
the range 0.0001 < [G] < 0.01 M, eventually achieving a
limiting value of Alift-off ) 52 Å2/sulfonate beyond this
concentration range (Figure 4). This area agrees with that
expected, based on 3-D single-crystal structures, for either of
the planar GS networks in Figure 2, signaling a 1:1 hydrogen-
bonded GS monolayer for [G] > 0.01 M. The collapse pressure
of the hydrogen-bonded monolayer was lower than in the
absence of G, possibly reflecting the coexistence of crystalline
and amorphous phases just prior to collapse. A specific
hydrogen-bonding role of the G ion in the expansion of the
C16BPS monolayer was substantiated by isotherms collected
over aqueous 0.01 M Na2CO3 or 0.1 M KCl, which produced
extrapolated molecular areas of Amol ) 28 Å2/sulfonate, identical
with the value for water alone. Furthermore, C16BPS am-
phiphile spread over a subphase of aqueous 0.02 M G+Cl-
exhibited an isotherm with identical shape and lift-off area as
the isotherm collected over a subphase containing 0.01 M
G2CO3 (equivalent guanidinium concentrations).
ing homologous guest molecules. The molecular area of a GS
unit, on the basis of single-crystal structures of compounds with
flat hydrogen-bonded GS sheets, is 48 Å2. Assuming a cross-
sectional area of 21 Å2 for an alkyl chain and 23 Å2 for a phenyl
ring, the cross-sectional area of an inclusion cavity in a GS
monolayer with biphenylalkane substituents would be roughly
25 Å2, sufficient for intercalation of an aliphatic or aromatic
guest molecule (Chart 1).
Herein we report the formation and characterization, princi-
pally with GIXD, of ternary inclusion monolayers that result
from intercalation of guest molecules into a two-dimensional
hydrogen-bonded host network of guanidinium ions and orga-
nomonosulfonate amphiphiles. These inclusion monolayers,
consisting of G, 4′-hexadecylbiphenyl-4-sulfonate (C16BPS),
and various biphenylalkanes 1-5 (Chart 2), possess certain
structural features observed in their 3-D crystalline counterparts.
Surface pressure-area isotherms and GIXD data combined
revealed a structure-enforcing role for the GS network, which
stabilized the inclusion monolayers toward collapse and loss
of guest molecules. In contrast, guests 6 and 7 formed host-
guest monolayers with structures that precluded formation of
an ordered GS network in a manner that suggested an important
role for interactions between host and guest biphenyl groups.
These results illustrate that the existence of ternary inclusion
monolayers, with the GS network intact, requires guest mol-
ecules that are structurally homologous with the hydrophobes
of the host, in this case biphenylalkanes. The observation of
GIXD scattering patterns for C16BPS on pure water (Figure
3b), measured at selected points along the π-A isotherm in the
range 25 < Amol < 50 Å2/sulfonate, exhibited two Bragg peaks
(27) Gerson, A. R.; Roberts, K. J.; Sherwood, J. N. Acta Crystallogr. 1991,
B47, 280.
(28) Charbonneau, G. P.; Delugeard, Y. Acta Crystallogr. 1977, B33, 1586.
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Crystalline Ternary Inclusion Compounds
A R T I C L E S
Figure 3. (a) π-A isotherms of C16BPS on pure water (black) and C16BPS on 0.01 M G2CO3 (red). GIXD patterns shown as two-dimensional contour
plots of the scattered intensity, I(qxy,qz), measured for (b) C16BPS on pure water at Amol ) 30 Å2/sulfonate (the GIXD pattern was essentially unchanged
within the range 25 Å2 < Amol < 50 Å2/sulfonate) and (c) C16BPS on 0.01 M G2CO3 at Amol ) 30 Å2/sulfonate (a similar signal resulted throughout the
range Amol < 42 Å2/sulfonate). The schematics of the monolayers depict an amorphous (no diffraction observed) (G)C16BPS monolayer at low pressures
(denoted on the isotherm by the red asterisks) and a crystalline C16BPS monolayer at higher pressures, where the G ion is forced out of the air-water
interface so close packing of the chains can be achieved.
sulfonate, however, no diffraction pattern was observed. This
is not surprising; formation of the GS network at the air-water
interface, which is supported by the lift-off area of Alift-off
)
52 Å2/sulfonate, would preclude close packing of the C16BPS
hydrophobes to the extent that these scattering centers would
be disordered. Compression of the film would expel the G ions
from the GS network into the subphase, thereby allowing
formation of the guanidinium-free C16BPS monolayer.
Crystalline Monolayers with Biphenylalkane Intercalates.
A low-density monolayer like (G)C16BPS offers the prospect
for intercalation of nonamphiphilic guest molecules within the
cavities generated by the hydrophobes of the host framework.
The π-A isotherm for a 1:1 solution of C16BPS and 4-hexa-
decylbiphenyl (C16BP) guest spread over 0.01 M G2CO3
revealed Alift-off ) 52 Å2/sulfonate (Figure 5a), identical with
the behavior in the absence of C16BP. Grazing angle FT-IR of
a Langmuir-Blodgett (LB) monolayer on a gold-coated glass
substrate, prepared by transfer at a surface pressure of 10 mN/
m, revealed N-H bending modes at 1550-1750 cm-1. These
modes also were observed in bulk crystals of guanidinium
alkylbiphenylsulfonates, substantiating the existence of a hy-
drogen-bonded GS network. Further characterization based on
N-H stretching modes between 3100 and 3500 cm-1 was
prohibited by the presence of persistent water, probably bound
to the film through hydrogen bonding. Unlike its guest-free
counterpart, the isotherm of this (G)C16BPS:C16BP monolayer
was very steep, signifying an increase in monolayer rigidity
owing to inclusion of C16BP. Notably, the π-A isotherm for
a 1:1 mixture of C16BPS and C16BP on a pure water subphase
was somewhat similar, differing only with respect to the collapse
pressure. In the absence of other evidence, one would surmise
that the two isotherms in Figure 5 are associated with the
identical monolayers. GIXD measurements, however, revealed
that the monolayers over pure water and over 0.01 M G2CO3
adopt different packing arrangements. Furthermore, both struc-
tures differ from that of the crystalline phase observed in the
absence of the guest, as evidenced from comparison of the
diffraction patterns in Figures 3 and 5.
Figure 4. Dependence of the lift-off area on the G subphase concentration
for a C16BPS monolayer. The liftoff area achieves a plateau of 52 Å2/
molecule at [G] > 0.01 M, consistent with the area of a GS network. The
line shown is the logarithmic fit of the first five data points, with a correlation
factor R ) 0.99.
at qxy ) 1.52 and 1.70 Å-1. These two peaks can be indexed to
a pseudo-centered rectangular unit cell containing two molecules
and having lattice parameters a ) 4.99 Å, b ) 7.39 Å, and
Acell ) 36.8 Å2. This unit cell resembles those for typical
monolayers of aliphatic surfactants with alkyl chains aligned
perpendicular to the air-water interface.29 Analysis of the full
width at half-maximum (fwhm) of the Bragg peaks (qxy) and
Bragg rods (qz) indicated an average domain coherence length
of ca. 900 Å and a monolayer thickness of ca. 35 Å, respectively.
A weak Bragg peak also was observed at qxy ) 1.40 Å-1, but
the fwhm of this peak corresponded to a coherence length (ca.
45 Å) that was substantially smaller than the domains of the
rectangular cell, revealing that this peak was due to a second
phase with lower crystallinity. The observation of a single Bragg
peak suggests a hexagonal cell (a ) 5.16 Å), with a thickness
of ca. 14 Å, as deduced from the fwhm value of its Bragg rod.
At Amol < 42 Å2/sulfonate a C16BPS monolayer over 0.01
M G2CO3 produced a diffraction pattern (Figure 3c) similar to
that obtained over pure water (Figure 3b). At Amol > 42 Å2/
(29) Kuzmenko, I.; Kaganer, V. M.; Leiserowitz, L. Langmuir 1998, 14, 3882.
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A R T I C L E S
Plaut et al.
Figure 5. (a) π-A isotherms of C16BPS:C16BP on pure water (black) and 0.01 M G2CO3 (red). GIXD contour plots of the scattered intensity measured
from C16BPS:C16BP on (b) pure water at 48 < Amol < 70 Å2/sulfonate, (c) pure water at Amol < 38 Å2/sulfonate, (d) 0.01 M G2CO3 at 48 < Amol < 70
Å2/sulfonate, and (e) 0.01 M G2CO3 at Amol < 25 Å2/sulfonate.
a
Table 1. Structural Parameters of Uncompressed Crystalline C16BPS-Based Monolayers on Water and 0.01 M G2CO3
water only
0.01 M G2CO3
2
2
guest
none
a (Å)
b (Å)
γ (deg)
A
cell (Å )
t (deg)b
ψc
a (Å)
b (Å)
γ (deg)
A
cell (Å )
t (deg)b
ψ (deg)c
4.99
5.58
5.48
5.68
7.39
8.13
8.26
8.32
90
89.8
90
36.8
45.4
45.3
47.3
0
31
8
35
30
29
0
151
0
149
139
90
C16BP
5.58
5.51
5.56
7.83
8.15
8.11
90
90
90
43.7
44.9
45.1
26
8
31
0
0
151
C6BP
C16BP-Cld
90
C24
4.99
9.04
90
45.1
4.99
9.04
90
45.1
29
90
a Structural parameters determined from GIXD data acquired at a surface pressure of 0 mN/m. b t ) tilt angle of the alkyl chains from the normal to the
air-water interface (see Figure 6). c ψ ) azimuthal direction with respect to the a axis along which tilt, t, occurs (see Figure 6). d On water only, two phases
with identical unit cell parameters but slightly different t and ψ values were apparent in the Bragg rod. The two contributions were separated by least-square
fittings. See the Supporting Information.
GIXD (Figure 5b) of the C16BPS:C16BP mixed monolayer
on pure water, measured at surface pressures less than 5 mN/m
where Amol > 48 Å2/sulfonate, produced Bragg peaks (qxy) that
could be indexed to a near-rectangular unit cell, containing two
molecules, with lattice parameters a ) 5.58 Å, b ) 8.13 Å, γ
) 89.8°, and Acell ) 45.4 Å2 (Table 1). On the basis of the qz
position of the intensity maxima along the Bragg rods, I(qz),
the alkyl chains tilt 31° from the normal to the air-water
interface at an azimuthal direction 151° from the a axis. The
measured fwhm of the Bragg peaks corresponds to a coherence
length of 190 Å. The monolayer packing arrangement was
refined on the basis of X-ray structure factor calculations using
an atomic coordinate model of a unit cell based on known 3-D
crystal structures. Using the SHELX97 program adapted for 2-D
crystals, the packing arrangement for a unit cell containing one
C16BPS amphiphile and one C16BP guest was refined with
each molecule constrained separately as a rigid body. The best
refinement was achieved with the alkyl chain of C16BPS and
C16BP having C1-C2-C3-C4 and C3-C4-C5-C6 dihedral
angles locked to angles measuring 155° (C1 being the carbon
atom attached to the biphenyl), with the remaining portion of
the chains being all-trans (Figure 6). This results in a substantial
tilt of the biphenyl rings from the surface normal. Upon
compression of the C16BPS:C16BP mixed monolayer to Amol
) 38 Å2/sulfonate the {02} Bragg rod became skewed (Figure
5c), which signifies buckling of the monolayer.30,31 Furthermore,
the maximum position of intensity became qzmax ) 0 Å-1 for
all Bragg rods, indicating a transformation to a new phase in
which the alkyl chains were perpendicular to the air-water
interface. The diffraction data for this more compressed mono-
layer were indexed to a unit cell of a ) 5.6 Å, b ) 7.8 Å, γ )
90°, and Acell ) 43.7 Å2 and the domain coherence length was
unchanged.
The GIXD contour plots for the(G)C16BPS:C16BP mono-
layer (Figure 5d), collected over 0.01 M G2CO3 at Amol > 48
Å2/sulfonate, clearly revealed a crystal structure different from
the monolayer in the absence of G, Verifying the existence of a
ternary inclusion compound at the air-water interface. The
data support a rectangular cell with two molecules and lattice
dimensions a ) 5.58 Å, b ) 7.83 Å, and Acell ) 43.7 Å2. The
alkyl chains tilt along the a axis by 26° from the normal to the
air-water interface, as deduced from the qz position of the
intensity maximum along the (11) Bragg rod. The measured
fwhm of the Bragg peaks corresponds to a coherence length of
380 Å, a factor of 2 larger than that measured for the C16BPS:
C16BP monolayer without G in the subphase. Similar to the
C16BPS:C16BP monolayer, the GIXD pattern of the com-
pressed (G)C16BPS:C16BP monolayer at Amol < 25 Å2/
sulfonate (Figure 5e) displayed a skewing of the Bragg rods
associated with a buckling of a stiff monolayer.
Using the rectangular shifted ribbon hydrogen-bonding motif
of guanidinium 4′-ethylbiphenyl-4-sulfonate (GC2BPS) as a
(30) Kjaer, K.; Bouwman, W. G. X-ray 2D-Powder Diffraction Methods for
Films at Liquid Surfaces. In Annual Progress Report of the Department of
Solid State Physics 1 January-31 December 1994; Lindgård, P.-A.,
Bechgaard, K., Clausen, K. N., Feidenhans’l, R., Johannsen, I., Eds.; RISØ-
R-779(EN); Risø National Laboratory; Roskilde, Denmark; Jan 1995; p
79.
(31) Howes, P. B., Kjaer, K. X-ray Diffraction from Curved Thin Films. Annual
Progress Report of the Department of Solid State Physics 1 January-31
December 1996; Jørgensen, M., Bechgaard, K., Clausen, K. N., Feidenhans’l,
R., Johannsen, I., Eds.; RISØ-R-933(EN); Risø National Laboratory;
Roskilde, Denmark; Jan 1997; p 71.
9
15926 J. AM. CHEM. SOC. VOL. 125, NO. 51, 2003

Crystalline Ternary Inclusion Compounds
A R T I C L E S
Figure 6. (a) Experimental (black dots) and calculated (blue lines) Bragg rod intensity profiles, I(qz), for C16BPS:C16BP over pure water. Refined packing
arrangements of the C16BPS:C16BP monolayer viewed down the (b) a and (c) b axes. C16BP molecules are depicted in red. The tilt angle, t, and the
azimuthal angle, ψ, of the alkyl chains are defined according to the panel at the right.
Figure 7. Rectangular (a) shifted ribbon and (b) rotated shifted ribbon GS hydrogen-bonded motifs used in the refinement of the (G)C16BPS:C16BP
monolayer. The hexadecylbiphenyl groups are omitted for clarity, and the N-H‚‚‚O-S hydrogen bond distances are denoted (in Å). Experimental (black
dots) and calculated (blue lines) Bragg rod intensity profiles, I(qz), for (G)C16BPS:C16BP with (c) the shifted ribbon motif and (d) the rotated shifted-ribbon
motif. The refinement of the {11} Bragg rod is improved in (d). Panels (e) and (f) depict the refined packing arrangements of the (G)C16BPS:C16BP
monolayer with a “rotated shifted ribbon” motif, as viewed down the (e) a and (f) b axes. C16BP guests are depicted in red. The angles t and ψ are defined
in Figure 6.
guide (Figure 2, above), an atomic coordinate model of the
(G)C16BPS:C16BP monolayer packing arrangement was con-
structed and refined with the GIXD data. The refinement was
performed with an asymmetric unit consisting of a guani-
dinium-sulfonate amphiphile ion pair and a C16BP guest
molecule, each constrained as separate rigid bodies. Refinements
based on a shifted ribbon GS motif like that observed in bulk
crystals, but with the hydrogen-bonding distances adjusted to
compensate for the different unit cell dimensions (smaller a and
larger b), produced acceptable fits (Figure 7c). An alternative
motif with the G and S ions rotated so that some hydrogen bonds
are unfulfilled (Figure 7d), however, improved the fit to the
{11} Bragg rod. Although this variant of the shifted ribbon motif
has not been observed in bulk crystals, water from the subphase
may perturb the typical GS motifs through hydrogen bonding
with the G and S ions at the air-water interface, possibly
becoming part of the hydrogen-bonded network. The structure
was refined with all-trans alkyl chains for both the host and the
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J. AM. CHEM. SOC. VOL. 125, NO. 51, 2003 15927

A R T I C L E S
Plaut et al.
Figure 9. (a) π-A isotherms of C16BPS:C6BP on pure water (black) and
on 0.01 M G2CO3 (red). GIXD contour plots of the scattered intensity
measured from C16BPS:C6BP on (b) 0.01 M G2CO3 from 30 < Amol < 70
Å2/sulfonate, (c) pure water at 60 and 40 Å2/sulfonate, and (d) pure water
at 22.5 Å2/sulfonate. The diffraction pattern observed at the molecular area
corresponding to the asterisk is illustrated in Figure 11 as a 1-D Bragg
peak representation.
interactions both contribute to stability and properties of the
inclusion monolayers.
The influence of guest alkyl chain length on monolayer
structure and stability was probed in detail for a selected member
of the biphenylalkane series, C6BP (3). The GIXD patterns
measured from (G)C16BPS:C6BP (Figure 9b) on 0.01 M
G2CO3 consisted of Bragg peaks that could be indexed to a
Figure 8. π-A isotherms of (a) C16BPS:CnBP on pure water and (b)
C16BPS:CnBP on 0.01 M G2CO3 with guest molecules C1BP (1), C4BP
(2), C6BP (3), C10BP (4), and C16BP (5).
rectangular unit cell of a ) 5.51 Å, b ) 8.15 Å, and Acell
)
44.9 Å2. The alkyl chains tilt along the a axis by 8° from the
normal to the air-water interface, as calculated from the qz
position of the intensity maxima along the Bragg rods. The
measured fwhm of the Bragg peaks corresponded to a coherence
length >1200 Å. The structure of the (G)C16BPS:C6BP
monolayer was deduced by refinement of an atomic coordinate
model in a unit cell containing the (G)C16BPS ion pair and
the C6BP guest molecule, each constrained as separate rigid
bodies. The best refinement produced a structure with the rotated
shifted ribbon GS hydrogen-bond motif and a pseudo-her-
ringbone arrangement between the biphenyl moieties and alkyl
chains of the host and guest (Figure 10). Notably, the GIXD
pattern associated with the (G)C16BPS:C6BP inclusion mono-
layer did not change when compressed to areas in the range 30
< Amol < 60 Å2/sulfonate.
guest, with pseudo-herringbone29 alkyl-alkyl and biphenyl-
biphenyl host-guest motifs (Figure 7e,f).
To probe the contribution of host-guest interactions above
the air-water interface to the stability of these ternary inclusion
monolayers, the biphenylalkane guests 4-CnH2n+1-C6H4-C6H5,
where n ) 1, 4, 6 and 10, denoted CnBP, were examined (Chart
2). The isotherm of the C16BPS:C10BP monolayer (no G
present) resembles that of the C16BPS:C16BP monolayer
with Alift-off ) 52 Å2/sulfonate and a steep increase of sur-
face pressure upon compression (Figure 8a). In contrast,
the isotherms of the C16BPS:C1BP, C16BPS:C4BP, and
C16BPS:C6BP mixed monolayers are essentially identical with
the isotherm of the C16BPS monolayer without guest intercalate,
with similar slopes upon compression and nearly identical
molecular areas at collapse (ca. 20 Å2/sulfonate). The isotherms
therefore signify robust 1:1 mixed monolayers for guests 4 and
5, which have long alkyl tails, but substantially less stability
for guests 1-3.
The π-A isotherms for the (G)C16BPS:CnBP monolayers
(Figure 8b) parallel those observed in the absence of G, albeit
with some noticeable differences. The collapse pressures are
generally lower. Furthermore, the (G)C16BPS:CnBP monolay-
ers with guests 1-3 exhibit a perceptible Alift-off at ca. 50 Å2/
sulfonate and have larger extrapolated limiting areas, features
that are consistent with the GS sheet and host-guest inclusion
at low surface pressures. The slopes of the isotherms increase
in the order 1 < 2 < 3 < 4 < 5, reflecting increasing monolayer
stiffness due to increasing alkyl chain length and packing
density. The isotherms in Figure 8b, particularly those for guests
1-3, indicate that GS hydrogen bonding and host-guest
At Amol > 40 Å2/sulfonate, a C16BPS:C6BP mixed monolayer
on pure water produced a GIXD pattern (Figure 9c) similar to
that observed for (G)C16BPS:C6BP, suggesting similar host-
guest packing in the two monolayers. The integrated intensities
of all the Bragg peaks, however, are 5 times smaller for
C16BPS:C6BP, and the fwhm values of the Bragg peaks
correspond to a smaller coherence length (700 Å). Both features
signify reduced crystallinity for the C16BPS:C6BP monolayer.
Furthermore, unlike (G)C16BPS:C6BP, the GIXD pattern of
C16BPS:C6BP changed substantially upon compression (Figure
9d). This observation further supports the structure-reinforcing
role of GS hydrogen bonding and explains the observation of
different isotherms for the two monolayers.
The evolution of the structural changes during compression
of the guanidinium-free C16BPS:C6BP monolayer is apparent
from new Bragg peaks (qxy) that emerge at various surface
pressures (Figure 11). The data obtained upon compression to
9
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Crystalline Ternary Inclusion Compounds
A R T I C L E S
Figure 12. The herringbone layers of C6BP displayed in the 3-D single-
crystal structure. The alkyl chains in (b) have been omitted for clarity.
Although the limited amount of data precluded definitive
structural assignments of these transient phases, a plausible
explanation for this behavior is the pressure-induced expulsion
of the C6BP guests, initially producing C6BP inclusions at the
top of the monolayer followed by a C6BP multilayer on top of
the guest-free C16BPS monolayer. The presence of a multilayer
is supported by narrow Bragg rods which exhibited a fwhm
corresponding to a film thickness of 140 Å. The Bragg peaks
at qxy ) 1.375 and 1.530 Å-1 associated with these rods were
indexed to a rectangular unit cell with in-plane dimensions of
5.50 and 8.21 Å. Notably, C6BP single crystals grow from
methanol as extremely thin plates. Single-crystal X-ray diffrac-
tion32 at -173 °C revealed P21/c symmetry (a ) 31.558 Å, b
) 5.377 Å, c ) 7.943 Å, â ) 93.216°) and (100) bilayers of
C6BP molecules in a herringbone arrangement within each layer
(Figure 12). Powder X-ray diffraction (PXRD) of bulk C6BP
at 0 °C, near the 4 °C operating temperature of the GIXD, also
revealed a monoclinic lattice but with slightly expanded lattice
dimensions (a ) 32.50 Å, b ) 5.59 Å, c ) 8.24 Å, â ) 93.56°)
compared with -173 °C owing to thermal expansion. The bulk
b and c cell dimensions measured by PXRD at 0 °C are
essentially identical with the in-plane lattice constants deter-
mined for the multilayer by GIXD. The GIXD Bragg peaks at
qxy ) 1.530 Å-1 and qxy ) 1.375 Å-1 correspond to the (002)
and (011) reflections of bulk C6BP, respectively. The (001) and
(010) reflections, which have d spacings within the measured
Figure 10. (a) Experimental (black dots) and calculated (blue lines) Bragg
rod intensity profiles, I(qz), for (G)C16BPS:C6BP as refined with the rotated
shifted ribbon (Figure 6b) hydrogen bonding motif. The refined packing
arrangement of the (G)C16BPS:C6BP monolayers with the rotated shifted
ribbon motif as viewed down the (c) a and (d) b axes. The C6BP guest
molecules are depicted in red. The angles t and ψ are defined in Figure 6.
qxy range, are not allowed by the P21/c space-group symmetry.
The agreement of the PXRD and GIXD lattice parameters and
the proclivity of C6BP to organize as bilayers with a thin plate
crystal morphology are consistent with the formation of a C6BP
multilayer. Unfortunately, corroboration of this putative mul-
tilayer with atomic force microscopy (AFM) of LB films on
mica substrates was precluded by the low melting point of C6BP
(31 °C), near the operating temperature of the AFM.
Figure 11. Bragg peaks of C16BPS:C6BP on pure water at (a) 40 < Amol
<60 Å2/sulfonate, (b) Amol ) 30 Å2/sulfonate, (c) Amol ) 30 Å2/sulfonate,
after 7 min, and (d) Amol ) 23 Å2/sulfonate. The Bragg peaks are tentatively
assigned to the following phases: (solid circle) parent C16BPS:C6BP
monolayer; (solid square) an intermediate phase, possibly with two guests
alligned vertically end-to-end between the amphiphiles; (open square)
C16BPS monolayer; (solid triangle) C6BP multilayer; (open triangle)
monolayer beneath C6BP multilayer. The unit cell dimensions of the C6BP
multilayer are similar to those of bulk C6BP.
Tetracosane Intercalate. To examine the importance of
biphenyl-biphenyl host-guest interactions to monolayer struc-
ture and stability, tetracosane (C24), which has a length
comparable with C16BP but lacks the biphenyl group,
was examined as a guest molecule. The π-A isotherm of
C16BPS:C24 spread over pure water was similar to the isotherm
obtained over 0.01 M G2CO3, with Alift-off ) 50 Å2/sulfonate,
a transition to a second phase at 46 Å2/sulfonate, and a plateau
30 Å2/sulfonate suggest multiple phases, including the guest-
free C16BPS monolayer (Figure 3c) and two new phases with
unit cell dimensions differing slightly from the original C16BPS:
C6BP monolayer (each phase can be identified from the sets
of Bragg peaks with fwhm values corresponding to an equal
coherence length). The relative amounts of these phases, as
deduced from the Bragg peak intensities, change with time while
the surface pressure is held constant at 30 Å2/sulfonate. As the
monolayer is compressed further to <25 Å2/sulfonate, the Bragg
peaks associated with one of the new phases disappears.
(32) Single-crystal X-ray structure data for 4-hexylbiphenyl (5): formula C18H22;
formula weight 238.36; crystal system monoclinic; space group P21/c; a
) 31.558(2) Å; b ) 5.377(5) Å; c ) 7.943(5) Å; â ) 93.216(2); V )
1345.8(2) Å3, temperature 100(1) K; Z ) 4; R1(I > 2σ(I)) ) 0.1527; wR2-
(I > 2σ(I)) ) 0.3604; GOF ) 1.071.
9
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A R T I C L E S
Plaut et al.
and 700 Å, respectively. The refined structure for phase II is
consistent with packing of all-trans alkyl chains organized in a
parallel arrangement (Figure 14e,f). Like phase I, the scattering
intensity could be attributed solely to the close-packed alkyl
chains.
It also is reasonable to suggest that at higher surface pressures
the alkane guest can be expelled from the C16BPS monolayer.
The Bragg peaks for phase II at qxy ) 1.52 and 1.69 Å-1 at
Amol < 32 Å2/sulfonate are identical in position with those
produced by both a C16BPS monolayer alone (Figure 3) and
also with those reported for a crystalline C24 bilayer33 in the
presence of 10 mol % C28H57OH. The scattering pattern
produced by a heterobilayer composed of C24 overlayer on an
underlying C16BPS monolayer at the air-water interface thus
would consist of a combination of coinciding Bragg peaks
corresponding to the identical unit cell dimensions of the two
individual crystalline layers. The intensities of the Bragg peaks
for phase II are substantially larger (ca. 50× at Amol ) 32 Å2/
sulfonate) than those observed for the C16BPS monolayer alone
(Figure 15), indicating that phase II is more crystalline. Tapping
mode AFM of LB films of phase II, transferred to a mica
substrate at 50 mN/m, revealed the presence of a 28 Å thick
film. This is consistent with the existence of a mixed monolayer
rather than a heterobilayer, which would be expected to exhibit
a thickness of 60 Å. The 28 Å thickness is consistent with that
expected for a C16BPS monolayer with vertically aligned
chains. This is not unexpected, as the segments of the C24
molecules that protrude from the upper surface of the C16BPS
monolayer are most likely disordered such that the AFM tip
penetrates to the surface where the C16 and C24 alkyl chains
become close-packed.
These observations combined argue that the structure of this
mixed monolayer, for which the biphenyl group is absent in
the nonamphiphilic intercalate, is governed by alkane-alkane
interactions to produce a packing motif that is incompatible with
formation of the GS network. The absence of an ordered GS
sheet and the pressure-induced expulsion of the alkane guest
molecules illustrate that biphenyl-biphenyl interactions play
an important role in the formation and stabilization of inclusion
monolayers based on this host. This suggests that the achieve-
ment of a stable ternary inclusion monolayer relies on guests
that are homologous with the hydrophobe of the organosulfonate
amphiphile. The importance of the biphenyl-biphenyl interac-
tions are corroborated by GIXD measurements on monolayers
containing p-chlorohexadecylbiphenyl guest within the (G)-
C16BPS host, for which GIXD data revealed that the chloro
substituent excluded G from the air-water interface, apparently
to preserve the biphenyl-biphenyl interactions (see the Sup-
porting Information).
Figure 13. (a) π-A isotherms of C16BPS:C24 on pure water (black) and
on 0.01 M G2CO3 (red). GIXD contour plots of the scattered intensity
measured for C16BPS:C24 on pure water at (b) 50 < Amol < 70
Å2/sulfonate, where the crystalline phase is characterized by tilted alkyl
chains, (c) 40 Å2/sulfonate, where two phases are evident, and (d) 32 Å2/
sulfonate, corresponding to a second phase with alkyl chains that are oriented
normal to the air-water interface. The GIXD contour plots measured for
C16BPS:C24 on 0.01 M aqueous G2CO3 (not shown) are identical with
the patterns above at corresponding positions on the isotherm.
at ca. 30 Å2/sulfonate (Figure 13a). The transitions at 46 and
ca. 30 Å2/sulfonate occur at lower surface pressures over pure
water compared with 0.01 M G2CO3.
The GIXD patterns for C16BPS:C24 on pure water and 0.01
M G2CO3 were identical. At 50 < Amol < 70 Å2/sulfonate
(Figure 13b), these data support the presence of a single
crystalline phase (a ) 4.99 Å, b ) 9.04 Å, γ ) 90°, Acell
)
45.1 Å2) and alkyl chains tilted 29° from the normal along the
b axis (herein referred to as phase I). The C16BPS:C24 packing
arrangement was determined by refinement based on an asym-
metric unit containing C16BPS and C24, each constrained as
separate rigid bodies. Molecular models revealed that the shorter
a axis and longer b axis of phase I, relative to the aforemen-
tioned inclusion monolayers, promote dense packing of the host
and guest alkyl chains. The a and b lattice parameters of phase
I, however, are inconsistent with any reasonable in-plane
hydrogen bonding for a GS sheet like that observed in bulk
crystals and the aforementioned monolayers. The refined
structure was consistent with all-trans C24 and C16 alkyl chains
organized in a pseudo-herringbone arrangement (Figure 14c,d),
with the C24 chains elevated above the biphenyl groups and
protruding from the upper surface of the C16BPS layer. Notably,
the structural refinement indicated that the GIXD data could
be attributed solely to scattering from close-packed C24 and
C16 alkane chains, i.e., by ignoring scattering contributions from
the biphenyl and sulfonate moieties of the C16BPS amphiphiles.
This is consistent with disorder of the biphenylsulfonate moiety
of the amphiphile due to loose packing of the biphenyl groups
and the absence of an ordered GS network.
Summary
Crystalline ternary inclusion monolayers consisting of a two-
dimensional hydrogen-bonded host network of guanidinium (G)
ions and organosulfonate (S) amphiphiles, and biphenylalkane
guests, can be generated at the air-water interface through
synergistic structural enforcement by hydrogen bonding and
host-guest packing. The structure of these monolayer inclusion
compounds resembles the interdigitated bilayer structures of
The diffraction pattern collected at Amol ) 40 Å2/sulfonate
(Figure 13c), i.e., at a molecular area below the phase transition,
revealed the emergence of a second crystalline phase (herein
referred to as phase II) that coexists with phase I but has a
smaller unit cell (a ) 4.99 Å, b ) 7.43 Å, γ ) 90°, Acell
)
37.1 Å2) and alkyl chains perpendicular to the air-water
interface. Further compression to 32 Å2/sulfonate afforded a
GIXD pattern attributable to only phase II (Figure 13d). The
coherence lengths for the low- and high-pressure phases are 200
(33) Weinbach, S. P.; Weissbuch, I.; Kjaer, K.; Bouwman, W. G.; Nielsen, J.
A.; Lahav, M.; Leiserowitz, L. AdV. Mater. 1995, 7, 857.
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Crystalline Ternary Inclusion Compounds
A R T I C L E S
Figure 14. Experimental (black dots) and calculated (blue lines) Bragg rod intensity profiles, I(qz), for as refined for (a) phase I C16BPS:C24 at 50 < Amol
< 70 Å2/sulfonate and (b) phase II C16BPS:C24 at Amol < 32 Å2/sulfonate. The refined packing arrangement of the phase I and phase II as viewed down
the (c, e) a and (d, f) b axes, respectively. The C24 molecules are depicted in red. The angles t and ψ are defined in Figure 6.
Figure 15. GIXD surface plots of the scattered intensity measured for (a) the C16BPS:C24 monolayer on pure water and compressed to 40 Å2/sulfonate
and (b) C16BPS on pure water over all areas measured. These GIXD patterns display identical peak positions at qxy ) 1.52 and 1.70 Å-1, but the intensity
in (a) is ca. 50 times greater than that in (b).
(G)C1BPS and (G)C2BPS (Figure 2), with the nonamphiphilic
guests supplanting the organosulfonate residues of the opposing
GS sheet. Interestingly, attempts to crystallize bulk inclusion
compounds based on (G)C6BPS and (G)C10BPS with their
isomorphous guests were not successful, as only guest-free
compounds34 with a continuously interdigitated layer architec-
ture35 were obtained. This indicates that the topology of the
monolayer inclusion compounds, in which all the organosul-
fonate residues project from the upper side of the GS sheet, is
unique to the air-water interface. The ability to sustain ternary
host-guest phases with the GS network intact appears to require
guests that are structurally homologous, in the strictest sense,
with the organosulfonate substituents. It is important to note
that the absence of GIXD peaks due to separate monolayer
phases or multilayers of biphenylalkane guests with n > 6
proves that these guest molecules are included within the host
monolayer in a 1:1 stoichiometry, as expected from the
composition of the spreading solution. The observation of these
monolayer inclusion compounds suggests an approach for the
design and synthesis of functional monolayers in which non-
amphiphilic molecules adorned with desired functionality can
be anchored at the air-water interface through intercalation in
host inclusion cavities with well-defined metrics. Such an
approach offers the prospect of separating the function of the
monolayer (i.e., the guest) from the inherent structure of the
host, avoiding the requirement of designing functional molecules
that are also amphiphilic.
Experimental Section
Materials Synthesis and General Procedures. 4-Chlorobiphenyl
was purchased from TCI America and used as received. Tetrahydrofuran
(Aldrich) was distilled under argon over potassium and benzophenone
prior to use. All other solvents and starting materials were purchased
as ACS grade from Aldrich and were used directly as received.
Guandinium tetrafluoroborate, which was used for the synthesis of
guanidinium biphenylalkanesulfonate salts, was synthesized by com-
bining equimolar amounts of guanidinium carbonate and tetrafluroboric
acid in acetone, followed by filtration to yield (G)BF4 as the precipitate.
All reactions were performed under a nitrogen or argon atmosphere.
1H NMR spectra were recorded on a Varian Inova 500 spectrometer.
Mass spectrometry (MS) was performed with a high-resolution Finnigan
MAT 95 instrument. Grazing angle infrared spectra of Langmuir-
Blodgett films were recorded with a Nicolet Magna 550 FT-IR equipped
with a Harrick Seagull reflectance accessory and an MCT detector, at
an incident angle of 86° with unpolarized light and 4 cm-1 spectral
resolution. The LB films for FT-IR studies were prepared by transfer
(34) Single-crystal X-ray structure data for (G)(4′-hexylbiphenyl-4-sulfonate):
formula C19H27N3O3S; formula weight 377.51; crystal system orthorhombic;
space group P212121; a ) 7.4862(11) Å; b ) 7.6555(12) Å; c ) 33.449(5)
Å; V ) 1917.0(5) Å3, temperature 173(2) K; Z ) 4; R1(I > 2σ(I)) ) 0.0420;
wR2(I > 2σ(I)) ) 0.1112; GOF ) 1.105. Single-crystal X-ray structure
data for (G)(4′-decylbiphenyl-4-sulfonate): formula C23H35N3O3S; formula
weight 433.61; crystal system monoclinic; space group P21/c; a ) 7.668-
(9) Å; b ) 7.923(8) Å; c ) 38.321(3) Å; â ) 92.89(3); V ) 2325.7(0) Å3,
temperature 173(2) K; Z ) 4; R1(I > 2σ(I)) ) 0.1110; wR2(I > 2σ(I)) )
0.2138; GOF ) 1.095.
(35) Horner, M. J.; Holman, K. T.; Ward, M. D. Angew. Chem., Int. Ed. 2001,
40, 4045.
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A R T I C L E S
Plaut et al.
of Langmuir monolayers to gold-coated glass microscope slides. Atomic
force microscopy (AFM) was performed in tapping mode with a
Nanoscope IIIa Multimode system (Digital Instruments, Santa Barbara,
CA), a scanner with a maximum 15 µm scan range, and a 90 µm Si
cantilever tip (MikroMasch, Tallinn, Estonia) with an Al-coated
backside, force constant of 14 N/m, and resonant frequency of 270
kHz. All data were acquired in height mode with a scan rate of 2 Hz.
The LB films for AFM measurements were prepared by transfer of the
Langmuir monolayers to a mica substrate (Ted Pella Inc.). Only of the
mica substrate was coated, so that the film thickness could be measured
relative to the bare mica surface.
of p-dibromobenzene and 0.17 g of NiCl2(dppf) (0.21 mmol) in 25
mL of tetrahydrofuran at 4 °C. The resulting reaction mixture was stirred
for 1 h at 4 °C, warmed to room temperature, and heated at reflux
overnight. The reaction mixture was quenched in 50 mL of water,
acidified with an aqueous HCl solution to pH 1, and extracted twice
with 20 mL of methylene chloride. The combined organic layers were
dried over magnesium sulfate and filtered, and the solvent was removed
under reduced pressure. The product was purified from by flash
chromatography with hexane to yield the product as a white powder
1
after removal of the hexane solvent: total yield 6.9 g, 96%. H NMR
(CD2Cl2, 500 MHz): δ 7.40 ppm (d, 2H, Ar H), 7.08 (d, 2H, Ar H),
2.57 (t, 2H, CH2-1′), 1.58 (m, 2H, CH2-2′), 1.27 (m, 26H, CH2), 0.89
(t, 3H, CH3).
4-n-Hexadecylbiphenyl (C16BP). Hexadecylmagnesium bromide,
prepared from 5.18 g (0.22 mol) of magnesium turnings and 40 mL
(0.13 mol) of n-hexadecyl bromide in 250 mL of tetrahydrofuran, was
added dropwise via cannula to a stirred tetrahydrofuran solution of 27.16
g (0.13 mol) of 4-chlorobiphenyl and 138 mg (0.26 mmol) of NiCl2-
(dppe) in 500 mL of tetrahydrofuran at 4 °C. The hexadecylmagnesium
bromide solution was transferred hot in order to prevent precipitation
in the cannula. The resulting yellow-green reaction mixture was stirred
for 2 h at 4 °C, warmed to room temperature over 3 h, and heated at
reflux overnight. The reaction mixture was quenched in 500 mL of
water, acidified to pH 1 with concentrated aqueous HCl (12 M), and
extracted twice with 200 mL of methylene chloride. The combined
organic layers were dried over magnesium sulfate and filtered, and the
solvent was removed under reduced pressure. The resulting orange solid
was passed through a plug of silica gel to remove the colored impurities
associated with the NiCl2(dppe) catalyst. The product, which is a solid
at room temperature but melts at 66 °C, was isolated from the reaction
mixture by fractional distillation to yield a white powder that was
sufficiently pure and did not require recrystallization (total yield 22 g,
4′-Chloro-4-n-hexadecylbiphenyl (C16BP-Cl). Using a procedure
identical with that for 1-bromo-4-hexadecylbenzene, C16BP-Cl was
prepared by the addition of 1-(magnesium bromide)-4-hexadecylben-
zene to an equimolar amount of 1-bromo-4-chlorobenzene in tetrahy-
drofuran. The product was purified by flash chromatography with
hexane to yield the product as a white powder after removal of the
hexane solvent: total yield 0.95 g, 77%; mp 75 °C. 1H NMR (CD2Cl2,
500 MHz): δ 7.56 ppm (d, 2H, Ar H), 7.50 (d, 2H, Ar H), 7.42 (d,
2H, Ar H), 7.28 (d, 2H, Ar H), 2.66 (t, 2H, CH2-1′), 1.65 (m, 2H,
CH2-2′), 1.28 (m, 26H, CH2), 0.90 (t, 3H, CH3). MS: m/z 412.29. Anal.
Calcd for C28H41Cl: C, 81.41; H, 10.01; Cl, 8.58. Found: C, 81.71;
H, 9.86; Cl, 8.63.
4′-n-Hexadecylbiphenyl-4-sulfonic acid (C16BPS). Chlorosulfonic
acid 0.97 mL (14.5 mmol) was added dropwise to a stirred solution of
5 g (13 mmol) of 4-n-hexadecylbiphenyl in 125 mL of chloroform.
The mixture was stirred under nitrogen for 5 h, after which the solvent
was removed under reduced pressure. The product was obtained as a
white powder upon recrystallization from acetonitrile: total yield 6 g,
1
45%): mp 66 °C, bp 220/4 mmHg. H NMR (CD2Cl2, 500 MHz): δ
1
7.61 ppm (d, 2H, Ar H), 7.53 (d, 2H, Ar H), 7.44 (t, 2H, Ar H), 7.34
(t, 1H, Ar H), 7.28 (d, 2H, Ar H), 2.66 (t, 2H, CH2-1′), 1.65 (m, 2H,
CH2-2′), 1.28 (m, 26H, CH2), 0.90 (t, 3H, CH3). MS: m/z 378.32. Anal.
Calcd for C28H42: C, 88.82; H, 11.18. Found: C, 88.77; H, 11.04.
4-n-Decylbiphenyl (C10BP). Using a procedure analogous to that
for C16BP, 5 mL of n-decyl bromide was converted into 4-n-
decylbiphenyl. The product was purified by fractional distillation and
recrystallized from hexane to obtain a white powder: total yield 3.9 g,
90%. H NMR (DMSO, 500 MHz): δ 7.63 (d, 2H, Ar H), 7.57 (d,
2H, Ar H), 7.56 (d, 2H, Ar H), 7.26 (d, 2H, Ar H), 2.59 (t, 2H, CH2-
1′), 1.5 (m, 2H, CH2-2′), 1.22 (m, 26H), 0.84 (t, 3H, CH3). Anal. Calcd
for C28H42O3S: C, 71.32; H, 9.23; S, 6.99. Found: C, 71.13; H, 9.50;
S, 6.52.
Guanidinium 4′-Decylbiphenyl-4-sulfonate ((G)C10BPS). Using
a procedure similar to that employed for C16BPS, 4′-n-decylbiphenyl-
4-sulfonic acid was prepared from 4-decylbiphenyl. The guanidinium
salt was prepared by treatment of the sulfonic acid with a saturated
acetone solution of guanidinium tetrafluoroborate (roughly a 5-fold
molar excess of guanidinium), which afforded the product as a white
precipitate that was isolated by filtration and recrystallized from
1
55%; mp 55 °C, bp 210 °C/2 mmHg. H NMR (CD2Cl2, 500 MHz):
δ 7.61 ppm (d, 2H, Ar H), 7.53 (d, 2H, Ar H), 7.44 (t, 2H, Ar H), 7.34
(t, 1H, Ar H), 7.28 (d, 2H, Ar H), 2.66 (t, 2H, CH2-1′), 1.64 (m, 2H,
CH2-2′), 1.29 (m, 14H, CH2), 0.90 (t, 3H, CH3). Anal. Calcd for
C22H30: C, 89.73; H, 10.27. Found: C, 88.57; H, 10.28.
1
methanol as colorless needles: mp 177 °C. H NMR (DMSO, 500
4-n-Hexylbiphenyl (C6BP). Using a procedure analogous to that
for C16BP, 8.2 mL of n-hexyl bromide was converted into 4-n-
hexylbiphenyl. The product was purified by fractional distillation and
recrystallized from methanol to obtain extremely thin plates with crystal
dimensions 0.06 mm × 0.03 mm × 0.002 mm: total yield 5 g, 40%;
mp 31 °C, bp 150 °C/3 mmHg. 1H NMR (CD2Cl2, 500 MHz): δ 7.63
ppm (d, 2H, Ar H), 7.56 (d, 2H, Ar H), 7.46 (t, 2H, Ar H), 7.36 (t, 1H,
Ar H), 7.30 (d, 2H, Ar H), 2.68 (t, 2H, CH2-1′), 1.68 (m, 2H, CH2-2′),
1.37 (m, 6H, CH2), 0.94 (t, 3H, CH3). Anal. Calcd for C18H22: C, 90.70;
H, 9.30. Found: C, 90.79; H, 9.22.
MHz): δ 7.68 (d, 2H, Ar H), 7.62 (d, 2H, Ar H), 7.61 (d, 2H, Ar H),
7.31 (d, 2H, Ar H), 6.95 (s, 6H, (NH2)3), 2.63 (m, 2H, CH2), 1.61 (m,
2H, CH2), 1.27 (m, 14H, CH2), 0.82 (t, 3H, CH3).
Guanidinium 4′-Hexylbiphenyl-4-sulfonate ((G)C6BPS). Using
a procedure similar to that employed for C16BPS, 4′-n-hexylbiphenyl-
4-sulfonic acid was prepared from 4-hexylbiphenyl. The guanidinium
salt was prepared by treatment of the sulfonic acid with a saturated
acetone solution of guanidinium tetrafluoroborate (roughly a 5-fold
molar excess of guanidinium), which afforded the product as a white
precipitate that was isolated by filtration and recrystallized from
1
4-n-Butylbiphenyl (C4BP). Using a procedure analogous to that
for C16BP, 6.26 mL of n-butyl bromide was converted into 4-n-
butylbiphenyl. The product was purified by fractional distillation to
methanol as colorless needles: mp 244 °C. H NMR (DMSO, 500
MHz): δ 7.64 (d, 2H, Ar H), 7.58 (d, 2H, Ar H), 7.56 (d, 2H, Ar H),
7.26 (d, 2H, Ar H), 6.91 (s, 6H, (NH2)3), 2.59 (m, 2H, CH2), 1.57 (m,
2H, CH2), 0.85 (t, 3H, CH3).
1
obtain a clear liquid: total yield 4 g, 33%; bp 140 °C/3 mmHg. H
NMR (CD2Cl2, 500 MHz): δ 7.62 ppm (d, 2H, Ar H), 7.54 (d, 2H, Ar
H), 7.45 (t, 2H, Ar H), 7.35 (t, 1H, Ar H), 7.29 (d, 2H, Ar H), 2.68 (t,
2H, CH2-1′), 1.66 (m, 2H, CH2-2′), 1.41 (m, 2H, CH2), 0.98 (t, 3H,
CH3).
1-Bromo-4-hexadecylbenzene. A hot solution of n-hexadecylmag-
nesium bromide, prepared from 0.84 g of magnesium turnings and 7.13
mL (23.1 mmol) of n-bromohexadecane in 25 mL of tetrahydrofuran,
was added dropwise via cannula to a stirred solution of 5 g (21 mmol)
Guanidinium 4′-Ethylbiphenyl-4-sulfonate ((G)C2BPS). Using a
procedure similar to that employed for C16BPS, 4′-n-ethylbiphenyl-
4-sulfonic acid was prepared from 4-ethylbiphenyl. The guanidinium
salt was prepared by treatment of the sulfonic acid with a saturated
acetone solution of guanidinium tetrafluoroborate (roughly a 5-fold
molar excess of guanidinium), which afforded the product as a white
precipitate that was isolated by filtration and recrystallized from
1
methanol as colorless needles: mp 296 °C. H NMR (DMSO, 500
9
15932 J. AM. CHEM. SOC. VOL. 125, NO. 51, 2003

Crystalline Ternary Inclusion Compounds
A R T I C L E S
MHz): δ 7.63 (d, 2H, Ar H), 7.57 (d, 2H, Ar H), 7.56 (d, 2H, Ar H),
7.28 (d, 2H, Ar H), 6.90 (s, 6H, (NH2)3), 2.62 (q, 2H, CH2), 1.86 (t,
3H, CH3).
structural refinements based on these models and the GIXD data were
performed using SHELX-97.38 Although this software package is
designed for bulk single crystals, refinement of a 2-D structure can be
achieved with the following procedure.39,40 The continuous Bragg rod
Guanidinium 4′-Methylbiphenyl-4-sulfonate ((G)C1BPS). Using
a procedure similar to that employed for C16BPS, 4′-n-methylbiphenyl-
4-sulfonic acid was prepared from 4-methylbiphenyl. The guanidinium
salt was prepared by treatment of the sulfonic acid with a saturated
acetone solution of guanidinium tetrafluoroborate (roughly a 5-fold
molar excess of guanidinium), which afforded the product as a white
precipitate that was isolated by filtration and recrystallized from
2
intensities distributed along qz, |F2(h,k,qz)| , were transformed into
2
discrete reflection data sets, |F(h,k,l)| , wherein a particular Miller index
l is associated with a virtual c axis calculated as c ) 2π/∆q, where qz
2
is sampled in steps of -∆qz. The superposition of the |F(h,k,qz)| and
2
|F(-h,-k,qz)| Bragg rods, a property of the measured GIXD powder
pattern, was simulated in SHELX-97 by imposing a twinning of the
crystal about the ab plane. Because of the limited amount of diffraction
data, refinement could only be performed with the molecular constitu-
ents constrained as rigid bodies, fixed within the measured unit cell
dimensions. Following the refinement, the calculated structure factor
|F2(h,k,l)| was converted to the calculated Bragg rod intensity profile
1
methanol as colorless needles: mp 294 °C. H NMR (DMSO, 500
MHz): δ 7.65 (d, 2H, Ar H), 7.59 (d, 2H, Ar H), 7.55 (d, 2H, Ar H),
7.26 (d, 2H, Ar H), 6.94 (s, 6H, (NH2)3), 2.33 (t, 3H, CH3).
Surface Pressure-Area (π-A) Isotherms and Langmuir Blodgett
(LB) Films. Langmuir isotherms were recorded on a Nima Langmuir
trough (Model No. 611) (trough area 10 cm × 30 cm) at a compression
rate of 5 mm/min and a subphase temperature of 20.0 ( 0.1 °C. The
aqueous subphase was prepared with deionized water (purified to 17.9
MΩ cm with a Barnstead water purifier) and the selected solute, if
any. Surface pressure was recorded with a Wilhelmy balance and a
paper plate. All monolayers were prepared by the introduction of
chloroform solutions containing 1% methanol (v/v), C16BPS (0.4-
0.8 mg/mL), and an equimolar amount of CnBP, C16BP-Cl, or C24
(if any). Before isotherms were collected, monolayers formed from
mixed spreading solutions were annealed by compressing the barriers
until a surface pressure of 1 mN/m was achieved and then expanding
the barriers to their initial position.
2
|F(h,k,qz)| , which was then compared with the experimental data. This
comparison allows direct visual inference of the quality of the refined
structure.
Single-Crystal X-ray Structural Analysis. Experimental parameters
pertaining to the single-crystal X-ray analyses of the various guani-
dinium 4′-alkylbiphenyl-4-sulfonates are provided in the references.
CIF files are included as Supporting Information. Data were collected
at the X-ray Crystallographic Laboratory in the Department of
Chemistry at the University of Minnesota with a Bruker CCD platform
diffractometer with graphite-monochromated Mo KR radiation (λ )
0.710 73 Å) at 173(2) K. The structures were solved by direct methods
and refined with full-matrix least-squares/difference Fourier analysis
using the SHELX-97 suite of software. All non-hydrogen atoms were
refined with anisotropic displacement parameters, and all hydrogen
atoms were placed in idealized positions and refined with a riding
model. Data were corrected for the effects of adsorption using
SADABS. Single-crystal structural analysis of 4-hexylbiphenyl crystals,
grown from methanol as very thin plates (0.06 × 0.03 × 0.002 mm;
see above), was performed by Dr. Victor Young using the APS
synchrotron at sector 15-C with a frame time of 3 s and a detector
distance of 4.9 cm with the Bruker Kappa/SMART 6000 microdif-
fractometer and λ ) 0.5594 Å at 100(1) K. A randomly oriented region
of reciprocal space was surveyed to the extent of 2.0 hemispheres and
to a resolution of 0.84 Å with ψ scans. The intensity data were corrected
for absorption and decay (SADABS41). Final cell constants were
calculated from 3155 reflections from the actual data collection after
Grazing Incidence X-ray Diffraction. GIXD for the monolayers
was performed using the liquid surface diffractometer on the undulator
beam line BW1 in HASYLAB at DESY (Hamburg, Germany).36
A
sealed and temperature-controlled Langmuir trough (4 °C), equipped
with a Wilhelmy balance for surface pressure measurement of the films,
was mounted on the diffractometer. Monochromatic synchrotron
radiation (λ ) 1.3037 Å) was produced by Bragg reflection from a Be
crystal. Surface sensitivity was optimized by adjustment of the incident
grazing angle of the beam, with respect to the monolayer surface, to
Ri ) 0.85Rc (where Rc ) 0.138°, the critical angle for total external
reflection), which creates an evanescent wave with a minimal penetra-
tion depth and reduces scattering due to the subphase. The footprint of
the incident X-ray beam at the air-water interface was 5 mm × 50
mm, and the diffracted radiation was recorded with a one-dimensional
position-sensitive detector (PSD), which resolved the vertical component
of the X-ray scattering vector, qz = (2π/λ) sin Rf (where Rf is the angle
between the diffracted beam and the horizon), over the range 0.0 e qz
e 1.4 Å-1. The PSD was mounted vertically behind a horizontally
collimating Soller slit with a horizontal resolution width ∆(qxy) ) 0.007
Å-1. The GIXD patterns were measured by scanning over a range of
the horizontal component of the X-ray scattering vector, qxy = (4π/λ)
sin θxy, where 2θxy is the horizontal scattering angle between the beam
and the Soller collimator. The GIXD diffraction data can be presented
in three ways: (1) as a two-dimensional intensity distribution I(qxy,qz)
in a surface or contour plot, (2) as Bragg peaks I(qxy) obtained by
measuring the scattered intensity over the range of qxy and integrating
over the entire qz range of the PSD, and (3) as Bragg rod intensity
profiles I(qz) obtained from the scattered intensity recorded in channels
along the PSD and integrated over the qxy range corresponding to a
specific Bragg peak.
integration (SAINT42). The structure was solved using SIR-97.43
A
direct-methods solution was calculated, which provided most non-
hydrogen atoms from the E map. Full-matrix least-squares/difference
Fourier cycles were performed, which located the remaining non-
hydrogen atoms. All non-hydrogen atoms were refined with isotropic
displacement parameters. All hydrogen atoms were placed in ideal
positions and refined as riding atoms with relative isotropic displacement
parameters.
Acknowledgment. This work was supported by the IHP-
Contract HPRI-CT-1999-00040/2001-00140 of the European
Commission, the United States-Israel Binational Science
Foundation (BSF), the Danish Foundation for Natural Science,
and the National Science Foundation (DMR-0305278). We
thank HASYLAB, DESY, Hamburg, Germany, for beam time
Molecular models of 2-D crystalline monolayers were constructed
using the CERIUS2 or Molecular Studio computational packages37 and
previously determined bulk crystal structures as a guide. Least-squares
(38) Sheldrick, G. M. SHELX-97 Program for Crystal Struture Determination;
University of Go¨ttingen, Go¨ttingen, Germany, 1997.
(39) Weissbuch, I.; Baxter, P. N. W.; Kuzmenko, I.; Cohen, H.; Cohen, S.; Kjaer,
K.; Howes, P. B.; Als-Nielsen, J.; Lehn, J. M.; Leiserowitz, L.; Lahav, M.
Chem. Eur. J. 2000, 6, 725.
(36) Jensen, T. R.; Kjaer, K. Structural Properties and Interactions of Thin Films
at the Air-Liquid Interface Explored by Synchrotron X-ray Scattering. In
NoVel Methods to Study Interfacial Layers; Moebius, D., Miller, R., Eds.;
Studies in Interface Science 11; Elsevier Science: Amsterdam, 2001; pp
205-254.
(40) Rapaport, H. Kuzmenko, I.; Lafont, S. Kjaer, K.; Howes, P. B.; Als-Nielsen,
J.; Lahav, M.; Leiserowitz, L. Biophys. J. 2001, 81, 2729.
(41) Blessing, R. Acta Crystallogr. 1995, A51, 33.
(42) SAINT V6.1, Bruker Analytical X-Ray Systems, Madison, WI.
(43) SIR-97: Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. (1997).
(37) CERIUS2 (IRIX platform) and Materials Studio (Windows platform)
computational packages, Accelrys, San Diego, CA.
9
J. AM. CHEM. SOC. VOL. 125, NO. 51, 2003 15933

A R T I C L E S
Plaut et al.
for GIXD experiments, Victor G. Young, Jr., and the X-ray
Crystallographic Laboratory in the Department of Chemistry
at the University of Minnesota for assistance with single-crystal
X-ray structural analysis, and ChemMatCARS at Argonne
National Laboratory for beam time made available for the
collection of the C6BP crystal data. ChemMatCARS Sector 15
is supported principally by the National Science Foundation/
Department of Energy under Grant No. CHE0087817 and by
the Illinois Board of Higher Education. The Advanced Photon
Source is supported by the U.S. Department of Energy, Basic
Energy Sciences, Office of Science, under Contract No. W-31-
109-Eng-38.
Supporting Information Available: Complete GIXD data for
all crystalline Langmuir monolayers, summary of GIXD data
for (G)C16BPS:p-chlorohexadecylbiphenyl, measured and cal-
culated X-ray powder diffraction data for C6BP, unit cell index
and refinement report for C6BP, GIXD analysis for C16BPS:
C16BP-Cl on water, atomic force microscopy data for the
C16BPS:C24 LB film (Phase II), and single-crystal X-ray
diffraction data (CIF format) for all reported bulk crystals. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA0371404
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15934 J. AM. CHEM. SOC. VOL. 125, NO. 51, 2003