Contrasting two- and three-dimensional crystal properties of isomeric dialkyl phthalates

Article abstract of DOI:10.1021/ja0744798

Competitive adsorption occurs whenever a solution containing multiple components is in contact with a surface, and the relative adsorption strengths affect both the properties of the interface and the solution. To investigate this phenomenon, the factors

Full text of DOI:10.1021/ja0744798

Published on Web 11/15/2007
Contrasting Two- and Three-Dimensional Crystal Properties of
Isomeric Dialkyl Phthalates
Katherine E. Plass, Keary M. Engle, and Adam J. Matzger*
Contribution from the Department of Chemistry and the Macromolecular Science and
Engineering Program, UniVersity of Michigan, Ann Arbor, Michigan 48109-1055
Received June 19, 2007; E-mail: matzger@umich.edu
Abstract: Competitive adsorption occurs whenever a solution containing multiple components is in contact
with a surface, and the relative adsorption strengths affect both the properties of the interface and the
solution. To investigate this phenomenon, the factors influencing relative adsorption strengths of a series
of isomeric dialkyl phthalates were determined from their competitive adsorption behavior and interpreted
in the context of the thermodynamic properties of the bulk crystals and the structural features of the
monolayers. The order of stabilities of the two-dimensional crystals paralleled that of the three-dimensional
crystals, as determined by the trends in melting point, enthalpy of melting, and solubility. The magnitude
of the stability differences, however, could only be understood through examination of the structures of the
monolayers. Thus, the important process of adsorption of solutes from solution must be viewed as a
microscopic phenomenon; adsorption strengths are influenced not only by molecular and macroscopic
properties but also by the distinct assembly mode of the adsorbed layer.
Introduction
dicarbamates with an even number of methylene groups
separating the functional groups had higher melting points and
Whenever solutions containing multiple solutes are in contact
with a surface, as is the case in most industrial, biological, and
environmental processes, competitive adsorption takes place.
When these solutes adsorb into ordered monolayers, they can
be directly imaged at the liquid/solid interface on metal, mineral,
and carbon substrates.1-4 The monolayers that result at the solid
interface from a mixed solution either consist of one preferen-
tially adsorbed component that completely displaces the others
or consist of different coexisting components. Which component
is preferentially adsorbed depends upon the strength of the
intermolecular interactions between the adsorbed molecules and
the interactions of these molecules with the surface, as well as
the relative amounts of each component and their concentrations
in solution. The composition of the adsorbed film affects its
properties as well as those of the solution when preferential
depletion occurs; therefore, the prediction of competitive
adsorption behavior based on easily obtainable bulk crystal
properties of the components is an appealing approach that could
prove valuable in designing purification and surface-coating
processes, particularly where in-depth characterization of ad-
sorption processes is not feasible. The viability of such an
approach was suggested by the parallels between the two- and
three-dimensional crystals of a series of dicarbamate molecules,5
where both systems exhibited an odd-even effect; those
greater adsorption strengths than similarly sized compounds with
an odd number of methylenes. Comparisons of two-dimensional
and three-dimensional crystal properties have been few and are
primarily limited to structural comparisons. Here we present a
detailed comparison of two- and three-dimensional crystal
stabilities which is essential to understanding adsorption pro-
cesses at a molecular level and establishing a unified framework
for self-assembly phenomena across dimensions.
A particularly challenging situation, for which the outcome
is important in a variety of industrial processes, is the competi-
tive adsorption of positional isomers. These compounds have
the same elemental constitution but vary, for example, in the
substitution about an aromatic ring. Such isomers are produced
as a mixture through reactions such as Friedel-Crafts alkylation
or acylation and are found as components in petroleum products.
The competitive adsorption behavior determines whether these
isomers can be separated via chromatographic methods based
on their differing affinities for a solid stationary phase.
Examination of the competitive adsorption of disubstituted
benzene isomers not only offers an opportunity to examine the
effect of geometry on two-dimensional crystal packing stability
controlled for the effect of molecular size and functionality, but
also enables comparison between the relative adsorption strengths
and the well-established melting point trend in this class of
materials. For the disubstituted benzenes in general, the para
compounds melt at higher temperature than the meta and ortho
compounds.6-8
(1) Giancarlo, L. C.; Fang, H. B.; Rubin, S. M.; Bront, A. A.; Flynn, G. W. J.
Phys. Chem. B 1998, 102, 10255-10263.
(2) Uemura, S.; Samor´ı, P.; Kunitake, M.; Hirayama, C.; Rabe, J. P. J. Mater.
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(3) Perronet, K.; Charra, F. Surf. Sci. 2004, 551, 213-218.
(4) Kaneda, Y.; Stawasz, M. E.; Sampson, D. L.; Parkinson, B. A. Langmuir
2001, 17, 6185-6195.
(6) Holler, A. C. J. Org. Chem. 1948, 13, 70-74.
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(8) Pinal, R. Org. Biomol. Chem. 2004, 2, 2692-2699.
(5) Kim, K.; Plass, K. E.; Matzger, A. J. J. Am. Chem. Soc. 2005, 127, 4879-
4887.
9
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J. AM. CHEM. SOC. 2007, 129, 15211-15217
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A R T I C L E S
Plass et al.
Table 1. Two-Dimensional Crystal Structure Parameters, Both
Observed by STM and Calculated by Computational Modeling, for
All Observed Phases
experimental
computed
plane
group
a
(nm)
b
(nm)
R
(deg)
a
b
R
(deg)
molecule
Z
′
(nm) (nm)
17-meta
0.5 cm
6.1(1) 0.84(1) 90(3) 5.99 0.88 90.0
18-meta
17-ortho
18-ortho
17-para
18-para phase I 0.5 p2
18-para phase II 1.0 p2
0.5 p2mg 6.7(2) 0.80(5) 87(3) 6.25 0.88 90.0
1.0 p2
1.0 p2
3.1(2) 1.9(2) 110(5) 2.86 1.78 100.4
3.2(2) 2.0(2)
98(5) 2.95 1.77 98.2
99(5) 5.90 0.88 89.9
86(3) 3.09 0.88 87.7
89(2) 2.42 2.34 83.2
0.5 p2gg 5.6(1) 0.9(1)
3.3(2) 0.9(1)
2.4(2) 2.4(3)
etry and the length of the alkyl chains (Table 1). The patterns
adopted by meta- and para-substituted molecules (Figures 2 and
3) are related; both are close-packed, interdigitated structures
wherein molecules adopt a flat conformation retaining the
maximal molecular symmetry.16-18 Thus, meta isomers sit on
mirror planes, in the plane groups cm or p2mg, while para
isomers sit on two-fold rotation axes in p2 or p2gg (Table 1).
Such retention of two-fold molecular symmetry is very common
in monolayers formed at the liquid/graphite interface, but
retention of molecular mirror planes is not.18 In fact, mirror
planes are not generally incorporated into two-dimensional
crystal symmetry in any case; thus, the meta isomers represent
an unusual class of compounds that form symmetric, achiral
monolayers. Selection between the two possible plane groups
for a given substitution position is based on the parity of the
number of carbons in the alkyl chainsan “odd/even” ef-
fect.5,16,19,20 In the meta series, the odd isomer forms in cm,
while the even isomer forms in p2mg. This is a result of a two-
fold rotation between adjacent columns that allows the densest
packing. The extra methylene group present in each chain in
the even isomers would induce either steric crowding or empty
space if they adopted the same packing motif as the odd isomers,
depending on the respective shift of the methylenes. The flipping
of adjacent columns in the even isomers structure enables a
packing density comparable to that of the odd isomer structure.
Evidence for this arrangement is seen in the contrast difference
between adjacent columns of aromatic rings observed for 18-
meta monolayers, but not for 17-meta monolayers.16,17 The same
phenomenon is seen in the para isomers, but it is the odd isomer
Figure 1. Compounds belonging to both isomeric series for which two-
and three-dimensional crystal stabilities were compared and two-dimensional
crystal structures determined. The isomers of diheptadecyl phthalate, which
have 17 carbons in each alkyl chain, are termed the “odd isomers”, and the
isomers of dioctadecyl phthalate, which have one more methylene in each
alkyl chain, are the “even isomers”.
The adsorption properties of a series of alkyl-substituted
phthalate isomers (Figure 1) were examined, and the results were
interpreted by comparison of their bulk properties and monolayer
structural features. The self-assembly of these compounds has
implications for their use as surfactants, lubricants, and polymer
additives. This class of molecules is also a valuable model for
understanding various alkyl phthalate-based polyesters. The
monolayer structures of these compounds at the solution-graphite
interface were examined, as was the competitive adsorption
between ortho, meta, and para isomers. Graphite is a particularly
relevant substrate because it is a model for important carbon-
aceous adsorbents, such as activated carbon and carbon black.
Furthermore, the solution-graphite interface is an excellent
system for studying competitive adsorption processes because
the molecules that make up the physisorbed monolayer are in
dynamic equilibrium with molecules in solution and the struc-
ture of the adsorbed phase can be examined with high-resolu-
tion using scanning tunneling microscopy (STM). The stabilities
of the two-dimensional crystals of these compounds are
determined by competitive adsorption experiments. In order to
establish a relationship with three-dimensional crystal prop-
erties, the melting points, the change in enthalpy upon melting,
the change in entropy upon melting, and solubilities were
compared among isomers and with the two-dimensional crystal
data.
(9) Lackinger, M.; Griessl, S.; Markert, T.; Jamitzky, F.; Heckl, W. M. J. Phys.
Chem. B 2004, 108, 13652-13655.
(10) Gomar-Nadal, E.; Abdel-Mottaleb, M. M. S.; De Feyter, S.; Veciana, J.;
Rovira, C.; Amabilino, D. B.; De Schryver, F. C. Chem. Commun. 2003,
906-907.
(11) Feng, C. L.; Zhang, Y. J.; Jin, J.; Song, Y. L.; Xie, L. Y.; Qu, G. R.; Jiang,
L.; Zhu, D. B. Surf. Sci. 2002, 513, 111-118.
Results and Discussion
(12) Grim, P. C. M.; Vanoppen, P.; Ru¨cker, M.; De Feyter, S.; Valiyaveettil,
S.; Moessner, G.; Mu¨llen, K.; De Schryver, F. C. J. Vac. Sci. Technol. B
1997, 15, 1419-1424.
The surface-adsorbed and bulk crystalline properties of the
ortho, meta, and para isomers of dioctadecyl phthalate (termed
the even isomers: 18-ortho, 18-meta, and 18-para) and dihep-
tadecyl phthalate (termed the odd isomers: 17-ortho, 17-meta,
and 17-para), were examined (Figure 1). These names describe
the number of carbons in each alkyl chain and the ring-
substitution pattern.
Two-Dimensional Crystal Structures. The variation in the
two-dimensional structures formed by isomers offers an oppor-
tunity to explore the effects of molecular symmetry and structure
on packing.9-15 In the dialkyl phthalate isomers examined here,
the packing motif depends both on the ring-substitution geom-
(13) Vanoppen, P.; Grim, P. C. M.; Ru¨cker, M.; De Feyter, S.; Moessner, G.;
Valiyaveettil, S.; Mu¨llen, K.; De Schryver, F. C. J. Phys. Chem. 1996,
100, 19636-19641.
(14) Stevens, F.; Dyer, D. J.; Walba, D. M. Langmuir 1996, 12, 436-440.
(15) Meier, C.; Ziener, U.; Landfester, K.; Weihrich, P. J. Phys. Chem. B 2005,
109, 21015-21027.
(16) Plass, K. E.; Kim, K.; Matzger, A. J. J. Am. Chem. Soc. 2004, 126, 9042-
9053.
(17) Plass, K. E.; Engle, K. M.; Cychosz, K. A.; Matzger, A. J. Nano Lett.
2006, 6, 1178-1183.
(18) See Plass, K. E.; Grzesiak, A. L.; Matzger, A. J. Acc. Chem. Res. 2007,
40, 287-293 for a discussion of the relationships between molecular and
monolayer symmetry in two-dimensional crystals and a comparison between
two- and three-dimensional crystal structures.
(19) Tao, F.; Bernasek, S. L. Chem. ReV. 2007, 107, 1408-1453.
(20) Wei, Y.; Kannappan, K.; Flynn, G. W.; Zimmt, M. B. J. Am. Chem. Soc.
2004, 126, 5318-5322.
9
15212 J. AM. CHEM. SOC. VOL. 129, NO. 49, 2007

Crystal Properties of Isomeric Dialkyl Phthalates
A R T I C L E S
Figure 2. Two-dimensional crystals of the meta isomers, 17-meta (A,B) and 18-meta (C,D). (A,C) STM images with overlaid molecular models and (B,D)
computed packing patterns with unit cell. These two packing motifs, formed by molecules that have a one methylene difference in both the attached alkyl
chains, are distinguished by the relative orientation of adjacent columns.
that exhibits differently oriented adjacent columns, adopting a
p2gg symmetry rather than p2 (Table 1). The difference in ring-
substitution geometry allows a slightly denser packing for the
para isomers than for the meta, as indicated by the computational
models. Both sets of structures can be understood on the basis
of the tendency toward close-packing, given that the lowest-
energy conformation for these molecules is an all-trans carbon
backbone with the ester functionality at 0° torsion angle with
respect to the benzene ring.16
In addition to this packing motif, 18-para has a pseudopoly-
morph, phase II (Supporting Information). This alternate packing
is characterized by the interdigitation of dimers. Both packing
structures often coexist, but as the solutions are diluted, phase
II ceases to form, and only phase I is observed. When the
solution ratios approach the transition from adsorption of 18-
para to adsorption of either 18-ortho or 18-meta, it is phase I
that is in competition with the phases formed by the other
isomers, and therefore, it is the relative stability of this more
stable form that is measured in competitive adsorption experi-
ments.
The two-dimensional crystals of the ortho isomers differ
significantly from those of the meta or para isomers due to the
constraints on the molecular conformation set by the ring-
substitution geometry.17 The ortho isomers, unlike the meta or
para compounds, cannot lie flat on the surface due to the
proximity of the substituents, thus reducing the interaction
between the molecules and the substrate. The steric interaction
of the ester functionalities on adjacent carbons prevents them
from both adopting the energetically preferred 0° torsion angle
with respect to the benzene ring. Furthermore, the proximity of
the alkyl chains enables extensive intramolecular contacts. This
results in a hairpin conformation, similar to that seen with other
ortho-substituted molecules,17,21 that precludes interdigitation
with the alkyl chains of neighboring molecules. Instead, these
monolayers are characterized by dimers of molecules forming
angled columns. The partially desorbed benzene rings can
overlap slightly with the methyl group from a neighboring
molecule, providing a stabilizing C-H‚‚‚π interaction. The
model and STM images are given in Figure 4. In contrast to
the meta and para structures, no marked differences were
observed between monolayers consisting of ortho-substituted
molecules with 17 or 18 carbons in the alkyl chains.
Competitive Adsorption. The para isomers preferentially
adsorbed to the surface when in competition with an equal
number of the corresponding ortho isomers, forming a mono-
layer indistinguishable from that obtained from pure compound.
Thus, the para isomers form more stable two-dimensional
crystals on the graphite substrate with more negative free
energies of adsorption.22 As the ratio of ortho to para in solution
was increased, there was a sharp transition from adsorption of
solely the para isomer to solely the ortho isomer, such that co-
adsorption was not observed. Only upon increase of the solution
ratio of ortho:para (80:1 for the even isomers and 99:1 for the
odd isomers) was the ortho monolayer observed. From the solute
(21) Schuurmans, N.; Uji-i, H.; Mamdouh, W.; De Schryver, F. C.; Feringa, B.
L.; Van Esch, J.; De Feyter, S. J. Am. Chem. Soc. 2004, 126, 13884-
13885.
(22) The phase observed to form at the surface from multicomponent solutions
during the competitive adsorption experiments were not metastable,
kinetically favored phases. This was determined by displacement experi-
ments, where a monolayer was formed from pure solution and then rapidly
displaced by addition of another isomer.
9
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A R T I C L E S
Plass et al.
Figure 3. Two-dimensional crystals of the para isomers, 17-para (A,B) and 18-para (C,D). (A,C) STM images with overlaid molecular models and (B,D)
computed packing patterns with unit cell. The para compounds pack very similarly to their meta isomers, with the same molecular conformations and contact
with the surface; however, the odd-even effect is reversed.
ratios at which pure monolayers form,5,23,24 it was calculated
that the ortho monolayers are less stable than the para mono-
layers by greater than 2.5 kcal mol-1 (Table 2).
contrast difference between the columns, and random mixing
was difficult to rule out due to the similarities in shape and
conformation. High-resolution images obtained for 1:1 solutions
(Supporting Information) confirmed that the para isomers are
preferentially adsorbed and hence constitute the more stable
monolayer. This is in agreement with the observation that it
requires more ortho molecules to displace the para isomers than
it does to displace meta isomers. Approximate values of the
free energy difference between para and meta are obtained by
subtracting the results obtained from the competition between
para/ortho and ortho/meta. Thus, the two-dimensional crystals
of the meta isomers are less stable than those of the para isomers
by no more than 0.5 kcal mol-1 (Table 2).
Overall, the ortho isomers of both alkyl chain lengths formed
the least stable two-dimensional crystals, and those formed
by the para isomers were the most stable. It is well-known
that molecules with larger molecular weights often displace
smaller molecules,25 and that molecules with stronger inter-
molecular interactions will adsorb preferentially to those
without.5 The differences in the free energies of adsorption
for the various isomers are, therefore, particularly significant
because of the identical molecular functionality and molec-
ular weights. The origin of this behavior will be discussed below
in the context of the bulk properties and two-dimensional
structure.
When in competition with an equal concentration of meta
isomers, the ortho isomers are displaced, just as they were when
competing with para isomers; thus, the meta isomers are
preferentially adsorbed. For the odd isomers, coadsorption was
not observed because of the sharp transition from one phase to
the other as the solution ratios were increased. For the even
isomers, however, cocrystallization was observed in a narrow
solute ratio range (ratios of 40:1 and 50:1 18-ortho:18-meta).17
This 1:1 cocrystal exhibits a very unusual structure, with a much
larger periodicity than the crystals formed by either of the pure
compounds. The cocrystal results from an energetic compromise
wherein the changing solution composition is accommodated
by formation of a phase that is of intermediate stability to either
pure form. Solution ratios of ortho:meta of 80:1 (odd isomers)
and 60:1 (even isomers) were required in order for the ortho
monolayer to adsorb. Therefore, the meta monolayers are
between 2.2 and 2.5 kcal mol-1 more stable than those of the
ortho isomers (Table 2).
Monitoring the competitive adsorption between the para and
meta isomers was more difficult than for the ortho/para and
ortho/meta mixtures due to the similarity in the two-dimensional
crystal structures. Very high-resolution STM images were
required to distinguish one phase from another based on the
Bulk Properties. The substitution geometries of disubstituted
benzene isomers have been observed to affect melting points,
(23) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99,
6608-6619.
(24) Yablon, D. G.; Ertas, D.; Fang, H. B.; Flynn, G. W. Isr. J. Chem. 2003,
43, 383-392.
(25) Castro, M. A.; Clarke, S. M.; Inaba, A.; Arnold, T.; Thomas, R. K. J. Phys.
Chem. B 1998, 102, 10528-10534.
9
15214 J. AM. CHEM. SOC. VOL. 129, NO. 49, 2007

Crystal Properties of Isomeric Dialkyl Phthalates
A R T I C L E S
Figure 4. Two-dimensional crystals of the ortho isomers, 17-ortho (A,B) and 18-ortho (C,D). (A,C) STM images with overlaid molecular models and
(B,D) computed packing patterns with unit cell. The packing motifs adopted by the ortho compounds are very different than those of the meta and para
isomers, with increased intramolecular contact and decreased contact between the molecules in the monolayer and partial desorption from the graphite
surface.
Table 2. Calculated Free Energy of Adsorption Differences
between the Two-Dimensional Crystals of Different Isomers
Table 3. Solubilities, Melting Points, Heats of Melting (∆Hm), and
Entropies of Melting (∆Sm) for Both the Odd and Even Isomersa
differences in the free energy
onset
1
of adsorption (kcal mol-
)
solubility in
temperature
phenyloctane of the melting
∆
Hm
∆Sm
more stable isomer
less stable isomer
odd isomers
even isomers
1
1
1
(mg mL-
)
curve (
°
C)b
(kcal mol-
)
(kcal mol-1 K-
)
para
meta
para
ortho
ortho
meta
2.7 ( 0.1
2.5 ( 0.1
∼0.2
2.5 ( 0.1
2.2 ( 0.3
∼0.3
17-ortho
17-meta (form I)
17-para
160 (20)
87 (3)
5.8 (8)
50.1
54.0
76.9
22.2
24.3
27.9
0.072
0.074
0.080
order for odd isomers o > m > p o < m < p o < m < p o < m < p
melting enthalpies, melting entropies, and solubilities.6-8,26-30
For the general class, the para isomers have the highest melting
points, greatest enthalpies of melting, smallest entropies of
melting, and lowest solubilities compared to the ortho and meta
isomers, all suggesting that the para isomers have the lowest
free energy of crystallization and therefore form more stable
crystals. This behavior is attributed to both the smaller difference
in entropy between a symmetric molecule in the solid and liquid
states, which is indicated by the entropy of melting,8,29 and
greater crystal cohesion for more symmetric molecules, indicated
by the enthalpy of melting.28 These factors affect both the
melting points and the solubilities.8,30 While the relationship
between meta and ortho isomers is variable, depending on the
chemical composition,6 examination of the melting points of
18-ortho
18-meta
18-para
122 (6)
60 (15)
2.4 (4)
54.2
54.5
85.5
23.4
31.3
33.2
0.071
0.096
0.089
order for even isomers o > m > p o ≈ m < p o < m < p o < p < m
a The orders of solubility, melting points, and heats of melting are in
accordance with the trends for shorter-chain dialkyl phthalate isomers,
whereas the entropies of melting exhibit no consistent trend. b Measured
by DSC at 10 °C min-1
the short-chain phthalate esters (Supporting Information) reveals
that for these compounds, para has the highest melting point,
and ortho generally has the lowest, although the melting points
of the ortho and meta isomers become similar as the alkyl chain
length increases.
The melting points, melting enthalpies, melting entropies, and
solubilities of the even and odd isomers (Table 3) were
compared to determine if they conform to the established trends.
For these properties, the relationships among the various isomers
were similar to the shorter chain analogues. Among the even
(26) Carnelley, T. Philos. Mag. 1882, 13, 112-130.
(27) Carnelley, T. Philos. Mag. 1882, 13, 180-193.
(28) Gavezzotti, A. J. Chem. Soc., Perkin Trans. 2 1995, 1399-1404.
(29) Wei, J. Ind. Eng. Chem. Res. 1999, 38, 5019-5027.
(30) Abramowitz, R.; Yalkowsky, S. H. Pharm. Res. 1990, 7, 942-947.
9
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A R T I C L E S
Plass et al.
isomers, the 18-para crystal was most stable, having the lowest
solubility, highest melting point, and greatest enthalpy of
melting, as anticipated from the behavior of this general class.
The solubilities and enthalpies of melting indicated that 18-
ortho was the least stable, but the melting points of 18-ortho
and 18-meta were essentially identical. In the odd isomer series,
17-meta exists in two different bulk crystalline forms. Because
it is the more stable polymorph, the properties of form I were
compared to those of the other isomers.31 The solubilities,
melting points, and melting enthalpies followed the trend
expected from the melting points of the small molecules and
the even isomers: 17-para had the lowest solubility, the highest
melting point, and the greatest melting enthalpy, while the 17-
ortho properties were the reverse. For both the odd and even
isomers, the bulk properties of these isomers indicate that the
para isomers form the most stable crystals and the ortho isomers
form the least stable crystals.
As stated above, the melting point trend of the disubstituted
benzene isomers has been related to the effect of molecular
symmetry on the entropy in the solid and liquid states,8,29 and
greater crystal cohesion for more symmetric molecules.28 From
the data gathered in this case, however, the entropy does not
contribute to the trend in three-dimensional crystal stability.
Generally, it is reported that the most symmetric isomer has
the smallest change in entropy upon melting, but that is not the
case here. Instead, entropy of melting was lowest for the ortho
isomers. This small entropy change in the ortho isomers may
be due to intramolecular contact of the alkyl chains in both the
bulk and in solution, which would minimize the conformational
entropy change between the solid and liquid, the increase of
which is a contributing factor to the behavior of other isomer
sets. This smaller entropy difference would also explain the
decrease in the melting point difference between ortho and meta
isomers as the alkyl chain length increases.
ortho isomers are much closer to each other than to those of
the para isomer. The two-dimensional crystalline structures of
17-meta, 18-meta, 17-para, and 18-para in phase I were
analogous; all consist of flat molecules with oppositely extended
interdigitated all-trans alkyl chains. The molecular conforma-
tions, intermolecular interactions, and interactions with the
surface are quite similar for both isomers. This is not the case
with the bulk structures. Powder X-ray diffraction data indicate
that among a set of isomers, none of the three-dimensional
crystal structures is isomorphous. The failure of bulk properties
to predict the adsorption strengths can be attributed to the lack
of structural similarity between the meta and para isomers in
the bulk. This underscores the necessity of studying two-
dimensional structural properties and developing the means to
predict and control this packing. In comparison to the meta and
para isomer packing motifs, the two-dimensional crystal struc-
tures of the ortho isomers exhibit several features that make
them much less stable than both the meta and para crystals.
The ortho isomer has conformational barriers to lying flat on
the surface, resulting in a hairpin conformation where the
benzene ring is partially desorbed. Thus, the interaction with
the substrate is not as stabilizing for the ortho monolayers as it
is with the other isomers. The contact area with the surface is
decreased, and the possible π‚‚‚π interactions between the
benzene ring and the graphite are attenuated. The two-
dimensional structures generated by the ortho isomers are also
less stable because a molecular conformation is adopted that is
strained relative to the energy-minimized gas-phase conforma-
tion and because intermolecular contacts are greatly reduced
due to the hairpin conformation.32 Taking this two-dimensional
structural information into account, the small difference in the
stabilities of the monolayers of para and meta isomers and the
great reduction in stability of the ortho isomers monolayers can
be rationalized.
Comparison between Two- and Three-Dimensional Crys-
talline Properties. The possibility of using bulk crystal proper-
ties to predict adsorption behavior of the isomeric dialkyl
phthalates investigated can be assessed on the basis of the
relative stabilities of two- and three-dimensional crystals
determined here. The general stability trend for these isomers,
wherein para compounds form the most stable crystals and ortho
compounds form the least stable crystals, is retained during
surface adsorption: para molecules are most strongly adsorbed
while ortho molecules are least strongly adsorbed. Despite the
persistence of this trend, the magnitudes of the two-dimensional
crystal stability differences can only be interpreted by examina-
tion of the structural features of the monolayers, such as the
contact area, molecular conformations, and densities of the
monolayers. This is an important caveat when attempting to
predict adsorption behavior. The small energy difference
between the meta and para isomers on the surface, for example,
would not have been predicted from their disparate properties
in the bulk. The melting points and solubilities of the meta and
Three-dimensional crystal properties can now be compared
with the relative adsorption strengths for two different systems,
the phthalate isomers and the previously reported series of
dicarbamates.5,33 In the dicarbamate system, the order of stability
of the two-dimensional crystals parallels the orders of the
melting points and the enthalpies of melting, but unlike in the
dialkyl phthalate isomer system, the solubilities are not related
to the monolayer stabilities. No single bulk property could
reliably predict the magnitudes of the adsorption strengths in
both systems. The universality of any parallels between two-
and three-dimensional crystal stabilities is dubious and will
likely depend more on structural similarities between the packing
patterns in both systems.
(32) See ref 5 and (a) Eichhorst-Gerner, K.; Stabel, A.; Moessner, G.; Declerq,
D.; Valiyaveettil, S.; Enkelmann, V.; Mu¨llen, K.; Rabe, J. P. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 1492-1495. (b) Mena-Osteritz, E. AdV. Mater.
2002, 14, 609-616. (c) Mu, Z.-C.; Kong, J.-F.; Wang, Y.; Ye, L.; Yang,
G.-D.; Zhang, X. ChemPhysChem 2004, 5, 202-208. (d) Constable, E.
C.; Guntherodt, H. J.; Housecroft, C. E.; Merz, L.; Neuburger, M.;
Schaffner, S.; Tao, Y. Q. New J. Chem. 2006, 30, 1470-1479 for
comparisons of the two- and three-dimensional crystal packing of various
compounds.
(31) Two polymorphs of 17-meta were discovered: form II was obtained from
recrystallization from methyl ethyl ketone, and cooling from the melt of
form II produced form I. The relative stabilities were determined by
allowing saturated solutions of the two polymorphs of 17-meta in
phenyloctane to equilibrate. These solutions were filtered, and the
precipitates were compared by optical microscopy and powder X-ray
diffraction, which indicated that both solids were form I. The same process
was monitored in acetone using optical microscopy. The transformation of
form II to form I indicates that form I is the more stable polymorph of
17-meta.
(33) The solubilities and enthalpies of melting for dicarbamates were compared
to the previously reported relative adsorption strengths and melting points.
1,12-Diyl-bis(octylcarbamate)dodecane, (mp ) 111 °C, ∆Hm ) 22.5 kcal
mol-1, solubility ) 0.3(1) mg mL-1) was the most strongly adsorbed
dicarbamate, forming a two-dimensional crystal 0.16(3) kcal mol-1 more
stable than that formed by 1,8-diyl-bis(octylcarbamate)octane (mp ) 109
°C, ∆Hm ) 17.9 kcal mol-1, solubility ) 1.1(1) mg mL-1) and 0.21(3)
kcal mol-1 more stable than that formed by 1,10-diyl-bis(octylcarbamate)-
decane (mp ) 98 °C, ∆Hm ) 11.9 kcal mol-1, solubility ) 0.9(1) mg
mL-1).
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Crystal Properties of Isomeric Dialkyl Phthalates
A R T I C L E S
ortho/17-para. Different concentrations were used, depending upon the
pair of isomers examined to ensure that monolayers of each compound
would be observable in the absence of the other component.
Differential Scanning Calorimetry. Thermograms of all samples
were measured on a TA Instruments DSC Q10 with Universal Analysis
2000 software (version 4.0C). Samples were weighed to the nearest
microgram and enclosed in a hermetically sealed aluminum pan (TA
Instruments). Thermal behavior of the samples was studied under a
nitrogen purge at a rate of 10 °C min-1 while heating in the range of
30-120 °C. The melting temperature was obtained from the peak of
these curves, and the enthalpy of melting was derived from the peak
area. Entropies of melting were calculated by dividing the enthalpies
of melting by the peak temperature of the melting curve. Melting
entropy calculations assume that the free energy difference is zero when
the liquid and solid are in equilibrium, and the melting peak is the best
estimate of the temperature at which this equilibrium occurs.
Solubility. Saturated solutions in phenyloctane containing two
isomers of the same alkyl chain length were allowed to sit for several
weeks to ensure equilibrium. These solutions were then filtered, and
15.0 µL was combined with 15.0 µL of a 1,2,4,5-tetramethylbenzene
solution in phenyloctane (9.00 mg/mL). The proton NMR spectra of
these solutions diluted in CDCl3 were obtained using a 400 MHz
spectrometer. The relative solubilities were confirmed by comparing
the areas of peaks due to the aromatic hydrogens, and the absolute
solubilities were measured with respect to the internal standard,
correcting for the different number of protons contributing to each peak.
The solubility of the para isomers were sufficiently low that detection
by NMR was difficult; thus, the volume of phenyloctane required to
dissolve a known mass of material was measured instead to obtain the
solubility.
Conclusion
The substitution position of isomeric dialkyl phthalates
affected the structure and stabilities of the two-dimensional
crystals formed at the liquid-solid interface, as well as the bulk
solubility and melting temperature, enthalpy, and entropy. The
stability ordering of isomeric dialkyl phthalate crystals is the
same in two- and three-dimensions, indicating that similar
factors may be influencing crystal stabilities. Isomers with higher
melting points and enthalpies of melting and lower solubilities
formed more strongly adsorbed monolayers. However, detailed
knowledge of the packing structure at the liquid/solid interface
and examination of the structural features that affect stability
was requisite for interpreting the magnitudes of the stability
differences. Only through comparison of the intermolecular
interactions, the interaction with the surface, and the density of
packing could the nearly equivalent adsorption strengths of the
para and meta isomers and the much lower adsorption strengths
of the ortho isomers be rationalized; none of the bulk thermo-
dynamic properties suggested this result. Thus, the properties
of two-dimensional crystals are specifically dependent on
monolayer structure, and a general extrapolation of bulk
properties to these systems is not valid, making adsorption
strength prediction from bulk properties inadvisable. While these
findings make apparent a disconnect between two- and three-
dimensional properties of the same compounds, they provide a
unified context for understanding crystallization phenomena
based on structural analysis. The broad applicability of this
paradigm can be seen in the aspects of molecular structure, in
addition to the molecular weight and functionality, that affect
adsorption strengths revealed here, which must be employed to
predict selective removal from solution, a ubiquitous purification
technique.
Acknowledgment. We thank the National Science Foundation
(CHE-0616487) and the Rackham Graduate School at the
University of Michigan for financial support.
Supporting Information Available: Synthesis and charac-
terization of compounds, experimental details, STM image of
the two-dimensional crystal of 18-para in phase II, table of the
solution ratios examined by STM and the identity of the resulting
monolayers, STM images of 1:1 meta:para solution mixtures,
melting points of various alkyl phthalate esters, table of the bulk
properties of the less stable polymorph of 17-meta, DSC curves
of 17-meta polymorphs, and DSC curves of all reported
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
Experimental Methods
Competitive Adsorption. For the competitive adsorption experi-
ments, solutions in phenyloctane containing two solutes of the same
alkyl chain length were prepared. For each set of compounds, the ratios
of the solutes were varied, while the total concentration was held
constant. Table S1 (Supporting Information) contains all of the ratios
of ortho:meta and ortho:para examined and the identity of the resulting
monolayer. The total concentration was 20.0 mg/mL in phenyloctane
for the ortho/meta comparisons, 1.0 mg/mL for the meta/para com-
parisons, 1.0 mg/mL for 18-ortho/18-para, and 5.0 mg/mL for 17-
JA0744798
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