Hydrogen-bond-induced hysteresis in the compression/relaxation of monolayer films

Article abstract of DOI:10.1021/ja803491y

Two stereoisomers of surfactants were synthesized in which a diketopiperazine ring was inserted between a hydrocarbon chain (of variable length) and an anionic headgroup. It was found (by HPLC, conductivity, surface tension, and diffusion NMR) that these compounds have low solubilities in water, remarkably high Krafft temperatures, and low critical micelle concentrations. Of particular interest were the pressure/area isotherms of insoluble monolayers of nonionic analogues of the amphiphiles. These isotherms showed an unprecedented hysteresis but only during the first compression/relaxation cycle. Additional cycles were normal in that they lacked the original hysteresis. These results were attributed to diketopiperazine rings that initially lie flat on the water surface at the air/water interface. Compression flips the diketopiperazine rings so that they now reside roughly perpendicular to the interface in a stable hydrogen-bonded orientation where the rings occupy far less space. Copyright

Full text of DOI:10.1021/ja803491y

Published on Web 07/12/2008
Hydrogen-Bond-Induced Hysteresis in the Compression/Relaxation of
Monolayer Films
Jennifer L. Sorrells and Fredric M. Menger*
Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322
Received May 15, 2008; E-mail: menger@emory.edu
Morphologies of self-assemblies, like the laws of legislative
assemblies, are established when opposing forces reach a compro-
mise. For example, a micellar structure reflects a balance between
attractive hydrophobic forces among internal chains and repulsive
electrostatic forces among external ionic groups. Self-assembly can
be affected by tinkering with either component. Over the years we
have made use of this fact while synthesizing a large variety of
structural modifications in amphiphilic systems. Thus, the hydro-
phobic region has been rigidified,1 interconnected,2 subdivided,3
functionalized,4 and hyperextended.5 And the ionic surface has been
provided with catalytic groups,6 metals,7 sugars,8 macrocycles,9 and
zwitterions.10 In the present Communication, an amphiphile has
Figure 1. Synthesis of 1A.
been endowed with yet another variation: a diketopiperazine ring
inserted between a chain and an ionic headgroup (1). The question
arose as to how such a ring, known to effectively hydrogen-bond
with itself in linear oligomeric arrays,11 perturbs the spherical
assembly normally created by the hydrophobic chains. After
characterizing the properties of 1 in aqueous solutions, we examined
monolayer films at the air/water interface where the diketopiperazine
has a profound effect on the compression isotherms.
Figure 2. X-ray structure of monobenzyl-protected diketopiperazine made
from serine. Cyclic amide-to-amide hydrogen-bonding is seen at the top of
the array.
The synthesis of the anionic (2S,5S)-cyclo(serine/serine) surfac-
tant (1A) is given in Figure 1. The corresponding (2S,5R)
stereoisomer (1B) was synthesized by a related procedure (provided
in the Supporting Information). Consistent 1H and 13C NMR,
HRMS (ESI-), and elemental analyses were obtained following
multiple chromatographies on a Sephadex LH-20 size-exclusion
column. An X-ray structure of the monobenzylated serine/serine
diketopiperazine (Figure 2), an intermediate in the synthesis of 1B,
shows the correct relative stereochemistry plus the expected ring-
to-ring hydrogen-bonding within the solid-state assembly.
Surfactants 1A have water-solubilities of below observability for
n ) 13; 0.4 mM for n ) 11; and 8 mM for n ) 9 (measured at 22
°C via quantitative HPLC with evaporative light scattering detec-
tion). Surfactants 1B have water-solubilities of 0.94 mM for n )
11 and 12 mM for n ) 9. Since sodium dodecyl sulfate (SDS) has
more than a 10-fold greater solubility than any of these values,
intermolecular hydrogen-bonding of the diketopiperazines, as in
Figure 2, clearly favors the solid state.
Figure 3. (Left) Conductivity vs concentration at 33 °C for 1B (n ) 9).
(Right) Surface tension vs concentration at 33 °C for 1B (n ) 9).
11), 65-69 °C; 1A (n ) 9), 59 °C; 1B (n ) 11), 59 °C; 1B (n )
9), 27 °C. These are also the temperatures where solid suspensions
could be seen to clarify. Given that short-chained anionic surfactants
usually have subambient Tk values, a Tk of 59 °C for 1A (n ) 9)
is uncommonly high for a surfactant with a mere 9-carbon chain.
Thus, a conventional surfactant with a 10-carbon chain, sodium
decyl sulfate, has a Tk of only 9 °C.13 Plots of conductivity vs
concentration for 1B (n ) 9) at ambient temperature (22 °C) are
linear up to saturation, indicating an absence of micellization. When,
however, the temperature was raised above the Tk to 33 °C, the
plot shows an abrupt change in slope, corresponding to a critical
micelle concentration (cmc) for 1B (n ) 9) of 16 mM (Figure 3,
left).
Surface tension for 1B (n ) 9) declines linearly with the
log[conc] at 22 °C, showing again that no micellization occurs
below saturation at this temperature. Application of the Gibbs
equation to the linear data14 gave an area-per-molecule of only 40
Å2. Since this is the identical area displayed by the far less bulky
SDS, one can presume that spontaneous packing at the planar air/
water interface is aided by diketopiperazine hydrogen-bonding. At
The low solubilities at 22 °C encouraged us to explore potential
Krafft phenomena in our system. Below the Krafft temperature,
Tk, surfactant in solution is exclusively monomeric, whereas above
Tk one observes micellization and hence a much greater solubility.12
Using the fact that conductivity vs temperature plots show a break
at Tk, we estimated the following Krafft temperatures: 1A (n )
9
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10.1021/ja803491y CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
cycles). Moreover, all additional compressions of the alcohol, once
the first compression/relaxation sequence has been completed, now
show a more normal rise at 22 Å/mol instead of 40 Å/mol (see
Figure 4B for compression no. 2).
The hysteresis is understandable in terms of horizontal-to-vertical
molecular reorientation at the air/water interface. Thus, during
compression no. 1 the diketopiperazine rings hydrogen-bond to each
other as they initially lie flat on the air/water interface and, thereby,
occupy an unusually large area for surfactant with only a single
tail. When pressures near 25 mN/m are reached, the rings flip so
that they are now more-or-less perpendicular to the interface where
they can also hydrogen-bond but in a much smaller space. This is
a stable ring orientation that remains vertical when the monolayer
is expanded during relaxation no. 1, and hence the hysteresis. Since
the vertical orientation is already in place throughout compression
no. 2, the first phase of the compression seen in compression no.
1 is avoided. The model implies the absence of a rapid equilibrium
between the vertical and horizontal states. Rearranging film
molecules by a chemist, like rearranging puzzle pieces by a child,
can obviously be an engaging activity.
Figure 4. (A) Compression no. 1 and relaxation no. 1 of monolayer film
of 1A (n ) 12) alcohol showing hysteresis. (B) Repeat cycle upon
completion of cycle A. All subsequent cycles are identical to cycle B!
33 °C there is an abrupt change of slope at a cmc of 16 mM (the
same value obtained by conductivity) (Figure 3, right). Normally,
surface tension plots level off above the cmc, but in our case the
slope turns positive. Although such behavior has in the past been
attributed to a surface-active impurity,15 quantitative NMR of our
1B (n ) 9) sample shows that decanoic acid, if present at all, cannot
exceed 0.5 wt % (an impurity level that was shown to have no
effect on SDS). We believe the aberrant surface tension plot results
from formation of suspended aggregates destined to become
precipitated solid (an explanation consistent with a haziness that
develops in the solutions just above the cmc). Thus, the picture
that emerges for 1B (n ) 9) is a rather insoluble surfactant that is
monomeric at low concentrations, micellar at the cmc, and a solid
phase soon thereafter. Rather than hydrophobicity and hydrogen-
bonding acting in concert, they seem to play independent roles,
with hydrophobicity important in micellization, and hydrogen-
bonding dominating precipitation (i.e., the solid state) as the
concentration is increased above the cmc.
Acknowledgment. We thank Dr. Kenneth Hardcastle and Dr.
Shaoxiong Wu for their assistance with the X-ray analyses and
diffusion NMR, respectively. We also thank Mr. Lei Shi, Dr. Dan
Lundberg, and Dr. Syed Rizvi for useful discussions and Prof. Kevin
Caran and Stephanie Torcivia for use of a Nima Tech DST 9005
tensiometer. This work was supported by the National Institutes of
Health.
Supporting Information Available: Analytical and synthetic
procedures including spectroscopic data and instrumentation. This
material is available free of charge via the Internet at http://pubs.acs.org.
References
The question remained as to the nature of the micellar phase.
Since 1H NMR spectra showed no significant line-broadening above
the cmc, the micelles are likely small and spherical as opposed to
elongated assemblies. Pulse-gradient spin-echo NMR (i.e., diffusion-
NMR)16 in D2O at 33 °C, coupled with the Stokes-Einstein
equation, gave a hydrodynamic radius for 1B (n ) 9) of 3.29 nm
as compared with 2.45 nm for SDS. The modest increase in the
diameter of 1B (n ) 9) over SDS is likely related to the former’s
longer and bulkier headgroup as well as hydration of the dike-
topiperazine ring.
The most revealing data came from compression of monolayer
films17,18 of the nonsulfonated alcohol corresponding to 1A (n )
11). Selecting the alcohol assured that pressure/area (π/A) isotherms
were not perturbed by transport from the air/water interface into
the aqueous subphase while the insoluble film was being com-
pressed. Figure 4A shows a π/A isotherm at 20 °C in which the
film was compressed at 4.0 Å2/mol/min up to 35 mN/m pressure
(a value insufficient to destroy the monolayer). A transition from
expanded to condensed phases is clearly visible in compression
no.1. The “lift-off” area near 40 Å/mol is twice that expected for
a single-chained amphiphile, indicating that intermolecular assembly
at the interface begins at a particularly low surface concentration.
Even more striking is the unprecedented hysteresis seen in Figure
4A upon the relaxing the monolayer back to its original low
pressure. (By way of comparison, stearic acid monolayers show
only a minor 2 Å/mol shift between compression and relaxation
(1) Menger, F. M.; Ding, J. Angew. Chem., Int. Ed. 1996, 35, 2137.
(2) Menger, F. M.; Brocchini, S.; Chem, X. Y. Angew. Chem., Int. Ed. 1992,
31, 1492.
(3) Menger, F. M.; Peresypkin, A. V. J. Am. Chem. Soc. 2001, 123, 5614.
(4) Menger, F. M.; Shi, L. J. Am. Chem. Soc. 2006, 128, 9338. Menger, F. M.;
Chlebowski, M. E. Langmuir 2005, 21, 2689. Menger, F. M.; Galloway,
A. L. J. Am. Chem. Soc. 2004, 126, 15883.
(5) Menger, F. M.; Yamasaki, Y. J. Am. Chem. Soc. 1993, 115, 3840.
(6) Menger, F. M.; Whitesell, L. G. J. Am. Chem. Soc. 1985, 107, 707.
(7) Menger, F. M.; Gan, L. H.; Johnson, E.; Durst, D. H. J. Am. Chem. Soc.
1987, 109, 2800.
(8) Barragan, V.; Menger, F. M.; Caran, K. L.; Vidil, C.; More`re, A; Montero,
J-L. Chem. Commun. 2001, 85.
(9) Menger, F. M.; Bian, J.; Sizova, E.; Martinson, D. E.; Seredyuk, V. A.
Org. Lett. 2004, 6, 261.
(10) Menger, F. M.; Peresypkin, A. V.; Caran, K. L.; Apkarian, R. P. Langmuir
2000, 16, 9113.
(11) Hanabusa, K.; Matsumoto, M.; Kimura, M.; Kakeki, A.; Shirai, H. J. Colloid
Interface Sci. 2000, 224, 231.
(12) Ohta, A.; Ozawa, N.; Nakashima, S.; Asakawa, T.; Miyagishi, S. Colloid
Polym. Sci. 2003, 281, 363.
(13) Osipow, L. I. Surface Chemistry; Reinhold Publishing Co.: New York, 1962;
p 190.
(14) Holmberg, K.; Jo¨nsson, B.; Kronberg, B.; Lindman, B. Surfactants and
Polymers in Aqueous Solution, 2nd ed.; John Wiley & Sons: Chichester,
England, 2002; p 251.
(15) Lin, S. Y.; Lin, Y. Y.; Chen, E. M.; Hsu, C. T.; Kwan, C. C. Langmuir
1999, 15, 4370–4376.
(16) Lundberg, D.; Unga, J.; Galloway, A. L.; Menger, F. M. Langmuir 2007,
23, 114534.
(17) Menger, F. M.; Wood, M. G., Jr.; Richardson, S.; Zzhou, Q.; Elrington,
A. R.; Sherrod, M. J. J. Am. Chem. Soc. 1988, 110, 6797.
(18) Birdi, K. S. Self-Assembly Structures of Lipids and Macromolecules at
Interfaces; Kluwer Academic/Plenum Publishers: New York, 1999.
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