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Langmuir 1989,5, 998-1005
Effects of Added Achiral Components on Chiral Recognition in Monolayers of Stearoylserine Methyl Ester Noel G. Harvey and Edward M. Arnett* Department of Chemistry, Duke University, Durham, North Carolina 27706 Received March 1, 1989 Additions of palmitic, stearic, and arachidic acid were made to mixtures of enantiomeric and racemic stearoylserine methyl ester (SSME). Interaction between the chiral and achiral components was studied by various monolayers techniques: surface pressure versus area isotherms, monolayer stability limits, surface shear viscosity, and the effect in prespread films of the other components on the equilibrium spreading pressures of each component when brought to equilibrium with its crystalline phase. By all of these criteria, the multicomponent films of palmitic acid and SSME demonstrate chiral recognition only when the chiral component is in excess and when the film system is in a condensed or collapsed state. No chiral recognition was detectable in the rheological or thermodynamic properties under conditions where the monolayers were clearly in a fluid phase. This is interpreted to mean that chiral discrimination when detected in these mixed films is due to segregation of the less stable enantiomericfilm and its slow collapse to a quasi-crystalline surface state. In addition, it is shown for the first time that the rheological properties of enantiomeric and racemic monolayers may differ drastically and that the flow properties may be further altered by the addition of an achiral Surfactant component which acts, in effect, as a "two-dimensional" solvent that assists the spreading of SSME from its crystalline phase.
Introduction Chiral compounds are of special value for the study of intermolecular forces since enantiomers are exactly alike in every way except for their symmetry properties. Interactions between chiral molecules may be expressed physically in terms of their asymmetry.'-' Stereorecognition has been demonstrated experimentally for over 100 years in the melting point vs composition diagrams for mixtures of crystalline enantiomers and diastereomers, but has been exceedingly difficult to detect in less tightly aggregated liquid and gaseous states.&12 However, variation of surface pressure and average molecular area in monolayers at the air-water interface may be used to control directly the aggregation, orientation, and even conformation of chiral surfactant species, so that the physical properties of their monolayers on the surface of water reflect directly the influence of molecular asymmetry on intermolecular for~es.'~-'' These highly ordered systems provide a unique means by which intermolecular chiral interactions may be modeled by molecular symmetry effects on hydrogen-bonding, van der Waals, dipolar, and electrostatic (1)Mason, S.F.Molecular Optical Actiuity and the Chiral Discriminations; Cambridge University Press: Cambridge, 1982. (2)Schipper, P. E.;Harrowell, R. J. Am. Chem. SOC.1983,105,723. (3)Salem, L.J. Am. Chem. SOC.1987,109, 2887. (4)Craig, D. P.; Elsum, I. R. Chem. Phys. 1982,73,349. (5)Amaya, K.Bull. Chem. SOC.Jpn. 1961,34,1689. (6)Amaya, K.Bull. Chem. SOC.Jpn. 1961,34,1803. (7)Amaya, K.BuZl. Chem. SOC.Jpn. 1962,35,1794. (8)Atik, Z.;Ewing, M. B.; McGlashan, M. L. J.Phys. Chem. 1981,85, 3300. (9)Lepori, L.;Mengheri, M.; Mollica, V. J.Phys. Chem. 1983,87,3520. (10)van den Oord, R. J.; Hermans, L. J. F.; Beenaker, J. J. M. J. Chem. Phys. 1986,85,2193. (11)Atik, Z.;Ewing, M. B.; McGlashan, M. L. J. Chem. Thermodyn. 1983,15,159. (12)Horeau, A.; Guette, J. P. Tetrahedron 1974,30,1923. (13)(a) Stewart, M. V.; Arnett, E. M. Top. Stereochem. 1982,13. (b) Amett, E. M.; Harvey, N. G.; Rose, P. L. 'Stereochemistry and Molecular Recognition of 'Two Dimensions'". Acc. Chem. Res. 1989,22,131. (14)Georges, C.; Lewis, T. J.; Llewellyn, J. P.; Salvagno, S.; Taylor, D. M.; Stirling, C. J. M. J.Chem. SOC.,Faraday Trans. I 1988.84.1531. (15)Boul&ssa, 0.; Dupeyrat, M. Biochim.-Biophys. Acta 1988,938, 395. (16)Harvey, N.;Rose, P. L.; Porter, N. A.; Huff, J. B.; Arnett, E. M. J. Am. Chem. SOC.1988,110,4395. (17)Arnett, E. M.; Harvey, N.; Rose, P. L. Langmuir 1988,4,1049.
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Recently, we reported a study of stereoselective interactions in monolayers spread from N-stearoylserine methyl ester (SSME), a chiral surfactant which exhibits striking chiral molecular recognition not only in such bulk physical properties as melting point and heats of fusion but also in every surface monolayer property which we have mea~ u r e d . ~Force-area ~ isotherms, equilibrium spreading pressures, and surface shear viscosities were determined at several temperatures for films cast from either the pure enantiomers or their racemic mixture. In every case, the racemic films demonstrated a higher degree of surface expansion and greater fluidity at higher surface pressures than films cast from either pure antipode. The dependence of these monolayer properties on temperature and enantiomeric purity revealed that the chiral molecular recognition displayed in these films was highly dependent on the surface phase (i.e., solid-, liquid-, or gaslike) of the monolayer system. Differences in the viscoelastic and quasi-thermodynamic properties of racemic and enantiomeric SSME monolayers were shown to occur as the result of a slow collapse of the less stable enantiomeric film to a crystalline or quasi-crystalline surface state under conditions in which the racemic film remained in a stable, fluid monolayer phase. Epifluorescence microscopy of racemic and enantiomeric films in situ at the air-water interface allowed direct observation of the dense packing arrangement in the enantiomeric films vs the random packing in the racemic monolayer. Transmission electron and scanning-tunneling microscopy of the collapsed films transferred to graphite substrates demonstrated stereoselective packing in the condensed film phases. In sum, these results suggest a significant degree of hydrogen bonding between amino acid methyl ester headgroups, especially in condensed monolayer states. (18)Andelman, D.; Brochard, F.; de Gennes, P.-G.; Joanny, J. F. C. R. Acad. Sci., Ser. 2 1985,301,675. (19)Andelman, D.; Brochard, F.; Joanny, J. F. J . Chem. Phys. 1987, 86,3673. (20)McConnell, H. M.; Moy, V. T. J. Phys. Chem. 1988,92, 4520. (21)Andelman, D.; DeGennes, P.-G. C.R. Acad. Sci. 1988,307,233. (22)Andelman, D. Private communication. (23)Harvey, N. G.; Mirajovsky, D.; Rose, P. L.; Verbiar, R.; Arnett, E. M. J. Am. Chem. SOC.1988,Ill, 1115. (24)Langmuir, I. J . Am. Chem. SOC.1917,39,1848.
0 1989 American Chemical Society
Langmuir, Vol. 5, No. 4, 1989 999
Chiral Recognition in Monolayers
Interestingly, it was noted that incorporation of the opposite enantiomer into the solidlike film of a pure antipode resulted in an expansion and consequent fluidization of the homochiral monolayer, which in turn effected a loss of detectable stereoselectivity in molecular packing as reflected by monolayer properties. As in regular, three-dimensional chiral liquids, the arrangement of SSME molecules in the more highly fluid monolayer state is apparently neither homo- nor heterochiral but a nearly random array in which stereoselective hydrogen bonding is not a significant factor in molecular packing. The question may then be raised as to whether the fluidization of the pure enantiomeric films might also be accomplished to an equal extent by addition of an achiral component which has been doped into the monolayer matrix without a severe disruption of the closely packed hydrogen-bondingnetwork which appeared to be necessary for the expression of enantiomeric discrimination. The effects of a third, achiral component on the interaction between two chiral surfactants have been approached recently from theory.22 However, there has been no systematic exploration of stereoselective interactions in threecomponent monolayers consisting of two enantiomers R and S and a third, achiral component P. The interactions between enantiomer R and component P must be exactly the same as those between P and enantiomer S . It is therefore reasonable to ask whether the interactions between enantiomers will be sufficient for chiral discrimination to be detected in the presence of achiral component
P. Chiral recognition in the presence of achiral species is central to phenomena as diverse and fundamental as the miscibility of chiral solids in achiral liquids, crystallization, resolution from solution, polymorphism, and adsorption, yet the clarification of these interactions has barely been addressed.2 From the standpoint of regular solution theory, the feasibility of determining the degree of recognition between chiral species in an achiral solvent system rests on several variables. External pressure, temperature, concentration, diffusion, local fluctuations in the population of chiral species, and the properties of each of the solution components on both the macroscopic and molecular scale must be taken into account for a complete understanding of the forces controlling chiral molecular re~ognition.~-'J~ The difficulties of manipulating these variables independently in a regular solution experiment preclude a complete knowledge of the subtle forces governing the recognition. As shown in Figure 1,the Langmuir-Gibbs model of the surfactant-covered water surface presents an opportunity to control such an achiral/chiral matrix in a highly ordered array that is not possible in a normal three-dimensional solution.24 If the headgroup of the achiral component is also capable of hydrogen-bonding interactions with the chiral component, it is conceivable that chiral recognition between SSME headgroups may arise even when the chiral centers on each headgroup are separated by one or more small, achiral carboxylate headgroups of the added achiral fatty acid component (Figure 1A). Alternatively, the recognition between chiral centers may be reduced beyond the limit of detection if the achiral headgroups effectively mask the stereochemistry of SSME. Segregation of the chiral components from the achiral fatty acid matrix should allow direct contact of the chiral SSME headgroups (Figure 1B) and stereoselective packing might then be preserved and expressed in monolayer properties. If the composition of the multicomponent system is varied across a series of well-defined fatty acid/SSME ratios at constant
A.
"MIXED" FATTY ACID I SSME MATRIX I
h
'SEGREGATED' FATTY ACID I SSME MATRIX
B
5 - oh0 t P7 25
WATER UR
oi
on
O'C,
O'C,
-
on
on
~~-~~ Finally, the ESP of the palmitic acid component is independent of monolayer composition, regardless of whether the film was cast from racemic or enantiomeric material. The equilibrated system is therefore composed of palmitic acid crystal, palmitic acid monolayer, and collapsed SSME. In addition, the ESP of the palmitic acid crystal is higher than either of the SSME crystals. The ESP order in the phase diagram (palmitic acid > racemic SSME > enantiomeric SSME) suggests that as the film is compressed, the reorganization of the components would most likely arise from collapse of the component with the lowest ESP. Table I1 summarizes the ESP data obtained for prespread films of stearic and arachidic acid. These data were not taken as a function of varying film composition, but they still point out the obvious differences in the spreading of the racemic vs enantiomeric SSME. The results obtained for the spreading of the SSME components are the same as those obtained in the palmitic acid system, within experimental error. Note that the ESPs of both of the fatty acid components are lower than those obtained for (40)Eliel, E. Stereochemistry of Carbon Compounds;McGraw-Hill: New York, 1962. (41)Jacques, J.; Collet, A.; Wilen, S. H. Enantiomers, Racemates, and Resolutions; Wiley: New York, 1981; pp 105-144. (42) Gershfeld, N. L.; Pagano, R. E. J . Phys. Chem. 1972, 76, 1231. (43)Gershfeld, N.L.;Pagano, R. E. J . Phys. Chem. 1972,76,1238. (44)Gershfeld, N. L.;Pagano, R. E. J . Phys. Chem. 1972,76, 1248.
Table 111. Effect of Achiral Fatty Acid Added to Chiral Films of SSME at 25 O C monolayer property n / A isotherms
addition of fatty acid steady condensation of monolayer, isotherms are identical when fatty acid is in excess monolayer stable to higher surface pressures; stability limit independent of stereochemistry when fatty acid is in excess ESPs in prespread increased ESP for racemic crystals; no rise films in ESP of enantiomeric crystals compression/expansion compression/expansion hysteresis decreases when fatty acid is in excess hysteresis surface shear viscosity of racemic systems nearly viscosities independent of composition; viscosity of enantiomeric systems decreases with increasing fatty acid; fatty acid rich films independent of stereochemistry
palmitic acid; these values agree with those obtained on clean water ~urfaces.4~ The lower ESPs of the fatty acids apparently do not affect the spreading of the SSME, presumably because the fatty acid monolayers are metastable above their E S P S ~ ~and , ~do ’ not collapse if SSME spreads to higher surface pressures. In addition, the ESPs of racemic and enantiomeric SSME are the same (within experimental error) as those obtained in pure palmitic acid films, indicating that the chain length of the achiral component in the C,,-C2, range does not affect spreading to an extent detectable by our method. Taken together, these data imply that the monolayer stability limits given in Table I do not reflect the reorganization of a stable monolayer system but are most likely a result of the collapse of the least stable film component to a crystalline or quasi-crystalline surface phase. As was found for the pure enantiomeric and racemic SSME systems, this collapse always occurs at lower surface pressures for the enantiomeric systemz3and reflects the greater ease of formation of a collapsed nucleation site for the pure enantiomers. Placing a crystal on these monolayers provides a “seed” for the collapse of a given component which is not formed readily in the spread monolayer alone. The enhanced stabilities of the SSME components above their individual ESP limits, as determined here, are therefore not due to an ideal or near-ideal miscibility of the monolayer components but most likely to a time-dependent diffusional or energetic barrier to collapse. Comparison of the ESP data and the area/composition diagram for the film spread from solution demonstrates the significant differences between films spread from solution and those allowed to equilibrate with the crystals of one component. When spread from solution, these films display chiral recognition only when compressed past the stability limit of the less stable enantiomeric component. When allowed to equilibrate with crystals, however, films of racemic or enantiomeric SSME display large differences in their equilibrium thermodynamic states which can be attributed specifically to the stereochemical configuration of the chiral component and the thermodynamic stability of the crystalline phase46,48at every film composition. These observations correlate nicely with the fact that the flow properties of the palmitic acid/SSME mixtures diverge only when the chiral component is in excess and when the surface pressure at which the flow rate was obtained is above the monolayer stability limit. A summary of the effect of the achiral fatty acid on the interaction (45)Boyd, G. E. J . Phys. Chem. 1958,62,536. (46)Gershfeld, N. L.J . Colloid Interface Sei. 1982,85,28. (47)Rabinovitch, W.; Robertson, R. F.; Mason, S. G. Can. J . Chem. 1960, 38, 1881. (48)Erikson, J. C.J . Colloid Interface Sci. 1971,37, 659.
Langmuir 1989,5, 1005-1008 between the enantiomers of SSME is given in Table 111. Although the experiments presented here do not explicitly take into account the contribution of the subphase water, possible changes in the structure of the vicinal water layer, or the degree of headgroup solvation, they do reduce all possible interactions between film components to a set of well-defined composition variables which differ only through the stereochemistry of the SSME headgroup. Taken as a whole, the I I / A isotherms, monolayer stability limits, area/composition diagrams, surface shear viscosities, and equilibrium spreading pressures indicate that any detectable chiral discrimination in these multicomponent films must arise from direct contact between chiral headgroups (Figure 1B).
Conclusions By every criteria we have devised, multicomponent films of palmitic acid and enantiomeric and racemic SSME demonstrate chiral molecular recognition only when the chiral component is in excess and when the film system is in a condensed or even collapsed state. No chiral recognition could be detected in any viscoelastic or thermodynamic monolayer property under conditions where the films were distinctly fluid monolayers. The chiral discrimination as detected in these chiral/achiral systems is therefore due to a segregation of the less stable enantiomeric film component from the fatty acid matrix and
1005
to a slow collapse to a different, solidlike surface state. The experiments presented here have demonstrated several phenomena unprecedented in the study of chiral interactions in ordered systems. It has been shown here for the first time that the flow properties of enantiomeric and racemic monolayers may differ drastically and that these flow properties may be altered by the addition of other surfactant components while maintaining a stereoselective packing pattern. In addition, the dependence of the surface shear viscosities on composition indicates that the added surfactant is capable of breaking the two-dimensional lattice of the enantiomeric films much in the same manner as does the addition of the opposite enantiomer. It has also been demonstrated by ESP measurement that a prespread film at the air/water interface may act as a two-dimensional "solvent", enhancing the spreading of a monolayer of SSME from the crystal; this dilution effect is different for racemic and enantiomeric crystals, as is often the case in three-dimensional systems.
Acknowledgment. This work was supported by a generous grant from AT&T. We thank Philip L. Rose and Jonathon Heath for helpful discussions and criticisms and Marjorie Richter for technical assistance. Registry No. SSME, 118319-49-6; palmitic acid, 57-10-3; stearic acid, 57-11-4; arachidic, 506-30-9.
Reverse Micelles of Aerosol-OT in Benzene. 4. Investigation of the Micropolarity Using 1-Methyl-8-oxyquinoliniumBetaine as a Probe M. Uedal and Z. A. Schelly* Center for Colloidal and Interfacial Dynamics, Department of Chemistry, The University of Texas at Arlington, Arlington, Texas 76019-0065 Received October 24, 1988. I n Final Form: February 21, 1989 The aggregation and the micropolarity of Aerosol-OT reverse micelles in benzene are investigated by using a new absorption probe, 1-methyl-8-oxyquinolinium betaine (QB)at 25 "C, as a function of the surfactant concentration and the water content of the solutions. QB is preferentially partitioned (>500:1) in the aqueous pool of the aggregates, and its transition energy EQBis shown to have a linear relationship with Kosower's 2 values and Dimroth et al.'s ET(30)values.
Introduction Solutions of surfactants in nonpolar solvents always contain at least a small amount of water which cannot be neglected because it promotes the association of the amphiphile to reverse micelles and accumulates as a pool in the polar core of the aggregates. With increasing water content of the solution, the pools swell, which successively leads to the formation of swollen reverse micelles and w/o microemulsions. Due to the rather peculiar chemical and physical properties2 of the polar interior of reverse micellar aggregates, substantial efforts have been focused on the investigation (1) R.A. Welch postdoctoral fellow. On leave from the Osaka Municipal Technical Research Institute, Japan. (2) Fendler, H. J. Membrane Mimetic Chemistry; Wiley: New York, 1982.
of the state of water in the pools. For this purpose, absorption and fluorescence probes such as TNS? ANS; pyranine derivatives: vitamin BI2,6acridine orange: picric acid?yQ TCNQ,'OJ1 pyrene derivatives,12 etc., have been commonly used. In utilizing probe molecules, ideally, one (3) Menger, F. M.; Donohue, J. A.; Williams, R. F. J. Am. Chem. SOC. 1973, 95, 286. (4) Wong, M.; Thomas, J. K.; Gratzel, M. J. Am. Chem. SOC.1976,98, 2391. (5) Kondo, H.; Miwa, I.; Sunamoto, J. J. Phys. Chem. 1982,86, 4826. (6) Fendler, J. H.; Nome, F.; VanWoer, H. C. J.Am. Chem. SOC.1974, 96, 6745. (7) Herrmann, U.; Schelly, Z. A. J. Am. Chem. SOC.1979, 101, 2665. (8) Tamura, K.; Schelly, Z. A. J. Am. Chem. SOC.1981, 103, 1013. (9) Tamura, K.; Schelly, Z. A. J. Am. Chem. SOC.1981, 103, 1018. (10) Muto, S.; Meguro, K. Bull. Chem. SOC.Jpn. 1973, 46, 1316. (11) Harada, S.; Schelly, Z. A. J.Phys. Chem. 1982,86, 2098. (12) Verbeeck, A.; Galade, E.; DeSchryver, F. C. Langmuir 1981, 2, 448.
0743-7463/89/2405-1005$01.50/00 1989 American Chemical Society
