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Dendritic Crystal Growth in N-Dodecylgluconamide Monolayers at the Air-Water Interface D. Vollhardt,*?tT. Gutberlet,tv$G. Emrich,? and J.-H. Fuhrhops Max-Planck-Institut fur Kolloid- und Grenzflachenforschung,Rudower Chaussee 5, 0 - 1 2 489, Berlin, Germany, Institut fur Kristallographie, Freie Universitat Berlin, Berlin, Germany, and Institut fur Organische Chemie, Freie Universitat Berlin, Berlin, Germany Received January 11, 1995. In Final Form: April 3, 1995@ Monolayers of pure enantiomers and the racemic mixture of the amphiphilic N-dodecylgluconamide were investigated at the air-water interface using Brewster angle microscopy and surface pressure measurements. A striking chiral discrimination is demonstrated comparing the pure enantiomeric forms and the racemic mixtures. The monolayer morphology obtained on compression was visualized and studied by Brewster angle microscopy. “he enantiomeric N-dodecylgluconamides form regular ramified growth pattern of high stability in a shape characteristic for dendritic crystallization, while the racemic mixtures show isotropical solidification. The chiral discrimination effect is supported by differences in the surface pressure isotherms and the constant surface pressure relaxations of the pure enantiomeric forms and the racemic mixtures. “he dendriticcrystallization found experimentallyis discussedin respect of the molecular organization at the air-water interface.
Introduction Monolayers of long-chain amphiphiles with a chiral polar head group provide a unique system to study the importance of chirality-dependent interactions under defined conditions. Such systems have been considered as potent models of biological membranes and surfaces. The potential of monolayer techniques as applied to the comparison of isomers and their mixtures that differ only in shape or symmetry properties has been reviewed recently.1,2 In the solid state, chiral discrimination effects are wellknown for melting points or fusion enthalpies3 and have been explained by differences in the interaction potential between molecules of the same chirality (D:D or L:L) and molecules of opposite chirality (D:L). Recently, striking evidence has been provided by Fuhrhop et al.4-6that the formation of three-dimensional aggregates of long-chain amphiphilic N-alkylaldonamides in aqueous solutions and gels is based on chiral discrimination effects. The aggregates formed in aqueous solutions as platelets, tubes, ribbons, and complex helices depend on the stereochemical configuration of the polyolic head groups of this class of molecule^^^^ so that a “chiral bilayer effect” was stated as a dominant force in determining the superstructural arrangements of these molecule^.^ The pronounced effects inducing chirality in three-dimensional arrangements should also be aware of the arrangement in two dimensions. In the crystalline state, a complex packing of the sugar head group of the enantiomeric N-alkylaldonamides by a cyclic network of hydrogen bonds between four neighboring Max-Planck-Institutfur Kolloid- und Grenzflachenforschung. Institut fur Kristallographie. 8 Institut fur Organische Chemie. @Abstractpublished in Advance A C S Abstracts, June 15,1995. (1)McConnell, H. M. Annu. Rev. Phys. Chem. 1991,42,171-195. (2)Heath, J. G.;Amett, E. M. J . A m . Chem. SOC.1992,114,45004514. (3)Jacques, J.;Collet, A.; Wilen, S. H. Enantiomers, Racemates, and Resolutions; Wiley: New York, 1981. (4)Fuhrhop, J.-H.; Schnieder, P.; Rosenberg, J.;Boekema, E. J . Am. Chem. SOC.1987,109,3387-3390. ( 5 ) Fuhrhop, J.-H.; Schnieder, P.; Boekema, E.; Helfrich, W. J . Am. Chem. SOC.1988,110,2861-2867. (6)Fuhrhop, J.-H.;Boettcher,C. J . A m .Chem.Soc.1990,112,17681776.
molecules was f~und.~-lOThe orientation and development of a hydrogen bond system in the crystal structures is closely related to the chiral environment of the sugar head groups, e.g., up to date it has been not possible to obtain suitable single crystals of racemic N-alkylaldonamides.1° The recent development of Brewster angle microscopy offers the use of a powerful method to visualize the morphological structure of monolayers without influencing the monolayer with additional trace components.11J2 The objective of this work is to study the morphological properties of Langmuir monolayers of long-chain amphiphiles with a chiral polar head group. We report here evidence for the chiral discrimination effect visualized by the morphological monolayer structures of the amphiphilic pure enantiomers N-dodecyl-D- and -L-gluconamide and the racemic mixture of these N-dodecyl-D- and -L-gluconamides at the air-water interface by using the Brewster angle microscopy. In particular, the formation of crystalline ramified growth patterns is observed during isothermal compression experiments with the enantiomeric monolayers. The subject of our investigation includes also the corresponding surface pressure-area isotherms combined with representative constant surface pressure relaxation curves.
Experimental Section N-Dodecyl-D-gluconamideand N-dodecyl-L-gluconamidewere prepared as described elsewhere.6 They were used without further purification and dissolved in CHC13/ethanol/water (7.0/ 3.0/0.1(v/v/v)).The spreadingsolventsused were obtained from Merck (Darmstadt,Germany). The m clear solutionsofthe enantiomericN-dodecylgluconamideswere spread on a Langmuir trough filled with doubly distilled water by a syringe. The solutions of the racemic mixture were prepared by 1:l mixing of appropriate amounts of the two enantiomeric solutions. After evaporation of the spreading solvent, the molecules remaining (7)Tinant, B.; Declercq, J.-P.; van Meersche, M. Acta Crystallogr., Sect. C 1986,42,579-581. (8)Zabel, V.; Miiller-Fahmow, A.; Hilgenfeld, R.; Saenger, W.; Pfannemiiller, B.; Enkelmann, V.; Welte, W. Chem. Phys. Lipids 1986, 39,313-327. (9)AndrB, C.; Luger, P.; Svenson, S.;Fuhrhop, J.-H. Carbohydr.Res. 1992,230,31-40. (10)AndrB, C. Thesis, Free University, Berlin, 1993. (11)Honig, D.;Mobius, D. J . Phys. Chem. 1991,95,4590-4592. (12)HBnon, S.;Meunier, J. Rev. Sci. Instrum. 1991,62,936-939.
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at the air-water interface were compressed and expanded at a rate of 0.04-0.10 nm2 molecule-' min-I. The surfacepressurearea (n-A) isotherms were registered with the automatic Langmuir film balance (FW2)equipped with a temperingsystem from Lauda (Konigshofen, Germany). By use of Brewster angle microscopy," the morphological characteristics of the obtained monolayers were visualized and inspected. Therefore the Brewster angle microscope BAM 1from NFT (Gottingen,Germany)was mounted on the Langmuir film balance. To avoid disturbances by impurities and strong convection, it is necessary to shelter the microscope and film balance in a box. Details of the Brewster angle microscope and the experimental setup are described e1se~here.I~ Images of the monolayer morphology were stored using a video system including a video recorder, monitor, and a video printer.
o.2 0
(13)Vollhardt, D.; Gehlert, U.; Siegel, S. Colloids Surf., 1993,76, 187-195. (14)Vollhardt, D.Adu. Colloid Interface Sci. 1993,47,1-23. (15)Kato, T.; Hirobe, Y.; Kato, M. Langmuir 1992,7,2208-2212.
1 0
2M)O
4000
6000
t/s
8000
Figure 3. Constant surface pressure relaxation at n = 10 mN m-' observed for monolayers of the enantiomericN-dodecyl-Dor -L-gluconamide and the racemic mixture (D:L = 1:l) at 10 and 25 "C.
Results To obtain fundamental information on the two-dimensional behavior of the enantiomeric N-dodecylgluconamides and their racemic mixtures, the morphological features of the monolayers have been compared with the corresponding surface pressure-area (n-A) isotherm. 1. Isotherms. As a result of careful study of the surface pressure properties ofN-dodecylgluconamide, it has been found that the shape of the n-A isotherms measured at temperatures between 10 and 40 'C is strongly affected by the compression rate. The drastic change of the isotherm shape is coupled with definite relaxation phenomena which can be measured at constant surface pressures or constant molecular areas. In addition, in a simple way the effect of monolayer relaxation on the isotherm shape was checked by stopping the barrier at a certain value. Then, the surface pressure rapidly decreased and, on further compression, the isotherm was shifted to a lower molecular area. The effect of the relaxing processes on two-dimensional monolayers is smaller the higher the compression rate of the monolayer.14J5 Therefore, the n-A isotherms of the enantiomeric N-dodecyl-D- or -L-gluconamide shown in Figure 1for 10 "C and in Figure 2 for 25 "C were measured at a quite high compression rate of 0.10 nm2 molecule-l min-l. A striking chiral discrimination can be observed comparing the pure enantiomeric forms and the racemic mixtures (Figures 1and 2). At low temperatures (Figure 1for 10 "C) the molecular area of the racemic mixtures is decreased toward that of compression isotherm of the enantiomeric monolayers. At 10 "C and 20 mN m-l, the racemic mixture of N-dodecyl-D,L-gluconamidehas a
30
'
molecular area of 0.201 nm2 while the enantiomers have a value of 0.222 nm2. However, the situation seems to be more complex than that reported for other amphiphilic monolayers in literature.2J6 Considering the isotherms of the enantiomeric and racemic structures for both temperatures (Figures 1 and 2), it can be seen that the isotherms are strongly affected by the temperature. Changing the temperature of the aqueous bulk phase from 10 to 25 "C, the molecular areas are shifted to lower values for corresponding surface pressures. Furthermore, the shape of the isotherms of the racemic form is obviously changed by the temperature. The surface pressure for irreversible collapse depends strongly on the conditions for the monolayer compression. Although the monolayers can be compressed up to approximately 50 mN m-', the compression isotherms were investigated only up to 20 mN m-l. Defined constant surface pressure relaxations at 10 mN m-l for an enantiomeric and the corresponding racemic mixture are presentedin Figure 3. For both temperatures, the chiral discrimination can also be observed. Again the relaxation of both enantiomeric forms agrees with each other within the accuracy of measurements, while the racemic form relaxes more rapidly. The absolute relaxation rate and also the differences in the relaxation of the enantiomeric and racemic forms increase considerably with the temperature. Thus it is clear that the isotherm shape measured, such as the shift to lower area values at higher temperatures, is largely affected by the relaxation kinetics. (16) Stine, K. J.;Uang, J. Y.-J.; Dingman, S. D. Langmuir 1993,9, 2112-2118.
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Figure 5. Dendritic crystallization in a N-dodecyl-L-gluconamide monolayer on compression to 0.23 nm2 molecule-l(3 mN m-l) at 25 "C observed by Brewster angle microscopy. Time interval between micrograph a and b is 40 s. Figure 4. Development of a dendritic growth pattern from a nucleation center, formed in a N-dodecyl-L-gluconamidemonolayer a t 0.28 nm2molecule-l and 25 "C. Time interval between a and b is 10 s.
Evidence could be provided that these relaxation effects are not caused by an increased solubilityof the monolayer material into the bulk. Repeated compressioddecompression cycles performed under equivalent experimental conditions do not shift substantially the isotherms to lower area values. However, at temperatures of 240 "C, the monolayer material dissolves in the subphase within a few minutes so that no surface pressure can be detected on further compression. A low solubility of monolayer material could be also observed a t temperatures (40 "C if the monolayer was maintained at high molecular areas for a certain time. 2. BrewsterAngle Microscopy. The first condensed aggregates of the enantiomeric N-dodecyl-D- and -Lgluconamideat the air-water interface have been formed even after spreading and have been visualized using the Brewster angle microscopy. Already in this state, the dendritic shape of the aggregates can be recognized. Also small two-dimensional trees or snowflakes have been observed. Compression of the monolayer of the enantiomeric N-dodecyl-D- or -L-gluconamidemolecules up to a surface pressure of a few mN m-l results in the growth of condensed aggregates like those presented in the representative BAM images of Figures 4 and 5. No differences in the dendritic growth and aggregation of the D- and L-enantiomers have been noticed although the morphological properties have been compared very carefully. Typical growth patterns of N-dodecyl-L-gluconamide developed from the nucleation center and formed a t 0.28 nm2molecule-l and 25 "C are presented in Figure 4. Here,
two growth stages of a dendritic structure have been visualized for a time interval of 10 s. As the growth proceeds, only two of the four dendritic main branches are developed, obviouslyaccompanied by an asymmetrical front propagation of side branches. Figure 5 shows a typical dendritic growth pattern of N-dodecyl-L-gluconamide a t 0.23 nm2 molecule-l (3 mN m-l) and 20 "C where the barrier motion was stopped as soon as the formation of dendrites could be observed. The dendrites grow with straight main axes under formation of numerous side branches developing themselves preferentially into one direction along the main chain. In general, the branches are oriented at an angle of 90" toward the main axis, but also lower angles up to 60" are observed (Figure 5). The growth rate achieves values of up to 100 p d s dependent on the supersaturation of the surface concentration of the amphiphiles in a fluid monolayer state, i.e., the local surface pressure used a t a given temperature. Obviously, the angles formed by the side branches and the main axes depend on the growth rate of the main axis. The smaller angles are realized a t high growth rates of the main axes (left-handside of Figure 5), while preferential side branching with angles of about 90"toward the main axis is developed at low growth rates (right-hand side of Figure 5). The effect of temperature on the morphological properties of the enantiomeric monolayers has not been studied in detail. We found, however, that in contrast to the higher temperatures (25 "C), it was not easy to visualize the growth kinetics of the dendritic structures at lower temperatures (10 "C). Here, all the dendritic structures visualized during compression of the monolayer did not change in any way. It is interesting to note that the surface pressure decreases during the growth of the dendritic aggregates a t constant molecular area. After the dendritic growth
2664 Langmuir, Vol. 11, No. 7, 1995
Figure 6. Dissolution of broken dendritic aggregates on decompression of N-dodecyl-L-gluconamidemonolayers to ' 29 nm2 molecule-l at 20 "C.
Figure 7. Isotropic morphology of 1:l racemic N-dodecyl-D,Lgluconamide monolayers at 4 mN m-l.
was finished at a selected molecular area, a further small compression of the monolayer induced a renewed growth. The dendritic structures of the enantiomers have been found to be very brittle. At progressive monolayer compression, they broke into pieces at impingement on each other. On the other hand, they could be kept stable for hours if the molecular area was maintained constant. Monolayer decompression led to a stop of the dendritic growth and subsequently to a continuous dissolution of the broken dendritic aggregates (Figure 6) into the fluid monolayer phase within several minutes. During the compression of the enantiomeric monolayer up to 50 mN m-l, the branches of the dendritic aggregates formed initially were broken increasingly, and, finally, an isotropic solid layer was obtained. On decompression, the continuous solid layer was disintegrated into platelike pieces. In opposite, the morphological images of the 1:lracemic N-dodecyl-D,L-gluconamidemonolayers are less pronounced. The condensed aggregates formed after spreading and at further compression are isotropically structured (Figure 7) at microscopic scale. During compression, the morphological changes of the racemic mixture of Ndodecyl-D,L-gluconamide monolayers were less obvious, i.e., growth of condensed aggregates could not be observed. The isotropic layer formed at high surface pressures has collapsed above 50 mN m-l.
Vollhardt et al.
Discussion MonolayerProperties ofN-Dodecylgluconamides. N-Dodecylgluconamide monolayers demonstrate a representative two-dimensional system for chiral discrimination indicated impressively by the morphological differences (Figures 4-7), but also the differences in the isotherms (Figures 1and 2) and in the constant surface pressure relaxations (Figure 3). The BAM images show chiral discrimination in the monolayer morphology between the pure enantiomers N-dodecyl-D- and -L-gluconamideand the racemic mixture of this N-alkylaldonamide a t 25 "C at the air-water inter€ace. The enantiomeric N-dodecylgluconamidemonolayers form crystalline ramified growth patterns with a striking growth anisotropy of the side branching (Figures 4 and 5). In contrast to the dendritic crystallization of the enantiomeric monolayers, the racemic monolayers do not exhibit the growth of noticeable morphological forms (Figure 6). The isotropic growth observed a t compression monolayers sugof racemic N-dodecyl-D,L-gluconamide gests that a less ordered structure is formed. In spite of a careful inspection of the BAM images produced under very different experimental conditions, we have not been able to find differences in the morphological appearance of the D- and L-enantiomer. These crystalline dendrites differ by high stability in the shape from the few dendritic structures which to date have been observed a t other amphiphilic mon01ayers.l~ The grown dendrites of the enantiomeric monolayers remain stable at a given area per molecule. The monolayer morphology of N-dodecylgluconamide developed in the region of low surface pressure shows distinct chiral discrimination visualized by the dendritic crystallization of the enantiomers and an isotropic solidification of the racemic mixture. The dendritic crystallization is accounted for by supersaturation of the monolayer system initializing the nucleation and immediate growth of the different solid aggregates. Chiral discrimination of the monolayer morphology of the N-dodecylgluconamides is observed at all temperatures investigated. However a t lower temperatures (10 "C)the dendritic crystallization of the enantiomeric monolayers is not as well developed as that at higher temperatures (25 "C). The differences in the surface pressure isotherms of the pure enantiomeric forms and the racemic mixtures (Figures 1and 2) support the striking chiral discrimination evidenced by the morphological behavior. Surface pressure isotherms show the racemic monolayer to be less expanded thgn the enantiomeric monolayer a t 10 "C. This suggests heterochiral discrimination. In a theoretical study, Andelmads calculated the chiral discrimination of various types of intermolecular interaction, such as van der Waals, dipoles, and charges, for particular tripodalshaped molecules. A preferred heterochiral behavior was predicted for van der Waals interactions, while homochiral discrimination was calculated for dominant electrostatic interactions. As discussed subsequently in more detail, additionally the hydrogen bonding between the molecules must be considered for N-dodecylgluconamide monolayers. At higher temperatures (25 "C),however, the situation is reversed over a wide surface pressure interval between 0 and about 15 mN m-l. Formally, for equilibrium isotherms homochiral discrimination seems to be indicated. It is necessary to note that at high stdace pressures for low temperatures (10 "C)the molecular areas correspond to reasonable values of a tight monomolecular (17)Akamatsu, S.; Bouloussa, 0.;To, K.; Rondelez, F. Phys. Rev. A 1992,46, R4504-R4507. (18)Andelman, D. J.Am. Chem. SOC.1989,111,6536-6544.
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packing while for higher temperatures (25 "C) the of the alkyl chains toward the aqueous phase is detected adequate area values are too low for a monomolecular in both systems. packing, which, obviously, can be traced back on the At further compression, the grown dendrites of the effective relaxation kinetics. enantiomeric N-dodecylgluconamide monolayers break into pieces if they touch to each other and fill up the gaps In contrast to the isotherms, the chiral discrimination between them. Finally, the dendritic structures can be of the constant surface pressure relaxation at 10 mN m-l, compressed so tightly that they appear as a dense solid especially observed for higher temperatures (25 "C), has the same tendency as that described for monolayers of monolayer homogeneously reflecting. Under these condiN-stearoylserine methyl ester;16 i.e., the racemic form tions, the racemic monolayer of N-dodecylgluconamide relaxes more rapidly. shows a similar pattern at the air-water interface. At least two features of the relaxation kinetics affect As demonstrated by electron microscopic studies, chiral the isotherms of N-dodecylgluconamide in a complex discrimination was preserved in supported collapsed way: (i)the increasing relaxation rate with temperature monolayers of enantiomeric and racemic 12-hydroxystearic a ~ i dand ~ stearoylserine ~ $ ~ ~ methyl ester.26 Brewand (ii) the chiral discrimination effect of the relaxation ster angle microscopic investigation of collapsed Nrate. The effect of both aspects can explain the quite flat dodecylgluconamide monolayers did not visualize any surface pressure increase of the racemic compression differences in the patterns obtained. In principle, the isotherm at 25 "C. The maximum compression rate is not ability to detect three-dimensional structures grown by high enough, so that the effect of the relaxation shifts the relaxation of monolayers or by collapse using Brewster molecular area with increasing compression time to lower angle microscopy has been demonstrated r e c e n t l ~ . ~ ~ , ~ ' values. The isotherm presented for a low temperature (10 "C)is affected least by the relaxation kinetics as both Hydrogen Bonding in the Two-DimensionalAgthe absolute relaxation rate and the differences in the gregates of N-Dodecylgluconamide. The importance relaxation of the enantiomeric and racemic N-dodecylof strong hydrogen bonds between secondary amide groups gluconamides increase considerably with the temperature. has been recently demonstrated for the enantiomeric N-alkylaldonamides both at the formation of a variety of Over the last decade, similar relaxation of various superstructure assemblies in aqueous gels, such as longamphiphilic monolayers has been studied. A large number chain tubes, rods, ribbons, or complex helices, and for the of amphiphilic monolayers shows a wide region of superrectangular molecular orientation in the crystalline state. saturation leading to this monolayer relaxation. At present, it is evident that the relaxation involves the The formation of the three-dimensional structures found transformation of monolayer material to overgrown threein the aqueous gels of the enantiomers was traced back dimensional structures which has been quantitatively to the cooperative amide hydrogen bond chains formed by described by different nucleation-growthm e c h a n i ~ m s . l ~ - ~ ~the amide group of the molecules and by a certain bent conformation of the head groups allowing additional It seems reasonable to suppose that the relaxation of hydrogen bonding resulting in a so-called "chiral bilayer N-dodecylgluconamide monolayers is also caused by a e f f e ~ t " . ~In- ~ the ~~ crystalline ~ state of the enantiomeric transformation of monolayer material into three-dimenN-alkylaldonamides,a complex cyclic network of hydrogen sional structures. The special shape of the relaxation bonds ofthe hydroxyl groups in 1,3-syndiaxial-orientation curves suggests, however, another growth mechanism was evidenced.'-1° These hydrogen bonds stabilize a than the compact growth evidenced for simple single chain rectangular orientation of the molecules toward each other. amphiphiles.20,22 It is interesting to note that the crystalline racemic The relaxation mechanism of the N-dodecylgluconamide N-alkylglyconamidesform molecular racemates, i.e., demonolayers is not yet completely clear. There are neither mixture of the enantiomers and mutual dissolution of both an indication of a random nucleation and immediate enantiomers can be excluded. Experimental evidence for growth of three-dimensional compact structuresz2nor a a formation of molecular racemates has been provided by partial dissolution of molecules expelled from the monotheir higher melting enthalpies compared to the other layer into the aqueous ~ u b p h a s e .However, ~~ it seems to possibilities.28 Up to now the crystal structure of the be quite reasonable to assume that as a consequence of racemic N-alkylgluconamide has not been solved.1° the 2D-3D relaxation the enantiomeric monolayersform multilayered dendritic crystals, while the racemic monoTwo-dimensionaldendritic crystallization can occur in layers are transformed into multilayered structures the N-dodecyl-D- or -L-gluconamide monolayers already isotropically distributed. The packing of N-octyl-D-gluafter spreading. The formation of this solid phase seems conamide in the crystalline state shows a head-to-tail to be less affected by hydrophobic interactions of the alkyl layered arrangemenL8 Thus, the epitactical formation of chains, but rather by the interactions between the polyolic molecular layers with an adequate head-to-tail arrangechiral head groups. The preferred occurrence of hydrogen ment of the molecules and a thickness of more than one bonds between the amide groups of the gluconamidesand layer on the water subphase is probable. between syn-oriented hydroxyl groups, as depicted in the crystalline state of homologous alkyl gluconamides,8J0has It is interesting to note that in aqueous gels the racemic been stated as an important feature in the formation of mixture of N-dodecylgluconamide aggregates only as an superstructures of these amphiphiles in aqueous gel^.^,^ unstructured precipitate.6 The isotropic solidification at It can be expected that equivalent interactions are realized the air-water interface could indicate molecular interacalso at the air-water interface. tions equivalent in both environments. No efficient packing of the head groups with opposite chirality occurs The orientation of the molecules in the solid state of a and a random solidification due to the hydrophobic effect monolayer can be considered as comparable to that in the (19)Vollhardt, D.; Retter, U. J. Phys. Chem. 1991,95,3723-3727. (20)Vollhardt, D.; Retter, U. Thin Solid Films 1991,199,189-199. (21)Vollhardt, D.;Ziller, M.; Retter, U. Langmuir 1993,9, 32083211.
(24)Uzu, Y.; Sugiura, T. J . Colloid Interface Sci. 1976,51,346-349. (25)Tachibana, T.; Hori, K. J. Colloid Interface Sci. 1977,61,398400. (26)Harvey, N.G.; Mirajovsky, D.; Rose, P. L.; Verbiar, R.; Amett, E.M. J . A m . Chem. SOC.1989,111,1115-1122. (27)Gutberlet, T.;Vollhardt, D.Submittedfor publicationinColloids Surf. (28)Svenson, S. Ph.D. Thesis, Free University, Berlin, 1993.
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2666 Langmuir, Vol. 11, No. 7, 1995
Figure 8. Suggested orientation of N-dodecylgluconamide at the air-water interface. The orientation and conformation of the molecules are as found in crystal structure analysis of homologous ~ - g l u c o n a m i d e s .The ~ ~ ~configuration ~ of the Lgluconamide was modeled by exchange of the corresponding atoms and groups of the D-gluconamidestructure. Regions with hydrogen bonds are underlaid gray. (a)The enantiomeric form N-dodecyl-D-gluconamideviewed on top of the amide hydrogen bond chain in the left side and in direction of this chain in the right side. The hydrogen bonds between the hydroxyl groups of the sugar head groups continue parallel to the amide hydrogen bond chain in a zigzag configuration. (b) The racemic form N-dodecyl-D,L-gluconamidein the same view. While the amide hydrogen bond chain is not affected, the cycle of hydrogen bonds within the sugar head groups of the four molecules is largely disturbed.
crystalline state, the more so as the enantiomeric Nalkylgluconamidesare packed in an unsymmetric layered head-to-tail orientation exceptionally for amphiphiles.8 The resulting hydrogen bonds in the plane of the head groups are shown in Figure 8a. Parallel to the air-water interface, the chain of the hydrogen bonds between the amide groupsis located, while the hydrogen bonds between the hydroxyl groups of the sugar head groups are lined up rectangularly in a zigzag configuration. Thus the coordinated formationof the hydrogen bonds between four enantiomeric polyolic head groups dominate the aggregation a t the air-water interface, while in this respect the hydrophobic van der Waals interactions of the aliphatic chains will play only a minor role. It seems reasonable to suppose that the geometry of the visualized dendrites correlates with the development of the intermolecular hydrogen bonds as discussed above. Thus, the dendritic growth at the molecular level would be directly related to the development of the intermolecular hydrogen bonding cycle, and the molecular orientation order of the investigated enantiomeric N-dodecylgluconamides a t the air-water interface would be identical as found in the crystalline state, showingthe growth of solid
aggregates of N-dodecyl-D- or -L-gluconamide as twodimensional crystalline dendrites. The difference in packing of the chiral molecule with itself (enantiomeric form) and with its mirror image (racemic form) is sketched in Figure 8b. In the racemic mixture, the cyclic network of hydrogen bonds is disturbed by packing a D- together with a L-enantiomer. Thus, no well-ordered crystalline growth can be developed in the racemic mixture. The chiral discrimination and, therefore, the difference in the two-dimensional aggregation of the enantiomeric N-dodecylgluconamide and the racemic N-dodecyl-D,bgluconamideunder similar experimental conditions can be traced back to the molecularscale within this model. The remaining hydrophobic interactions of the alkyl chains are looked upon as reason for a compact aggregation of isotropic two-dimensional morphology of the racemic N-alkylgluconamide. Dendritic Crystallization in N-Dodecyl-D- or -Lgluconamide Monolayers. Dendritic crystallization, i.e., growth and structure of regular patterns, has been an actual subject of three-dimensional nonlinear nonequilibrium systems and its appearance has found also increasing experimental and theoretical interest in twodimensional structure^.^^^^^ The theoretical analysis has been simplified by reduction of the dimensionality to afford insight into the mechanism of the dendrite f~rmation.~‘ However, the corresponding experiments on dendritic crystal growth in quasi-two-dimensional systems, such as a narrow gap between parallel plates, were only partially in accord with theoretical prediction^.^^ A nonequilibrium model of dendritic growth is known as diffusion limited a g g r e g a t i ~ n .Here, ~ ~ the diffusion of latent heat during dendritic growth has been proposed to be the rate limiting step. Aggregation and growth mechanisms of dendritic structures can be understood on the basis of statistical mechanical models and have been simulated by random walks within two-dimensional lattice
model^.^^^^^ Amphiphilic monolayers seem to be ideal systems to realize two-dimensionaldendritic structures, the more so as they can be visualized by new microscopic techniques with good resolution. Such dendritic structures were visualized by fluorescence microscopy.16J7 Therefore the fractal patterns have been linked to the presence of the fluorescent probe35p36and the mechanism has been discussed a t large. Other fractal-like patterns, very different to the crystalline dendritic structures of the enantiomeric N-alkylaldonamide monolayers, have been formed by steplike monolayer compression into the two-phase coexistence region. If the compression is stopped, these nonequilibrium patterns are transformed into the quasiequilibrium domains.36 This morphological study of N-dodecylgluconamide monolayers presents the first example of dendritic crystallization in true 2D systems visualized by BAM. The enantiomeric N-dodecylgluconamide monolayers form regular ramified growth patterns of high stability in the shape characteristic for dendritic crystallization. During the formation kinetics, the system has a first-order phase transition referring to the existence of a line tension. The morphological behavior of the enantiomeric N(29) Langer, J. S. Rev. Mod.Phys. 1980,52, 1-28. (30) Brener, E. A.; Mel’nikov, V. I. Adu. Phys. 1991,40, 53-97. (31) Suresh, K. A.; Nittmann, J.; Rondelez, F. Europhys. Lett. 1988, 6,437-443. (32) Glicksman, M. E.; Schaefer, R. J.; Ayers, J. D. Metall. 2”runs.A 1976, 7A, 1747-1759. (33) Witten, T. A.; Sander, L. M. Phys. Rev. A 1983,27,5686-5697. (34) Nittmann, J.; Stanley, H. E. Nature 1986,321, 663-668. (35) Miller, A; Mahwald, H. J . Chem. Phys. 1987,86,4258-4265. (36),Knobler,C . M. Science 1990,249,870-874.
Dendritic Crystal Growth dodecylgluconamide monolayers deviates strikingly from the dendritic structures observed for most of other amphiphilic monolayers. All analytical data given in recent paper^^,^ provide evidence for the high purity of the enantiomers used, so that any effect of natural impurities on the growth mechanism can be excluded. It is interesting to note that the use of the Brewster angle microscopy allows one to investigate the two-dimensional morphology without addition of impurities in the form of fluorescent probes. Considering Figure 5, we can observe two different dendritic pattern of the enantiomeric N-dodecylgluconamide monolayers. On the left-hand side of the micrograph, the growth pattern resembles the tip-stable type for two-dimensional dendritic crystal growth, while on the right-hand side, noticeable dendritic side branches are formed. These growth patterns depend on the twodimensional supersaturation regime of the surrounding fluid phase which is caused by the monolayer compression rate. According to theoretical work, the tip-stable type is assigned to a moderate supersaturation regime. Growth patterns, as presented on the left-hand side of Figure 5 and characterized by noticeable dendritic side branches, are usually interpreted as resulting from the combined effect of a selective amplification of noise from the tip region and the advection of the perturbations away from the tip.37,38In this case, dendritic side branches are developedby small, noisy perturbations near the tip which have produced more or less large deformations away from the tip as a reason of the corresponding side branches. Small supersaturation reduces the tip growth nearly completely and permits only a slow growth of the side branches. The simultaneous appearance of both growth patterns could be accounted for by local differences in the supersaturation regime. A striking phenomenon of the growth kinetics of the enantiomeric N-dodecylgluconamides is the front propagation of the side branches. In recent paper^,^^,^^ side branch evolution of low anisotropic dendrites has been considered theoretically as a front propagation problem. It is discussed that the presence ofkinetic anisotropy would encroach the side branch front propagation. Indeed the growth kinetics of the enantiomeric N-dodecylgluconamides reveal two mechanisms of such a front propagation of the side branches (Figure 5). In the two-dimensional monolayers at the air-water interface, the thermally controlled instability can be assumed as completely suppressed because the heat can diffuse vertically away into the liquid substrate. Consequently the mass diffusion is completely confined to the two-dimensional monolayer and the unstable growth of two-dimensional dendritic structures is due to mass concentration gradients in the monolayers. The dendritic crystallization observed in the twodimensional enantiomeric N-dodecylgluconamide monolayers at the air-water interface is characterized by regular growth patterns of which the anisotropic growth shape is determined by the molecular chirality. The growth direction of the dendrites should be related to the axes of the two-dimensional symmetry arising out of the molecular packing at the air-water interface. It is conceivable that in the crystalline monolayer state the amphiphilic molecules are arranged in a hexagonal, (37)Langer, J. S.Science 1989,243,1150-1156. (38)Langer, J. S.Phys. Rev. A 1987,36,3380. (39)Ben-Jacob, E.;Brand, H. R.; Dee, G.;Kramer, L.; Langer, J. S. Physica D 1985,14348. (40)van Saarloos, W.; Caroli, B.; Caroli, C. J . Phys. I1993,3,741751.
Langmuir, Vol. 11, No. 7, 1995 2667 monoclinic, or orthorhombic lattice41leading to different monolayer structures. The dendritic crystallization is realized in preferential growth directions which agree with outstanding lattice directions. An orthorhombic molecular packing at the air-water interface having two lattice axes rectangular to each other may be indicated by the patterns found for slow growth in the dendritic structures of enantiomeric N-dodecylgluconamides with side branches rectangular to the main branch. The specific hydrogen bonds, as found in the crystalline state, favor a packing of the amphiphiles in such a rectangular lattice which renders possible also a monoclinic subcell in the crystal structure of N-octyl-Dgluconamide.8 Pattern formation and crystallization growth of enantiomeric N-dodecyl-D- or -L-gluconamide monolayers are characterized by the anisotropic dendritic growth shape due to the chiral amphiphilic head group. The enantiomeric N-dodecylgluconamide monolayers show always an asymmetrical growth toward one side of the main axis independent of the compression rate or the temperature of the subphase. The reason of this observation can be attributed to a depletion in the diffusion field along the growth direction related to the anisotropy of the chiral interactions. Further outstanding features of the dendritic crystalline structures are high stability in the shape independent of their growth rate and the complete morphological agreement between both enantiomeric forms. These features are in contrast to the chiral discrimination effect reported recently for the morphological properties of N-stearoylserine methyl ester monolayers which show differences in the orientation of the enantiomeric monolayers.16
Conclusions A representative two-dimensional system for chiral discrimination is demonstrated by N-dodecylgluconamide monolayers. Chiral discrimination of N-dodecylgluconamide monolayers is indicated by morphological differences and also differences in the isotherms and in the constant surface pressure relaxation. Brewster angle microscopy is a new advantageous method to visualize the chiral discrimination of the monolayer morphology. The enantiomeric N-dodecyl-D-or -L-gluconamide monolayers develop dendritic crystal growth, while an isotropic solidification of their racemic mixtures occurs. This behavior corresponds to the chiral discrimination in aqueous gels. The dendritic crystallization can be accounted for by supersaturation of the monolayer system initializing nucleation and succeeding growth of solid aggregates. An asymmetrical growth toward one side of the main axis is a characteristic feature of the enantiomeric N-dodecylgluconamide monolayers. Experimental evidence revealing two mechanisms for the front propagation of the dendritic side branch growth is provided. On a molecular scale, the distinct formation of hydrogen bonds between neighboring hydroxyl groups and amide groups of enantiomeric N-dodecylgluconamide molecules is favored. It seems reasonable to suppose that as a consequence of the 2D-3D relaxation the enantiomeric monolayers form multilayered dendritic crystals. In contrast, the racemic monolayers are transformed into multilayered structures isotropically distributed. In accordance to the crystalline state, a head-to-tail arrangement of the successive layers formed during the monolayer relaxation is probable. (41)Bibo, A. M.; Knobler, C. M.; Peterson, I. R. J . Phys. Chem. 1991, 95,5591-5599.
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2668 Langmuir, Vol. 11, No. 7, 1995
Similar to the superstructures in aqueous gel^,^,^ the chiral discrimination of the monolayer morphology can be related to the stereospecific configuration and conformation of the sugar head groups of the gluconamides. Hence the chirality of like and unlike molecule pairs influences the growth of dendrites in two dimensions at the air-water interface. The direction of growth depends on the molecular packing in the two-dimensional lattice. At the molecular level, the dendritic growth can be directly traced back to the development of an intermolecular hydrogen bonding cycle which is disturbed by packing a D- together with a L-enantiomer. The molecular organi-
zation in the dendrites may be closely related to the packing and orientation in the crystalline state of the enantiomeric N-alkylgluconamides.8-10
Acknowledgment. We thank Ms B. Kling for the preparation of the N-dodecylgluconamides. Financial assistance from the Deutsche Forschungsgemeinschaft (SFB312 “Gerichtete Membranprozesse”)and the Fonds der chemischen Industrie is gratefully acknowledged. LA9500179
