J. Phys. Chem. 1992,96, 15-16
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A Correlation betwemn Surface Wettablltty and Solvent Effect on Crystal Growth. The N-n-OctyCD-gluconamide/Methanol System Jim-Lung Wang, Leslie Leiserowitz,* and Meir Lahav* Department of Materials and Interfaces, The Weizmann Institute of Science, Rehovot 76100, Israel (Received: September 16, 1991)
Different growth rates on the opposite hemihedral faces of polar, platelike crystals of N-n-octyl-D-gluconamide were observed. One face is hydrophobic and the other hydrophilic, in keeping with contact angles of various solvents on these faces which were measured. It was found that the different growth rates can be explained by the hydrophilicity and hydrophobicity of the crystal surfaces.
Although solvent has a strong influence on the structure and habit of crystals, its effect is still not well understood. At present, there are two distinct and apparently diametrically opposed a p proaches to clarify the effect of solvent-surface interactions. Calculations based on “surface-roughening”considerations predict that favorable interactions between solute and solvent on specific faces will lead to reduced interfacial tension, causing a transition from a smooth to a rough interface, and a concomitant faster surface growth.’V2 This approach is supported, in part, by the effect of water on the growth of (R,S)-alanine and y-gly~ine.~ It was deduced that if a solvent at a particular face is strongly bound at a subset of surface sites, and repelled, or very weakly adsorbed, at the remaining surface sites, the crystal face may be exposed to a cycle of solvent binding at a subset of surface sites, solute adsorption at the “free sites”, followed by solvent expulsion, leading to relative fast growth of this face. Alternatively, it has been proposed that the preferential adsorption of solvent molecules at specific faces will inhibit growth of those faces as removal of a bound solvent molecule poses an additional energy barrier for continued In some of these studies, the binding of polar solvents was estimated by calculations of the electrostatic potential at the crystal surface and interpreted in terms of crystal surface hydrophobicity and -phili~ity.~-~ This approach is compatible with the observed effect on molecular crystal morphology arising from the presence in solution of Ytailor-made” molecular additives, which have a structure very similar to that of the molecule to be crystallized, but for an altered moiety.8 The change in morphology was interpreted in terms of stereospecific adsorption to specific faces such that the altered moiety of the additive emerges from the crystal surface, causing an inhibition of growth perpendicular to the surface, generally resulting in an increase of its surface area relative to the unaffected faces. In order to better bridge the gap between the effect of tailor-made additives and of solvents, these studies were extended to examine the effect on the growth of crystal solvates by “tailored” solvents. For example, the change ~~
~
(1) (a) Bennema, P.; Gilmer, G. In Crysral Growth; An Introduction; Hartman, P., Ed.;North Holland: Amsterdam, 1973; p 272. (b) Elwenspoek, M.; Bennema, P.; van der Eerden, J. P. J. Crysr. Growrh 1987,83,297. (c)
Bennema, P.; van der Eerden, J. P. In Morphology of Crysrals; Terra Scientific Publishing Co.: Tokyo, 1987; pp 1-75. (2) Bourne, J. R.; Davey, R. J. J . Cryst. Growrh 1976, 36, 278, 287. (3) Shimon, L. J. W.; Vaida, M.; Addadi, L.; Lahav, M.; Leiserowitz, L. J . Am. Chem. Soc. 1990, 112, 6215-6220. (4) (a) Wells, A. F. Philos. Mag. 1946, 37, 184. (b) Wells, A. F. Discuss Faraday Soc. 1949,5, 197. (5) Wireko, F. C.; Shimon, L. J. W.; Frolow, F.; Berkovitch-Yellin, Z.; Lahav, M.; Leiserowitz, L. J . Phys. Chem. 1987, 91, 472. (6) Berkovitch-Yellin, Z. J. Am. Chem. Soc. 1985, 107, 8239. (7) (a) Davey, R. J. J. Crysr. Growrh 1986, 76, 637. (b) Davey, R. J.; Milisavljevic, E.; Bourne, J. R. J . Phys. Chem. 1988, 92, 2032. (8) Weissbuch, I.; Addadi, L.; Lahav, M.; Leiserowitz, L. Science 1991, 253, 637-645.
in crystal habit of asparagine monohydrate and of rhamnose monohydrate was examined by the added presence of methanol in s ~ l u t i o n .It~ was found that such tailored solvents behave in a manner akin to tailor-made additives. Here, we present further evidence in favor of the idea that strong solvent-surface interactions retard crystal growth, by making use of crystal surface wettability measurements to provide an experimented measure of the difference in adhesion of the solvent to the different crystal faces. Recently, we have observed a difference in wettability at the opposite hemihedral faces of the platelike crystals of N-n-alkylgluconamide (CH3(CH2),,NHCO(CHOH)4CH20H, n = 6-9).9 The structure of one face of the plate is hydrophobic and that of the opposite face hydrophilic, in keeping with the wettability measurements. Within the crystal series (n = 6-9), we found that the octyl derivative ( n = 8) yields large well-shaped crystals and so was used as a model system for the study of the effect of solvent on crystal growth. N-n-Octyl-D-gluconamide crystallizes in space group P2,with two molecules in the unit cell (Figure The molecules form layers by translation symmetry only. Within each layer the molecules are interlinked by hydrogen bonds at the gluconamide moiety and by methylene-methylene contacts at the alkyl side. The layers are stacked head-to-tail in a polar arrangement along the twofold screw symmetry axis b. The crystals form plates whose (0101 faces are parallel to the molecular layers. Thus the wmethylene group at the hydrophobic end of the molecule emerges at one (010)face of the crystal plate and the terminal CH20H moiety at the opposite face, the OH group oriented such that its oxygen lone pair electrons protrude from the crystal surface. For sugars of (D) configuration the (010) face of the crystal plate is hydrophobic and the opposite (010) face is hydrophilic. The reverse situation holds for molecules of (L) configuration. Crystals of octyl-D-gluconamide were grown from methanol at room temperature by slow evaporation. Crystals with wellexpressed surfaces were chosen for the contact angle measurements and the effect of solvent on crystal growth. The advancing contact angles on the hemihedral [OlO)faces of the crystals were measured with four different solvents and one solution mixture: glycerol, water, ethylene glycol, methylene iodide, and a methanol solution saturated with octylgluconamide (the mother solution of the crystal). The intrinsic property of the crystal to cleave along the (010)face provided two clean and fresh surfaces, one hydrophobic and the other one hydrophilic, yet of similar texture for the contact angle measurements. The results are given in Figure 2. Following the contact angle measurements, some of the smaller single crystals, which had been cleaved to be of equal thickness, were (9) Wang, J.-L.; Lahav, M.; Leiserowitz, L. Angew. Chew., Inr. Ed. Engf. 1991, 30, 696. (10) Zabel, V.; Miiller-Fahrnow, A.; Hilgenfeld, R.; Saenger, W.; Pfannemiiller, B.; Enkelmann, V.; Welte, W. Chem. Phys. Lipids 1986, 39, 313-327.
0022-365419212096-15$03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry. Vol. 96. No. I . 1992
Letters
Figure 1. Stereoview of packing arrangement of N-n-OCtyl-Dgluconamide crystal.
Figure 3. Morphology of Iv-n-octylgluconamide crystal grown from methanol. The seeds can be observed as an opaque shadow at the bottom of the crystals. Thickness of added material on the hydrophobic face is denoted as B, and that added on the hydrophilic surface is denoted as L. For IO experiments, the B/L ratios have an average value of about 5, with the ratio ranging from 4 to 7.
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B/L = 6
A
A O
A
O
0 40-
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used for the crystal growth experiments. The pairs of crystals were placed on the bottom of the solution vessel with their fresh faces, hydrophobic (010)and hydrophilic (OIO), exposed to the solution. The cleaved crystals were allowed to grow at room temperature for five days, after which they were removed. Contact angle measurements on these crystals confirmed that they kept their original wettability and so their original polarity.” There were dramatic differences in the added thickness of the two crystals within each pair and thus in the growth rate at the two-cleaved hemihedral faces. Crystals with the hydrophobic (010) face exposed grew about 5 times faster than their counterpart whose hydrophilic (010) face was exposed to the solution (Figure 3). ( 1 1) In this way we showed that the crystals did not form twins about the
exposed face. Moreover. twinning was not observed in any of the crystals measured according to X-ray diffraction measurements.
We correlate this difference in growth rate at the hydrophilic and hydrophobic faces with the measured difference in contact angle at the two types of surfaces (Figure 2), in particular the angles of 2 5 O and 52’ of the saturated methanol mother solution. The contact angle between a liquid drop and a flat solid surface reflects the difference between cohesion forces of the liquid and the adhesion forces to the solid surface.’* Consequently, during crystal growth of octylgluconamide, the methanol solvent has a stronger affinity to the (010) crystal face which exposes the sugar moieties than the (010)face which exposes the u-methyl groups. Therefore, we deduce that the more tightly bound the solvent is to the surface of the growing crystal, the slower the growth of the face.I3 We qualify this statement with the observation that all the surface sites at each of the opposite hydrophobic and hydrophilic faces of octylgluconamide can make the same contact with solvent. If this condition does not hold, as would be the case for many crystals, the situation may be less clearcut. For example, the crystals of (R, S)-alanine and of y-glycine, grown in aqueous solution, do indicate a different set of rules may apply. Nevertheless, the general effect of tailor-made additives or tailor solvents on a variety of crystals is certainly in keeping with the findings presented here.
Acknowledgment. We thank the US-Israel Binational Science Fundation (BSF), Jerusalem, for financial support. ( 12) Zisman, W. A. In Conracr angles, Wefrabiliry and Adhesion; Fowkes,
F. M.. Ed.; Adv. Chem. Ser. No.43; American Chemical Society: Washington, DC. 1964; pp 1-5 1. ( I 3) We have assumed that growth rates of the hydrophobic (010) and hydrophilic (010) faces are the same in the absence of solventsurface interact ions.
