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On the Swelling of Amphiphiles in Water R. W. Corkery and S. T. Hyde* Applied Mathematics Department, Institute of Advanced Studies, Australian National University, Canberra, 0200, Australia Received August 12, 1996. In Final Form: September 26, 1996X We present evidence supporting the proposition that amphiphiles displaying lyotropic mesomorphism (and are thus swelling in water) predominantly have double layers of polar headgroups running through lamellae of its pure, crystalline form. In contrast, we propose that amphiphiles NOT displaying lyotropic mesomorphism (and are thus nonswelling in water) predominantly have monolayers of polar headgroups running through the lamellae of its pure, crystalline form. Water can simply penetrate the double-layered headgroup sheets in the “swelling” amphiphiles, an impossibility in the “nonswelling” class. The latter chemicals are thus expected to be efficient scavengers of hydrophobic fluids in the presence of water.
It is well-known that lipids fall into two classes: swelling and nonswelling in water. Thus, triglycerides (triacylglycerols) and many diglycerides (diacylglycerols) are devoid of lyotropic mesomorphism, whereas related monoglycerides (monoacylglycerols) and some diglycerides exhibit a rich variety of liquid crystalline phases in water.1-3 The diglycerides straddle both classes, some exhibit lyotropic mesomorphism, for example, mono- and digalactosyl diglyceride and monoglucosyl diglyceride,3 and therefore belong to the swelling class, while the 1,3 diglyceride of 3-thiadecanoic acid does not swell (Kåre Larsson, private communication). A similar dichotomy can be found among the salts of fatty acids: alkali-metal soaps display lyotropic mesomorphism,3,4 while most alkaline-earth, transition-metal, heavy-metal, and rareearth soaps (metallic soaps) do not.5 Thus a standard preparation of the metallic soaps involves precipitation of the soap from an aqueous solvent.5 In contrast, thermotropic mesophases can be found among members of all four chemical classes. Further, all such amphiphiles readily swell in hydrophobic solvents. To our knowledge, the variation in water uptake, often remarked upon, has yet to be explained. A very simple answer may lie in the following observation of differences between crystalline forms of swelling and nonswelling lipids and soaps. A number of different crystal structures are to be found in each class; however the disposition of the hydrophobic aliphatic chains relative to the polar headgroups of the molecular crystals apparently falls into two groups. In all known crystal structures of swelling lipids and soaps, the chain or chains associated with each molecular headgroup pack adjacent to each other, so that (in the case of double-chain amphiphiles) the headgroup lies at one end of the hairpin-shaped molecule. The molecules assemble in the crystal to form bilayers, joined end-to-end at their aliphatic tails, and bounded on both sides by polar headgroups1,2,6 (simplified in Figure 1A). Crystal structures of nonswelling lipids and soaps are rarer. However, all examples we have located exhibit a distinct “splayed” chain structuresthe hairpin is straightX Abstract published in Advance ACS Abstracts, November 1, 1996.
(1) Larsson, K. Lipids-Molecular Organization, Physical Functions and Technical Applications; The Oily Press: Dundee, 1994. (2) Small, D. M. The physical chemistry of lipids: from alkanes to phospholipids; Plenum Press: New York and London, 1986. (3) Fontell, K. Prog. Chem. Fats Other Lipids 1978, 16, 145-162. (4) Luzzatti, V.; Mustacchi, H.; Skoulios, A.; Husson, F. Acta Crystallogr. 1960, 13, 660-667. (5) Kirk, R. E.; et al. In The Kirk-Othmer Encyclopedia of Chemical Technology; Mark, H. et al. Eds.; Wiley Interscience: New York, 1977; pp 34-49. (6) Pascher, I.; Lundmark, M.; Nyholm, P-G.; Sundell, S. Biochim. Biophys. Acta 1992, 1113, 339-373 and references therein.
S0743-7463(96)00794-9 CCC: $12.00
Figure 1. (A-C) Schematic view of the swelling mechanism for single- and double-chained amphiphiles in water. (D, E) Idealized drawing of the “splayed-chain” structure of nonswelling (in water) double- and triple-chain amphiphiles.
ened. Examples include the following: diglycerides (1,3diglyceride of thiododecanoic acid;1 1,3-diglyceride of 11bromoundecanoic acid7); ceramides (24-pSp8 ; h218-pSp9); triglycerides (β-trilaurin,1 β′-triundecanoin1 (“two-up, onedown” conformation)); metallic soaps10 and derivatives (anhydrous copper(II) decanoate;11 and, rare-earth (III) soaps (e.g., triple-chain lanthanide octanoates);12 and, cobalt stearate-pyridine complex13). Here the chains (7) Hybl, A.; Dorset, D. L. Acta Crystallogr. 1971, B27, 977-981. (8) Dahle´n, B.; Pascher, I. Acta Crystallogr. 1972, B28, 2396-2404. (9) Pascher, I.; Sundell, S. Chem. Phys. Lipids 1992, 61, 79-86. (10) Vold, R. D.; Hattiangdi, G. S. Ind. Eng. Chem. 1949, 41, 23112320. (11) Lomer, T. R.; Perera, K. Acta Crystallogr. 1974, B30, 29122913. (12) Mehrotra, K. N.; Shukla, R. V.; Chauhan, M. Bull. Chem. Soc. Jpn. 1995, 68, 1825-1831. (13) Corkery, R. W.; Hockless, D. C. R. Single-crystal x-ray structure. Submitted to Acta Crystallogr. C.
© 1996 American Chemical Society
Letters
associated with a single headgroup lie on opposing sides of the headgroup (Figure 1D, E). Thus the molecules are bounded at both ends by hydrophobic chains in the crystalline state, and the molecular layers are linked by van der Waals bonding between these hydrophobic chain ends. Each molecular layer contains a single polar sheet running through the headgroups, sandwiched by two aliphatic layers, in contrast to water-swelling amphiphiles, that contain two adjacent polar layers held together in the crystal by van der Waals interactions. Addition of polar solvents to form lyotropic liquid crystals in the latter case (above the Krafft temperature of the constituent molecules) is thus possible by separating the polar sheets (Figure 1B,C). Swelling of “splayed-chain” molecules can only be achieved by (i) breaking covalent linkages between chains and headgroups, (ii) flipping chains, or (iii) intercalating water between aliphatic chains. All these possibilities are unlikely at the low temperatures typically required to form lyotropic mesophases (typically less than 100 °C). The second option is perhaps the least unlikely; however no calorimetric evidence can be found to support this possibility. We suggest that those molecules whose crystals exhibit the splayed-chain architecture resist the uptake of water, are nonswelling in water, and are thus
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devoid of lyotropic mesomorphism. Such molecules typically contain two or more chains, so that the total chain cross-sectional area (associated with each headgroup) exceeds significantly that of the headgroups. Slight mismatches in area can be accommodated in the molecular crystal by inclination of the chains with respect to the axis normal to the headgroup layers, in the water-swollen liquid crystal, the mismatch results in warping of the layers. Larger area differences result in the splayed chain disposition, thereby reducing the area mismatch. Thus ideal defect-free crystals of multivalent metallic soaps (containing one chain per charge) and triglycerides of fatty acids are water-resistant, and the molecular features of water-resistant coatings containing such amphiphiles must share these features: bulky chains and suitable bonding geometry (or coordination about metal ions) to accommodate the splayed chain geometry. A useful corollary of this behavior is that such chemicals may be used to selectively scavenge hydrophobic liquids in the presence of hydrophilic liquids, for example, in the recovery of waste and spilled oil on water or the removal hydrophobic vapors from humid air. LA960794O
