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, 1996
X
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 (triacyl-
glycerols) 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 rare-
earth 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, thermo-
tropic 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 headgroups
1,2,6
(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 straight-
ened. Examples include the following: diglycerides (1,3-
diglyceride of thiododecanoic acid;
1
1,3-diglyceride of 11-
bromoundecanoic acid
7
); ceramides (24-pSp
8
; h218-pSp
9
);
triglycerides (-trilaurin,
1
′-triundecanoin
1
(“two-up, one-
down” conformation)); metallic soaps
10
and derivatives
(anhydrous copper(II) decanoate;
11
and, rare-earth (III)
soaps (e.g., triple-chain lanthanide octanoates);
12
and,
cobalt stearate-pyridine complex
13
). Here the chains
X
Abstract published in Advance ACS Abstracts, November 1,
1996.
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and Technical Applications; The Oily Press: Dundee, 1994.
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pp 34-49.
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Biophys. Acta 1992, 1113, 339-373 and references therein.
(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, 2311-
2320.
(11) Lomer, T. R.; Perera, K. Acta Crystallogr. 1974, B30, 2912-
2913.
(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.
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 non-
swelling (in water) double- and triple-chain amphiphiles.
5528 Langmuir 1996, 12, 5528-5529
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