J. Phys. II France
Volume 6, Numéro 7, July 1996
Page(s) 999 - 1022
DOI: 10.1051/jp2:1996113
J. Phys. II France 6 (1996) 999-1022

Buckling of Amphiphilic Monolayers Induced by Head-Tail Asymmetry

J.-G. Hu and R. Granek

Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel

(Received 2 October 1995, received in final form and accepted 27 March 1996)

Packing asymmetry between head-group and tail-chain of amphiphiles may induce buckling modulations in monolayers at air-water or oil-water interfaces. We consider three different cases associated with the head-tail asymmetry: (i) spontaneous curvature, (ii) molecular tilt divergence, and (iii) local composition variation in mixed monolayers. For a pure monolayer with non-zero spontaneous curvature, we find that, below some critical surface tension, an hexagonal array of "long-fingers" becomes more stable than the flat surface. This "long-finger" structure is not expected to remain stable against multilayer formation in the case of a Langmuir monolayer, but is relevant for monolayers at the oil-water interface. When the molecular tilt is non-zero, as often is the case in the liquid condensed phase of Langmuir monolayers, the coupling between curvature and tilt can also give rise to a first-order buckling transition. Considering a binary mixture monolayer, we find that it can easily buckle to periodic structures following composition modulations. For the latter case we find two kinds of buckling structures. One involves a very large amplitude, a counterpart of the "long-finger" structure, and is dominated by the average spontaneous curvature. The other structure is of much smaller amplitude, and results from the curvature-composition coupling. Implications for the process of spontaneous emulsification are also briefly discussed.

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