Buckling of Amphiphilic Monolayers Induced by Head-Tail Asymmetry

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Abstract
Packing asymmetry between head-group and tail-chain of anlphiphiles may in- duce 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, (it) molecu- lar tilt divergence, and (iii) local composition variation in nlixed monolayers. For a pure mono- layer with non-zero spontaneous curvature, we find that, below sortie 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 nlixture 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 nluch snlaller anlplitude, and results from the curvature-composition coupling. Implications for the process of spontaneous emulsification are also briefly discussed.

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