Buckling of amphiphilic monolayers induced by head-tail asymmetry

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.