Liquid phases of Langmuir monolayers

Abstract

Monte Carlo simulation of the solid to super liquid phase transition of Langmuir monolayers using crosssection potentialsThe liquid phases of Langmuir monolayers are studied in a simple continuum model in which molecules are treated as flexible chains made up of rodlike segments, with the head confined to the air-water interface. The model is studied within mean-field approximation. We show that both properties of chain flexibility and of segment rigidity, with concomitant anisotropic rodlike interactions, are needed to produce two stable liquid phases. Uniform tilt can occur in the liquid-condensed phase, but requires a mismatch between the size of the head segment and that of the others comprising the chain. The chain conformational order, and the effect of chain length on phase behavior, are also discussed.

Short grafted chains: Monte Carlo simulations of a model for monolayers of amphiphiles

“…The transition from a hexatic to a fluid phase is usually believed to be continuous. In the case of amphiphile monolayers, however, we have argued that it can be driven first order, as an effect of the interplay between chain entropy and chain packing [17]. We have already noted that the melting transition in our system is mainly driven by the chains, which enhances the likelihood of such a scenario.…”

“…Grafted rigid rods exhibit tilt transitions [12][13][14], molecules with non-circular cross-sections show rotator transitions [15]. For the transition between the liquid condensed and the liquid expanded phase, however, the conformational degrees of freedom of the chains play a crucial role [10,16,17]. They have been incorporated in a heuristic way as "internal degeneracies" in Ising-type two-dimensional lattice models for monolayers and bilayers, e.g., in the Pink model [18,19].…”

Influence of interaction energy in fluid-fluid phase transitions on Langmuir monolayers

“…The second reason may be the assumption of rigid rods for the hydrophobic tails driven out of the surface into the air under compression. Schmid and Schick [16] and Schmid [17] showed that the two phase transitions occur only for two sufficiently flexible chains above the surface and that the increase of rigidity gives rise to only a single transition from a gaseous to a condensed phase. Therefore one may conclude that pure lattice models are able to predict only one fluid-fluid phase transition on Langmuir monolayers.…”

Toy amphiphiles on the computer: What can we learn from generic models?

“…It is clear that the optimized amphiphile models discussed in the previous section are too simple to be a good starting point for studies of internal membrane transitions. Theoretical mean-field calculations [95] indicate that the main transition is driven by an interplay between the conformational entropy of the tails and their tendency to develop nematic order, hence a good model should take into account the chain character of lipids. Our membrane model [96] is based on an amphiphile model that has already been used successfully to study phase transitions in Langmuir monolayers [97][98][99][100][101][102][103] The lipids are modeled by semiflexible chains of 6 "tail" beads attached to one slightly larger "head" bead.…”