ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
A simple model for lipid monolayers on water surfaces at high spreading pressure is investigated in this work. In this model, the hydrophilic head group of the lipid molecules form a rigid regular triangular lattice, and the hydrophobic alkane chains (assumed to be in an all-trans state) are represented by rigid rods with two angular degrees of freedom (θ, cursive-phi). The rods consist of "effective monomers,'' and between the effective monomers on neighboring rods a Lennard-Jones interaction is assumed. The model is studied by exact ground-state calculations, mean-field theory, and Monte Carlo simulations. Basic parameters are rod length a and lattice constant b. The ground-state phase diagram shows the following phases: for small b, the rods are oriented perpendicularly to the monolayer plane (no-tilt phase, 〈θ〉=0); for somewhat larger b, a sixfold degenerate uniform-tilt state occurs with all rods tilted towards one of their next-nearest neighbors. For still larger b, the rods are tilted nonuniformly and form a "striped'' structure. These unexpected phases do not occur if we allow a rectangular distortion of the lattice. For T〉0, the simplest mean-field theory predicts a gradual disordering of the uniform-tilt state via a second-order phase transition. For the transition region, the Monte Carlo results disagree with this picture. Instead they show a strong asymmetric first-order phase transition with pronounced hysteresis. The transition temperature increases with increasing rod length a, qualitatively similar to experiment.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.462077
