Phase behaviour of model Langmuir monolayers

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Title Phase behaviour of model Langmuir monolayers
Author(s) Sheldon B. Opps
Date 2001
Publisher University of Guelph, 2000
Place published Ottawa :National Library of Canada
Abstract In this work, we examine some of the properties of a model Langmuir monolayer, which consists of surfactant molecules (amphiphillic organic molecules) deposited onto a water subphase. The surfactants are modeled as rigid rods composed of a head and tail segment. The tails consist of 'nt' ≈ 4-7 effective monomers representing CH2 methylene groups. These rigid rods interact via site-site Lennard-Jones potentials with different interaction parameters for the tail-tail, head-tail and head-head interactions. The present research has focused on some of the key physical parameters, such as chain length, bond length, and head diameter, responsible for driving tilting transitions from uniformly tilted to untilted phases and between different tilt-ordered phases. At zero temperature, by systematically increasing the head-tail diameter ratio and bond length, we have observed continuous transitions from untilted to tilted phases and first-order transitions from next-nearest-neighbor (NNN) to nearest-neighbor (NN) tilted phases. Some metastable phases have also been discovered where coexistence occurs between molecules with the same kind of tilt ordering. To investigate system properties at finite temperature, we performed Monte Carlo simulations in the constant molecular number N, area A, and temperature T (NAT) ensemble. Some of the earlier studies incorporated a continuum model to characterize the water subphase, but this led to unphysical results. Subsequent work, where the water was replaced by a hard surface, examined the effects on the tilting transition temperature 'T c' due to variations in density, chain length and head-tail diameter mismatch. In some recent studies, we have utilized a more realistic model for the surfactant-water interactions, developed by Karaborni and Toxvaerd, in order to re-examine the role which the water subphase has on tilt-ordering.

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