Monte Carlo studies of model Langmuir monolayers

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Title Monte Carlo studies of model Langmuir monolayers
Author(s) Sheldon B. Opps, B. Yang, C. G. Gray, D. E. Sullivan
Journal Physical Review E
Date 2001
Volume 6304
Issue 4
Start page 041602
End page 041602
Abstract This paper examines some of the basic properties of a model Langmuir monolayer, consisting of surfactant molecules deposited onto a water subphase. The surfactants are modeled as rigid rods composed of a head and tail segment of diameters sigma (hh) and sigma (tt), respectively. The tails consist of n(t)approximate to4-7 effective monomers representing 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. In a previous paper, we studied the ground-state properties of this system using a Landau approach. In the present paper, Monte Carlo simulations were performed in the canonical ensemble to elucidate the finite-temperature behavior of this system. Simulation techniques. incorporating a system of dynamic filters, allow us to decrease CPU time with negligible statistical error. This paper focuses on several of the key parameters, such as density, head-tail diameter mismatch, and chain length, responsible for driving transitions from uniformly tilted to untilted phases and between different tilt-ordered phases. Upon varying the density of the system, with sigma (hh) = sigma (tt), we observe a transition from a tilted (NNN)-condensed phase to an untilted-liquid phase and, upon comparison with recent experiments with fatty acid-alcohol and fatty acid-ester mixtures [M. C. Shih, M. K. Durbin, A. Malik, P. Zschack, and P. Dutta, J. Chem. Phys. 101. 9132 (1994): E. Teer, C. M. Knobler, C. Lautz, S. Wurlitzer, J. Kildae, and T. M. Fischer, J. Chem. Phys. 106. 1913 (1997)], we identify this as the L-2'/Ov-L-1, phase boundary. By varying the head-tail diameter ratio, we observe a decrease in T-c with increasing mismatch. However, as the chain length was increased we observed that the transition temperatures increased and differences in T-c due to head-tail diameter mismatch were diminished. In most of the present research, the water was treated as a hard surface, whereby the surfactants are only allowed to move within the plane of this surface. However, we have also utilized a more realistic model for the surfactant-water interactions, developed by Karaborni and Toxvaerd, in order to examine the role which the coupled effects of head group size and head group-subphase interactions plays in determining tilt ordering and on the stability of the monolayer. It is found that increasing the head diameter results in a widening of the air-water interface and an associated destruction of orientational order. Furthermore, the onset of capillary waves at lower temperatures for larger head diameters implies that the L-2-L-1 phase boundary for acids and acetates should move to lower temperatures relative to the L-2'/Ov-L-1 phase boundary for alcohols acid esters. This feature has yet to be seen in experimental studies.

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