Langmuir monolayers as disordered solids: Strain-tilt-backbone coupling and natural order parameters for the swiveling transitions

Authors

Tadeusz Luty, University of Nebraska - Lincoln
David R. Swanson, University of Nebraska - LincolnFollow
Craig J. Eckhardt, University of Nebraska - LincolnFollow

Date of this Version

February 1999

Comments

Published by American Institute of Physics. J. Chem. Physics Vol. 110, No. 5, 1 February 1999. ©1999 American Institute of Physics. Permission to use. http://jcp.aip.org/.

Abstract

Multistability of Langmuir monolayers, in particular those composed of fatty acids, is reflected by a very rich and complicated phase diagram. We argue that strain-tilt-backbone coupling determines the behavior described by that diagram. Following the solid state approach, we show that a natural order parameter set is defined by thermal averages of spherical harmonics and strain tensor components. In addition, we show that the backbone order parameter can be conveniently represented by an elastic dipole tensor. Treating Langmuir monolayers as disordered solids, we have derived an orientational entropy contribution to the free energy. The swiveling transition between L₂ (L_2h) and L′₂(L₂^*) phases is discussed in detail and is conveniently described in terms of the proposed order parameters. We discuss why the change in the distortion direction of the 2D unit cell tracks the change in tilt direction of the molecules, and also why the cell dimensions involved in the swiveling transition are the same in both phases.