Journal of Physical Chemistry, Vol.100, No.16, 6793-6800, 1996
Phase-Transitions in Quasi-2-Dimensional Molecular-Solids - Tilt-Induced Elastic Multipoles and Their Interaction and Ferroelasticity in Langmuir Monolayers
Using a solid state approach, elastic multipoles are demonstrated to be effective for modeling the tilting of molecules in Langmuir monolayers. As shown in a previous paper (Luty, T.; Eckhardt, C. J. J. Phys. Chem. 1995, 99, 8872), the effective rotational couplings between the molecules are determined by indirect interaction mediated by the elasticity of the medium supplemented by direct coupling. A detailed analysis of this indirect interaction and the idea of using local elastic multipoles to describe fluctuations in orientational degrees of freedom is the thrust of this work. Expressed in terms of spherical harmonics, elastic multipoles provide a convenient and powerful representation of the local stresses and of their interaction as mediated by the elastic field of the monolayer. It is shown that a strong interaction of the elastic dipoles leads to molecular orientational ordering wherein the ordered phase may be characterized as ferroelastic. In particular, it is concluded that monolayer phases with tilts toward nearest neighbor and next nearest neighbor are ferroelastic with the "swiveling" transition between them occurring as a strain reversal process that passes through an undeformed hexagonal lattice of vertical molecules.