In this paper we describe a lattice density functional theory for the
rotator-herringbone phase transition in a liquid supported monolayer s
ystem of long chain acids or alcohols. It is assumed that both phases
have long-range translational order and that the rotator phase is char
acterized by a uniform distribution of backbone orientation. We find t
hat orientational interactions among the backbone planes an sufficient
to drive a second order transition, and we characterize the phase of
the system by a parameter associated with the width of the peaks in a
distribution which describes the orientations of the backbones. We als
o briefly discuss generalizations of the theory to include lattice vib
rations and distortions. (C) 1995 American Institute of Physics.