LANGMUIR MONOLAYERS AS DISORDERED SOLIDS - STRAIN-STATE CALCULATIONS APPLIED TO STEARIC-ACID

This paper presents a calculational procedure to determine the equilib rium phase for a given surface pressure pi. The monolayer is treated a s orientationally free tails grafted to a two-dimensional net formed b y the head groups of the amphiphilic molecules. The head groups form a subsystem with translational degrees of freedom characterized by stra in variables in the plane of the surface, and the tail groups compose a subsystem characterized by rotational degrees of freedom. The order in the monolayer derives indirectly from the crystalline head groups t hrough translational-rotational coupling. A stress-strain relation is derived which shows the energetically most favorable path for reorient ation of the molecules due a a two-dimensional strain This set of stra in stares for a given symmetry (phase) allows a contribution to the st rain-state partition function to be computed. It is then straightforwa rd to calculate the strain-state contribution to the free energy for a given phase and estimate the transition temperature between phases. ( C) 1997 American Institute of Physics.