Statistical thermodynamics of monolayers of flexible-chain amphiphiles: Adding nearest-neighbor correlations to a Scheutjens–Fleer approach

In recent work, Wang and Rice have developed a lattice statistical thermodynamic model for amphiphile monolayers, based in part on the work of Scheutjens and Fleer. As in Flory–Huggins theory, their approach relies on a Bragg–Williams approximation to the probability of successful placement of a chain segment into a lattice site. This may be a poor approximation for typical monolayers, which contain relatively short hydrocarbon chains and are characterized by orientational anisotropy, particularly at high surface densities. A model is developed that incorporates intramolecular correlations in nearest-neighbor (bond) approximation and used to predict osmotic pressure and segment and bond distributions as a function of surface density. These results are compared with surface equations of state and segment distributions obtained in Scheutjens–Fleer approximation, i.e., in the absence of bond correlations. The two levels of approximation yield considerably different isotherms, although only minor differences in the segment distributions are predicted. In particular, inclusion of nearest-neighbor correlations is found to reduce greatly the magnitude of both entropic and energetic contributions to the lateral pressure.