PRIMITIVE MODELS OF CHEMICAL ASSOCIATION .1. THEORY AND SIMULATION FOR DIMERIZATION

The structure and thermodynamic properties of a model of associating p articles that dimerize into fused-sphere dumbbells are investigated by MC simulation and by integral-equation theory. The model particles, i ntroduced by Cummings and Stell, associate as a result of shielded att ractive shells. The integral equation theories are of two types. The f irst is an extension of Wertheim's associative Percus-Yevick (APY) equ ation to the case of the shielded sticky shell model, which is the lim iting case of the shielded attractive shell model that can be handled analytically. The second is the extended mean spherical approximation (EMSA) of Zhou and Stell applied to the shielded sticky shell model. I n the case of partially associated systems, the EMSA requires as input the equilibrium association constant, which is obtained here using an exact relation between monomer density and a cavity correlation funct ion, together with an equation of state due to Boublik. The structure obtained from the EMSA is in good agreement with the predictions of th e MC simulation over a substantial density range that includes liquid- state densities, while the thermodynamic input from Boublik's equation is in excellent agreement with the simulation results for all densiti es. Predictions of the APY approximation are also in good agreement wi th the simulation results as long as the density of the system-is rela tively low or, at high density, when the hard-core volume of a dimer i s noe substantially less than that of the two free monomers from which it is formed. There is an intermediate density range in which neither integral-equation theory gives correlation functions of high quantita tive accuracy.