Features of the dimerization equilibrium in square-well fluids

We have analyzed chemical equilibria in dimerizing fluids where the pair interactions between monomers and dimers are described by square-well potentials of varying depths and widths. The association is limited by the dimerization stage, in which the dimer is a sphere circumscribed around overlapping monomers. The distances between their centers correspond to the bond lengths. The dissociation energy is considered as a parameter of the theory. Within the statistical-thermodynamic model of the chemical reaction in fluid phase inevitable volume changes in associative equilibria were also taken into account by means of a consistent equation of state (EoS). We show that the attractive part of the intermolecular interaction can manifest itself in deviations of the chemical equilibrium from ideality alternative with temperature. The influence of the attraction and repulsion between monomers and dimers on the position of associative equilibrium is different at high and low temperatures. The stronger the attraction between monomers and (or) the weaker the attraction between associates, the more the equilibrium shifts towards dissociation (especially at moderate temperatures). The changes in density were analyzed as a function of the variation of the square-well depth and width in the associated mixture as well. The increase in the packing fraction is most pronounced with an increase in the width, describing the attraction between the dimers, particularly in the case of small bond lengths. With this, the square-well attraction, in particular between the dimers, contributes to an additional ordering of the fluid. (C) 2018 Elsevier B.V. All rights reserved.