Chemical association in simple models of molecular and ionic fluids. IV. New approximation for the cavity function and an application to the theory of weak electrolytes

A multiplicative interpolation scheme (MIS) for the cavity function of associating hard‐core particles is proposed and applied to the case of our shielded sticky‐point model in which the particles associate into fused hard‐sphere diatomic molecules (hard dumbbells). The results indicate that the MIS is a significant improvement over the simple interpolation scheme (SIS) of our earlier paper when the bonding length is much less than the hard‐core diameter. The MIS is also applied to the case of a weak electrolyte modelled by shielded sticky‐point particles that are charged, which associate into hard dipolar dumbbells. Using a near‐exact hard‐sphere cavity function along with the mean spherical approximation (MSA) for the electrostatic part of the activity coefficient, we find that the resulting version of the MIS is already in reasonable agreement with the experimental data for monochloroacetic acid, despite the equal‐diameter restriction of the simple version of the model that we are using.