A molecular theory of the hydration force in an electrolyte solution

A model, consisting of a pair of large macroions in a dipolar hard sphere-p oint ion electrolyte, is considered in order to evaluate the hydration forc e (solvent-mediated) contribution to the force between colloidal particles, which is missing in the DLVO theory. Using the mean spherical approximatio n (MSA), an explicit expression for this force is obtained. It is shown tha t the force consists of the hard-core exclusion term that was proposed rece ntly by Henderson and Lozada-Cassou (HLC) [J. Colloid Interface Sci. 121, 4 86 (1988)], and a dipole alignment contribution that originates from the or ientational ordering of the solvent molecules near the colloidal particles. The long-range asymptotic form of the total force is given by a Coulomb co ntribution and is described by the Poisson-Boltzmann or Derjaguin-Landau-Ve rwey-Overbeek (DLVO) result. The hydration force is short-ranged and extend s about ten solvent layers and is responsible for the oscillations of the t otal force. The total force that we obtain is similar to the semiempirical result of HLC. The comparison with the experimental results for a 10(-3) M KCl electrolyte solution is discussed. (C) 1999 Academic Press.