Forces between like-charged walls in an electrolyte solution: A comparisonof McMillan-Mayer results for several models

The interaction of like-charged walls immersed in aqueous solutions with mo novalent counterions is investigated at the McMillan-Mayer (MM) level of de scription. The net pressure acting between the walls is obtained by applyin g the anisotropic hypernetted-chain theory. The MM approach requires solven t-averaged ion-ion potentials of mean force as input. Results based on "rea listic" models for Na+ and Cl- in water are available in the literature and these are used in the present calculations. The wall-wall interactions obt ained can differ dramatically from the primitive model (dielectric continuu m solvent) case. For some models attractive wall-wall forces are observed a t small separations. The MM theory is found to be rather sensitive to detai ls of the counterion-counterion potential of mean force, and different mode ls for the same counterion can give qualitatively different results. At pre sent it is difficult to evaluate the relative accuracy of the different mod els that have been proposed. However, the results presented here give at le ast an idea of the interesting possibilities that lie in the physically rea listic range. (C) 2000 American Institute of Physics. [S0021-9606(00)50331- 6].