CAVITY FORCES AND CRITICALITY IN ELECTROLYTES

It has been suggested that observations of classical criticality (with beta = nu = 1/2, gamma = 1) in place of Ising-like behavior (beta congruent-to 0.32, etc.) in certain electrolytes might be associated with charge-cavity, ion-ion interaction potentials varying with distance as 1/r4. We report that such forces are strongly screened at nonzero ionic density, decaying as exp[-2kappar]/r2 when the direct Coulomb coupling is screened, as usual, by the factor exp [-kappar]. Within Debye-Huckel theory, furthermore, representing the ions as spheres of different dielectric constant does not change any critical parameters. Consequently, other explanations of the experiments seem called for.