Electrolytic depletion interactions

We consider the interactions between two uncharged planar macroscopic surfa ces, immersed in an electrolyte solution, which are induced by interfacial selectivity. These forces are taken into account by introducing a depletion free-energy density functional, in addition to the usual mean-field Poisso n-Boltzmann functional. The minimization of the total free-energy functiona l yields the density profiles of the microions and the electrostatic potent ial. The disjoining pressure is obtained by differentiation of the total fr ee energy with respect to the separation of the surfaces, holding the range and strength of the depletion forces constant. We find that the induced in teraction between the two surfaces is always repulsive for sufficiently lar ge separations, and becomes attractive at shorter separations. The nature o f the induced interactions changes from attractive to repulsive at a distan ce corresponding to the range of the depletion forces. [S1063-651X(99)14012 -1].