Structure, Fluctuations, and Forces in Overlapping Electric Double Layers at Calcite (10.4)

Poisson–Boltzmann theory treats the solvent at a mineral surface as a uniform continuum with a single dielectric constant, whereas molecular dynamics simulation of calcite (10.4) gives an interfacial permittivity that oscillates in phase with the water density. Here, single and overlapping double layers at calcite (10.4) are treated by classical density functional theory with this permittivity profile, the corresponding ion self-energy, and measured dielectric decrements. In CaCl2, the mean potential changes sign within the first hydration layer: at 2 M, it is +21 mV at the contact plane, one chloride radius from the surface, while the surface is at −48 mV. The surface charge at which this reversal sets in lies inside the range reported for calcite. Specific adsorption is not included in the model, and the potential at the contact plane does not reverse in NaCl or when the ion self-energy is removed, so valence and solvation are sufficient to reverse the potential. The local compressibility, not previously reported for a mineral surface, peaks at 9.7 times its bulk value and decreases with added salt. The mean-field contribution to the disjoining pressure is attractive while the net pressure is repulsive, so a DLVO fit to these surfaces assigns the wrong sign to the electrostatic term. From 0.5 M upward, the CaCl2 film is two to five times more repulsive than the NaCl film at the same ionic strength, which is the ordering of the measured adhesion; below 0.5 M, the ordering is reversed. A surface complexation model gives a regulated charge between −0.01 and +0.11 C m−2, at which the CaCl2 film remains the more repulsive at 0.5 and 1 M, and the ordering inverts at 2 M. Using the permittivity of pure water instead of that of the solution lowers the maximum disjoining pressure by up to 44% in CaCl2 and raises it by up to 84% in NaCl.