Dlvo and hydration forces between mica surfaces in Mg2+, Ca2+, Sr2+, and Ba2+ chloride solutions

Abstract The total force between curved sheets of muscovite mica was measured in a range of concentrations of Mg2+, Ca2+, Sr2+, and Ba2+ chloride solutions as a function of separation. In each case an increase in concentration caused a reduction in Debye length in close agreement with theory and, in addition, a reduction in double-layer potential consistent with weak adsorption of the divalent cation at the negatively charged mica surface. At concentrations above about 10−3 M the weak double-layer repulsion was completely overcome by van der Waals forces such that the total force was attractive at all distances, corresponding to “coagulation” of the mica surfaces. However, at higher concentrations (⩾1.0 M) the divalent cations become firmly bound to the interacting mica surfaces and then give rise to strong, short-range, repulsive forces which prevent coagulation in a primary minimum. These forces are qualitatively similar to those previously observed for alkali metal ions (Li+, Na+, K+, and Cs+ chloride solutions) and appear to be due to the residual hydration shells of the bound cations. The main, qualitative, difference between divalent and monovalent cations is that the former ions are more strongly hydrated and therefore do not easily shed their hydration layers in order to bind to the mica surface. Divalent ions bind and give rise to deviations from DLVO theory at higher concentrations than do monovalent cations.

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