K+–Na+ exchange in phlogopite on the scale of a single layer

Abstract

Hydrothermal atomic force microscopy (HAFM) was used to investigate K+–Na+ ion exchange in phlogopite in-situ. The exchange of K+ for Na+ caused the interlayer distance to swell by approximately 5 Å. A distinct reaction front could be resolved between the K+-areas and the swollen (hydrated) Na+-areas, indicating a single reaction step mechanism. Although the fronts revealed kinematic variability due to inhomogeneities, the data indicate a diffusion mechanism within the interlayers. Diffusion coefficients ranged between 2 × 10−8 and 35 × 10−8 cm2/s, depending on the depth of the interlayer, the solution composition, and temperature. An activation energy of 15 kJ/mol was calculated from the temperature dependence of the diffusion coefficients. In addition to the regular 5 Å swelling, bulge-shaped irregular swelling of up to 200 nm could be observed. This irregular swelling might be an initial stage of delamination.

Reducing the Na+-concentration in the solution at a constant K+-concentration was found to reduce the exchange rate. The exchange ceases completely when the equilibrium ratio r(K+/Na+) of the solution is reached. The measured r(K+/Na+) of 0.013 indicates a lower K+-selectivity for interlayers that are closer to the surface. This lower selectivity is most likely related to a lower strain energy associated with the expansion of interlayers close to the surface.

Reversing the exchange reaction caused the interlayers to shrink to their original height. The kinematics of the front of the reverse reaction were significantly enhanced. In parts, swollen Na+-areas were engulfed and trapped by the shrunken K+-areas. No morphological indications of remnant alterations other than these trapped islands and the irregular swelling were observed.