RELATIONSHIP BETWEEN CATION-EXCHANGE SELECTIVITY AND CRYSTALLINE SWELLING IN EXPANDING 2 1-PHYLLOSILICATES/

A theoretical model describing the interaction between crystalline swe lling and cation exchange selectivity is proposed for expanding 2:1 ph yllosilicates. The model is based on the assumption that changes in ba sal spacing of a clay are phase changes, and that each phase of a clay has a different selectivity constant for a particular cation exchange reaction. Energy barriers stabilize the various phases over a limited range of interlayer ionic composition. These energy barriers cause hy steresis in crystalline swelling, which in turn causes hysteresis in c ation exchange. Results are presented for an experiment involving Ba-M g exchange on a synthetic fluoro-hectorite. The results demonstrate ke y aspects of the proposed model, including a correlation between measu red selectivity coefficients and basal spacings (R-2 = 0.85), an abrup t change in basal spacing that corresponds with an abrupt change in se lectivity and corresponding hysteresis in crystalline swelling and cat ion exchange selectivity. The results also demonstrate increased selec tivity for the preferred cation (Ba) at high solution mole fraction of the preferred cation. This trend is opposite of that observed for het erogeneous natural smectites but consistent with predictions of the mo del for a homogeneous smectite.