Clay behaviour in suspension is related to the ionicity of clay–cation bonds

The behaviour of four clays viz. an illite, a bentonite and two soil clays in relation to their dispersivity, zeta potential and mean particle size was investigated to test the hypothesis that water interaction with clays is dictated by the degree of ionicity of clay–cation bonds. The clay particle being considered as an anion with large size and high charge compared to the cations, the polarizability of the cations determine the degree of ionicity or covalency in a clay–cation bond. The ionicity indices of the cations Li+, Na+, K+, Mg2+, Ca2+, Sr2+ and Ba2+ were derived using their ionisation potentials and charge. In all homoionic clays, high and significant relationships between ionicity indices of cations in clay–cation bonds and the clay behaviour such as dispersibility (measured as turbidity of suspensions) and zeta potential confirm that the degree of ionicity in these bonds dictate the water interaction with clay particles leading to their separation from the clay aggregates. This is further confirmed by the relationship between the mean particle sizes of homoionic clays, measured in four types of clays, and the ionicity indices. Transmission electron micrographs of homoionic illites confirm the aggregation of particles in relation to the ionicity of clay–cation bonds. The strong relationship between relative zeta potential and relative turbidity suggests that the surface charge on clays is responsible for the variations in correlations between ionicity indices and clay behaviour among the four types of clays. The derived ionicity indices in the present study predict the clay behaviour in water better than either ionic potentials or Misono softness parameters.

Research highlights
► Ionicity of clay-cation bonds dictates the water interaction with clay particles.
► Ionisation potentials and the charge determine the ionicity index of a cation.
► Dispersion, zeta potential and size of clays depend on the ionicity of cations.
► Surface charge determines the relation between cation ionicity and clay behaviour.