EFFECTS OF pH and ELECTROLYTE CONCENTRATION ON PARTICLE INTERACTION IN THREE HOMOIONIC SODIUM SOIL CLAY SUSPENSIONS

The relation between shear stress and shear rate was determined with a Fann viscometer at each of several values of clay concentration, NaCI concentration, and pH for suspensions of Na-saturated clay extracted from Cecil, Putnam, and Hoytville soils. The predominant clay minerals in these soils were kaolinite, montmorillonite, and illite, respectively. Then values of the extrapolated shear stress were obtained by extrapolating the linear portions of the respective shear curves. Theory and practice indicate that extrapolated shear stress is a criterion of particle-particle interaction; for the Cecil kaolinitic soil clay, these values decreased with increasing NaCI concentration at low pH values but increased with increasing NaCl concentration at high pH values. As a result, curves of extrapolated shear stress versus pH at different NaCl concentrations intersected at or near a common point. However, for the Putnam (smectitic) and Hoytville (illitic) soil clays, extrapolated shear stress increased with increasing NaCl concentration and decreased with increasing pH. Moreover, these curves of different NaCl concentrations tended to converge with increasing pH but did not intersect. The findings were interpreted to mean that the edges of the Cecil clay were amphoteric with an isoelectric point at the point of intersection of the curves whereas the edges of the Putnam and Hoytville clays were negatively charged with a charge density that increased with increasing pH. Consequently, the particle arrangement in the Cecil kaolinitic soil clay changed from edge-to-face at low pH values to edge-to-edge at high pH values, and the particle arrangement in the Putnam and Hoytville 2:1 soil clays was edge-to-edge at all pH values.