Electrosurface properties of Na-bentonite particles in electrolytes and surfactants solution

Abstract

The results of study of the impact of pH, alkali metal, calcium and lanthanum chlorides, alkali metal sulphates as well as sodium dodecyl sulphate and cetyltrimethylammonium bromide on the zeta-(ζ) potential and electrical conductivity of suspensions containing Na-bentonite particles are presented. The behaviour of the suspension in an external electric field can be summarized as follows: ζ-potential of particles show low sensitivity to pH and they display relatively high ζ values (from −35 mV to −52 mV) both in water and in supporting electrolytes solution; the ζ(C) dependencies for RbCl and CsCl are characterized by a constant ζ-potential at C 10−5–10−3 mol/L, and for LiCl, NaCl and KCl by a maximum at C 10−3–10−2 mol/L; the relative position of the ζ(C) curves corresponds to the regular Hofmeister series; ζ-potential in alkali metal sulphates solutions was found to be higher compared to that for chlorides at equivalent concentration of salts; addition of electrolytes with bi-and trivalent counter-ions or cationic surfactant caused a substantial reduction in ζ and surface charge reversal, respectively; a regular decrease in the surface conductivity of particles was measured while moving from LiCl to CsCl in the lyotropic series.The observed regularities are explained by accounting for two opposite trends related to the polarization of the electrical double layer (EDL) in the external electric field (increase of ζ-potential at low concentration) and the effect of the EDL compression (decrease of ζ-potential at high concentration) as well as the increase in filling up the Stern-layer with alkali metal ions as their radius increases. We connect the bigger ζ-potential values for sulphates compared to that for chlorides with dehydration effect of sulphate co-ions which results in the thinning the surface hydrate layer and shift of the shear plane toward the surface. The “electrokinetic charge” calculated from the ζ-potential comprises a small part of the cation exchange capacity of the mineral, i.e. the counter-ions are located mainly in the hydro-dynamically immobile part of the EDL.