LINKAGE BETWEEN SIGMA-POTENTIAL AND ELECTRON DONICITY OF CHARGED POLAR SURFACES .1. IMPLICATIONS FOR THE MECHANISM OF FLOCCULATION OF PARTICLE SUSPENSIONS WITH PLURIVALENT COUNTERIONS

Studies were carried out on the surface properties of montmorillonite (SWy-1), glass and calcite particles by means of parallel measurements of electrophoretic mobilities and contact angles, as a function of the concentration of La3+ or Ca2+ ions. The investigations indicated that with a decrease in zeta-potential a concomitant strong increase in hydrophobicity takes place, for example, in aqueous NaCl solutions (ionic strength GAMMA/2 = 0.015, pH 7.5), SWy-1 has a zeta-potential of -66 mV, an electron-acceptor surface tension parameter gamma+ = 0.2 mJ m-2 and an electron-donor surface tension parameter gamma- = 34 mJ m-2, which is higher than that of water. This causes these particles to be very hydrophilic and to form an extremely stable suspension in water, owing to an electrostatic energy (15%) and to a net Lewis acid-base (polar) repulsion energy (85%). However, in the presence of 1.6 mM LaCl3 (GAMMA/2 = 0.024, pH 7.5), zeta = -13 mV, gamma+ = 0.15 mJ m-2 and gamma- = 5.51 mJ m-2, which makes the particles very hydrophobic and causes them to flocculate: the energies of attraction in this case are caused by caused by hydrophobic interactions (87%) and van der Waals forces (13%).