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%).
