Effects of sort and concentration of salts on the electrosurface properties of aqueous suspensions containing hydrophobic and hydrophilic particles: Validity of the Hofmeister series

The results of systematic study of the impact of electrolytes (in a wide range of electrolyte concentration, C = 10(-5)-10(-1) M) with alkali metal ions (Li+, Na+, K+, Rb+, Cs+) containing chlorides and sulphate anions on electrosurface properties of aqueous suspensions containing hydrophobic (polystyrene latex with surface sulphate or carboxylic groups) and hydrophilic (aerosil, zirconium dioxide, bentonite, Laponite RD (R)) particles are presented. The behaviour of zeta potential (zeta-potential) and electrical conductivity was studied. The observed zeta(C) dependencies revealed marked maxima at C = 10(-4)-10(-3) M for hydrophobic and at C = 10(-3)-10(-2) M for hydrophilic particles. This phenomenon was explained accounting for two opposite trends related to the polarization of the electrical double layer (EDL) in the external electric field (increase of zeta-potential at low concentration) and the effect of the EDL compression (decrease of zeta-potential at high concentration). In concentrated solutions (C> 10(-3)-10(-2) M) zeta--potentials always decreased regularly in the direct Hofmeister series from Li+ to Cs+ cations. However, in more diluted solutions, the indirect Hofmeister series for some particles were also observed. Such behaviour can reflect the impact of different properties of the surface including hydrophilic or hydrophobic nature, charge density, and partial dehydration. The sort of anions also influenced the value of zeta-potential. This phenomenon was explained by dehydration effect of sulphate ions, thinning the surface hydrate layer and shift of the shear plane toward the surface. (C) 2018 Elsevier B.V. All rights reserved.