Hofmeister effects in the stability and electrophoretic mobility of polystyrene latex particles

Specific ionic effects on numerous solid-aqueous solution interfaces have usually been ranked in the so-called Hofmeister series. Ion specificity has experimentally been manifested in many ways. In this work, the colloidal stability and electrophoretic mobility of three different polystyrene latex samples are analyzed in the presence of different electrolytes. The effects on their colloidal stability and electrokinetic behavior provoked by four anions (SO42-, Cl-, NO3-, and SCN-) and three cations (NH4+, Na+, and Ca2+) located in different positions in the Hofmeister series are shown. Some of these ions specifically modified the pair potential interaction of the colloidal particles yielding results not explained by the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory. A modification in the repulsive electrostatic term of this theory is then proposed to consider the ionic specificity. In addition, general but qualitative explanations about the subjacent mechanisms involved in the Hofmeister effects are also summarized.