Hofmeister Effects in Colloidal Systems: Influence of the Surface Nature

The original Hofmeister series concerned the effectiveness of different salts in precipitating negatively charged proteins. Although the relative position of the species in the series has been demonstrated to be almost universal, reversed Hofmeister sequences have been observed in many systems. Most of them are directly related with the sign of charge (positive or negative) of the surface studied. In this paper, we show experimental evidence of alterations in the Hofmeister order induced not only by changing the sign of the surface charge, but also by varying the hydrophobic/hydrophilic character of the surface. We have worked with a wide variety of systems showing different hydrophilic/hydrophobic character and having a pH-dependent charge sign. Particularly, the effect of these two factors (the surface charge and the hydrophilicity/phobicity) on the ion ranking has been analyzed by studying the colloidal stability of the systems. The typical inversions induced by modifying the charge sign of the particle were observed; in addition and more noteworthy, the direct series observed with hydrophobic systems turned into reversed series as the surface became more hydrophilic for surfaces of identical sign of charge. The correlation found between these inversions and restabilization processes promoted by hydration forces at high salt concentrations evidenced the solvent-structural nature of such inversions. Useful conclusions about the origin of these specific ion effects have been drawn from a comparative analysis among our systems.