Silica is a very interesting system that has been thoroughly studied in the last decades. One of the most outstanding characteristics of silica suspensions is their stability in solutions at high salt concentrations. In addition to that, measurements of direct-interaction forces between silica surfaces, obtained by different authors by means of surface force apparatus or atomic force microscope (AFM), reveal the existence of a strong repulsive interaction at short distances (below 2 nm) that decays exponentially. These results cannot be explained in terms of the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, which only considers two types of forces: the electrical double-layer repulsion and the London-van der Waals attraction. Although there is a controversy about the origin of the short-range repulsive force, the existence of a structured layer of water molecules at the silica surface is the most accepted explanation for it. The overlap of structured water layers of different surfaces leads to repulsive forces, which are known as hydration forces. This assumption is based on the very hydrophilic nature of silica. Different theories have been developed in order to reproduce the exponentially decaying behavior (as a function of the separation distance) of the hydration forces. Different mechanisms for the formation of the structured water layer around the silica surfaces are considered by each theory. By the aid of an AFM and the colloid probe technique, the interaction forces between silica surfaces have been measured directly at different pH values and salt concentrations. The results confirm the presence of the short-range repulsion at any experimental condition (even at high salt concentration). A comparison between the experimental data and theoretical fits obtained from different theories has been performed in order to elucidate the nature of this non-DLVO repulsive force.

In this paper the stability domains of immunoglobulin ͑IgG fragment͒ F͑abЈ) 2 -polymer systems have been examined using a low-angle scattering technique. The rates of aggregate formation are expressed in terms of a stability ratio as a function of electrolyte concentration. After the usual rapid aggregation achieved at a certain ionic strength ͑critical coagulation concentration͒, an abnormal stabilization is observed with increasing ionic strength. This exceptional stability at high electrolyte concentration cannot be explained by the Derjaguin, Landau, Verwey, and Overbeek ͓B. V. Derjaguin and L. Landau, Acta Physicochim. USSR 14, 633 ͑1941͒; E. J. W. Verwey and J. Th. G. Overbeek, Theory of the Stability of Lyophobic Colloids ͑Elsevier, Amsterdam, 1952͒, Vols. 1 and 2͔ theory, which attributes the colloidal stability to the London-van der Waals attraction and the electrostatic repulsion. Effects of electrolyte concentration, counterion valence, pH, protein coverage, and time on the experimental stability are investigated. A possible explanation based on the so-called ''hydration forces'' is proposed. ͓S1063-651X͑97͒05804-2͔

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Under certain conditions ion-ion correlations play a crucial role in the description of the electrical double layer of colloidal particles. In fact, in many instances, the inclusion of the short range correlations between ions in the study of the ionic distribution leads to quite different results with respect to the classical treatment (where ions are assumed to be points). In particular, these discrepancies become more noticeable for highly charged particles in the presence of moderate or highly multivalent counterion concentrations. Moreover, it can be shown that the existence of an electrolyte mixture consisting of multi-and monovalent counterions may cause that system to become overcharged, a feature that cannot be predicted from a classical point of view based on the Boltzmann distribution function. Precisely this aspect has recently produced an enormous interest in the field of biophysics since small variations in the physiological conditions of biocolloidal systems (e.g. the addition of a multivalent salt) can induce important changes in their behaviour. In order to determine the relevance of ion correlations in electrolyte mixtures, we present some experimental results on the electrophoretic mobility of latex particles in the presence of different 1:1 and 3:1 salt mixtures. Likewise, these results are analysed within the so-called hypernetted-chain/mean spherical approximation where ion size correlations are taken into account.