The discovery that the Poisson-Boltzmann (PB) description of the electrical double layer may fail in the presence of multivalent ions triggered renewed interest in the interaction of multivalent ions with interfaces and their influence on interparticle forces. Number of investigations show that in the presence of multivalent ions forces between similarly charged water-solid interfaces may become attractive, whereby the classical PB theory predict purely repulsive forces in such situations.
To clarify the importance of such additional forces, direct force measurements between different types of particles were carried out in aqueous solutions of various inorganic salts containing mono- and multivalent cations with the multiparticle colloidal probe technique based on the atomic force microscope. The theory of Derjaguin, Landau, Verwey and Overbeek (DLVO) is capable to describe well the force profiles in the presence of mono- and divalent cations down to distances of a few nm. For the short-ranged distances (below 3 nm) deviations from the DLVO theory were observed [1, 2]. For more highly charged ions, an additional and more long-ranged (non-DLVO) attractive force was observed, which is attributed to surface charge heterogeneities. Detailed analysis of the intermolecular interactions was performed using direct force measurements for carboxylate latex [1] and silica particles.
[1] F. J. Montes Ruiz-Cabello et al. J. Phys. Chem. B 2013, 117, 11853-11862
[2] P. Sinha et al. J. Phys. Chem. Lett. 2013, 4, 648−652
Francisco.Montes@unige.ch
