Non-DLVO colloidal interactions: excluded volumes, undulation interactions, depletion forces and many-body effects
A long time ago, Stern noted that the traditional assumption that the ions interact only with a "mean" electrical field (the Poisson - Boltzmann approach) leads to an ionic density in the vicinity of the interface that exceeds the available volume. A simple way to avoid this difficulty is to consider that the ions are hydrated, and therefore there are fewer positions available to them in the vicinity of charged surfaces [4.1]. When compared to the traditional Poisson-Boltzmann result, this correction leads to an increase in the repulsive force at short separations [4.1]. This approach is extended to account for the ion competition for hydrating water molecules [4.2]. A position in the water lattice is considered available to an ion only when it is "free" and has a certain number of "free" neighboring water molecules (i.e., water molecules which do not participate in the hydration of other ions). Whereas at high surface potentials, this "excluded" volume effect increases the repulsion as compared to the traditional double layer force, at low surface potentials it can either increase or decrease the repulsion (the latter case occurs when the hydrated coion is larger than the hydrated counterion). The strong dependence of this effect on the hydration number of ions provides a possible explanation for ion-specific effects, which will be examined in the next chapter.
KeywordsParticle Volume Fraction Excess Chemical Potential Undulation Interaction Depletion Force Depletion Interaction
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