Thermal compression of colloidal crystals: Paradox of the repulsion-only assumption

It is demonstrated that the Sogami potential, which contains both repulsive and attractive components, can account for the experimental data on photothermal compression of colloidal crystals much more satisfactorily than the purely repulsive DLVO potential. The Sogami theory gives a natural explanation for the non-space-filling nature of the crystals with an experimentally realistic value for the effective surface charge on the particles. A fundamental paradox of the DLVO theory is that in the case of thermal compression, when the whole dispersion is heated, it predicts no contraction at all, in contradiction to the experimental results.