Macroion shielding and state-dependent pair potentials in colloidal suspensions

We determine radial distribution functions for charge-stabilized colloidal suspensions for various volume fractions. By means of two independent inversion routines based on (i) the Ornstein–Zernike equation and (ii) the inverse Monte Carlo method, we extract effective colloid–colloid pair potentials from the distribution functions. Working at a fixed salinity, these potentials would not depend on the colloid density if the interactions between the colloids were purely pairwise additive. Our potentials however show a density dependence; agreement with the Yukawa form of DLVO (Derjaguin–Landau–Verwey– Overbeek) theory is found only at low densities, but not at high densities. Such a state dependence of pair potentials reveals the density-dependent impact of many-body terms in the total interaction energy of the system. We also perform primitive model calculations, with parameters simulating our experiment, and find qualitatively similar deviations of the effective pair potentials from the Yukawa form, which thus confirms our experimental findings. Using results from a calculation of three-body interactions in colloidal suspensions, we are able to trace the observed density dependence of the pair potentials back to a shielding effect of the macroions. This interpretation is consistent with other experimental studies observing attractive parts in the pair potentials which were interpreted as ‘like-charge attraction’. Our results, however, suggest that this attraction is a many-body effect.