Macroion shielding and state-dependent pair potentials in colloidal suspensions

Abstract

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