Strong electrostatic interactions in spherical colloidal systems

Abstract

We investigate spherical macroions in the strong Coulomb coupling regime within the primitive model in salt-free environment. We first show that the ground state of an isolated colloid is naturally overcharged by simple electrostatic arguments illustrated by the Gillespie rule. We furthermore demonstrate that in the strong Coulomb coupling this mechanism leads to ionized states and thus to long range attractions between like-charged spheres. We use molecular dynamics simulations to study in detail the counterion distribution for one and two highly charged colloids for the ground state as well as for finite temperatures. We compare our results in terms of a simple version of a Wigner crystal theory and find excellent qualitative and quantitative agreement.

Computer Simulations of Charged Systems

“…In all our results for τ versus √ N c we observe a linear dependence for a wide range of values for N c , τ ∝ √ N c , which again can be explained by applying the WC hole picture [110].…”

“…We also showed in Ref. [110] thatᾱ reaches the perfect WC value of 1.96 if the colloid radius a gets very large at fixed c, or when c becomes large at fixed a.…”

“…The physical reason is that a counterion can gain more energy by overcharging the colloid with c A then it loses by ionizing colloid B. A straight forward application of the procedure outlined for the barrier calculation [107,110] yields a simple criterion (more specifically a sufficient condition), valid for large macroionic separations, for the charge asymmetry √ N A − √ N B to produce an ionized ground state of two unlike charged colloids with the same size:…”

Size correlations in colloidal suspensions

“…Recent simulations suggest that the attraction between like charges might be related to the mechanism that induces overcharging [11]- [13]. Therefore, understanding the mechanism that leads to overcharging will shed some light in the appearance of attraction between like-charged polyions.…”

Kinetics of charge inversion

“…The counterion mediated attraction is responsible for the DNA compaction inside the bacteriophages, viruses that infect bacteria [17,18], and for the organization of eukaryotic cytoskeleton [19]. Another "strange" electrostatic behavior which can occur in suspensions containing multivalent counterions is the reversal of the electrophoretic mobility [1,20,21,22,23,24,25]. The first thing that is learned in a course on electrostatics is that the force produced by the electric field on a charged particle is…”