Polyion monolayers and halos around large weakly-charged colloids

In this paper we present a theory for the structure of a suspension of highly charged spherical polyions near the surface of a much larger but more weakly charged spherical colloid. Starting from a level of description involving effective screened Coulomb potentials between the particles of both species, we calculate the radial distribution function of the polyions around isolated big colloids. Our theory is based on the use of the Ornstein–Zernike integral equation, complemented with the hyper-netted chain approximate closure, whose numerical solutions are selectively contrasted with the results of Monte Carlo simulations generated for this purpose. Our results indicate that, under certain conditions, our model suspension of polyions is predicted to adsorb onto the surface of the larger colloid, even if both species carry charges of the same sign. Under extreme conditions, the structure of the collection of adsorbed particles corresponds strictly to a monolayer, strongly bound to the surface of the larger particle by electrostatic forces. The complex formed by one large particle plus its adsorbed polyions then bears a large effective charge, which may enhance the electrostatic stability of the former. At some threshold condition, however, this monolayer structure becomes loose enough to be better described as a halo. One might thus associate this threshold condition with the threshold for the stability of the colloidal species formed by the larger particles. We discuss the possible connection of our theoretical results with the recent experimental observations that motivated this work.