Long range polarization attraction between two different likely charged macroions

TL;DR

This paper reveals a long-range polarization attraction between macroions with different surface charge magnitudes, which can dominate over van der Waals forces in salt-free and salty solutions, due to macroion polarization effects.

Contribution

It introduces the concept of long-range polarization attraction between macroions with different charge densities, extending understanding beyond short-range correlation forces.

Findings

Polarization attraction decays as a power law with distance.

Polarization force exceeds van der Waals attraction over a broad range.

Long-range polarization persists even in salty solutions under certain conditions.

Abstract

It is known that in a water solution with multivalent counterions (Z-ions), two likely charged macroions can attract each other due to correlations of Z-ions adsorbed on their surfaces. This "correlation" attraction is short-ranged and decays exponentially with increasing distance between macroions at characteristic distance A/2\pi, where A is the average distance between Z-ions on the surfaces of macroions. In this work, we show that an additional long range "polarization" attraction exists when the bare surface charge densities of the two macroions have the same sign, but are different in absolute values. The key idea is that with adsorbed Z-ions, two insulating macroions can be considered as conductors with fixed but different electric potentials. Each potential is determined by the difference between the entropic bulk chemical potential of a Z-ion and its correlation chemical potential at the surface of the macroion determined by its bare surface charge density. When the two macroions are close enough, they get polarized in such a way that their adjacent spots form a charged capacitor, which leads to attraction. In a salt free solution this polarization attractive force is long ranged: it decays as a power of the distance between the surfaces of two macroions, d. The polarization force decays slower than the van der Waals attraction and therefore is much larger than it in a large range of distances. In the presence of large amount of monovalent salt, when A/2\pi<< d<< r_s (r_s is the Debye-H\"{u}ckel screening radius), this force is still much stronger than the van der Waals attraction and the correlation attraction mentioned above.