Phase diagram of aggregation of oppositely charged colloids in salty water

TL;DR

This paper develops a quantitative phase diagram theory for aggregation of oppositely charged colloids in salty water, highlighting how Coulomb interactions and salt concentration influence aggregation behavior, with applications to DNA and chromatin systems.

Contribution

It introduces a new theoretical framework for understanding colloid aggregation in salty water, considering macroion shape and salt effects, with applications to biological systems.

Findings

Aggregation domain expands as salt screening increases.

Partial aggregation domains become smaller with decreasing Debye length.

Theory applies to DNA and chromatin-like systems.

Abstract

Aggregation of two oppositely charged colloids in salty water is studied. We focus on the role of Coulomb interaction in strongly asymmetric systems in which the charge and size of one colloid is much larger than the other one. In the solution, each large colloid (macroion) attracts certain number of oppositely charged small colloids ($Z$-ion) to form a complex. If the concentration ratio of the two colloids is such that complexes are not strongly charged, they condense in a macroscopic aggregate. As a result, the phase diagram in a plane of concentrations of two colloids consists of an aggregation domain sandwiched between two domains of stable solutions of complexes. The aggregation domain has a central part of total aggregation and two wings corresponding to partial aggregation. A quantitative theory of the phase diagram in the presence of monovalent salt is developed. It is shown that as the Debye-H\"{u}ckel screening radius $r_s$ decreases, the aggregation domain grows, but the relative size of the partial aggregation domains becomes much smaller. As an important application of the theory, we consider solutions of long double-helix DNA with strongly charged positive spheres (artificial chromatin). We also consider implications of our theory for in vitro experiments with the natural chromatin. Finally, the effect of different shapes of macroions on the phase diagram is discussed.