  This study uses a coarse-grained Monte Carlo algorithm to model and simulate
the coadsorption of a binary mixture of counterionic colloids in Gibbs
monolayers. These monolayers form at a idealized air-water interface, with one
non-soluble species confined at the interface and the second one partially
soluble in the aqueous phase. The investigation focuses on the effect of
colloidal size and charge on the thermodynamics and microstructure of the
monolayer. We find that the composition of the monolayer evolves non-trivially
with surface coverage, depending on the balance of steric and electrostatic
forces. When the electrostatic interactions are weak, the soluble species is
expelled from the monolayer upon compression, yielding a phase behaviour
particularly sensitive to the relative size of the soluble and non-soluble
colloids. By contrast, strong electrostatic interactions favour the
stabilization of the soluble particles in the monolayer and the formation of
quasi-equimolar fluids, with only a weak dependence on particle size. The
combination of these phenomena results in the formation of a number of
two-dimensional mesoscopic arrangements in the monolayer, ranging from diluted
gas-phase behaviour to domains of aggregates and percolates, and to incipient
crystalline structures.
