Heterogeneity of the electrostatic repulsion between colloids at the oil-water interface

The pairwise and multi-body interaction forces between polystyrene particles at an oil-water interface are measured. The electrostatic repulsive force has the expected dependence on particle separation for a dipole-dipole interaction, F-rep similar to r(-4), but exhibits a distribution of magnitudes in which the force depends on the particle pairs tested and sample preparation method. A gamma distribution accurately models this variation in the repulsion between pairs of particles. Despite this heterogeneity, the multibody interactions measured in small ensembles are pairwise additive. Good agreement is found for the two-dimensional equilibrium suspension structure between experiments and Monte Carlo simulations when a heterogeneous interaction potential is implemented in the latter. The heterogeneity and long-range of the repulsive interaction accounts for the lower apparent pair interaction potential derived from the suspension radial distribution function at dilute, but finite, surface concentrations when compared to the direct pair interaction measurements made with laser tweezers at nearly infinite dilution.