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It is general wisdom that the pair potential of charged colloids in a liquid may be closely approximated by a Yukawa interaction, as predicted by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. We experimentally determine the effective forces in a binary mixture of like-charged particles, of species 1 and 2, with blinking optical tweezers. The measured forces are consistent with a Yukawa pair potential but the (12) cross-interaction is not equal to the geometric mean of the (11) and (22) like-interactions, as expected from DLVO. The deviation is a function of the electrostatic screening length and the size ratio, with the cross-interaction measured being consistently weaker than DLVO predictions. The corresponding non-additivity parameter is negative and grows in magnitude with increased size asymmetry.

Dispersions of poly(methyl methacrylate) (PMMA) latexes were prepared in a low dielectric, nonpolar solvent (dodecane) both with and without the oil-soluble electrolyte, tetradodecylammonium-tetrakis(3,5-bis(trifluoromethyl)phenyl)borate. For dispersions with a high concentration of background electrolyte, the latexes become colloidally unstable and sediment in a short period of time (<1 h). This is completely reversible upon dilution. Instability of the dispersions is due to an apparent attraction between the colloids, directly observed using optical tweezers by bringing optically trapped particles into close proximity. Simple explanations generally used by colloid scientists to explain loss of stability (charge screening or stabilizer collapse) are insufficient to explain this observation. This unexpected interaction seems, therefore, to be a consequence of the materials that can be dispersed in low dielectric media and is expected to have ramifications for studying colloids in such solvents.T he Derjaguin−Landau−Verwey−Overbeek (DLVO) theory is one of the classic foundations in the field of colloid science; it describes the stability of colloids as a balance of attraction (van der Waals dispersion forces) and repulsion (electrostatic forces).1,2 Later, other ways to control colloidal interactions were developed, such as steric repulsion, for example, where nonionic macromolecules anchored to the surface overlap. 3Generating new ways to mediate colloidal stability and instability would be highly beneficial for controlling the properties of nanoparticles, particularly in nonpolar solvents, where charge numbers are typically low and van der Waals attractions are weak. In this Letter, a classic system in colloid science is used: poly(methyl methacrylate) (PMMA) latexes coated with poly(12-hydroxystearic acid) (PHSA) brushes in dodecane solvent. 4 This system is an extremely popular tool for experimental studies of hard spheres.5−12 The particles are always considered in the literature to be sterically stabilized. That the particles are sterically stabilized, however, does not necessarily mean that they are uncharged. 13−16 In this Letter, the effect of adding an oil-soluble electrolyte on the colloidal interactions is studied. If there were no interactions other than attraction (dispersion) and repulsion (steric and electrostatic), then the addition of salt would have no influence on stability. The electrolyte would reduce the screening length of the electrostatic interaction, but the polymer brushes would still act as steric stabilizers. As the results show, an effective attraction can be induced between the colloids in high concentrations of electrolyte. That the addition of an electrolyte destabilizes a colloidal dispersion is an unexpected observation that cannot be explained using existing simple theories of colloidal stability.Dispersions of PMMA latexes (AC12, containing DiIC 18 dye, a = 775 ± 25 nm) were prepared in dodecane at a volume fraction (ϕ) of 3.7 × 10 −4 . The oil-soluble compound tet...