(314a) Direct Measurements of the Effects of Salt and Surafactant on Interaction Forces Between Colloidal Particles at the Oil-Water Interface

Recently, detailed studies of the aggregation kinetics and surface rheology of colloidal particles at the oil-water interface have been reported [1]. In the interest of understanding and tailoring these phenomena, the forces between colloidal particles at a decane-water interface have been studied using laser tweezers. Experiments were performed in the presence of low concentrations of a monovalent salt (NaCl) and of the surfactant sodium dodecylsulfate (SDS) in the aqueous subphase. In the absence of electrolyte and surfactant, particle interactions exhibit a long-range repulsion, yet the variation of the interaction for different particle pairs is considerable. Averaging over several particle pairs was necessary to obtain reliable averages of the effects of salt and surfactant. It has previously been suggested that the electrostatic repulsion is consistent with electrostatic interactions between a small number of dissociated charges in the oil phase, leading to a decay with distance to the power -4 and an absence of any effect of electrolyte concentration. However, the present work demonstrates that increasing the electrolyte concentration does yield, on average, a reduction of the magnitude of the interaction force with electrolyte concentration. This implies that charges on the water side still contribute significantly to the electrostatic interactions. An increase in the concentration of SDS leads to a similar decrease of the interaction force. Moreover, the repulsion at fixed SDS concentrations decreases over time, providing a basis for understanding the aggregation kinetics of these 2D suspensions.

[1] S. Reynaert et al. Langmuir, 2006, 22, 4936.