Interaction forces in dispersions containing non-ionic surfactants
Rates and equilibrium positions of aggregation have been measured for dispersions of monodisperse polystyrene latex particles containing the non-ionic surfactant octaoxyethylene glycol dodecyl ether (C12E8). Measurements cover a range of particle sizes and temperatures. Rates are observed which, even in the presence of sufficient electrolyte largely to suppress electrostatic interactions, can be significantly below the diffusion-controlled ‘rapid’ rate. Data are interpreted in terms of both primary and secondary minima. The temperature sensitivity of the aggregation appears to arise from variations in the attractive rather than the repulsive contribution, these variations resulting from the effect of temperature on the level of adsorption. The attractive effects are of an essentially short-range nature, and because of this primary-minimum aggregation is more sensitive to temperature than secondary-minimum aggregation.
The data have been fitted to potential-energy functions consisting of an attractive van der Waals contribution and a repulsion arising from the surfactant. Making reasonable allowance for the attractive contribution leads to the conclusion that a weak repulsive force is operative at separations considerably in excess of the physical reach of the adsorbed surfactant. Here an arbitrary model for the repulsion has been used which comprises the sum of two exponential decays, a relatively strong short-range component (decay length 0.2 nm) and a weak long-range component (decay length 3.0 nm).