7.3 Existence of an Electrical or Steric Barrier to Coalescence on the Dispersed

Droplets The presence of a charge on the dispersed droplets constitutes an electrical barrier to the close approach of two particles to each other. This is believed to be a significant factor only in O/W emulsions. In O/W emulsions，the source of the charge on the dispersed droplets is the adsorbed layer of surfactant with its hydrophilic end oriented toward the water phase. In emulsions stabilized by ionic surfactants，the charge on the dispersed phase may arise either from adsorption of ions from the aqueous phase or from frictional contact between droplets and the aqueous phase.^③ In the latter case，the phase with the higher dielectric constant is charged positively. In W/O emulsions，there is very little charge，if any，on the dispersed particles and experimental data indicate no correlation between stability and any charge present. In fact，for water-in-benzene emulsions stabilized by oleate soaps of polyvalent metals，an anticorrelation was found between zeta potential and stability against coalescence. The true stabilizers in these systems are probably insoluble basic metal oleates produced by hydrolysis of the original metal oleates. Those metal oleates that do not stabilize water-in-benzene emulsions show no hydrolysis and have the highest zeta potentials. The hydrolysis products，if insoluble in both phases，accumulate at the interface and prevent the formation of an electrical double layer in the oil phase. Their accumulation at the interface stabilizes the W/O emulsion，since these basic metal oleates are preferentially wetted by the benzene and，in addition，form an interfacial film or layer that mechnically prevents coalescence of the water droplets.