Ultrafiltration of saline oil-in-water emulsions stabilized by an anionic surfactant: Effect of surfactant concentration and divalent counterions

The paper presents results of a direct visualization study of membrane fouling by hexadecane-in-water emulsions (0.1% v/v) stabilized by sodium dodecyl sulfate (SDS; 0.1 mM or 3 mM) in the presence of divalent counterions (Mg2+; 0 mM, 6.7 mM, or 42.6 mM). Direct Observation Through the Membrane (DOTM) tests were performed using an ultrafiltration membrane (Anopore; dpore =0.02 µm) and low wettability by hexadecane (contact angle, θ, in 3 mM aqueous solution of SDS is ~164°). Three emulsions employed in DOTM tests had different values of interfacial tension and surface charge and exhibited distinctly different behaviors of oil droplets on the membrane surface. In addition to decreasing the solubility of SDS, MgSO4 had two opposing effects on emulsion stability wherein both interfacial tension and ζ-potential of oil droplets decreased; the overall effect of MgSO4 was a more facile droplet coalescence that was further promoted by permeate flux and concentration polarization of oil. The dominant fouling mechanism was cake filtration with multilayer and sub-monolayer coverages observed for different conditions. Because of the relative oleophobicity of the membrane, the attached oil did not form contiguous oil films. Under conditions of extensive coalescence (high MgSO4), oil droplets reached a critical size and were then removed by the crossflow resulting in minimal membrane fouling.