SURFACTANT-INDUCED WETTING TRANSITIONS - ROLE OF SURFACE HYDROPHOBICITY AND EFFECT ON OIL PERMEABILITY OF ULTRAFILTRATION MEMBRANES

In this study the effect of surface hydrophobicity on adsorption of sodium oleate from a mixture of water and 2-propanol is investigated as well as consequences of this adsorption for wettability by oil or water/2-propanol, respectively. The surface hydrophobicity is varied by the use of compatible mixtures of a hydrophobic (polystyrene) and a more hydrophilic polymer (polyamide or poly (methyl methacrylate)). It appears that for the adsorption onto these surfaces three regions can be distinguished. In the case of a hydrophobic surface adsorption of the surfactant is due to van der Waals and/or hydrophobic interactions between the apolar tails of the surfactant and the surface. Upon an increase of the surface hydrophilicity a large region in which adsorption takes place is followed by a very narrow region of 0.5 mN m-1 in which no adsorption is found. A further increase in surface hydrophilicity results in an almost stepwise increase of the amount adsorbed, now due to polar interactions between the polar head groups of the surfactant and the surface, possibly followed by the adsorption of a second layer. The adsorption behavior in the three regions can be explained in a semi-quantitative way, considering interfacial free energies between different components present in the system. This result can be used in order to design an oil-selective ultrafiltration membrane for the separation of a system of two immiscible liquids (i.e., o/w or w/o emulsions). It was found that only in the region where no surfactant is adsorbed is the surface preferentially wetted by oil and that only then is an ultrafiltration (UF) membrane entirely permeable to oil and not to the aqueous phase. © 1990.