Role of surfactants in filtration and fouling of colloidal silica

Abstract

The objective of this study is to investigate the influence of three different types of surfactants (i) anionic sodium dodecyl sulphate (SDS), (ii) cationic (hexadecyltrimethylammonium bromide (CTAB), and (iii) non-ionic: Triton X-100 (Polyethylene glycol tert-octylphenyl ether) and the effect of surfactant concentration on ultrafiltration of colloidal silica nanoparticles. Due to the high surface area to volume ratio of nanoparticles, the role of surface interactions on the stability of silica suspensions is enhanced. The effects of adsorption of surfactants are studied by means of static light scattering and zeta potential measurements. The strongest particle-surfactant interaction is observed between oppositely charged CTAB with silica, followed by TX-100 and SDS. An ultrafiltration hollow-fibre membrane is used in a semi-dead end configuration to perform filtration of silica suspension with varying surfactant concentration to critical micelle concentration (CMC) ratio, csurfactant/ccmc in a flux-step mode. The effect of surfactants and process conditions (flux) on filtration process have been compared by evaluating the critical flux and total fouling rate. The occurrence of critical flux and evolution of fouling rates are also strongly affected by the surfactant concentration. This difference in filtration performance is attributed to various competing and complementary mechanisms: electrostatic and hydrophobic interactions between surfactant-membrane surface, electrostatic and hydrophobic interactions between particles as well as the hydrodynamic effect of fluid motion towards the membrane. A comparison of the overall fouling potential for surfactant-silica systems showed that SDS-silica systems showed fouling rates of an order of magnitude higher than those of CTAB-silica and TX100-silica systems at the same csurf/cCMC ratio. This was an unexpected finding, as we would expect stable colloidal systems such as SDS-silica systems would exhibit lower fouling than unstable colloidal systems (e.g. CTAB-silica systems).