Surface modification of activated carbon by surfactants mixtures

Adsorption of surfactants is widely used for surface modification of different materials. Using of the mixtures of the surfactants in many cases is more attractive compared with single surfactants, however composition of mixed adsorption layers at solid surface was not sufficiently studied yet. In this work, the mutual effect of anionic and nonionic surfactants on the composition of the mixed adsorption layer on the surface of activated carbon (AC) was studied. Sodium hexadecyl sulphate (SHS) and oxyethylated octylphenol Triton X100 (TX-100) were used as anionic and nonionic surfactants, respectively. Mixed SHS/TX-100 systems have been studied over a concentration range of 0.1-8.0 mmol/L and the molar fractions of SHS in the mixtures were within 0.2-0.8. It was shown that the adsorption isotherms of SHS or TX-100 surfactants at AC could be describe by the Langmuir equation. For surfactants mixtures at low equilibrium concentrations, it was found that adsorption of SHS increases when TX-100 is present in the solution. This might be explained by the decreasing of electrostatic interactions between the head groups of SHS molecules in the mixed adsorption layers due to incorporation of the molecules of the nonionic surfactant. At higher total surfactant concentrations, SDS adsorbed at AC surface is replaced by TX-100 molecules and as result the nonionic surfactants is preferentially adsorbed on AC.