Competitive adsorption and interactions between lysozyme and ionic surfactants in an aqueous/organic liquid system

This study investigates the behavior of mixed protein/ionic surfactant mixtures in an aqueous/p-xylene system. Tritium labeled compounds and the scintillation phase method were used to study the competitive adsorption and distribution of lysozyme, sodium dodecyl sulfate (SDS, an anionic surfactant) and dodecyltrimethylammonium bromide (DTAB, a cationic surfactant) in the aqueous/p-xylene system. The substitution of lysozyme by both surfactants was observed, even when the concentration of the surfactants was only about 7 × 10−8 mol/l, that is, four orders of magnitude below the surfactants’ critical micelle concentration (CMC). A radiochemical study indicated that both surfactants eject the lysozyme into the organic phase. The obtained experimental data were helpful in explaining the significant synergistic effect that was observed between lysozyme and DTAB at the aqueous/p-xylene interface. Based on the experimental data and calculations, a mechanism of the formation of lysozyme – ionic surfactants mixed adsorption layer at the aqueous/p-xylene interface was revealed. The experimental study was carried out for ionic surfactants concentrations that significantly lower than their CMC.