Interfacial tension in aqueous biopolymer–surfactant mixtures

The current study offers a first insight into the interfacial properties of pullulan-sodium dodecyl sulphate (SDS) aqueous two-phase systems (ATPS) in the presence of sodium chloride (NaCl). The effect of composition on the interfacial tension (sigma) in these ATPS was investigated over a wide range of pullulan, SDS and NaCl concentrations. An increase in the interfacial tension was observed with increasing pullulan and SDS concentrations and a small increase was also observed as the NaCl concentration was increased. In both cases the interfacial tension increases were closely related to the phase behaviour of these systems; as a consequence of increasing the pullulan, SDS and/or NaCl concentrations, the system moves further away from the critical point. In all systems interfacial tensions (of the order of muN/m) were comparable with those reported for polymer-polymer ATPS. Interfacial tensions sigma can be well correlated with the difference in pullulan and SDS concentrations between the phases (DeltaC pul and DeltaC SDS) and also the tie-line length (TLL); all yield straight lines on a log-log scale.