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 (σ) 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 μN/m) were comparable with those reported for polymer–polymer ATPS. Interfacial tensions σ   can be well correlated with the difference in pullulan and SDS concentrations between the phases (ΔCpulΔCpul and ΔCSDSΔCSDS) and also the tie-line length (TLL); all yield straight lines on a log–log scale.

Graphical abstractModified retracting drop method was used for measuring interfacial tension in biopolymer–surfactant ATPS. By analysing the drop retraction and plotting deformation parameter D (Eq. (1)) against time t (Fig. 1), interfacial tension can be calculated using Eqs. (2)–(3).D=a−ba+b(1),lnD(t)=lnD0−tτ(2),τ=μcR0σ(19λ+16)(2λ+3)40(λ+1)(3).Figure optionsDownload full-size imageDownload as PowerPoint slide