(Liquid + liquid) phase equilibrium of aqueous two-phase system containing (surfactant + sodium sulfate + water) at different temperatures

Abstract

The binodal data for an aqueous two-phase system (ATPS) containing nonionic surfactant (octylphenol polyethoxylene, Triton X-100) and sodium sulfate (Na2SO4) was determined at T = 293.15, 298.15, 303.15, 308.15, 313.15 K, respectively. The experimental binodal data was correlated by various experiential equations for the satisfactory fitting results. With the increase of temperature, the separated two-phase region areas of Triton X-100/Na2SO4 ATPS were expanded, and the salting-out abilities of Na2SO4 increased based on the Effective Excluded Volume (EEV) theory, while the phase disengagement of ATPS became difficult at the higher temperature. The experimental tie-line data of Triton X-100/Na2SO4 ATPS was further applied to fit the Othmer-Tobias and Bancroft equations at T = 293.15, 298.15, 303.15 K, respectively, which could successfully predict unknown tie-line data within the investigated tie line length (TLL) ranges. Moreover, the experimental mass ratios (Mr) of different TLL under various temperatures showed a good concordance with the predicted Mr values (standard deviation, SD < 0.1). The (liquid + liquid) equilibrium (LLE) data of Triton X-100/Na2SO4 ATPS containing 8% SDS (to total mass of surfactants, w/w) at 293.15 K was further studied according to the above method. It indicated that the addition of SDS could accelerate phase disengagement and expand the two-phase region. In the Othmer-Tobias and Bancroft equations of Triton X-100/Na2SO4 ATPS containing SDS, the SD value of TLL was less than 0.05 between experimental data and predicted values, which was generally lower than the corresponding SD value of Triton X-100/Na2SO4 ATPS without SDS.