Study on the miscibility of lithium tetradecyl sulfate and lithium perfluorooctane sulfonate in the adsorbed film and micelle

The surface tension of the aqueous solution of lithium tetradecyl sulfate (LiTS) and lithium perfluorooctane sulfonate (LiFOS) mixture was measured as a function of the total molality and composition of surfactants at 298.15 K under atmospheric pressure. The compositions in the adsorbed film and micelle were evaluated numerically by applying the thermodynamic equations derived previously. It was found that the adsorbed film is enriched in LiFOS molecules at a small concentration, while it is enriched in LiTS molecules at a large concentration, as compared to the aqueous solution, and both molecules mix nonideally with each other. On the other hand, they were proved to be partially immiscible in the micellar state and to form the two kinds of micelles which coexist in equilibrium only at the composition of the heterogeneous azeotropic (hetero-azeotropic) point. It was also observed that the LiTS molecules expel the LiFOS molecules from the micelles in a composition range where the mole fraction of LiFOS is small. Comparing the above results to those of a lithium dodecyl sulfate (LiDS) and LiFOS mixture, the chain length of hydrocarbon surfactant was concluded to have a considerable influence on the miscibility of hydrocarbon and fluorocarbon surfactants in the adsorbed film and micelle.