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

The surface tension of the aqueous solution of lithium dodecyl sulfate (LiDS) 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 by applying the thermodynamic equations to the experimental results. It was found that the LiDS and LiFOS molecules mix homogeneously with each other in the adsorbed film and the mole fraction of LiFOS in the total surfactant in the adsorbed film is larger than that in the solution. However, the molecules turned out 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 maximum of critical micelle concentration vs composition curve. This behavior was attributed to the repulsive interaction between the hydrocarbon and fluorocarbon chains of the surfactants. Further, it was suggested that the interaction between LiDS and LiFOS molecules in the micelle is weaker than that in the adsorbed film.