Macroscopic thermodynamic properties of ionic micellar solutions depending on physicochemical formulation for the sodium dodecyl sulfate/sodium sulfate/water/n-heptane/1-pentanol system

Abstract

This work characterized the macroscopic thermodynamic properties of ionic micellar solutions. Formulation-composition studies were done along one dimension by varying the salinity (S) or oil–water ratio (WOR) in mixtures of sodium dodecyl sulfate, sodium sulfate, water, n-heptane, and 1-pentanol at 25°C and 1 atm. Material balance and multiple regression models were used to get the partial, mixed, and excess molar volumes. The UNIFAC local composition model, coupled with Debye–Hückel theory, was employed to calculate activity coefficients and dimensionless Gibbs energies (partial, mixed, and excess). The equilibrium SOW systems exhibited phase transitions (WI-WIII-WII), with micellar solubilization increasing as salinity increased at constant WOR. Solubilization peaked at the optimal formulation, and further increases in WOR enhanced solubilization up to the formation of a single-phase system. Deviations from ideal behavior, in the thermodynamic properties between aqueous and oil micellar solutions, were mainly due to chemical interaction of solutes ($\gamma \ne 1$) respect to solvents ($\gamma \to 1$). Negative values of the Gibbs energy of mixing confirmed the stability of the liquid phases and their extension to the liquid–liquid equilibrium. The response surfaces V = f(WOR, S) and G^E/RT = f(WOR, S) represent the macroscopic thermodynamic behavior of micellar solutions as a function of physicochemical formulation. These results can be extended to other colloidal systems for the design of formulations with surfactants and anionic salts, oriented to the diagnosis and resolution of problems in real systems, both at laboratory and industrial level.