Molecular behavior of ionic and nonionic surfactants in saline medium

Abstract

The aim of this study was to investigate the molecular behavior of ionic and nonionic surfactants at different salt concentrations. The nonionic surfactants used in this work were nonylphenol 20 EO, nonylphenol 100 EO, lauryl alcohol 10 EO, and lauryl alcohol 23 EO, being EO the number of ethene oxide groups. The ionic surfactants were synthesized through saponification reactions using the following vegetable oils: coconut oil and castor oil. The cmc (critical micelle concentration) of each surfactant was determined using surface tension measurements at different surfactant and salt concentrations. The study of surfactant adsorption, using the Gibbs equation with factor n = 1 for nonionic surfactants and n = 2 for ionic ones, allowed to obtain the packing area per surfactant molecule, the surface excess, and the Gibbs free energy of micellization. The obtained results were compared and discussed in view of the hole of the type of surfactant and electrolyte influence. From the obtained results, it can be concluded that the ionic surfactants, due to electrostatic repulsion between ionic head groups, showed higher cmc values than those obtained for nonionic ones. When salt ions were added to the ionic surfactant solution, the cmc values were decreased due to the existence of electrostatic interactions between the ionic head groups of these molecules with the electrolytes. For nonionic surfactants, the increasing ethoxylation degree, the hydrophilic portion of surfactant molecule, and electrolyte addition diminishes cmc values. Concerning to the Gibbs free energy of micellization, the obtained results showed that the adsorption process was spontaneous for all studied surfactants, but it was thermodynamically more favorable for nonionic ones.