Solubility of ionic surfactants below their Krafft point in mixed micellar solutions: Phase diagrams for methyl ester sulfonates and nonionic cosurfactants

TL;DR

This study investigates the solubility behavior of ionic surfactants below their Krafft point in mixed micellar solutions, providing phase diagrams and a theoretical model to enhance their application in cleaning products.

Contribution

It introduces a new methodology for constructing phase diagrams of surfactant mixtures using minimal experimental data and develops a theoretical model to explain their phase behavior.

Findings

Phase diagrams include mixed micelles, crystallites, and solutions.

The methodology simplifies phase diagram construction with fewer measurements.

Results suggest expanded use of MES-surfactants with high Krafft temperatures.

Abstract

Many ionic surfactants with wide applications in personal-care and house-hold detergency show limited water solubility at lower temperatures (Krafft point). This drawback can be overcome by using mixed solutions, where the ionic surfactant is incorporated in mixed micelles with another surfactant, which is soluble at lower temperatures. The solubility and electrolytic conductivity for a binary surfactant mixture of anionic methyl ester sulfonates (MES) with nonionic alkyl polyglucoside and alkyl polyoxyethylene ether at 5 deg C during long-term storage were measured. Phase diagrams were established; a general theoretical model for their explanation was developed and checked experimentally. The binary and ternary phase diagrams for studied surfactant mixtures include phase domains: mixed micelles; micelles and crystallites; crystallites, and molecular solution. The proposed general methodology, which utilizes the equations of molecular thermodynamics at minimum number of experimental measurements, is convenient for construction of such phase diagrams. The results could increase the range of applicability of MES-surfactants with relatively high Krafft temperature, but with various useful properties such as excellent biodegradability and skin compatibility; stability in hard water; good wetting and cleaning performance.