Molecular-Thermodynamic (MT) Modeling of the Micellization of Binary Mixtures of Fluorocarbon-Based and Hydrocarbon-Based Surfactants In Aqueous Solution

Owing to fluorine’s position in the periodic table, fluorinated surfactants possess several unique attributes, including increased surface activity, lipophobicity, chemical inertness, and thermal stability. These attributes make fluorinated surfactants far superior than their hydrocarbon counterparts, resulting in their widespread use in paints, polishes, adhesives, and fire-fighting foams. However, the non-biodegradability and toxicity associated with certain fluorinated molecules has prompted the vigorous search for alternative surfactant formulations that have comparable performance characteristics while minimizing the use of fluorinated surfactants.

It is well known that mixtures of surfactants can exhibit superior properties than those of the individual surfactant components. Therefore, an interesting strategy to reduce the amount of fluorinated surfactant in a surfactant formulation, while maintaining or improving the formulation performance characteristics, is to mix them with other benign surfactants like hydrocarbon-based surfactants. Experimental studies of binary mixtures of fluorocarbon-based and hydrocarbon-based surfactants have revealed that, for certain mixtures, two types of mixed micelles: one rich in fluorocarbon-based surfactant and the other rich in hydrocarbon-based surfactant, co-exist in solution, while for certain other mixtures, only one type of mixed micelle exists. Accordingly, in order to predict the micellization properties of these surfactant mixtures, which will enable us to evaluate their performance characteristics, and hence their viability as potential replacements, it is necessary to develop a model that allows for the co-existence of more than one type of micelle in solution.

In this talk, we discuss the development of a Molecular-Thermodynamic (MT) framework that incorporates the co-existence of micelles in a surfactant solution. This model, when applied to mixtures of surfactants that interact antagonistically with each other, provides a physical understanding of the conditions (molecular structures of the constituent surfactants and composition of the surfactant formulation) required to observe two types of mixed micelles in solution. In addition, the practical applicability of the model is tested by making predictions of the critical micelle concentrations (CMCs) of various binary mixtures of fluorocarbon-based and hydrocarbon-based surfactants, including comparing the theoretically predicted CMCs with the experimental ones.