Surfactant formulations of practical utility typically consist of many surfactant components. pH-sensitive surfactants are often added as a secondary surfactant because they enhance performance properties, including solubility, foaming, and mildness to the skin or to the eyes. In addition, pH-sensitive surfactants may be used effectively in novel applications where pH variations can be utilized to control self-assembly, including controlled drug release, targeted gene delivery, and the fabrication of nanoscale materials for optics, electronics, and sensors.

A molecular-thermodynamic (MT) theory was developed to model the micellization of mixtures containing an arbitrary number of pH-sensitive and conventional surfactants. The inputs to the theory included molecular characteristics of the surfactants and the solution conditions (such as the type and amount of added salt, acid, or base). The theory molecularly predicts solution and micelle properties, including the solution pH, the critical micelle concentration (cmc), the micelle composition, the degree of counterion binding to micelles, and the shape and size of micelles.

Predicted cmc's compared favorably to experimental cmc's obtained from the literature for mixtures of conventional surfactants and pH-sensitive sufactants. For example, a commercial nonionic surfactant (Genapol UD-079) was modeled as a mixture of 16 surfactant components. The predicted cmc agreed remarkably well with the experimental cmc. As experimentally observed in related systems, the monomer concentration was also predicted to increase significantly above the cmc. The predicted solution pH of mixtures of pH-sensitive and conventional surfactants was also found to agree well with experimental data. In addition, the theory was validated using micellar titration data for varying compositions of mixed micelles containing dodecyldimethylamine oxide (DDAO) and a cationic, nonionic, or anionic surfactant. The MT theory accurately modeled the titration behavior of DDAO mixed with the nonionic surfactant. Although the MT predictions were qualitatively similar to experimental data for titrations in micelles containing the anionic and the cationic surfactants, the experimental data suggest that DDAO interacts more favorably with the anionic or cationic surfactant than can be predicted by the MT theory.

The MT theory represents the first molecular-based model of the micellization behavior of mixtures of three or more conventional surfactants and of mixtures of pH-sensitive surfactants and conventional surfactants. The MT theory resulted in qualitative and quantitative predictions of micellization properties for a variety of surfactant systems. The resulting MT theory provides fundamental, physical insight and may also decrease the need for the costly and time-consuming process of "trial-and-error" surfactant formulation.