(228f) A Molecular-Thermodynamic Theory of Micellization of Mixtures of pH-Sensitive and Conventional Surfactants

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.