Molecular-thermodynamic theories of micellization of multicomponent surfactant mixtures and of pH-sensitive surfactants

• Main Author: Goldsipe, Arthur Clayton
• Other Authors: Daniel Blankschtein.
• Format: Others
• Language: English
• Published: Massachusetts Institute of Technology 2006
• Subjects: Chemical Engineering.
• Online Access: http://hdl.handle.net/1721.1/34277

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2006. === This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. === Includes bibliographical references (p. 249-264). === This thesis focuses on two research areas that are particularly relevant to the practical application of surfactant science: (1) the micellization of multicomponent surfactant mixtures, and (2) the micellization of pH-sensitive (or amphoteric) surfactants. Surfactant formulations of practical utility typically consist of many surfactant components. In many practical applications, pH-sensitive surfactants are 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 eectively 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. First, a molecular-thermodynamic (MT) theory was developed to account for counterion binding to mixed micelles composed of ionic-nonionic and ionic-zwitterionic binary surfactant mixtures. The theory successfully predicted the degree of counterion binding ([beta]) of monovalent and multivalent ions to mixed micelles as a function of the micelle composition ([alpha]). === (cont.) The theory was also found to be consistent with the concept of critical counterion binding. An inection in the [beta vs. [alpha] curve was correlated to a micelle shape transition. Second, the MT theory was generalized to include pH eects in order to model the micellization of pH-sensitive surfactants. The theory was validated by comparing predictions of critical micelle concentrations (cmc's), micelle aggregation numbers, and micellar titration behavior to experimental data for alkyldimethylamine oxide surfactants, which are cationic in the protonated state (at low pH) and zwitterionic in the deprotonated state (at high pH). The MT theory qualitatively reproduced the minimum in the cmc and the maximum in the micelle aggregation number, which are both observed experimentally at intermediate pH values, resulting from the synergy between the two forms of the pH-sensitive surfactant in the micelle. This self-synergy, which was previously attributed by other researchers to the formation ofsurfactant-surfactant hydrogen bonds in the micelle, was rationalized instead in terms of electrostatic interactions operating between surfactants and bound counterions in the micelle. Very good quantitative agreement was obtained for the predicted cmc's in solutions containing no added salt. === (cont.) In particular, the experimentally observed maximum in the cmc, which originated from changes in the solution ionic strength, was reproduced by the MT theory but not by the empirical regular solution theory (RST). Micellar titration data were also examined in terms of the relative values of the micellar deprotonation equilibrium parameter (pK). The pK was related to the derivative of the electrostatic contribution to the free energy of micellization ( gelec) with respect to . The molecular model of gelec predicted pK > 0 in the limit of micelles composed entirely of the deprotonated form of the pH-sensitive surfactant, consistent with the experimental data. Third, a theory based on RST was developed to model the titration behavior of micelles containing a pH-sensitive surfactant and an arbitrary number of conventional surfactants. The conventional surfactants were successfully modeled as a single eective surfactant, thus considerably simplifying the theoretical analysis of multicomponent surfactant mixtures. The RST description was validated using experimental micellar titration data for single surfactant systems (obtained from the literature) and for binary surfactant mixtures (measured as part of this thesis). === (cont.) Experimental uncertainties in the micellar titration data were examined, and a new method was introduced to account for these uncertainties by using a weighted regression analysis. Fourth, a MT theory was developed to model the micellization of mixtures containing an arbitrary number of conventional surfactants. The maximum micelle radius was examined theoretically for a ternary surfactant mixture. Due to the limited availability of experimental data, only the predicted cmc's were compared with the experimental cmc's. Good agreement was obtained for the predicted cmc's, which were comparable to, and sometimes better than, the cmc's determined using RST. The MT theory was also used to model a commercial nonionic surfactant (Genapol UD-079), which was modeled as a mixture of 16 surfactant components. The predicted cmc agreed remarkably well with the experimental cmc. The monomer concentration was predicted to increase signicantly above the cmc. In addition, the monomer and the micelle compositions were predicted to vary signicantly with surfactant concentration. These composition variations were rationalized in terms of competing steric and entropic eects and a micelle shape transition near the cmc. === (cont.) Finally, the MT theory was further generalized to model the micellization behavior of mixtures of a pH-sensitive surfactant and an arbitrary number of conventional surfactants. Predicted values of the solution pH of mixtures of a pH-sensitive surfactant and an ionic surfactant, as well as of the cmc's of mixtures of two pH-sensitive surfactants, compared favorably with the experimental values. The MT theory was also validated using micellar titration data for varying compositions of mixed micelles containing dodecyldimethylamine oxide (C12DAO) and a cationic, nonionic, or anionic surfactant. The MT theory accurately modeled the titration behavior of C12DAO mixed with the nonionic surfactant. However, C12DAO appeared to interact more favorably with the anionic and the cationic surfactants than waspredicted by the MT theory. The MT theories presented in this thesis represent the rst molecular-based models of the micellization behavior of the following systems: (1) pH-sensitive surfactants, (2) mixtures of three or more conventional surfactants, and (3) mixtures of pH-sensitive surfactants and conventional surfactants. The MT theories resulted in qualitative and quantitative predictions of the micellization properties for a variety of surfactant systems. === (cont.) A simpler theory based on RST was also developed to model titrations of micelles containing pH-sensitive and conventional surfactants. In addition, this thesis resulted in the rst experimental study of the eect of micelle composition on the titration behavior of mixed micelles containing a pH-sensitive surfactant and a conventional surfactant. The resulting MT theories have provided fundamental, physical insight, and they may also decrease the need for the costly and time-consuming process associated with "trial-and-error" surfactant formulation. === by Arthur Clayton Goldsipe. === Ph.D.