title: Molecular-thermodynamic framework to predict the micellization behavior of mixtures of fluorocarbon-based and hydrocarbon-based surfactants.
abstract: We present a molecular-thermodynamic (MT) framework to predict the micellization properties of mixtures of fluorocarbon-based and hydrocarbon-based surfactants. Practically, this mixing reduces the use of fluorinated surfactants in the surfactant formulation, thereby addressing environmental concerns associated with the non-biodegradability and toxicity of fluorinated surfactants. The micellization properties of these mixtures are affected by the enthalpic interactions between the fluorocarbon and hydrocarbon surfactant tails. Consequently, the MT framework incorporates an enthalpy of mixing contribution estimated using regular solution theory (RST). The RST interaction parameter is estimated on the basis of phase equilibrium data. The MT framework also makes allowance for the coexistence of two types of micelles in solution to account for experimental findings which suggest that mixtures of fluorocarbon-based and hydrocarbon-based surfactants can form two types of micelles. Furthermore, the model used to calculate the packing free energy of binary mixtures of surfactant tails is generalized to incorporate the difference in the tail volumes, tail lengths, and conformational energies of the fluorocarbon and hydrocarbon tails. The MT framework is then used to predict micelle population distributions, critical micelle concentrations, and optimal micelle compositions for various mixtures of fluorocarbon-based and hydrocarbon-based surfactants, and the predictions are compared with the corresponding experimental values. While many of the predictions compare well with experiment, some of the experimentally observed trends are not reproduced by the MT framework. Ways to eliminate the discrepancy between theory and experiment are discussed. We also find that prediction of the micelle population distribution is very sensitive to the magnitude of the RST interaction parameter used to calculate the enthalpy of mixing, where an increase in the RST interaction parameter results in sharper peaks in the predicted bimodal micelle population distribution. In addition to the quantitative prediction of micellization properties, the MT framework provides useful physical insight about the reasons behind the differences in the micellization properties of various surfactant mixtures.
authors: Department of Chemical Engineering, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
authors: Jaisree
authors: J
authors: Iyer
authors: Daniel
authors: D
authors: Blankschtein