(383a) Predicting the Micellization Behavior of Perfluorinated and Partially-Fluorinated Surfactants in Dilute Aqueous Solution Using Computer Simulations and Molecular-Thermodynamic Theory

Authors: 
Mendenhall, J. D., Massachusetts Institute of Technology
Lin, S., Massachusetts Institute of Technology
Blankschtein, D., Massachusetts Institute of Technology


Fluorinated surfactants exhibit superior performance in many practical surface and bulk solution applications relative to their hydrogenated counterparts. An important example is the large reduction in surface tension achieved at a markedly lower surfactant concentration in aqueous solution. One main reason for the improved performance of this class of surfactants is the increased hydrophobicity of fluorinated alkyl groups relative to hydrogenated alkyl groups. In addition to the numerous applications resulting from the unique properties of fluorinated surfactants at surfaces (e.g., use in hydrophobic and oleophobic coatings, use in fire-fighting foams, and use in waxes, polishes, paints, and cleaners), fluorinated surfactants are of interest also for their useful bulk solution behavior. For example, fluorinated surfactants are used in emulsion polymerization applications and have also been investigated as potential artificial blood substitutes. Although the use of fluorinated surfactants is advantageous in many situations, concern has recently been raised regarding the safety of certain fluorinated surfactants, specifically in terms of their environmental toxicity and bioaccumulation. For example, in the case of perfluorooctanoic acid (PFOA), the EPA has created the 2010/15 PFOA Stewardship Program in order to encourage the reduction of industrial emission of PFOA, with the goal of 95% reduction by 2010 and elimination by 2015. In light of this effort, it would be desirable to identify replacement surfactants that have similar performance properties to perfluorinated surfactants, yet do not exhibit the accompanying negative characteristics. Such surfactants could possess architectural differences (e.g., branching or multiple tails) or chemical differences (e.g., partial hydrogenation or inclusion of biodegradable moieties such as esters) relative to their perfluorinated counterparts. Given the enormous investment required to synthesize proposed alternatives, including characterizing them experimentally and evaluating their performance behavior, it would be very advantageous to be able to predict the bulk solution properties of potential surfactant replacements molecularly, thus enabling the formulator to narrow his/her focus to those few candidates that exhibit the desired bulk solution properties. The recently developed combined Computer-Simulation/Molecular-Thermodynamic (CS-MT) theory of Stephenson, Beers, and Blankschtein [J Phys Chem B, 111, 1063-1075, 2007] has been demonstrated to accurately model the process of surfactant micellization in dilute aqueous solution for a variety of linear, fully-hydrogenated surfactants. Here, we demonstrate application of the CS-MT model to perfluorinated and partially-fluorinated surfactants. We first present a novel method that we have developed for general surfactant head/tail identification from Molecular Dynamics simulations. We then discuss general improvements that we have made to the computation of the change in hydration state of atomic groups within a surfactant molecule experienced upon incorporation into a surfactant micelle. Based on these and other molecular inputs, we compute crtitical micelle concentrations (CMC's) and micelle shapes and sizes for a variety of ionic and nonionic (ethoxylated) perfluorinated surfactants, and demonstrate very good agreement with the experimental values, when available. Finally, we present a sample design study illustrating the effect of subtle chemical differences on the bulk surfactant solution properties. In particular, it has long been known that fluorination of the ω-carbon of an otherwise fully-hydrogenated linear alkyl surfactant yields an anomalous 2-fold increase in the CMC [Muller and Birkhahn, J Phys Chem, 71, 952-962, 1967], where a decrease in CMC might be expected based on hydrophobicity arguments. We demonstrate that these types of effects can be captured using the CS-MT model with the addition of an enthalpic-mixing contribution to the free energy of micellization. We present predicted CMC's and micelle shapes and sizes for a series of partially-fluorinated surfactants with increasing fluorination ratio, and compare the predicted values to those available experimentally. In addition, we discuss briefly the application of the CS-MT model to other perturbations of the perfluorinated structure, including challenges posed by other chemical moieties, such as esters, as well as by chain branching.