Theoretical Prediction of Branched-Tail Surfactant Micellization In Dilute Aqueous Solutions Using Computer Simulations and Molecular Thermodynamics
Shangchao Lin, Jonathan D. Mendenhall, and Daniel Blankschtein. Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
Surfactants possessing branched lipophilic domains, or tails, are frequently encountered in industrial formulations due to their higher aqueous solubility at low temperatures, relative to linear surfactants. For example, secondary alkyl sulfonates (SAS's) are frequently used in cold-water detergent applications. Other situations in which branched surfactants arise include emulsification applications (including such surfactants as Triton X-100 and Silwet L-77) and in ethoxylated surfactant mixtures, as by-products of linear surfactant synthesis. The prediction of the micellization properties of such branched surfactants, including the critical micelle concentration (CMC), micelle shape and size, and micelle composition (for mixed micelles), is in general useful to formulators, who often must use trial-and-error experimentation to develop surfactant formulations exhibiting a desired solution behavior. Molecular-thermodynamic (MT) modeling is a successful approach that has been used to theoretically characterize both single- and mixed-surfactant systems based solely on surfactant molecular properties and molecular thermodynamics. In recent years, Stephenson, Beers, and Blankschtein [J Phys Chem B, 111, 1063-1075, 2007] have demonstrated the utility of combining Molecular Dynamics (MD) simulation results with MT modeling of single-surfactant systems, creating a hybrid CS-MT model that uses detailed microstructural hydration information from the MD simulations to more accurately compute the free energy of micellization. We present the key modifications to the CS-MT modeling approach that we have made to further extend the range of applicability of the model to single, branched surfactant systems, including refinement of the counting of water contacts and development of an improved model for surfactant tail packing in a micelle core. We present results for a wide variety of branched surfactants, including two series of SAS surfactants; aromatic branched surfactants, including linear alkylbenzene sulfonates (LAS); and the emulsifiers Triton X-100 and Silwet L-77.