(181g) Molecular Modeling of Alcohol Effects in Nonionic Surfactant Micelles with Density Functional Theory

Surfactant micelles have been of interest for the past several decades because of its importance in both scientific studies and industrial processes. Among the large number of different types of additives that have been tested and studied, alcohol is among the most frequently used. It is known that alcohol can act as co-solvent by altering solvent properties, and it can also act as co-surfactant by inserting themselves into the palisade layer of the micelle and co-aggregate with surfactants. In most cases alcohol act dualistically and the overall effect depends on alcohol carbon chain length. Several thermodynamic models have been developed to predict the critical micelle concentration (CMC) and size distribution of micelles, but few of them is able to fully capture the physics at molecular level and predict the dualistic role of alcohol additives.

We study the micelle formation of nonionic poly (ethylene oxide) alkyl ether surfactant using interfacial Statistical Associating Fluid Theory (iSAFT), a classical density functional theory. The model is used to predict the CMC, aggregation number, and distribution of solute into a single micelle. We also study the effect of adding linear 1-alcohols into the surfactant solution. Our model is able to capture both cosolvent and cosurfactant effects and effectively predict the trends of alcohol ranging from short-chain to long-chain in qualitative agreement with experimental data. The theory can provide detailed density profiles of each component in the system to illustrate the locus of alcohol inside a micelle. We observe that all alcohols studied are present in the palisade layer in different amounts. Partition coefficient, aggregation number, micelle size, and total number of surfactant and alcohol molecules are also predicted and discussed. Furthermore, results from different surfactant architecture are compared and analyzed.