All-Atom Molecular Dynamics Analysis of the Self-Assembly Process of Polyoxyethylene Alkyl Ethers in Water

The development of emulsified products, such as pharmaceuticals and cosmetics, often requires expensive and time-consuming experimental screening to optimize their physical properties owing to their complex mixtures of surfactants and additives. Given that industries are eager for a general-purpose in silico method for predicting aggregation properties, we investigated the applicability of all-atom molecular dynamics (AAMD) simulations for predicting the physical properties of aqueous solutions containing nonionic surfactants. This study focused on the self-assembly process of polyoxyethylene alkyl ethers (C_<i>m</i>EO_<i>n</i>), as they are popular components of emulsified products. We executed AAMD simulations starting from initial configurations with randomly mixed C_<i>m</i>EO_<i>n</i> in water to investigate the micelle formation dynamics. Surfactants of various lengths (<i>m</i> = 6-14 and <i>n</i> = 3-9) were investigated in a temperature range of 300 K-340 K. Within approximately 100 ns, C_<i>m</i>EO_<i>n</i> monomers self-aggregated with their hydrophilic chains facing outward toward the water phase and their hydrophobic chains directed inward, forming numerous less spherical, incomplete, small micelles. After 100 ns, the small micelles merged, forming larger and more spherical micelles. The aggregation number increased with increasing temperature and alkyl chain length and decreased with increasing polyoxyethylene chain length. The AAMD results were consistent with experimental observations, demonstrating the applicability of AAMD simulations in aggregation property prediction.