Atomic-level molecular modeling of the nonionic surfactant Triton X-100: the OPE9 component in vacuum and water

The commercially available nonionic surfactant Triton X-100 is a mixture of polyoxyethylene tert-octylphenyl ethers (OPEn) with an average of n = 9.5 oxyethylene (OE) units in the molecules, and the population maximum at n = 9. Thus, the OPEn = 9 component was chosen to be studied by atomic level molecular modeling, using second-generation force fields. The 1,000 conformers generated via random sampling of torsional angles around single bonds yielded 11 clusters based on geometrical similarity. Representatives of geometrically distinctly different clusters with significant populations were chosen from a narrow energy range around the most probable energy to be analyzed for interaction with water. The effect of water on the conformation of the OE chain was found to be modest, similar to the situation that had been reported earlier for the anionic surfactant Aerosol-OT (AOT). The number of bound water molecules is strongly dependent on the conformation of the OE chain and is affected by electrostatic as well as steric effects. Unlike the case of AOT, for which the length of the hydrophobic tail was found to govern the size of reverse micelles in CCl4, the size of reverse micelles of OPEn = 9 cannot be predicted from the dimensions of the hydrophilic tail.