Interactions of water with the nonionic surfactant polyoxyethylene glycol alkyl ethers studied by phase-sensitive sum frequency generation and molecular dynamics simulation

Abstract

Phase-sensitive sum frequency generation (SFG) vibrational spectroscopy and molecular dynamics (MD) simulation were used to study the interactions between water molecules and the surfactant polyoxyethylene glycol alkyl ether (C12E4) at its critical micelle concentration. The surfactant enhanced the positive peak of water's SFG spectrum suggesting that C12E4 was more anionic-like, even though the surfactant was overall neutral. MD simulations showed that the surfactant increased the depth of the surface anisotropic layer from 0.31 to 1.82nm and the average number of hydrogen bonds per water molecule from 2.7 to 3.1. For water molecules near the surfactant, their H and O atoms are confined in well-separated shells. Both the O and C atoms in the head group of the surfactant are surrounded by the H atoms, instead of the O atoms, of water indicating that the negatively charged O atoms of the surfactant play a more important role than the C atoms in determining the orientation of water. The simulation also showed that the orientation of surface water molecules was flipped in the presence of the surfactant, which was consistent with the observed SFG spectra.