Adsorption of poly(ethylene oxide) at the free water surface. A computer simulation study.

Molecular dynamics simulation of poly(ethylene oxide) (PEO) adsorbed at the free water surface are reported on the canonical ensemble at 298 K. The obtained results are analyzed in terms of the novel Identification of the Truly Interfacial Molecules (ITIM) method. To obtain a deep, molecular level insight into the origin of the adsorption process, structural, dynamical, and energetic aspects have been investigated. It is found that the vast majority, i.e., 82%, of the monomer units of PEO are immersed into the bulk liquid phase, 17% of them are anchored right at the liquid-vapor interface, while 1% of them penetrate into the vapor phase. Although the presence of a few monomer units in the vapor phase is clearly demonstrated, the average lifetime of these monomer units is found to be rather small, i.e., 3.1 ps, and they are found not to move far from the surface as their interaction energy with the water molecules is still not negligible. The partition of the monomer units between the bulk liquid phase, interface, and vapor phase is determined by the delicate interplay of entropic and energetic factors, mostly by the requirements of maximizing the configurational entropy of the polymeric chain and minimizing the interfacial energy surplus.