Hydrophilic interactions between organic and water molecules as models for monolayers at the gas/water interface

Model clusters of surfactant prototypes with small number of water molecules are calculated at different levels of theory. All approaches used yield correct trends in the variation of the dipole moment upon tail elongation or polar headgroup variation. Models including one, two, or more water molecules are optimized. The most stable structures are those with maximum number of atoms involved in hydrogen bonding. The normal components of the dipole moment prove to be less sensitive to the nature (aliphatic or aromatic) of the hydrophobic tail, in accord with findings from the phenomenological models. Values of the dipole moment approaching the experimental estimates required inclusion of sufficient aqueous environment (>20 water molecules per hydrophilic head) and of lateral intersurfactant interactions into the model.