Investigation of the saturated adsorption layer of 5-cyano-biphenyl and 5-cyano-terphenyl at the free water surface by Monte Carlo simulation

The saturated adsorption layer of the liquid crystal forming 5-cyano-biphenyl (5CB) and 5-cyano-terphenyl (5CT) molecules at the water liquid/vapor interface has been analyzed by Monte Carlo computer simulation at 300 K. The adsorbed molecules are found to be orientationally rather strongly ordered, preferring a tilted orientation relative to the interface normal axis. This orientation is also reflected in the preferred alignment of the hydrogen bonds between an adsorbed 5CB or 5CT and a water molecule, and also in that of water hydrogen bonded to 5CB or 5CT. Due to the fact that only a small fraction of the interfacial water is involved in such a hydrogen bond, their preferred orientation is markedly different from that of all the water molecules in the adsorption layer; the preferred orientation of the latter is imposed by the charge distribution of the polar part of the adsorbed 5CB or 5CT molecules.