Characterization of molecular interfaces in hydrophobic systems

Interactions between typical hydrophobic materials, such as hydrocarbon, fluorinated hydrocarbon and dimethylsiloxane, and water were investigated by contact angle and sliding angle as experimental methods and together with computational method. Results showed that interactions energy of the fluoro-polymers obtained by the experiments amounted to about three times higher than that of polydimethylsiloxanes, and it was coincident with the order of that obtained by the molecular orbital method. Generally, for the evaluation of hydrophobic properties, contact angle measurements are up to the present prevailing. Experimentally, however, antagonistic results, in which a water droplet hardly slides down on the surface of fluorinated polymers having a higher contact angle (theta; 117.0 degrees) than that of polydimethylsiloxanes having a lower one (theta; 95.8 degrees) have been observed. To clarify the reason of these phenomena, trial calculations for the stabilization energies in the optimization of geometry in the hydrophobic molecular/water systems using crystallized models of water, when its segment brings on the surface of water, were made. Bond angles, distances of O-H ... O of two water molecules were calculated. Results showed that interaction energies of FHC/water were very small. From the facts, a few contact points in this system will be predicted. The distance and bond angle of water molecules obtained by further calculations predicted the possibilities that the value of the contact angle is composed of enthalpy and negative excess entropy which structure enhancement of water molecules must influence. In consideration of enthalpy and entropy the interactions were discussed, and the contradictory phenomena between a and or could be elucidated. (C) 1997 Elsevier Science S.A.