Isotope Effects of Light and Heavy Water on Hydrophobic Interactions

Physiochemical properties of light water (H2O) and heavy water (D2O) differ to some extent due to the differences originate in isotope effects. To better understand the isotope effects on hydrophobic interactions, we report the direct experimental comparison of hydrophobic interactions in light water versus heavy water in extended molecularly smooth hydrophobized mica surfaces using a Surface Forces Apparatus (SFA). In this study, we synthesized molecularly smooth, easily reproducible and stable hydrophobic surfaces by deposition of perfluorodecyltrichlorosilane (FDTS) monolayers on plasma activated mica surfaces by using molecular vapor deposition (MVD) technique. The wettability and surface morphology of the FDTS-coated mica samples were directly monitored by using a contact angle cell and an atomic force microscope (AFM). Instability in the force-curve are observed during both approach and retraction of surfaces in SFA experiments and our preliminary force-spectroscopy experiments demonstrated that the magnitude of hydrophobic interaction between FDTS-coated mica surfaces was 20% stronger in heavy water than in light water. Our results indicate that the force-distance profiles obtained for both the cases of H2O and D2O could not be reasonably described by the classical Derjaguin−Landau−Verwey−Overbeek (DLVO) theory and the strong adhesion measured between the interacting FDTS-FDTS layers is dominated by the hydrophobic interactions.