Excess thermodynamic properties of thin water films confined between hydrophobized gold surfaces

Surface forces between gold surfaces were measured in pure water at temperatures in the range of 10–40 °C using an atomic force microscope (AFM). The surfaces were hydrophobized by self-assembly of alkanethiols (CnSH) with n = 2 and 16 in ethanol solutions. The data were used to determine the changes in excess free energies (ΔGf) of the thin water films per unit area by using the Derjaguin approximation [1]. The free energy data were then used to determine the changes in excess film entropy (ΔSf) and the excess film enthalpy (ΔHf) per unit area. The results show that both ΔSf and ΔHf decrease with decreasing film thickness, suggesting that the macroscopic hydrophobic interaction involves building some kind of structures in the intervening thin films of water. It was found that |ΔHf| > |TΔSf|, which is a necessary condition for an attractive force to appear when the enthalpy and entropy changes are both negative. That macroscopic hydrophobic interaction is enthalpically driven is contrary to the hydrophobic interactions at molecular scale. The results obtained in the present work are used to discuss possible origins for the long-range attractions observed between hydrophobic surfaces.

For the hydrophobic interaction between C16SH-coated gold surfaces at 20 °C, both the enthalpy (ΔHf) and entropy (TΔSf) changes are negative. Under these conditions, the free energy change (ΔGf) becomes negative (or attractive force appears) only when |ΔHf| > |TΔSf|.Figure optionsDownload high-quality image (61 K)Download as PowerPoint slideHighlights
► Hydrophobic forces were measured systematically at different temperatures.
► Macroscopic hydrophobic interaction entails decreases in both entropy and enthalpy.
► Macroscopic hydrophobic interaction is enthalpically driven.