Surface forces in thin liquid films of n-alcohols and of water-ethanol mixtures confined between hydrophobic surfaces

An atomic force microscope (AFM) was used to measure the surface forces in thin liquid films (TLFs) confined between gold surfaces hydrophobized by chemisorption of alkylthiols. The measurements were conducted in different H-bonding liquids and in water-ethanol solutions. Attractive forces stronger and longer-ranged than the van der Waals force were observed in water, ethanol, and 1-butanol in a descending order. The attractive force measured in methanol was much weaker. The surface forces measured in ethanol solutions varied with concentration. Initially, the attractive forces decreased rapidly upon ethanol addition, passed through a broad minimum centered around mole fractions in the range of 0.15-0.20, and then increased as the ethanol mole fraction was further increased above ~0.5. Thermodynamic analysis of the data suggests that clusters of water and ethanol may form in TLFs. Thus, the attractive forces may arise from changes in liquid structure. It appears that structuring is a consequence of H-bonding liquid to minimize its free energy in the vicinity of hydrophobic surfaces and in the presence of foreign species in solution.