Effects of Short-Chain Alcohols and Pyridine on the Hydration Forces between Silica Surfaces

Forces between fully hydroxalated silica surfaces were measured using an atomic force microscope. The measurements were conducted in Nanopure water and in solutions containing various organic solutes such as methanol, ethanol, trifluoroethanol (TFE), and pyridine. The results obtained in Nanopure water showed a strong short-range repulsive force at distances below 15 nm. This non-DLVO force can be fitted to a double-exponential force law with its longer decay length (D2) of 2.4 nm. On the other hand, the force curve obtained at 15% methanol by volume can be fitted to the DLVO theory perfectly, showing no signs of hydration force. These results suggest that the hydration force originate from the unique water structure in the vicinity of silica, which apparently is seriously disrupted in the presence of methanol. Methanol may adsorb on silica, displacing water molecules from the silanol groups and, thereby, breaking the H-bond network within the hydration sheath around silica. The displacement of water by methanol is thermodynamically possible because the latter is more basic than the former. In 10-20% ethanol solutions, D2 decreases to 1.1-1.2 nm, indicating that ethanol also adsorbs on silica but to a lesser extent than methanol. In TFE and pyridine solutions, the hydration force changes little, suggesting that these solutes cannot readily displace water molecules from silanol groups. The results presented in this communication may have a bearing on the intoxication of humans by alcohols, which may be related to the dehydration of lipid membranes. Copyright 1998 Academic Press.