Interfacial Force Microscopy of Humidity Dependent Solvation Forces of Water Between Bare and Fluorcarbon-Coated Silica Surfaces

Despite its ubiquity in living systems, water’s properties and behavior in small, confined spaces such as cell membranes is almost unknown. Due to the advancement of measurement techniques such as atomic force microscopy, recent research has been able to elucidate the interactions of water with surfaces at the nanometer scale. Studying small molecules such as water at this level is very difficult since experiments involving water molecules are delicate and easily disturbed. To this end, we recently developed an atomic force microscope technique called cantilever-based optical interfacial force microscope (COIFM). In a recent study using the COIFM, we observed remarkable oscillatory forces with amplitude of 60-90 nN and periodicity of 3–4 water molecule diameters in an ambient environment between two oxidized silicon surfaces as the tip-sample distance was decreased. However, the mechanism of these oscillatory forces is not fully understood. As an approach to discover the oscillatory behavior of water, we repeated the measurements with different oxidized silicon probe sizes and hydrophobic probes under varied relative humidities. We observed a small oscillatory force with a periodicity of 3–4 water diameters below 50% relative humidity, whereas the oscillatory forces disappeared at a higher humidity above 50% for both the oxidized silicon probe and the hydrophobic probe. The result suggests that the oscillatory pattern is present universally for water molecules confined between two surfaces in an ambient environment.