Gravimetric analysis of water vapor adsorption kinetics on hydrophilic and hydrophobic Si surfaces using microcantilevers

Understanding the nature of the adsorption of water molecules on a Si surface is of great interest in basic sciences as well as in broad range of industrial processes. Traditional techniques such as Fourier transform infrared spectroscopy cannot capture the adsorption phenomena on a larger surface area of Si. Herein, we report the adsorption kinetics of water molecules on hydrophilic and hydrophobic Si surfaces by means of a precision gravimetric method using microcantilevers (MC). Si MCs with hydrophilic and hydrophobic surfaces were synthesized by chemical route, and their resonance frequency with increasing Relative Humidity (RH) was measured. Shift in resonance frequency of MCs due to adsorption of water molecules was used to estimate the added mass on their surface. In hydrophilic MCs, added mass Vs RH data revealed three distinctive slopes, indicating variation in the molecular configuration of water molecules with increasing RH as predicted by spectroscopic techniques. These results are independently validated by measuring the adhesion force on the MC surface using force-distance curves. In contrast to this, in hydrophobic MCs, added mass due to adsorption was very low and found to increase marginally at higher RH values. An increase in MC surface roughness leading to the clustering of water molecules was shown to be the reason for these observations.