Water Structure at Superhydrophobic Quartz/Water Interfaces: A Vibrational Sum Frequency Generation Spectroscopy Study

Abstract

The water structure at superhydrophobic solid/bulk water interfaces has been investigated by vibrational sum frequency generation (SFG) spectroscopy. We have adapted a simple and facile chemical modification procedure (methyltrichlorosilane/toluene treatment followed by extraction with ethanol) to prepare transparent quartz crystal surfaces with varied wetting properties, from hydrophilic to hydrophobic and superhydrophobic. In comparison with those obtained on bare quartz, the SFG spectra on polymethylsiloxane-modified surfaces showed significant changes of the relative intensities of the two broad OH stretching modes of hydrogen-bonded water at ∼3200 and ∼3450 cm−1, which can be correlated with surface morphology and molecular variations. Intriguingly, on superhydrophobic quartz these bands are very weak and replaced by a characteristic “free OH” (not hydrogen-bonded) stretching (>3600 cm−1) band that is typically observed at water/air interfaces. These results suggest that hydrogen bonding between water molecules weakens as hydrophobicity increases on rough heterogeneous solid surfaces. More importantly, this study provides direct evidence for the existence of stable solid/air/water three-phase interfaces when a superhydrophobic solid is in contact with bulk water.