Molecular features of water films created with bubbles at silica surfaces

Sum frequency vibrational spectroscopy (SFVS) spectra indicate that a very ordered water structure exists in the stable water film at a hydrophilic silica surface during contact with a bubble and that the extent of hydrogen bonding increases with an increase in contact pressure. In contrast, the SFVS spectra of water at a hydrophobic silica surface show a lack of hydrogen bonding and are characterized by a distinct absorption at about 3700 cm⁻¹, similar to the spectrum of the air/water interface. These results suggest the presence of a water exclusion zone at the hydrophobic surface, as supported by X-ray reflectivity measurements reported in the literature, by AFM images, and by results from molecular dynamics simulations. Of course, the water film at the hydrophobic surface is unstable with film thinning and rupture upon bubble contact. Under these circumstances, it is shown that an attractive van der Waals force between a bubble and the hydrophobic surface can be expected when the water exclusion zone is taken into consideration. As the thickness of the water exclusion zone increases to a thickness corresponding to the size of nanobubbles, the calculated attractive van der Waals force increases. This analysis may help to explain the so-called ‘short-range’ and ‘long-range’ hydrophobic forces.