An X-ray and Neutron Reflection Study of Water Penetration into Fluorocarbon Doped Silica Gel Films

Abstract

Specular neutron and X-ray reflection have been used to investigate the penetration of water into silica gel films. The effect on water penetration of incorporating fluoroalkysilane (FAS) into the gel has also been measured. Gel films were deposited onto float glass substrates and exposed to either liquid water or an atmosphere of known relative humidity (between 0% and 100%). It was shown that, irrespective of FAS content, water penetrates throughout the gel film, and deeper into the glass substrate if the sample has prolonged contact with water. The X-ray reflection measurements have shown there are no changes in the structure of the gel film on glass on exposure to a humid atmosphere. This result is verified by the neutron reflection data and suggests that as water penetrates into the surface it occupies in the gel network. These voids can be filled or emptied by changing the relative humidity. The neutron reflectivity measurements have also revealed additional information about the distribution of the water. For the pure silica gel, with no added FAS, there is a water-rich layer (∼30 A thick) at the gel surface and a significant amount of water (volume fraction up to 18%) penetrated throughout the gel film. Gel films made with moderate amounts of FAS show no evidence for a water-rich layer on the surface. In fact, the water density is effectively zero within ∼20 A of the gel's surface. This suggests that the fluorinated materialformed a hydrophobic monolayer near the surface of the gel. For gels with much higher levels of FAS, the film surface becomes very rough and a detailed interpretation is not possible.