Hysteretic characteristics of 1/λ⁴ scattering of light during adsorption and desorption of water in porous Vycor glass with nanopores.

Porous Vycor glass with nanopores is transparent in the visible region and is often used in colorimetric chemical sensing when impregnated with selectively reacting reagents. However, it has some disadvantages in sensing, since changes in the humidity of ambient air strongly affect its transmission. In this work, by combining a humidity-controlled thermostatic chamber and an ultraviolet-visible and near-infrared spectrophotometer through fiber optics, we analyzed the effect of increasing and decreasing humidity in the ambient air on the transparency change of the nanoporous glass. The transparency response in the visible region to changes in humidity is analyzed to correlate the turbidity response of the glass with the amount of water in it. The turbidity is found to be dependent on the inverse fourth power of the wavelength (1/λ⁴), which implies that Rayleigh-type scattering takes place for both adsorption and desorption of water. We show that measures of the extent of the optical inhomogeneity that causes the scattering, such as the effective radius of scatterers and their number density, exhibit a pronounced hysteretic characteristic for the imbibition and drainage of water, while the absorption inherent to imbibed water also shows another type of hysteresis that is quite similar to the sorption isotherms of water. On the basis of the above observations, we show that the transitory white turbidity of nanoporous glasses during changes in humidity can be consistently interpreted and quantitatively analyzed by a simple Rayleigh scattering mechanism.