Fractal and spinodal-decomposed turbidities of nanoporous glass: fluctuation picture in turbid and transparent Vycor.

The light propagation and scattering in monolithic transparent nanoporous materials such as Vycor glasses exhibit two optical turbidities, both of which are slightly deviated from the λ-4 Rayleigh wavelength dependence in the visible region: one is a transient white turbidity τf, characterized by the convex-upward dependence on the inverse fourth power of wavelength, and the other is turbidity τsp inherent to the structural inhomogeneity, characterized by the convex-downward dependence. The former is attributed to a fractal-like percolation network of imbibed or drained pores as a consequence of transient imbibition or drainage of wetting fluid into or from the pore space. The latter is attributed to the structural inhomogeneities inherent to the original dry porous glass, which are produced by spinodal decomposition. In this paper, we develop a general scheme to estimate the transmittance spectra of Vycor through the turbidities τf and τsp in the visible region on the basis of the theory of dielectric constant fluctuations. We show the applicability and its limitation of the turbidity analysis of the photospectroscopically measured data as a method to study the correlation functions that characterize the pore space and the structural features of isotropic transparent nanoporous media, on the presupposition that there exists no light attenuation other than the scattering.

Optically observed imbibition and drainage of wetting fluid in nanoporous Vycor glass.

The light scattering and absorption of monolithic nanoporous Vycor glass during imbibition and drainage of wetting fluid such as water exhibit the following two optical hysteretic characteristics: one is the hysteretic response of the transient white turbidity in the visible (Vis) region and the other the hysteretic response of the absorbance peak in the near-infrared (NIR) region. We analyzed the effect of increasing and decreasing humidity in ambient air on the transmission in the glass with emphasis on its response to the humidity change, with or without holding at the maximum constant humidity, and its response to the humidity change up to various values of maximum attained humidity. We show that both the light scattering in the Vis region and the absorption in the NIR region are strongly affected by the duration and the maximum values of humidity, which implies that the amount of water in the pore space determines saturated and unsaturated responses of optical hystereses in both regions. We also show that the duration decreases the white turbidity while the immediate change of humidity from increasing to decreasing rather increases the turbidity. These facts verify that the nonequilibrium inhomogeneous distribution of the imbibed water in the pore space results in the optical inhomogeneity that causes the scattering, which is subsequently observed as the transient white turbidity, and that hysteresis loops of absorption are caused by the imbibed water in the pore space of Vycor glass. The existence of a threshold humidity of about 40% relative humidity (or corresponding pore-filling fraction of about 0.4) below which the two optical hystereses are suppressed and the fact that the maximum of optical turbidity occurs at about f=0.6 where the capillary condensation takes places imply that the appearance of a long-range capillary bridge between pores causes the transient white turbidity phenomenon.

Photospectroscopically observed pore-space correlations of a wetting fluid during the drying process in nanoporous Vycor glass.

We use light scattering to study spatial correlations in the pore space of Vycor glass upon draining a wetting fluid. We analyze the transmission spectrum of forward-scattered light on the basis of the theory of dielectric constant fluctuation, whereas conventional light scattering analyzes the scattered light at small angles of monochromatic incident light. Assuming that the drained pores, which are surrounded by filled pores, exhibit long-range correlations of a fractal dimension of 2.5, we analytically derive the corresponding turbidity. The slight deviation from the λ(-4) Rayleigh wavelength dependence directly provides the correlation length of the interconnected network of drained pores. The estimated length, ranging from 0.5 to 18 nm at most, is almost the same order as that indirectly estimated from our previous simple effective Rayleigh scatterer model.

1/λ4 scattering of light during the drying process in porous Vycor glass with nano-sized pores.

Porous Vycor glass with nano-sized pores is transparent in the visible region and is often used in colorimetric chemical sensing, when it is impregnated with selectively reacting reagents. However, it has some disadvantages in its use, since changes in the humidity of ambient air strongly affect the transmission. In this work, we analyzed the transparency change during the drying process to correlate the turbidity of the glass with the amount of water in it. The transparency change in the visible region takes place for the duration of the drying and is found to be dependent on the inverse 4th power of the wavelength (1/λ(4)), which implies that Rayleigh-type scattering takes place during the drying process. Based on the above observation, it is shown that the transitory white turbidity of nanoporous glasses during the drying process can be interpreted consistently and quantitatively analyzed by a simple Rayleigh scattering mechanism.

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.