Pore Size-Dependent Structure of Confined Water in Mesoporous Silica Films from Water Adsorption/Desorption Using ATR–FTIR Spectroscopy

Abstract

The local structure of water on chemically and structurally different surfaces is subject of ongoing research. In particular, confined spaces as found in mesoporous silica have a pronounced effect on the interplay between adsorbate-adsorbate and adsorbate-surface interactions. Mid-infrared spectroscopy is ideally suited to quantitatively and qualitatively study such systems as the probed molecular vibrations are highly sensitive to intermolecular interactions. Here, the quantity and structure of water adsorbed from the gas phase into silica mesopores at different water vapor pressures was monitored using mid-infrared attenuated total reflection (ATR) spectroscopy. Germanium ATR crystals were coated with different mesoporous silica films prepared by evaporation induced self-assembly. Quantitative analysis of the water bending vibration at 1640 cm-1 at varying vapor pressure allowed for retrieving porosity and pore size distribution of the mesoporous films. The results were in excellent agreement with those obtained from ellipsometric porosimetry. In addition, different degrees of hydrogen bonding of water as reflected in the band position and shape of the stretching vibrations (3000-3750 cm-1) were analyzed and attributed to high-density, unordered bulk and low-density, surface-induced ordered water. Thereby, the progression of surface-induced ordered water and bulk water as a function of water vapor pressure was studied for different pore sizes. Small pores with 5 nm diameter showed a number of two ordered monolayers, while for pores > 12 nm the number of ordered monolayers is significantly larger and agrees with the number observed on planar SiO2 surfaces.

A pocket-sized 3D-printed attenuated total reflection-infrared filtometer combined with functionalized silica films for nitrate sensing in water

“…The film density of =1.32 g cm -3 was calculated as the weighted averaged density from the density of bulk silica with a film porosity of 50 %. 45 With this, a maximum absorption capacity of 20 mgNO3-gSiO2 -1 (derived from qm,mass/ ) was obtained, which is in similar to ~30 mgNO3-gSiO2 -1 obtained for batch experiments after 1 h of equilibration time for similar materials 13,43 and in the range of commercially available anion exchange resins. 46…”

“…Mesoporous silica films with a pore size of 5 nm were prepared via the evaporation induced self-assembly process using CTAB as template as previously reported. 12,21,37,38 The formation of silica, complete removal of the template and successful surface functionalization was confirmed by FTIR measurements. The spectra obtained between each step of the synthesis are given in Figure 4.…”

“…These spectrometers are still considered as gold-standard for IR spectroscopy. The broad spectral coverage and robustness of FTIR spectrometers enabled applications in a variety of analytical tasks including quantitative analysis of liquids in off-line as well as in-line applications, [15][16][17][18][19][20] studying reactions at surfaces, [21][22][23] as well as clinical diagnositics. 24 An emerging, cost-effective alternative to FTIR spectrometers are filtometers that, in comparison with benchtop spectrometers, allow on-site measurements due to their small size and low energy consumption.…”

Anomalous Humidity Dependence in Photoacoustic Spectroscopy of CO Explained by Kinetic Cooling

“…One stream of nitrogen was humidified using a wash bottle filled with water. To achieve a reproducible humidity, the temperature of the wash bottle was stabilized at 40 • C in a water bath thermostat [15]. Using the three MFCs, the humidity was varied stepwise while keeping the concentration of CO constant.…”

Mid-IR sensing platform for trace analysis in aqueous solutions based on a germanium-on-silicon waveguide chip with a mesoporous silica coating for analyte enrichment

“…The coating refractive index was modeled using the Bruggeman Effective Medium Approximation (BEMA) for the effective dielectric permeability of a binary mixture [33] and assuming a porosity of 50%, corresponding to the data obtained in previous characterization of these coatings [34]. The simulated confinement factor and loss in the coating ranges from ∼1.0% and 0.6dB/cm (λ=6.5 µm, n=1.1) to ∼1.4% to 3.6dB/cm (λ=7.5 µm, n=0.97) for TE polarization.…”