Surficial Siloxane-to-Silanol Interconversion during Room-Temperature Hydration/Dehydration of Amorphous Silica Films Observed by ATR-IR and TIR-Raman Spectroscopy

Abstract

Silica has been frequently studied using infrared and Raman spectroscopy due to its importance in many practical contexts where its surface chemistry plays a vital role. The majority of these studies have utilized chemical-vapor-deposited films in vacuo after high-temperature calcination. However, room-temperature hydration and dehydration of thin silica particle films has not been well characterized in spite of the importance of such films as substrates for polymer and surfactant adsorption. The present study has utilized ATR-IR spectroscopy and thin silica particle films exposed to varying humidity to clearly show reversible conversion between surface siloxanes and hydrogen-bonded silanols without the need for semiempirical peak deconvolution. The IR spectra from corresponding hydration experiments on deuterated silica films has confirmed the vibrational mode assignments. The variation of humidity over silica films formed from silica suspensions of differing pH gave IR spectra consistent with the change in the relative populations of siloxide to silanol surface groups. In addition, total internal reflection Raman spectroscopy has been used to provide further evidence of room-temperature dehydroxylation, with spectral evidence for the presence of three-membered siloxane rings when films are dehydrated under argon. The confirmation of room-temperature siloxane-to-silanol interconversion is expected to benefit understanding in many silica surface chemical contexts.

Dehydration and Rehydration of an Ion-Leachable Glass Used in Glass-Ionomer Cements

“…A possible explanation for this is that replacing the molecular water on the particle surfaces is straightforward, but reversing the dehydration of silanol groups via hydration of siloxane bridges is more difficult. Recent studies have suggested that, on silica at least, both processes are able to occur [19], but our results suggest that the latter does not occur readily on this particular ionomer glass.…”

Reactivity of the Sterically Demanding Siloxanediol Mes2Si(OH)(μ‐O)Si(OH)Mes2 Towards Water and Ether Molecules

“…Solutions of 3 a in C 6 D 6 (0.01 m ) were treated with five equivalents of the respective donor molecules, and subsequently IR spectra were measured (Figure ). In C 6 D 6 , 3 a shows two broad absorption bands at 3676 and 3611 cm −1 for intramolecular hydrogen‐bridged silanol moieties . These absorption bands can be attributed to the cis conformer.…”

“…The spectrum for 3 a +H 2 O shows two absorption bands at 3590 and 3672 cm −1 for intramolecular hydrogen bridged silanol groups, which is in good accordance with the observations in solution. A broad band between 3300 and 3500 cm −1 is due to bulk water, which seemingly does not interact with 3 a . When D 2 O was used instead of H 2 O, absorption bands for the SiOH moieties (3595 and 3666 cm −1 ) were found, whereas bands at 2500 and 2660 cm −1 can be assigned to bulk D 2 O .…”

“…When a sample of 3 a in C 6 D 6 was treated with THF, a broad band at 3339 cm −1 was detected, which further supports the suggested hydrogen bonding to THF. In the presence of H 2 O, 3 a shows absorption bands for the intramolecular bridging silanol units at 3679 and 3609 cm −1 , whereas the dissolved water in C 6 D 6 results in a broadened band at 3300–3600 cm −1 . With D 2 O instead of H 2 O the bands for OH groups appear at 3671 and 3613 cm −1 .…”

A Thermodynamic Consideration on the Mechanism of Ultrasensitive Moisture Sensing by Amorphous Silica

“…1. Recently, it was pointed out by simulation [15] and experiments [16] that the chemical reaction described by Eq. 1 does occur at room temperature.…”