Study of Hydrated Structures on the Surface of Mesoporous Silicas and Carbosils by ¹H NMR Spectroscopy of Adsorbed Water

Abstract

1H NMR spectroscopy of adsorbed water was used to study mesoporous silica gel and carbosils synthesized with carbon in the range from 4 to 14.5% w/w in the surface layer. It has been revealed that both silica gel and carbosil are characterized by a rise in temperature of water freezing in an aqueous medium in mesopores. This phenomenon may be explained by contact of water adsorbed in pores with a continuous aqueous medium. The changes in free energy of water in pores due to adsorption have been calculated. It has been shown that carbonization of the mesopore silica gel surface leads to formation of carbon layers which are denser than those on a nonporous silica and the main types of water adsorption sites on the surface are basic graphite planes of carbon and oxygen-containing groups on the carbon part of the carbosil surface.

Radiolysis of Confined Water: Hydrogen Production at a High Dose Rate

“…[63][64][65][66] The assignments of various peaks of water, isolated and/or bound silanols in the 1 H NMR spectra have been elucidated [60][61][62][63][64] despite lines broadening arising from strong 1 H-1 H dipolar couplings and/or chemicalshift anisotropies. As shown in previous studies, [60,63] measurements of the spin-spin T 2 relaxation time allow the confirmation of the attributions of the various protons.…”

Features of acetylsalicylic acid hydration in tablets

“…As can be seen, the curves contain a vertical branch that corresponds to a constant concentration of unfreezing water retained in a broad range of DG, which implies that water adsorbed on the surface does not freeze in a broad range of temperatures below 273 K. It was shown for microporous and mesoporous materials [6,8] that the presence of such vertical branches is related to a strong interaction between adsorbed water molecules and the substrate surface. Since strongly bound water corresponds to a fraction freezing upon a significant decrease in the temperature [4], we can suggest that water adsorbed in aqueous ASA suspensions is strongly bound for C H O 2 < 60 mg/g in samples 1 and 2 and for C H O 2 < 38 mg/g in sample 3.…”

“…In contrast to the case of aqueous suspensions, where capillary phenomena are absent, the value of g s in air contains two main components, which correspond to (i) a decrease in the free energy of water caused by the interaction of water molecules with the primary and/or secondary adsorption centers (DG ads ) and (ii) a decrease in the free energy of a water film at the adsorbed water -air interface (DG liq ) [8]:…”

Changes in Hydration Properties of Silica Gel in a Process of Its Carbonization by Pyrolysis of Acetylacetone Zn (Ti) Acetylacetonates

“…In aqueous medium the magnitude of a free surface energy of adsorbents can be determined on the basis of a temperature dependence of intensity of the 1 H NMR signal of nonfreezing water at a temperature of T Ͻ 273 K (21)(22)(23)(24). This method is based on the fact that the freezing temperature of water is lowered by some values depending on the surface energy of adsorbent and on the distance of the adsorbed molecules toward the surface.…”

“…Its selection was determined by the fact that porous materials are less exposed to the polarization surface effects, promoting the appearance of a long-distance component of the surface forces (23). At the same time the changes in the structure of hydration layers observed during the modification process may be caused by the structural heterogeneity of the surface itself.…”