Application of H-1 NMR spectroscopy method for determination of characteristics of thin layers of water adsorbed on the surface of dispersed and porous adsorbents

The paper presents H-1 NMR spectroscopy as a perspective method of the stud ies of the characteristics of water boundary layers in the hydrated powders and aqueous dispergated suspensions of the adsorbents. The method involves measurements of temperature dependence proton signals intensity in the ads orbed water at temperatures lower than 273 K. Free energy of water molecule s at the adsorbent/water interface is diminished due to the adsorption inte ractions causing the water dosed to the adsorbent surface freezes at T < 27 3 K. Thickness of a non-freezing layer of water can be determined from the intensity of the water signal of H-1 NMR during the freezing-thawing proces s. Due to a disturbing action of the adsorbent surface, water occurs in the quasi-liquid state. As a result, it is observed in the H-1 NMR spectra as a relatively narrow signal. The signal of ice is not registered due to grea t differences in the transverse relaxation times of the adsorbed water and ice. The method of measuring the free surface energy of the adsorbents from the temperature dependence of the signal intensity of non-freezing water i s based on the fact that the temperature of water freezing decreases by the quantity which depends on the surface energy and the distance of the adsor bed molecules from the solid surface. The water at the interface freezes wh en the free energies of the adsorbed water and ice are equal. To illustrate the applicability of the method under consideration the series of adsorpti on systems in which the absorbents used differed in the surface chemistry a nd porous structure. In particular, the behaviour of water on the surface o f the following adsorbents is discussed: non-porous and porous silica (aero sils, silica gels); chemically and physically modified non-porous and porou s silica (silanization, carbonization, biopolymer deposition); and pyrogene ous Al2O3 and aluminasilicas. The effect of preliminary treatment of the ad sorbent (thermal, high pressure, wetting with polar and non-polar solvents) on the characteristics of the structurized water layers was discussed, The influence of the adsorbent porous structure on the free energy of the adso rbed water was also studied, The discussion of the obtained results was mad e, (C) 1999 Elsevier Science B.V. All rights reserved.