Water on silicas

A further thirty years were to pass before Kuhn and his co-workers (3) successfully repeated Tswetf s original work and separated lutein and xanthine from a plant extract. Nevertheless, despite the success of Kuhn et al and the validation of Tswett s experiments, the new technique attracted little interest and progress continued to be slow and desultory. In 1941 Martin and Synge (4) introduced liquid-liquid chromatography by supporting the stationary phase, in this case water, on silica in the form of a packed bed and used it to separate some acetyl amino acids. 

HPLC methods can be ntilized for the pre-concentration of aromatic amines from polluted waters on silica gel or octadecyl silica (ODS) colnmns [55], The determination is then performed by RP HPLC using ODS packings as the stationary phases and a mixture of methanol, isopropanol, and water as the mobile phase [55], RP HPLC with diode array detector (DAD) methods coupled on-line with a continnons seqnential anaerobic/aerobic reactor system have been employed in wastewaters treatments [56], A continnons monitoring of the possible presence of aromatic amines in azo dyes wastes is based on indncing in the waste, the reaction of a reduction of the dye, followed by HPLC/ UV or HPLC/MS analysis [57-59], The redncing agent solutions are sodium dithionite or tin(II) chloride in an aqneons acidic medinm at 70°C, followed by SPE [58,59], LLE [60,61], or SEE [60-62],... 

In chemical modifications of inorganic particles, the adsorbed water sometimes affects the reaction of the hydroxyl group with the modifier. According to Fripiat et al. (6), evacuation at 200°C and 10-7 mmHg for 24 h still leaves about 0.2 mmol/ g adsorbed water on silica (Aerosil 200). 

No cross ozonide was formed from unsymmetrical alkenes. The authors theorized628 that the carbonyl oxide zwitterionic species formed on wet silica gel immediately adds water followed by rapid decomposition of the intermediate hydroxyalkyl hydroperoxide to carboxylic acid and water. It means that water on silica gel acts as participating solvent. In the absence of adsorbed water, rapid recombination of the adsorbed aldehyde and carbonyl oxide due to a favorable proximity effect gives normal ozonide. The low mobility of adsorbed species on the silica surface accounts for the absence of cross ozonides. 

The adsorption of hafnium species on glass was found to increase with the solution pH and hafnium concentration. The effects on the adsorption of the solution preparation and age were studied and the equilibration time for the adsorption process was determined. The surface area of the glass sample was determined by the B.E.T. method using water vapor. The results are discussed in terms of the hydrolyzed hafnium(IV) species. At equilibrium, nearly monolayer coverage was obtained at pH > 4.5. Under these conditions hafnium is in the solution in its entirety in the form of neutral, soluble Hf(OHspecies. In the close packed adsorption layer the cross-sectional area of this species is 24 A which is nearly the same as for water on silica surfaces. 

Patrick,6 however, has found that the heat of adsorption of water on silica gel is positive below 4°. This is ascribed to the surface of the gel consisting largely of water, not dry silica. 

Nakashima M, Tachikawa E. (1983) Hydrogen evolution from tririated water on silica gel by gamma-irradiation. Radiochim Acta 33 217-222. 

Air at 25°C and 1 atm with a relative humidity of 25 % is to be dehumidified in an adsorption column packed with silica gel. The equilibrium adsorptivity of water on silica gel is given by the expression ... 

The first domain is used to determine the specific surface area (5 bet(H20)) and the BET constant values. Moreover, the large number of available experimental points allows the calculation of the corresponding adsorption energy distribution function of water on silica. 

Fig. 4. Adsorption energy distribution function (AEDF) of water on silica at 40 °C.

As discussed by Hobza et al. (1981), in studying the adsorption energetics of water on silica surfaces, the appropriate reaction to use is... 

Zhdanov (Institute of Silicate Chemistry, the U.S.S.R. Academy of Sciences, Leningrad) (154) showed (1949) that the adsorption of water vapor by SiC>2 (porous glasses, silica gels) strongly depends on the temperature of the preliminary thermal treatment of the adsorbent. Calcination of 300-500 °C resulted in a sharp decrease in the adsorption of H2O at low values of pressure over initial pressure p/p0 (<0.3), and the adsorption isotherms were found to be irreversible. On the other hand, the adsorption isotherms of water on silica subjected to calcination in vacuo at <500 °C (but after the sample was kept in contact with water vapor or liquid water at room temperature) again became reversible that is, the adsorption activity of Si02 was restored. 

When the potentials and Oj have the same sign but different orders of magnitude, as is the case for wetting films of water on silica, at some critical thickness the electrostatic forces change from repulsion at h > h, to attraction at h < h, (Derjaguin et al. 1987),... 

The polarization effects, isotopic effects (exchange of DjO for H2O), and non-ohmic behavior of adsorbed water on silica gel (surface area 660 mVg) are similar to those of an electrolyte solution [4]. The resistivity was found to decrease exponentially with water content and to increase roughly exponentially with OH content this was determined from a simultaneous measurement of resistivity and the near-infrared spectra of adsorbed water and surface hydroxyl groups. The adsorbed water on silica gel is immobile because the water molecule is hydrogen-bonded to two substrate hydroxyls, but the conductivity of silica gel increases as the amount of adsorbed water increases. 

In the extreme of normal phase HPLC, the stationary phase is highly polar adsorbed water on silica or alumina supports, so the eluants should be nonpolar, often hydrocarbon solvents. The mechanism of normal phase HPLC is often classified as adsorption rather than partition, as the analytes are essentially binding to a highly polar surface. 

It is generally accepted that hydroxyl groups of dehydrated silica surface are the main sites of adsorption of the molecules capable to form hydrogen bonds with these sites (water, alcohol, amines). This conclusion was made on the base of investigation of the shift of vibration bands of free Si—OH groups to low frequency region [49,50]. The following models are proposed, for example, for the adsorption complexes of water on silica surface ... 

FIGURE 3.15 Adsorption data from Cohen Stuart et al. (1980) for polyvinyl pyrrolidone in water on silica. The effect of polydispersivity is also shown. The weight fraction of component 2 in the mixture is 0.25, M = 1440 for component 1 and 10 for component 2. Reprinted with permission from John Wiley Sons. 

Zimmerman, J.R., Holmes, B.G., and Lasater, J.A. 1956. A study of adsorbed water on silica gel by nuclear resonance techniques. J. Phys. Chem. 60 1157-1161. 

For adsorption of water on silica, that is, hydration, water molecules first attach themselves to the silanol groups by hydrogen bonding, followed by H-bonding to the already adsorbed water. Hence the IR band is gradually lowered and broadened to 3400 cm For the same adsorbate, the frequency shift depends on the temperature as well as the amount adsorbed. The literature data on the frequency shifts by different adsorbate molecules have been discussed by Hair (1967) and by Kiselev and Lygin (1975) and are summarized in Table 6.1. The experiments for the permanent gases (e.g., N2 and CH4) were performed at low temperatures (-190 °C), while that for all others were at room temperature. The frequency shifts are denoted by Av (cm ), lowered from 3750 cm ... 

Gobmoh [67] has employed ethyl acetate-methanol (95 + 5), methyl ethyl ketone-toluene-methanol-acetic acid-water (80 +10 + 5 + 2 + 6) and ethyl acetate-chloroform (90 + 10) on silica gel G for Scilla glycosides and Steidlb [161, 162] has used butanone, saturated with water, on silica gel G, separating into zones, re-chromatographing with simitar... 

Ghezelayagh, H., and Gidaspow, D., Micro-macropore model for sorption of water on silica gel in a dehumidifier, Ghent. Eng. Sci., 37(8), 118I-1198 (1982). 

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