Water adsorption microcalorimetry

Toluene and water adsorption microcalorimetry experiments have shown the strictly hydrophobic nature of pure Si02 zeolite Beta synthesized in F" medium, while evidencing a slight increase in the hydrophilicity of the materials upon incorporation of Ti into the framework. This was found to be due to the relatively strong adsorption of precisely one H2O molecule per Ti site. In contrast, materials synthesized in OH medium showed an enhanced hydrophilicity [254]. 

The combined use of adsorption microcalorimetry and IR spectroscopy allows a precise evaluation of the hydrophilic or hydrophobic character of the silica powders investigated.10"12 The evolution of the enthalpy of adsorption of water with coverage (Figure 1) indicates the coverage values at which the enthalpy falls below the latent enthalpy of liquefaction which can be taken as a... 

It is now apparent that isolated silanols have relatively low affinity for water. Thus, the hydrophobic nature of silica is manifested after dehydroxylation when only the siloxane bridges and some isolated silanols (giving an IR band at c. 3750 cm-1) remain. On the dehydroxylated surface the net adsorption enthalpy for water is negative. In this case, the enthalpy of adsorption is lower than the normal enthalpy of condensation. Application of adsorption microcalorimetry has allowed an assessment to be made of the relative extents of the hydrophilic and hydrophobic areas of the surface (Bolis et al., 1991). On the hydrophilic surface, it appears that water is adsorbed via two hydrogen bonds to two silanols - one acting as the hydrogen donor and the other as the acceptor. In the case of the weaker attachment to the isolated OH, the attachment involves one hydrogen bond. 

Further work by the author [42] compared the adsorption of PtCle and AuCU ions on a basic ion exchanger and a graphitised carbon adsorbent and has confirmed that flow adsorption microcalorimetry can rapidly determine the capacity and the kinetics of sorption of the ions on such adsorbents under dynamic conditions. The rate of sorption of the Pt and the Au ions was limited by their diffusion into the resin matrix and the high affinity of the resin for water. High surface area graphitic carbons suffer much less from these effects and appeared to be much more effective adsorbents. 

The porous structure of active carbons can be characterized by various techniques adsorption of gases (Ni, Ar, Kr, CO ) [5.39] or vapors (benzene, water) [5,39] by static (volumetric or gravimetric) or dynamic methods [39] adsorption from liquid solutions of solutes with a limited solubility and of solutes that are completely miscible with the solvent in all proportions [39] gas chromatography [40] immersion calorimetry [3,41J flow microcalorimetry [42] temperature-programmed desorption [43] mercury porosimetry [36,41] transmission electron microscopy (TEM) [44] and scanning electron microscopy (SEM) [44] small-angle x-ray scattering (SAXS) [44] x-ray diffraction (XRD) [44]. 

Water adsorbed on the surface of pharmaceutical or food powders, even at low levels, has a significant impact on the behavior of such powders during processing, and ultimately on the quality and performance of the final product. Bakri (1993) reported the application of heat conduction microcalorimetry to study the adsorption of water vapor onto solid pharmaceuticals. The high sensitivity of the calorimeter permits detection, even at low humidities, of heat flow as a result of interaction of solvent vapor with the solid. The heat flow (Pj ) can be described by ... 

Results for water sorption obtained by isothermal microcalorimetry at various temperatures and RH values are shown in Figure 51.2, which shows that the metastable form. A, exhibits greater heat of adsorption than form B under all conditions of temperature and RH investigated. At the three lower temperatures (20, 25, and 35°C), form A shows a sharp increase in heat of adsorption as function of RH at RH values above the 70 -80% range. Form B also shows a similar sharp increase, but in this case the sharp raise is only observed only at 25°C. Figure 51.3 also shows that for each of the two polymorphs, the heat of adsorption reaches the highest values at 25°C in comparison with the other temperatures. 

Table 2 shows the heats of adsorption of benzene, cyclohexene and water on the catalysts at room temperature by flow microcalorimetry. n-Hexane for the two organic compounds and methyl ethyl ketone for water were used as the liquid carriers in measuring the heats. The heats of adsorption of water are overwhelmingly bigger than those of benzene and cyclohexene, namely, are about twenty times for the sol-gel catalysts and about ten times for the impregnation catalysts. 

In the case of water pollution, the estimation of adsorption affinity of potential solid adsorbent toward the specific pollutant can be done using the so-called liquid microcalorimetry. The instruments used for this purpose are differential heat flow microcalorimeters modified to allow continuous stirring of liquid samples. The adsorbate is added to both sample and reference cells simultaneously using a programmable twin syringe pump, linked to the calorimeter. The heat evolved as a result of adsorption can be obtained by integration of the area under the calorimeter signal, for each particular injection (dose). The output of typical microcalorimetric experiment of this type is shown in Fig. 10.9. 

Figure 10.10 shows the profiles of differential heats revealed as a function of the amounts of aqueous solution of nicotine adsorbed on different solids. Based on the values and profile of differential heats, it can be concluded that adsorption capabilities of P-zeolites are comparable with that of activated carbon, solid known to be effective adsorbent for remediation of wastewaters. There is evidence in the literature on the studies of other water pollutants adsorption, such as phenol, aldehydes and the ketones, done by the application of microcalorimetry [40]. 

The obtained information about the acidic/basic character can be correlated with the adsorption possibilities. The possibility of some solid material to adsorb (or modify) the atmospheric or water pollutant depends very often on these features. Therefore, there are literature sources that report the correlations between acidity/basicity of different materials and their abilities in pollutants abatement. For example, balanced acidic/basic properties are indispensable for an efficient conversion of NOx, that is, high activity and selectivity, over a broad temperature range. In case of alumina-supported indium oxide catalysts, microcalorimetry of ammonia and SO2adsorption helped to find effective formulations [41],... 

Adsorption isotherms. Isothermal microcalorimetry, in conjunction with an RH perfusion device, is a powerful method for mapping surface properties of solids and especially drugs [32]. The principle of the study is to adsorb and desorb water vapour onto and off the surface of a solid in small steps and measure the associated enthalpy change. At low RH values, monolayer water sorption conforms to a BET (Brunauer, Emmett and Teller) model and can therefore be used to determine surface properties. The analysis of the data can be achieved by plotting the water sorption isotherm as a function of RH and fitting to a modified BET type equation [33]. This can provide information about the surface affinity for water and the hydrophilic surface area, parameters... 

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