Enthalpies of sorption

This picture fits very well with the tendency of the sorption isotherm curvature (and hence of the site sorption mode) to disappear at T > Tg. On a more quantitative level, the above characterization of the Henry sorption mode is supported by the smooth temperature dependence of K, found in the PET-C02 system 12), which indicates a roughly unchanged enthalpy of sorption AH, above and below Tg. Additional support is provided by the correlation between K, and the Lennard-Jones parameter s/k characteristic of the gaseous penetrant, in accordance with... 

The solubility coefficients of gases in glassy polymers are not constant, but decrease with increasing concentration of the gas in the polymers (2). Calculations of the isosteric enthalpy of sorption in several gas-polymer systems confirm that the gas-polymer affinity is reduced with increasing sorbed gas concentrations (24, 25, 26). The change in the isosteric enthalpy of sorption is a result of the changes in the polymer matrix induced by the presence of the sorbed gas. 

The diamagnetic susceptibility is a measure of the averaged electronic distribution in bulk matter. Careri et al. (1977, 1980) showed that the differential diamagnetic susceptibility per gram of water adsorbed on lysozyme powders reached the bulk water value at 0.2 h. Lysozyme behaved as a normal diamagnetic substance. The diamagnetic susceptibility and the enthalpy of sorption for lysozyme change similarly at low hydration. 

Inverse gas chromatography, IGC, has been used to study water sorption of two poly (vinylidene chloride-vinyl chloride) and poly (vinylidene chloride-acrylonitrile) copolymers, at temperatures between 20 and 50°C and low water uptakes. It was found that the specific retention volume of water increases with decreasing amount of water injected, increases dramatically with decreasing temperature and strongly depends on the type of copolymer. Thermodynamic parameters of sorption namely free energy, entropy, enthalpy of sorption and activity coefficient were calculated. 

The total partial molar enthalpy of sorption (AHj=aH ) is related to V°g through the Clausius-Clapeyron (18) equation ... 

The enthalpy of sorption, a measure of the energy of attachment of sorbed molecules to the polymer, increases with increasing amount of water injected (Figure 3). 

TABLE II. Enthalpies of Sorption in Kcal/mole for the Three Temperature Regions 1 ... 

The enthalpy of sorption for each of the coals plotted versus carbon content is presented in Figure 6. The data were derived from a least squares linear regression of the IGC data taken above the temperature of the major transition. As with the temperature of the major transition, a trend is definitely indicated. However, several of the enthalpies are unrealistically high. This is probably due to loss of volatile matter from the coal in this temperature region. Since the actual mass of coal in the column at each... 

Overall, IGC appears to be a reproducible method for following the chemical and physical changes that occur when coals are heated in an inert atmosphere. Differences in the transition temperature and enthalpies of sorption can be observed for coals of various rank. PyHRMS results indicate that the minor transitions observed in the intermediate temperature region are a result of the loss of volatile matter from the coal. 

The solubility and diffusivity can each be treated separately as activated processes with relationships similar to Equation 15.3. The activation parameter for solution is called the Gibbs free energy for sorption (AGS), and consists of the enthalpy of sorption(AHs) and the entropy of... 

The thermodynamic functions of the solubilities of many gases in molecular liquids at room temperature have been tabulated [22]. The enthalpy of sorption is negative (exothermic) if the sorption energy exceeds the energy needed to make a hole of molecular size in the polymer or molecular liquid, and positive (endothermic) otherwise. In rough empirical correlations [21], S and AHS are usually related to the boiling temperature, critical temperature, or Lennard-Jones 6-12 potential energy parameter of the gas molecule. AHS can also be modeled atomistically [23-25], and by statistical thermodynamic equation-of-state theories (Section 3.E and Ref. [26]). 

FIGURE 1.12 Enthalpy of sorption as a function of the hygroscopic moisture content. (Egg white data by Nemitz potato data by Krischer.)... 

Gas Permeation Parameters and Other Physicochemical Properties 39 Table 2.5 Enthalpy of sorption, AHj, partial molar enthalpy of mixing, AHm, and partial... 

Sorption thermodynamics were studied using the IGC method for a number of solutes (n-aUcanes C3-C16, cyclohexane, methylcyclohexane) in the range 50-250 °C. For all the solutes, the linear dependencies of log S versus I IT enabled an estimation of the enthalpies of sorption AH. It was shown that when the size of the solutes (e.g. their critical volume Vc) increases the negative enthalpies of sorption also increase. This dependence can be presented by the equation ... 

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