Enthalpy sorption

Such NACs do not exhibit linear isotherms when they sorb under certain conditions with aluminosilicate clays (Fig. 11.66 Haderlein and Schwarzenbach, 1993 Hader-lein et al., 1996). Rather they show saturation behavior indicating an association with specific sites on the solid surfaces. This specific site interaction is also indicated by the observations of competitive effects among different NACs in sorption experiments. Further, the sorption enthalpies have been found to be much greater than excess enthalpies of aqueous solution of these sorbates (e.g., 4-methyl-2-nitro-phenol exhibits a sorption enthalpy of-41.7 kJ-mol"1). These data all indicate that there is a strong specific interaction of NACs with the aluminosilicate clay surfaces. 

Spurlock, F. C., Estimation of humic-based sorption enthalpies from nonlinear isotherm temperature dependence Theoretical development and applications to substituted phenylureas , J. Environ. Qual., 24,42-49 (1995). 

Using literature data, ten Hulscher and Cornelissen (1996) calculated sorption enthalpies with the following formula ... 

The development of conventional room-temperature hydrides based on interme-tallic compounds led to a large number of storage materials exhibiting very favorable sorption enthalpies with values of around 25 kj (mol H2), which can be operated in combination with conventional (80 °C operation temperature) polymer electrolyte membrane (PEM) fuel cells. However, their gravimetric storage capacity is limited to less than 3 wt.% H2. 

Values of isosteric sorption enthalpy, -AH, standard sorption entropy, AS°, and standard Gibbs free sorption energy, AG°, are calculated as dependences on n, cf., [4-8]. To calculate AG°, the boiling-point temperature of N2O is chosen as reference state. This choice provides a check for thermodynamic consistency of experimental data since dG°... 

Fig. 13.12. Interdependence of the sorption enthalpy and entropy for a series of n-alkanes on several...

Thermodynamically [4-6], the retention of a solute A can be expressed as a function of its free sorption enthalpy, AGm s (A), and the phase volume ratio, d> ... 

Both the In k values and the sorption enthalpies, AHm s, may be determined experimentally from the temperature dependence of retention. To calculate the sorption entropy, the phase volume ratio must be known. However, the thermodynamic data may be regarded simply as formal quantities, since the capacity factors correlate directly with AGm >s via the distribution coefficient according to Eq. (38), and since sorption exhibits both distributive and adsorptive character. 

The values thus derived are in the same order of magnitude as the sorption enthalpies determined by Mockel et al. [6] for various non-ionic inorganic and organic solutes. It is not possible to calculate the sorption entropy, ASm, s, which is normally derived from the intercept a by using Eq. (53)... 

The sorption enthalpy AHm >s for fluoride, chloride, nitrite, orthophosphate, and sulfate is small or zero, respectively, since no relation between In k and the reciprocal temperature could be established. The change in the molar sorption enthalpy is therefore... 

In contrast to the eluent pH value, the column temperature is seldom relevant for optimizing the separation. Retention can be somewhat reduced by raising the column temperature. Generally speaking, the viscosity of the mobile phase will be reduced and the chromatographic efficiency will be increased when the column temperature is raised. For mechanistic investigations, however, a variation in column temperature offers the possibility to determine the temperature dependence of the retention and to derive important thermodynamic quantities such as the sorption enthalpies (see also Section 3.2). 

Gabas, A.L., Menegalli, EC., and Telis-Romero, J. Water sorption enthalpy-entropy compensation based on isotherms of plum skin and pulp, /. Food Sci., 65, 680, 2000. 

The standard differential sorption volumes AV and the standard differential sorption enthalpy AH are hence practically identical with the actual differential sorption volumes and en alpies. 

Sorption isotherms, sorption enthalpies, diffusion coefficients and ermeabilities of water in a multilayer PEO/PAA polymer film [65]. 

Sorption isotherms, sorption enthalpies, and viscoelastic damping produced by water absorption in pharmaceutical film coat materials (in preparation). 

Figure 10 Two different polymorphic forms of a drug substance showing different vapour sorption enthalpy changes. Exploitation of different thermodynamic properties of material allows the identification of a form or mixture offorms (RH = relative humidity)...

The heat of phase change, q, also often called the sorption enthalpy, can be divided into the enthalpy of vaporization A/ lg and the bond enthalpy A/ g, as it was already presented in Chap. 2. 

The bond enthalpy decreases with increasing relative saturation, and for this reason also with rising moistnre content. In Fig. 10.2-3 the ratio q/Ah Q of the sorption enthalpy to the enthalpy of vaporization of potatoes is displayed in dependence of the moisture loading of the good (Gorling 1956). Figure 10.2-3 shows that above a moisture loading of 0.2, the sorption enthalpy differs ordy little from the enthalpy of vaporization. 

Fig. 10.2-3 Ratio of the sorption enthalpy q to the enthalpy of vaporization A/ lg of potatoes dependent on the moisture content X of the drying good...

SM1 Smith, A.L., Ashcraft, J.N., and Hammond, P.T., Sorption isotherms, sorption enthalpies, diffusion coeffidents and permeabilities of water in a multilayer PEO/PAA polymer film using the quartz ciystal microbalance/heat conduction calorimeter, Thermochim. Acta, 450, 118, 2006. 

The influence of temperature on the transport properties of CO2 in Pebax 1657 and 6100 extruded films was studied in the 20-40 C range. Their negative sorption enthalpies (-25 and-5kJmoF, for 1657 and 6100, respectively) can be expected from fundamentals of sorption thermodynamics in polymers [40], Negative sorption enthalpy reduces the membrane selectivity for gas treatments at higher temperature. 

The adsorption step is often modeled using the Langmuir isotherm, which is developed from two primary assumptions (1) only monolayer adsorption occurs and (2) the sorption enthalpy is the same for all sites regardless of the amount of surface coverage. This model was originally derived for adsorption of a gas onto a solid surface but it is developed here to model adsorption of a solution species to a solid surface. For the sorption process, the rate of change of fractional surface coverage (0 = T T, no units), which is the ratio of the surface concentration of occupied sites (T, mol/m ) to the surface concentration of all adsorption sites (T, mol/m ), is directly proportional to the adsorption rate constant k+, molal/sec), the concentration of adsorbate (m, molal), the number of surface sites, and the proportion of the surface that is not covered by adsorbate (1 - 0). The rate of desorption is directly proportional to the desorption rate constant k-, sec ), the number of surface sites, and the fraction of those sites that are covered by adsorbate. The overall rate is the difference between the adsorption rate and the desorption rate. 

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