Enthalpies aluminosilicate

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. 

The standard enthalpies of adsorption (Table 15) of these molecules on the thermally treated aluminosilicate support the former explanation. So the adsorption of benzene on H-473 is the most exothermic due to the specific interaction with the silanol groups. When the thermal treatment progresses, the process becomes less and less exothermic due to i) the partial dehydroxylation of the surface (H-673), ii) the change in the chemical surface groups (siloxane groups on H-873 and H-1073) and iii) the destruction of the chemical surface groups (H-1273). As a consequence of the latter process the standard enthalpy of adsorption of benzene on H-1273 is very similar to the heat of liquefaction. 

Standard enthalpy of adsorption on thermally treated aluminosilicates"... 

Meziani, M.J., Zajac, J., Douillard, J.-M., Jones, D.J., Partyka, S., and Rozi ie, J. 2001. Evaluation of surface enthalpy of porous aluminosilicates of the MCM-41 type using immersional calorimetry Effect of the pore size and framework Si Al ratio. J. Colloid Interface Sci. 233 219-226. 

In aluminosilicate materials, the enthalpy of formation for a given Al content become more negative as function of extra-framework cations in the sequence Ca, Li, Na, and K [OlNlj. The formation enthalpies of silica zeolites were reported in [93P1, OOPl],... 

Clay particles occur abundantly in the soil. They are mostly colloidal aluminosilicates. To learn how their extensive surfaces react with water, we have studied the following properties of water associated with montmorillonite and other clay minerals threshold gradient [1], thermal expansibility [2, 3], isothermal compressibility [4], frequency of O—H stretching [5, 6], molar absorptivity [7], freezing point depression [8, 9] specific volume [10], specific heat capacity [11], heat of compression [12], viscosity [13], and free energy, enthalpy and entropy [6,14,15]. Not all of these properties will discussed here. Instead, we will discuss only a few of them to illustrate the kind of results obtained. 

To improve their dispersibility, clays could be modified with organic surfactants, which are typically the quaternary ammonium salts of long fatty acid chains. These surfactants decrease the surface tension of the aluminosilicate particulates, which in turn reduces the endothermal enthalpy of mixing. Many organophilic nanoclays or organoclays, therefore, have already been studied, and some of their products are already marketed on an industrial scale (Matsuda et al., 2012 Park et al., 2002 Turri, Alborghetti, and Levi, 2008). 

Based on the assumption that surfactant aggregation on the solid surface is a prerequisite for the uptake of phenol and the aromatic molecules can be located only close to the free head-groups of the surfactant units within the interfacial aggregates, it may be helpful to compare the curves presenting the differential enthalpy of displacement as a function of the surface coverage by the surfactant adsorbate in the absence and in the presence of the additive. Figure 6.33 illustrates such a comparison for a cationic surfactant adsorbed onto ordered mesoporous aluminosilicate of the MCM-41 type. 

Fig. 6.33 Effect of phenol addition on the differential molar enthalpy of displacement upon adsorption of cationic Gemini C12C12C12 onto ordered mesoporous aluminosilicate of the MCM-41 type (Sbet = 860 m g mean pore diameter = 5 nm, Si Al = 32) from aqueous solution at 298 K and the initial pH 8 [114]. The ff enthalpy is plotted against the adsorption coverage of the sohd...

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