Parameters, water adsorption

Unavailable because experimental methods for estimation of this parameter for alicyclic hydrocarbons are lacking in the documented literature. Based on its high solubility in water, adsorption is not expected to be a dominant process. 

Table 2. Parameters of interface layer upon water adsorption onto fumed silicas synthesized under different conditions.

In some studies it has been found that Equation (9.3) is applicable to experimental water adsorption data in the region of 0.4

carbon surface and the pore structure (Carrott et al., 1991 Carrott, 1993). The range of fit has been extended by Barton et al. (1991,1992) by the addition of a fourth empirical parameter. 

By comparing almost 100 different silicas, Zhuravlev [86] has shown that the surface density of hydroxyl groups is a physicochemical constant for a fully hydroxylated surface (5.0 -OH per nm ). As the parameter cji is related mainly to this surface density, this means that the dispersion of water-surface interactions is also a constant value to a greater or smaller extent. Only in such a case peaks on the heat capacity curves of water adsorption on various silicas should be generally observed, as, in fact, the case is. 

Of course, water is not a chemical warfare agent. However, it plays a major role in the various adsorption mechanisms, either directly, in a benevolent role (e.g. hydrolysis), or indirectly, usually influencing in a negative way the adsorption behaviour of the carbon (see sections 6.4 and 6.5). Therefore it is quite important to have a good understanding of the water adsorption behaviour of carbons in general, and military carbons in particular. This behaviour is characterised by two parameters, the water uptake and the time to reach this uptake. The water uptake depends on the structure and surface chemistry of the carbon. This is illustrated ly the water adsorption isotherm (see Fig. 13), that presents a very distinctive shape, especially when compared to the isotherms of other vapours such as chloropicrin (see Fig.l4). Even when... 

The second parameter is the time to reach an equilibrium uptake for a given moisture level in the environment. Whereas there is still much debate regarding the exact nature of water adsorption and, consequently, tte interpretation of the isotherm, there is even less known about the kinetics of water adsorption. Up to now the only existing models are purely based on experimental data and, as such, they are completely empirical [93-97]. 

According to Fig. 7.3.1 the isotherm slopes are approximately equal in formamide and methanol whereas the slope for the aqueous system is considerably larger. The mutual repulsion of adsorbed anions is therefore evidently stronger in methanol and formamide than in water. The interaction parameter is also found to depend strongly on the anion for a given solvent. For example in the formamide system the second virial coefficient (which is directly related to the interaction parameter) for adsorption of 1 ions is 310 A /ion compared with 2000 A /ion for Cl ions. Thus the simple adsorption model of point charges undergoing lateral coulombic repulsion represents a considerable oversimplification in non-aqueous solutions as in aqueous solutions. Studies of adsorption of halide anions from mixed electrolyte solutions in formamide and methanoF reveal complex behaviour which cannot be explained in terms of a simple model. 

The equilibrium constant is 0.589. The system consists of an ethanol/water azeotrope containing 13.5% (molar base) of water. The water does not adsorb. Estimate the parameters of adsorption-desorption using the conversion as variable regression. Demonstrate what the best model. 

Among the two equations for describing water adsorption on carbon, the D-Se equation is a better choice as it reflects better the mechanism of adsorption. The DA equation, however, is the one to use if we wish to obtain the adsorption isotherm at different temperatures as the parameters E and n are almost temperature invariant (Stoeckli et al., 1994). To test these two isotherms, we use the following data of water on a sample of activated carbon. 

Parameters of Interface Layer upon Water Adsorption onto Nanosilicas Synthesized under Different Conditions... 

Stoeckli et al. (1990) related the parameters obtained from the adsorption isotherm of water on activated carbons with their enthalpies of immersion into water. In this way, they obtained a measure of the amount and type of active centers for water adsorption. 

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