Silica gels interaction energy

For NH3 adsorption, experimental evidence has shown that the main interaction mechanism is H bonding of Si-OH to the N atom in NH3 [124], Ammonia gas is a widely used chemical in industry, and it has to be removed to less than one ppm, for instance, from the gaseous effluents of ammonia fertilizer plants, urea plants, and other sources [127], It is evident that silica is an excellent adsorbent of NH3 [124-126], Also, adsorption of ammonia on silica gel has received considerable attention recently, owing to its potential use in solar energy cooling cycles [128],... 

It follows from the fit presented in Fig. 46 that Eb energies for all porous glass samples are about the same value of 33 kJ mol-1. However, for sample B the value of Eh is about 10% less than those for samples A and C. This fact can most likely be explained by the additional chemical treatment of sample B with KOH, which removes the silica gel from the inner surfaces of the pore networks. It is reasonable to assume that the defects generally form at the water interfaces, and only then penetrate into the water layer. Thus, it seems that the KOH treatment decreases the interaction between the water and inner pore surfaces and, consequently, decreases the defect formation energy Eb. 

Fig. 8. Possible models of symmetric intermediate states for the interaction of (a) acetylene (C,J and (b) ethylene (C3 ) with a surface OH group of silica gel. R and r correspond to the minimum of the total energy of the system.

Investigations of surface free energy (SFE) of controlled porosity glasses and silica gels carried out more recently showed certain similarity in the properties of bare materials and important differences caused by thermal treatment [49-56]. Dispersive interactions expressed as dispersive component of SFE (7 ) and polar interactions expressed as polar component of SFE (7 ) measured by means of hexane and toluene respectively are similar for both materials. The average value of 7 for silica gel equals 35.6 mj/m and for CPG 35.0 mJ/m. The mean values of 7P for silica gel and CPG are 159.8 mj/m and 159.2 mJ/m, respectively. The thermal treatment of both materials leads to a small increase of dispersive interactions and simultaneously causes a significant drop of polar interactions. 

Interaction energies (heats of adsorption) of test compounds on carbosil change in a. sequence characteristic of graphitized blacks [43] with which the compounds interact in a non-specific way q > q°t,B- That is why higher values of a were obtained in case of silica gel. PEG deposition on silica surface, however, causes selectivity decrease whereas the coefficient a increases on carbosil modified with glycol (Table 1). A considerable increase is observed after polyethylene glycol monolayer formation on the complex adsorbent surface. 

It is well known that the energy of interaction of an atom with the continuous solid is 2-3 times less than with the discrete (atomic) model (cf., e.g., Ref. [38], Figs. 2.2-2.4). Thus, to obtain the same Henry s Law constants with the two models, one has to increase e for the continuous model. This, however, does not discredit the continuous model which is frequently used in adsorption calculations. In particular, we can use the above mentioned results of Ref. [37] to predict the value of e for Ar which would have been obtained if one had carried out Henry s Law constant calculations for Ar in the AO model of Ref. [17] and compared them with experiment. One can multiply the value of e for CH4 obtained from AO model by the ratio of e values for Ar and CH4 in the CM model [36] to obtain tjk = 165A for Ar in the AO model. This is very close to the value of 160 K obtained in Ref. [21, 28] by an independent method in which the value of the LJ parameter e for the Ar - oxide ion interaction was chosen to match the results of computer simulation of the adsorption isotherm on the nonporous heterogeneons surface of Ti02. Considering the independence of the calculations and the different character of the adsorbents (porous and nonporous), the closeness of the values of is remarkable (if it is not accidental). The result seems even more remarkable in the light of discussion presented in Ref. [28]. Another line of research has dealt with the influence of porous structure of the silica gel upon the temperature dependence of the Henry constants [36]. 

Wu Y, Piekara-Sady L and Kispert LD(1991) Photochemically generated carotenoid radicals on Nafion film and silica gel An EPR and ENDOR study. Chem Phys Lett 180 573-577 Yeates TO, Komiya H, Chirino A, Rees DC, Allen JP and Feher G (1988) Structure of the reaction center from Rhodobacter sphaeroides R-26 and 2.4.1 Protein-cofactor (bacterio-chlorophyll, bacteriopheophytin, and carotenoid) interactions. Proc Natl Acad Sci USA 85 7993-7997 Young AJ (1991) The photoprotective role of carotenoids in higher plants. Physiol Plant 83 702-708 Young AJ and Frank HA (1996) Energy transfer reactions involving carotenoids Quenchingofchlorophyll fluorescence. J Photochem Photobiol B Biol 36 3-15... 

Carbon dioxide has been used as an alternative probe for characterizing carbonaceous materials (75) and homogeneous polymers (6,9). Carbon dioxide is about the same size as N2, but has a lower vapor pressure, allowing its isotherms to be determined at higher temperatures where the activation energy for diffusion in nanopores is overcome. Specific interactions of COj due to its quadrupole moment appear to play an unimportant role in its adsorption on carbonaceous and mineral surfaces (75 and references therein). In support of this, we have shown that adsorption of Nj and COj on a mesoporous silica gel are of similar magnitude (unpublished results). 

Figure 5.11 The relationship between log k and molecular interaction energy values on a Silica Gel G plate with a basic eluent.

Table 6.1 Molecular properties of pyridine and phenol, and their molecular interaction energies (kcal mol ) with model silica gels.

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