Water vapor adsorption models

Water vapor adsorption isotherms have been obtained on cotton from room temperature up to 150°C [303,304]. Theoretical models for explaining the water vapor sorption isotherms of cellulose have been reviewed [303]. Only adsorption theories will be discussed here at ambient temperatures. The shape of the isotherm indicates that multilayer adsorption occurs and thus the Brunauer, Emmett and Teller (BET) or the Guggenheim, Anderson and deBoer (GAB) theory can be applied. In fact, the BET equation can only be applied at relative vapor pressures (RVPs) below 0.5 and after modification up to a RVP of 0.8 [305]. The GAB equation, which was not discussed in the chapter in the book Cellulose Chemistry and Its Applications [303], can be applied up to RVPs above 0.9 [306]. Initially as the RVP... 

TPD experiments were performed using an NEVA NAG 110 mass spectrometer with a glass-made vacuum system. Prior to every TPD measurement the sample was heated up at 1173K for Ih under vacuum (10 torr) to remove the adsorbed impurities. The typical water vapor adsorption pressure was around 20 torr and the heating rate was 10 l min. The temperature programmer used in our experiments was Chino, model KP. 

Slawiiiski et al. [106] applied the so-called pore-solid-fractal (PSF) model to describe water retention curves that were measured for six peat soils in order to obtain their Du values. Furthermore, A values were determined by applying the FHH equation to water vapor adsorption and desorption and N2 adsorption isotherms. The PSF model gave Du values ranging from 2.88 to 2.94. In contrast to the results described above for Cambisols and Luvisols [108], A values obtained from N2 adsorption isotherms (2.25 < A < 2.575) were lower than those evaluated from water adsorption (2.58 < A < 2.70) and desorption (2.63 < A < 2.75) isotherms. 

Chou, C.L., Dynamic modelling of water vapor adsorption by activated alumina, Chem. Eng. Commun., 56,211-... 

In a recent review, Haruta has reassessed the mechanism of CO oxidation on the basis of the Bond and Thompson and Kung models, i.e. that the CO is activated by adsorption on Au° on the surface of the gold nanoparticles and that dioxygen is activated by the atoms at the periphery between the support and the gold nanocrystals (Fig. 4.7). The atoms at the periphery are proposed by Haruta to be cationic in nature, possibly Au(OH)s or Au(OH) formed by the presence of water vapor that is essential for the observed high activity catalysts. It is clear that the debate will continue for the immediate future. There are two reasons why finding an answer to the key question of the nature of the active site in gold catalysts for CO oxidation. The first is purely scientific... 

Early studies on adsorption of oxygen on catalysts are based on the so-called vapor phase models that only include a few molecules on top of a catalyst slab without participation of a water layer and any polymer atom. Correlation of the 0-0 and O-M bond strength to the experimental observations is revealed in these studies. 

Many studies have been conducted on predicting the role of packaging in moisture adsorption by dosage forms. Adsorption of moisture by tablets contained in polypropylene films was successfully modeled from storage temperature and the difference in water vapor pressure between the inside and outside of the packaging... 

Protein hydration has been the focus of attention for more than a century." Many theories have been suggested to explain protein hydration."" " These theories can be subdivided into three groups " (1) those based on the physical adsorption of water vapor on the protein surface, (2) those based on the stoichiometric binding of water molecules to specific functional groups of the protein, and (3) models that have considered the water—protein system as a simple aqueous solution. Our results regarding the excess number of water molecules in the vicinity of a protein molecule indicate that at low humidity ((p < 0.2— 0.3), the excess of water in the vicinity of a protein molecule is due mainly to the difference in the sizes of the water and protein molecules. 

The heat of adsorption of water vapor onto the surface of both polymorphs of the drug was measured under different RH conditions using an isothermal heat conduction microcalorimeter (Thermal Activity Monitor, TAM, Thermometric AB, Sweden). The integral heats of adsorption were measured using a TAM instrument fitted with a Thermometric RH perfusion ampoule (accessory Model 2255) adapted with Kalrez O-rings. The RH above the solid samples was controlled as originally described by Bakri (1993)... 

The second aspect of this paper is one of presenting a structural picture of the polymer-water interface, based on the potential-distortion adsorption model. Adjacent polymer chains Interact by non-bonded, that is, van der Waals forces, so that, locally, the surface should show resemblances to that of a molecular solid. A type B situation Is postulated, with local restructuring to from pockets into which water molecules have Inserted themselves, as depicted in Figure 12. As the water activity (vapor pressure, if vapor phase Is present) Increases, a layer several molecules thick builds up. The layer Is sufflclent-... 

The fractal approach was also used to investigate adsorption and desorption mechanisms of water vapor on active carbons that were derived from coconut shell, coal, coke and pitch fiber featuring a wide range of BET specific surface areas [78]. A values were measured for the water clusters adsorbed on primary carbon centers. Values ranging from 1.64 to 1.67 implied a diffusion-limited aggregation model on a pore wall plane, whereas higher A values (up to 1.86), measured at a relative pressure X = 0.95, implied the formation of water clusters that were partly merged vertically to the walls. 

The water vapor [23] and nitrogen gas [24] adsorption isotherms determined by using automatic adsorption apparatus have been reported. On the other hand, the manual adsorption method for the adsorption of water vapor and nitrogen gas is discussed in Refs. 25 and 4, respectively. The amount adsorbed is determined by using a Shibata moisture and surface area apparatus. Model P-850 [Shibata Scientific Technology, Ltd., Tokyo, Japan) [25]. The amounts of water vapor and nitrogen gas adsorbed are determined by the gravimetric method [25] and volumetric method [4], respectively. 

FIGURE 3.46 Incremental PSDV for carbon samples activated by (a) COj and water vapor in (b) fixed and (c) fluidized bed reactors, DPT method with the model of sUt-shaped pores. (With kind permission from Springer Science-iBusiness Media Adsorption, Activation and strnctnral and adsorption features of activated carbons with highly developed micro-, meso- and macroporosity, 17, 2011d, 453—460, Gun ko, V.M., Turov, V.V., Kozynchenko, O.P. et al. Copyright 2011.)... 

Rajniak, P., and Yang, R.T., A simple model and experiments for adsorption-desorption hysteresis Water vapor on silica gel, AlChE J., 39(5), 774-786 (1993). 

Impact of the Discovery of Fullerenes on Carbon Science, P. J. E. Harris Molecular Models of Porous Carbons, T. J. Bandosz, M. J. Biggs, K. E. Gubbins, Y. Hattori, T. liyama, K. Kaneko, J. Pikunic, and K. T Thomson Adsorption of Water Vapor on Activated Carbon A Brief Overview, D. Mowla, D. D. Do, and K. Kaneko... 

---