Water vapor desorption isotherms

The most fundamental manner of demonstrating the relationship between sorbed water vapor and a solid is the water sorption-desorption isotherm. The water sorption-desorption isotherm describes the relationship between the equilibrium amount of water vapor sorbed to a solid (usually expressed as amount per unit mass or per unit surface area of solid) and the thermodynamic quantity, water activity (aw), at constant temperature and pressure. At equilibrium the chemical potential of water sorbed to the solid must equal the chemical potential of water in the vapor phase. Water activity in the vapor phase is related to chemical potential by... 

Duplessix et al. used water vapor pressure isotherm (i.e., water uptake vs external relative humidity) data combined with simultaneous isotherm differential microcalorimeter analysis to determine the average heat of absorption per water molecule for 1200 EW acid form samples. Hysteresis was seen between sorption and subsequent desorption curves at 25 °C, and nonzero water content remained at zero relative humidity, indicating the presence of tightly... 

Figure 5 summarizes the low pressure end of water vapor adsorption isotherms obtained on quartz A. Bound water loss, above the surface condition resulting from 400° C. outgassing in vacuum, is indicated by displacement of the adsorption origin to reflect total water content (bound water plus adsorbed water) as a function of water vapor relative pressure. Vapor desorption curves show that... 

Fig. 9 Water vapor sorption isotherms for a-, P-, and y-CD at 40 C. (Dashed line adsorption, solid line desorption). (Adapted from Ref...

Water sorption-desorption isotherms can be carried out by using thermobalances. Now specific instruments allow to measure water sorption-desorption isotherms at different constant temperatures (e.g., dynamic vapor sorption instrument (DVS), Surface Measurement Systems Ltd., Monarch Beach, US). 

Water Sorption. The water vapor sorption isotherm (293 K) is given in Figure 3. The general sigmoidal shape is evident, and the hysteretic phenomenon is quite pronounced, even when desorption is instigated from intermediate pressures (about 0.6 Pq), and vacuum closure... 

Fig 3 Left panel water adsorption / desorption isotherms in Vycor versus chemical potential at different temperatures. Lines are guides to the eye. Right panel Isosteric heat of adsorption versus water coverage at different temperatures (300, 350, 400, 500 and 650K from top to bottom) symbols this work horizontal lines latent heat of vaporization of bulk water. 

Information on the free energy, heat, and entropy of water adsorption on clays during the subsequent stages of the adsorption and desorption process can be calculated from water-vapor sorption isotherms obtained at different temperatures. Alternatively, these quantities can be determined by combining the data of a single isotherm with data for the heats of adsorption obtained directly. In appropriate calorimeters, one can measure the heat of adsorption of increments of vapor admitted to the sample, or one can measure the heats of immersion of samples that are previously equilibrated with water vapor at various relative vapor pressures. The heat of desorption can also be obtained from the peak areas of differential thermal analysis curves of partially and completely hydrated samples (Barshad [1952]). 

Fig. XVI-7. Dielectric isotherms of water vapor at 15°C adsorbed on a-FeiOa (solid points indicate desorption). A complete monolayer was present at P/P = 0.1, and by P/P = 0.8 several layers of adsorbed water were present. (From Ref. 110.)...

Fig. 10. Adsorption ( , O)"desorption (A, A) isotherms of water vapor on poly(vinylidene chloride) (PVDC) carbon before (filled symbols) and after...

Fig. 3 Water vapor sorption and desorption isotherms for ipratropium bromide at 20°C ( ), 2 month sorption (A), 5 month sorption ( ), 2 and 5 month desorption [Note All 2 month sorption results, except at 53% and 63% relative humidity, were verified at 5 months.]...

Fig. 4 Water vapor sorption ( ) and desorption (A) isotherms for microcrystalline cellulose at 20°C.-----, GAB fit to sorption data ------, GAB fit to desorption data.

The desorption isotherm describes, under equilibrium conditions, the amount of water absorbed on the product at a given temperature as a function of water vapor pressure, as shown in Fig. 1.96. To approximate the equilibrium at a given temperature in a short time, the pressure during SD should be small compared with the equilibrium vapor pressure,... 

Teng, C.D., Zarrintan, M.H., and Groves, M.J. (1991). Water vapor adsorption and desorption isotherms of biologically active proteins. Pharm. Res., 8(2), 191-195. 

Fig. 1.85.6. Desorption isotherms of the residual moisture content (% w/w) as a function of water vapor pressure (1) 20 (2) 40 (3) 60 °C...

Mesoporous melamine-formaldehyde and phenolic-formaldehyde resins were synthesized in the process of polymerization in the presence of fumed silica as an inorganic template. The surface and structural characteristics of the obtained sorbents were investigated using XPS technique and sorption from gas phase. The parameters characterizing porous structure of the synthesized resins in a dry state were determined from nitrogen adsorption/desorption isotherms. The sorption processes of benzene and water vapor accompanied by simultaneous swelling of both polymers were also studied. 

Figure 2. Benzene and water vapor adsorption/desorption isotherms on PHD and MEA resins.

The operation pressure can influence the second part of the desorption mechanism. If the partial vapor pressure inside the product is equal to the value of the desorption isotherm at that temperature, no desorption can take place. Therefore, the pressure in the chamber has to be small compared to this equilibrium pressure. The necessary pressure drop from the place of desorption to the pressure in the chamber is difficult to evaluate theoretically because water molecules will be transported not only by diffusion in the capillaries but by migration on the solid structure. To transport the possible volume of water vapor would require a high chamber pressure. On the other hand, a large gradient in concentration for the diffusion of molecules would require a very low pressure in the chamber. In most practical cases the pressure limit given by the condenser temperature (T ) will be used, e.g., = -55°C limits the chamber pressure to 0.02 mbar. This may not be... 

Interactions between water vapor and amorphous pharmaceutical solids were evaluated using isothermal microcalorimetry. " The desorption of water from theophylline monohydrate has been investigated using microcalorimetric approaches.The properties of surfactants and surface-active drugs in solution were studied by Attwood et al. " using calorimetry, while titration microcalorimetry has been utilized to elucidate the nature of specific interactions in several pharmaceutical polymer-surfactants systems. " Drug decomposition was evaluated as a function of different... 

Figure 3. Adsorption-desorption isotherms for water vapor by spruce at...

The amount of water sorbed in a solid also decreases as the solid is heated. Complete removal of this type of moisture at 100°C is by no means a certainty, however, as indicated by the drying curves for an organic compound shown in Figure 35-4. After this material was dried for about 70 min at 105°C, its mass apparently became constant. Note, however, that additional moisture was removed by further increasing the temperature. Even at 230°C, dehydration was probably not complete. Commercial vapor sorption analyzers can automate the acquisition of moisture sorption and desorption isotherms. 

Figure 5. Part a Nitrogen (77 K) (O and ), argon (77 K) (U) and water vapor (298 K) (A and A) isotherms for Silicalite I. Part b detailed low relative pressure data. Clear symbols denote adsorption dark symbols denote desorption.

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