Desorption water vapor sorption

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.

FIG. 1.—Water-vapor sorption-desorption hysteresis for maize starch at 40°C (from Ref. 19). 

Water Vapor Sorption, Desorption, and Diffusion in Excised Skin Part I. Technique... 

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. 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... 

Water-vapor sorption/desorption hysteresis in concentrated food polymer systems. 

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]). 

In an open sorption storage system air is transporting water vapor and heat in and out of the packed bed of solid adsorbents (see Figure 235) or a reactor where the air is in contact with a liquid desiccant. In desorption mode a hot air stream enters the packed bed or the reactor, desorbs the water from the adsorbent or the salt solution and exits the bed cooler and saturated. In adsorption mode the previously humidified, cool air enters the desorbed packed bed or the... 

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... 

The amounts of water absorbed as function of relative water vapor pressure (relative humidity) for HA and its esters are reported in Tables 1-4. HA absorbed the highest amount of water at all humidity levels compared to its esters. The ethyl ester (Hyaff ) absorbs more water than the other two, and the dodecyl ester (Hyaff73) absorbs more water than the benzyl ester (Hyaffll). A small percentage of water absorption hysteresis, between sorption and desorption, was found for the four different materials analyzed. No significant differences in the percentage of hysteresis was found among the HA and the three esters. 

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... 

Basically, perecnl relative humidiiy is an indicator ol the water vapor saturation deficit of ihe gas mixture, rather ihun an indicator of sorption, desorption, comfort, or evaporation. A measurement of RH without a corresponding measurement of dry bulb temperature is not of particular value, since the water vapor content cannot be determined from % RH alone... 

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. 

Tt is generally recognized that water plays an important role in main-taining skin in a healthy state with desirable mechanical properties (I). This work describes a technique for generating information on the state of water in the stratum corneum in vitro, with particular emphasis on the mobility of water in the corneum matrix and the eflEect of stratum corneum components on the characteristics of water diffusion. The characteristics of water diffusion in the stratum corneum are derived from sorption and desorption kinetics by using a gravimetric technique which allows determination of the amount of water vapor sorbed or desorbed continuously from an air stream of any given relative humidity. 

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). 

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. 

In the preformulation study, the comprehension of physicochemical properties regarding water-solid surface interaction is beneficial to the handling, formulation, and manufacture of the finished products. Data on sorption/de-sorption isotherm, hydration of salts of drug product, water sorption of pharmaceutical excipients, and kinetics of water adsorption or desorption of a substance can be obtained effectively by the dynamic vapor sorption method. The knowledge may be utilized for dosage form design and supports the understanding of the mechanism of action. 

The second stage features the moisture sorption of fibers, which is relatively slow and takes a few minutes to a few hours to complete. In this period, water sorption into the fibers takes place as the water vapor diffuses into the fabric, which increases the relative humidity at the surfaces of fibers. After liquid water diffuses into the fabric, the surfaces of the fibers are saturated due to the film of water on them, which again will enhance the sorption process. During these two transient stages, heat transfer is coupled with the four different forms of liquid transfer due to the heat released or absorbed during sorption/desorption and evaporation/condensation. Sorption/ desorption and evaporation/condensation, in turn, are affected by the efficiency of the heat transfer. For instance, sorption and evaporation in thick cotton fabric take a longer time to reach steady states than in thin cotton fabrics. 

---