Vapor sorption method

A relatively fast and less labor-intensive process is the dynamic vapor sorption method. In this automated process, the sample is placed on a microbalance within the environment of a given relative humidity at a constant temperature. The relative humidity is changed continuously with a device controlled by a computer. The change in sample weight is monitored as a function of time or as a function of relative humidity, and the event is recorded. The weight gained (adsorption) or weight lost (desorption) versus relative humidity may... 

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. 

Studies of the diffusion of benzene in natural rubber represent some of the earliest detailed examinations of the interaction of an organic solvent with a polymer. Hayes and Park carried out measurements at low concentrations by the vapor sorption method (1), and at higher concentrations by determining the concentration distribution using an interferometric method (2). Complementary measurements by vapor transmission to determine the diffusion coefficient from time-lag data were carried out at low concentrations by Barrer and Fergusson (3). The main results of these studies have been summarized in Fujita s review (4) of organic vapor diffusion in polymers above the glass transition temperature. However, the problems with these measurements were not referenced. 

The piezoelectric sorption technique is a method that is especially suitable for the low solvent concentration range. It is the most sensitive solvent vapor sorption method. A resolution of nanograms can be realized. Measurements can also be made as a function of time... 

In the study of blends of a crystalline polymer with an amorphous one, the use of the vapor sorption method... 

Inverse gas chromatography (IGC) and vapor sorption methods utilize solvent probes to ascertain the interactions existing in polymer blends These methods [357-359] employ the following equation for the activity coefficient of the probe, a, in the polymer blend ... 

The most sensitive solvent vapor sorption method is the piezoelectric sorption detector. The amount of solvent vapor absorbed by a polymer is detected by a corresponding change in frequency of a piezoelectric quartz crystal coated with a thin film of the polymer because a frequency change is the response of a mass ehange at the surface of such a crystal. The frequency of the crystal decreases as mass increases when the crystal is placed in a gas or vapor medium. The frequency decrease is fairly hnear. The polymer must be coated onto the crystal from a solution with some care to obtain a fairly uniform film. Measurements can be made at dynamic (vapor flow) or static conditions. With reasonable assumptions for the stabihty of the crystal s base frequency and the precision of the frequency counter employed, the piezoelectric method allows the detection of as little as 10... 

The formation of niclosamide hydrates, and the effect of relative humidity on the solvatomorphs obtained from acetone and ethyl acetate has been studied [79], The acetone and ethyl acetate solvatomorphs could be desolvated, and exposure to elevated humidity resulted in the formation of two hydrate structures. Each hydrate could be dehydrated into a different anhydrate phase, but only the hydrate formed from the acetone desolvate could be rehydrated to form a hydrate phase. Dynamic vapor sorption has been used to develop a method for determining the onset relative humidity of a glass transition and associated crystallization process [80]. 

The experimental basis of sorption studies includes structural data (SANS, SAXS, USAXS), isopiestic vapor sorption isotherms,i and capillary isotherms, measured by the method of standard porosimetry. i 2-i44 Thermodynamic models for water uptake by vapor-equilibrated PEMs have been suggested by various groupThe models account for interfacial energies, elastic energies, and entropic contributions. They usually treat rate constants of interfacial water exchange and of bulk transport of water by diffusion and hydraulic permeation as empirical functions of temperature. 

Felder R. M., Huvard G.S. 1980, Permeation, diffusion and sorption of gases and vapors in Methods of experimental physics, 16c, 315-377, Academic Press Inc. 

Differential Vapor Pressure. Gravimetric Sorption, and Piezoelectric Sorption Methods... 

Before describing the details of the new technique, we shall present a concise review of the most popular methods used to study water vapor sorption and permeability of the stratum comeum. 

Figure 6. Equilibrium water vapor sorption isotherms obtained on female stratum corneum (age 29) using salt solutions ( ) and the dynamic method (O) at 23.3 0.5°C...

Mantovani F, Grassi M, Colombo I, Lapasin R. A combination of vapor sorption and dynamic laser light scattering methods for the determination of the Flory parameter chi and the crosslink density of a powdered polymeric gel. Fluid Phase Equilib 2000 167(1) 63— 81. 

The evaluation of the commercial potential of ceramic porous membranes requires improved characterization of the membrane microstructure and a better understanding of the relationship between the microstructural characteristics of the membranes and the mechanisms of separation. To this end, a combination of characterization techniques should be used to obtain the best possible assessment of the pore structure and provide an input for the development of reliable models predicting the optimum conditions for maximum permeability and selectivity. The most established methods of obtaining structural information are based on the interaction of the porous material with fluids, in the static mode (vapor sorption, mercury penetration) or the dynamic mode (fluid flow measurements through the porous membrane). 

There are several advantages to the dynamic vapor sorption device. First, any humidity value can be dialed in, whereas salt solutions are not available for every humidity value and some are quite toxic. Second, since the weight is monitored as a function of time, it is clear when equilibrium is reached. The dynamic devices also give the sorption/desorption rates, although these can easily be misused (see the drying kinetics section later). The salt solution method, on... 

In summary, IGC is an experimentally attractive method for obtaining polymer-polymer interaction parameters in polymer blends at temperatures above Tm for a crystalline blend, and above Tg for an amorphous blend. This technique yields interaction parameters that are generally consistent with data obtained with other techniques such as vapor sorption, melting point depression, neutron scattering, and small-angle X-ray scattering (40). Advances in IGC of polymer blends will require increased experimental precision in order to improve the consistency of the data, as well as refinements of thermodynamic models to allow better interpretation of interactions in ternary solutions. 

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