Vapor sorption/pressure measurement

In eomparison with all methods of determination of solvent aetivities from swelling equilibrium of network polymers, the gravimetrie vapor sorption/pressure measurement is the easiest applieable proeedure, gives the most reliable data, and should be preferred. 

The experimental values reported for the differential vapor pressure, gravimetric sorption, and piezoelectric methods are the weight fraction of solvent in the polymer solution and the pressure of the solvent vapors in equilibrium with the polymer solution. Differential vapor pressure measurements are performed by adding a known amount of solvent to a weighed polymer sample and measuring the vapor pressure of the solvent over the polymer solution (Bawn et al., 1950). 

Sorption isotherms, measurements of the weight of water adsorbed by a protein sample tis a function of the partial pressure of water in the vapor phase at constant temperature, are among the earliest descriptions of protein hydration [for references to early work, see Bull and Breese (1968a) and McLaren and Rowen (1951)]. 

Many methods of measuring water activity have been developed by researchers. These include direct vapor pressure measurement, equilibration with a stable hygroscopic substance that has a known sorption behavior, and various types of hygrometers (Doe, 1998). 

Enthalpy data from light scattering, osmometry, vapor pressure or vapor sorption measurements, and demixing experiments can be found in the literature. However, agreement between enthalpy changes measured by calorimetry and results determined from the temperature dependence of solvent activity data is often of limited quality. In this Handbook, data for AmHa°° determined by inverse gas-liquid chromatography (IGC) have been included. 

In summary, die decision for a special equipment depends to some extend on concentration, temperature and pressure ranges one is interested in. From the experience of the author, the combination of isopiestic vapor pressure/vapor sorption measurements for the determination of solvent activities with infinite dilution IGC for the determination of Henry s constants provides good experimental data and covers a temperature range that is broad enough to have a sufficient data basis for thermodynamic modeling. If one is interested in both solvent solubiUty and diffusion data, finite concentration IGC or piezoelectric sorption techniques should be applied. 

One method to obtain solvent activities in swollen polymer networks in equilibrium is to apply vapor pressure measurements. This is discussed in detail above in the Subchapter 4.4.3.1.1 and most methods can be used also for network systems, especially all sorption methods, and need no further explanation. The VPO-technique can be applied for this purpose, e.g., Amdt. IGC-measurements are possible, too, if one realizes a definitely erosslinked polymer in the column, e.g., Refs. " " ... 

Most of the entries in the table below were obtained from osmotic pressure, vapor sorption, or inverse gas chromatography measurements [5]. 

Chemical potentials for the solvent relative to tiie pure liquid can be obtained from vapor pressure or vapor sorption measurements. The characteristic molar volumes for eachjcomponent are needed for the expression of the excess chemical potential, Vj = NauJ ... 

The water vapor sorption experiments were generally performed in a Cahn D-200 electronic microbalance (with a sensitivity of lO g) enclosed in a thermostated reactor (Figure 5). The sample is placed in a pan and dried at 0% humidity. After reaching a plateau, the dry mass is achieved. Thereafter, the sample is exposed to vapor pressure and the mass gain is measured as a function of time until reaching the equihbrium state. The mass equilibrium is obtained at each humidity level tested. 

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