Vapor sorption measurements

In the alternative method (85), a second sampler, consisting of a Teflon particle filter followed by a quartz filter, is operated in parallel with a quartz-filter sampler. The carbon recovered from the quartz backup filter of this second sampler is subtracted from the organic C determination of the first sampler (Q - TQ strategy). The rationale for this alternative approach is to minimize the extent of sorption of vapor-phase carbon on the first (particle-collection) filter, which, otherwise, might significantly diminish the amount retained on the backup quartz filter. The need for an additional sampler and very accurate air volume measurements is a disadvantage. 

Prior to sorption measurements, zeolite samples were activated by evacuation at elevated temperatures. There is frequently some question as to how precisely one can establish the mass of a zeolite sample from which all zeolitic water, but no water arising from collapse of structural hydroxyl groups, has been removed (l f ). In order to establish that the (zeolitic-water-free) masses of the activated zeolite samples used here are well defined, the following stepwise activation procedure was used. Each sample was first heated in vacuo at 300°C. When the pressure had dropped to below about 10 torr, the balance was isolated from the pumps, the rate of pressure increase measured, and evacuation resumed. This process was repeated until the rate of pressure increase fell to below 5 X 10 torr min l, a duration of time which was from 15 to 30 minutes. This is a rate such that were the increase due to water vapor alone, and were the rate to remain constant, the weight loss would still be undetectable after 2h hrs., a duration seldom exceeded in activating zeolites. 

Water vapor at room temperature will not penetrate well-defined crystallites but will be adsorbed in the amorphous regions. Consequently, moisture sorption measured gravimetrically at a given relative vapor pressure and temperature has been used to determine order in cellulosic materials. In the case of Valentine [252] and Jeffries [253], the fraction of ordered material was obtained by correlating moisture sorption with values obtained by the deuterium... 

Vapor sorption measurements yield equilibrium composition and fugacity or chemical potential the isopiestic version (19) is used to determine the uptake of a pure vapor by a nonvolatile material. This technique determines equilibrium composition of a phase which cannot be separated quantitatively from the liquid phase in equilibrium with it. In our application, a nonvolatile crystalline surfactant specimen S is equilibrated with vapor of V, which is, in turn, at equilibrium with a system of S and V consisting of two phases, one rich in S, and one rich in V. At equilibrium, the Gibbs-Duhem relation guarantees. that the initial specimen of S takes up enough V from the vapor phase that the chemical potential of S, as well as of V, is the same as in the biphasic system, and so the composition of the phase formed by vapor sorption is the same as that of the S-rich phase. This composition is easily determined by weight measurement. If the temperature were a triple point, i.e. three phases at... 

For vapor sorption measurements a thermostatted vapor sorption apparatus (Worden Quartz Products, Inc.) was used (26). 

SORPTION MEASUREMENTS. Immersion experiments were carried out in toluene at room temperature using a blot drying technique. Samples for vapor sorption studies were thoroughly extracted before use, resulting in a 3 percent weight loss. Vapor sorption measurements were carried out using a standard vacuum sorption system and quartz... 

Data of Leung and Eichinger (52) Data of Hammers and DeLigny (48) Data of Newman and Prausnitz (51) Vapor sorption measurements (42)... 

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. 

Figure 4.4.15. Experimental data of the system toluene + polystyrene, = 1380 g/mol, at 323.15K, isopiestic vapor pressure/sorption measurement (fiill circles), VPO at higher concentrations (gray circles), data from authors own work.

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. 

The key feature of all acoustic wave sensors for detecting vapors is that measurable characteristics of the acoustic wave is altered as a result of adsorption on the surface of a receptive layer or absorption into the bulk of a thin layer (Figure 3). After sorption of the vapor by a thin film on top of the acoustic resonator equilibrium conditions are established and as a consequence of the increased mass or more accurately the change in the phase velocity of the acoustic wave a signal is created. Surface coatings generally enhance the sorption of vapors with the key properties of selectivity and sensitivity while affording reversibility. Typically, rubbery polymers were used on SAW devices such as polyisobutylene or substituted polysiloxanes but also self-assembled... 

Fig. 3. Schematic diagram of differential solvent vapor sorption measurement apparatus. (Reprinted from Yen, L. Y., Eichinger, B. E. J. Polym. Sci., Polym. Phys. Ed. 16, 121, 1978)...

Flory has derived the elastic free energy of dilation of a network with account of restrictions of fluctuations of junctions. Quantitative agreement has been reported for vapor sorption measurements. Particularly impressive is reproduction of the observation that the product of the linear expansion ratiok and the elastic contribution (pi — p.i)e, to the chemkal potential of the dilumt in a swollen network exhibits a maximum with increase in k, which is contrary to previous theory It is convenient to compare the phantom modulus obtained by stress-str measurements to that obtained from swelling equilibrium studies... 

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 ... 

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