Isopiestic pressure/sorption measurement

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. 

Methods of measurement have been reviewed (Kuntz and Kauzmann, 1974 McLaren and Rowen, 1951 Poole and Finney, 1986). Hydration levels are often established by isopiestic equilibration of protein samples against concentrated salt or sulfuric acid solutions of known water vapor pressure. A difficulty with this method is the long equilibration time, possibly several days, which is likely a consequence of the sample size (typically 100 mg or larger). Wilkinson et al. (1976) have described an automated sorption isotherm device transducers are used for the measurement of vapor pressure and sample weight. Gascoyne and Pethig (1977) used a resonating quartz crystal microbalance to study the hydration of bovine serum albumin and other proteins. Rao and Bryan (1978)... 

Equation-of-state approaches are preferred concepts for a quantitative representation of polymer solution properties. They are able to correlate experimental VLE data over wide ranges of pressure and temperature and allow for physically meaningful extrapolation of experimental data into unmeasured regions of interest for application. Based on the experience of the author about the application of the COR equation-of-state model to many polymer-solvent systems, it is possible, for example, to measure some vapor pressures at temperatures between 50 and 100 C and concentrations between 50 and 80 wt% polymer by isopiestic sorption together with some infinite dilution data (limiting activity coefficients, Henry s constants) at temperatures between 100 and 200 C by IGC and then to calculate the complete vapor-liquid equilibrium region between room temperature and about 350 C, pressures between 0.1 mbar and 10 bar, and solvent concentration between the common polymer solution of about 75-95 wt% solvent and the ppm-region where the final solvent and/or monomer devolatilization process takes place. Equivalent results can be obtained with any other comparable equation of state model like PHC, SAFT, PHSC, etc. 

The concentrated polymer solution between 30 and 85 wt% is controlled by vapor pressure measurements which were usually performed by various isopiestic sorption methods. The ultracentrifuge can also be applied for solutions up to 80 wt% polymer, but this was only scarcely done in the literature. 

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