Comparison of experimental VLE-methods

The general aim of all experiments is to measme solvent activities in polymer solutions over the complete concentration range and for all desired temperatiues (and pressures). Additionally, the dependence on molar mass of the polymer has to be taken into account. As is clear from all explanations above, there is no really universal method to fulfill all piuposes. 

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 

Head-space gas chromatography is a modem tool for the measurement of vapor pressures in polymer solutions that is highly automated. Solutions need time to equilibrate, as is the case for aU vapor pressure measurements. After equilibration of the solutions, quite a lot of data can be measured continuously with reliable precision. Solvent degassing is not necessary. Measurements require some experience with the equipment to obtain really thermodynamic equilibrium data. Calibration of the equipment with pine solvent vapor pressures may be necessary. HSGC can easily be extended to multi-component mixtures because it determines aU components in the vapor phase separately. 

In summary, the 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 solubility and diffusion data, finite concentration IGC or piezoelectric sorption techniques should be applied. 

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The method can be applied for saturated fatty acids, unsaturated fatty acids, fatty esters, fatty alcohols and acyl-glycerols. The regression is based on 1200 data points. The absolute deviation in predicting vapor pressure is 6.82%. Another advantage of Eq. (14.1) is the capability of predicting the VLE of mixtures of fatty acids and esters by using the UNIFAC model for liquid activity. The comparison with experimental data shows good accuracy not achieved by other methods [40]. 

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