Handbook of Solvents - George Wypych - ChemTech - Ventech!

178 Christian Wohlfarth
solvent concentrations may be used. Finite concentration IGC can be extended to
multi-component systems.
Head-space gas chromatography is a modern tool for the measurement of vapor pressures
in polymer solutions that is highly automated. Solutions need time to equilibrate, as is
the case for all 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 pure solvent vapor pressures
may be necessary. HSGC can easily be extended to multi-component mixtures because it
determines all 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.
4.4.3.2 Other measurement methods
This subchapter summarizes all other experimental methods mentioned in subchapter 4.4.1
in order of their special importance and use regarding the determination of solvent activities
in polymer solutions.
4.4.3.2.1 Membrane osmometry
Apart from VLE-measurements, membrane osmometry is the next important method that
has been used for measuring solvent activities in polymer solutions. This follows from the
tables in Refs. 1,2,5,8 according to its occurrence
in comparison to the other methods.
Most of these measurements were made in
the dilute solution regime; only a small
number of papers dealt with high-pressure
osmometry where one also can measure
solvent activities for concentrated solutions
with polymer concentrations up to about 50
wt%, e.g. Refs. 139-145
Laboratory designed instruments
were developed in the 40’s and 50’s, e.g. by
Zimm 145 or by Flory. 144 Later on, high speed
membrane osmometers are commercially
available, e.g., from Knauer, Hewlett-
Packard or Wescan Instruments. External
Figure 4.4.16. Principle scheme of a membrane
osmometer: 1 - solvent, 2 - polymer, π - osmotic pressure,
Δh - hydrostatic height difference, P 0 - ordinary pressure
or measuring pressure, V 1- partial molar volume of the
solvent in the polymer solution.
pressures may be applied to balance the osmotic
pressure if necessary, e.g., Vink. 140
The principle scheme of a membrane
osmometer together with the corresponding