Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.4 Measurement of solvent activity 159
Figure 4.4.6. Schematic of the experimental arrangement
for an isopiestic measurements. [Reprinted with permission
from Ref. 77, Copyright 1995, Elsevier Science].
evaporates and condenses (this is the isothermal
“distillation” process) between
them as long as the chemical potential of the
solvent is equal in all solutions. At least one
solution serves as reference system, i.e., its
solvent activity vs. solvent concentration
dependence is precisely known. After an
exact determination of the solvent concentration
in all equilibrated solutions (usually
by weighing), the solvent activity in all
measured solutions is known from and
equal to the activity of the reference solution,
Equations [4.4.1 to 4.4.5]. This
method is almost exclusively used for aqueous
polymer solutions, where salt solutions
can be applied as reference systems. It is a
standard method for inorganic salt systems.
Examples of this technique are given elsewhere.
76-80 Figure 4.4.6 provides a scheme
of the experimental arrangement for isopiestic
measurements as used by Grossmann
et al. 77 to illustrate the common principle.
The complete apparatus consists of six removable stainless steel cells placed in hexagonal
pattern in a copper block. The copper block is mounted in a chamber which is
thermostated. Each cell has a volume of about 8 cm 3 and is closed by a removable lid. During
the experiment, the cells are filled with about 2 cm 3 polymer solution (or reference solution)
and placed into the copper block. The chamber is sealed , thermostated and evacuated.
The lids are then opened and solvent is allowed to equilibrate between the cells as explained
above. After equilibration, the cells are closed, removed from the chamber and weighed
precisely. Equilibrium requires usually a couple of days up to some weeks. During this time,
the temperature of the thermostat does not fluctuate by less than ±0.1 K, which is realized by
the copper block that works as a thermal buffer.
Temperature lowering at specified isobaric or isochoric conditions is the most often
used technique for the determination of solvent vapor pressures or activities in polymer solutions.
The majority of all measurements is made using this kind of an isopiestic procedure
where the pure solvent is used as reference system. The equilibrium condition of equal
chemical potential of the solvent in the polymer solution as well as in the reference system is
realized by keeping the pure solvent at a lower temperature (T 1) than the measuring temperature
(T 2) of the solution. At equilibrium, the vapor pressure of the pure solvent at the lower
temperature is then equal to the partial pressure of the solvent in the polymer solution, i.e.,
s
P1 (T1)=P1(T2). Equilibrium is again established via the common vapor phase for both subsystems.
The vapor pressure of the pure solvent is either known from independent data or
measured additionally in connection with the apparatus. The composition of the polymer
solution can be altered by changing T1 and a wide range of compositions can be studied (between
30-40 and 85-90 wt% polymer, depending on the solvent). Measurements above
85-90 wt% polymer are subject to increasing errors because of surface adsorption effects.