Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.4 Measurement of solvent activity 169
Figure 4.4.14. Principle scheme of a vapor-pressure osmometer of the hanging
drop type. T - measuring temperature, ΔT - obtained temperature difference
(time dependent), measurements are made at atmospheric pressure where T determines
the partial vapor pressure of the solvent P 1 in air.
the solution is equal to
that of the pure solvent,
i.e., at infinite dilution.
The obtained temperature
difference is very
small, about 10 -5 K. Because
solvent transfer effects
are measured, VPO
is a dynamic method.
This leads to a time-dependent
measurement of
ΔT. The principle scheme
of a VPO apparatus is
given in Figure 4.4.14.
Today, vapor-pressure
osmometers are
commercially available
from a number of producers.
They can be divided
into two basic
types: those that employ conventional hanging drop thermistors as in Figure 4.4.14 and
those that use vertical thermistors. The vertical thermistors automatically control drop size
to ensure more reproducible response. The hanging drop design requires the operator to
manually monitor and control drop size. Furthermore, commercial instruments have been
developed which utilize vertical thermistors having cups or pieces of platinum gauze to control
drop size in a highly reproducible manner. More details about instrumentation and techniques
can be found in the reviews given by Glover, 34 Mays and Hadjichristidis. 40 A very
recent presentation can be found in a new book edited by Pethrick and Dawkin. 26
Depending on technical details of the equipment, on the sensitivity of the temperature
detector, on measuring temperature, solvent vapor pressure and polymer concentration in
the solution drop, a steady state for ΔT can be obtained after some minutes. The value of ΔT st
is the basis for thermodynamic data reduction (see below). If measuring conditions do not
allow a steady state, an extrapolation method to ΔT at zero measuring time can be employed
for data reduction. Sometimes a value is used that is obtained after a predetermined time;
however, this may lead to some problems with knowing the exact polymer concentration in
the solution. The extrapolation method is somewhat more complicated and needs experience
of the experimentator but gives an exact value of polymer concentration. Both methods
are used within solvent activity measurements when polymer concentrations are higher
and condensation is faster than in common polymer characterization experiments. A way to
avoid these problems is discussed below.
Experience has shown that careful selection of solvent and temperature is critical to
the success of the VPO experiment. Nearly all common solvents, including water (usually,
there are different thermistor sensors for organic solvents and for water), can be used with
VPO. The measuring temperature should be chosen so that the vapor pressure of the solvent
will be greater than 6,000 Pa, but not so high as to lead to problems with evaporation from
the chamber. Solvent purity is critical, especially volatile impurities, and water must be