Handbook of Solvents - George Wypych - ChemTech - Ventech!

168 Christian Wohlfarth
successful experiment. Commercially sold ebulliometers have seldom been used for polymer
solutions. For application to polymer solutions, the operating systems have been individually
constructed. The above-mentioned reviews explain some of these in detail which
will not be repeated here as ebulliometry is not really a practiced method to obtain solvent
activities and thermodynamic data in polymer solutions. However, ebulliometry is a basic
method for the investigation of vapor-liquid equilibrium data of common binary liquid mixtures,
and we again point to the review by Williamson, 55 where an additional number of
equilibrium stills is shown.
Ebulliometers have traditionally been classified as either simple, in which only a single
temperature is measured, or differential, in which the boiling temperatures of the pure
solvent and of the solution were measured simultaneously. In differential ebulliometers,
two independent temperature sensors can work, or a single differential temperature measurement
is done. Essentially, all ebulliometers for polymer solutions are of the differential
type. The manner in which the reference boiling temperature of the pure solvent is provided
differs, however. Establishment and maintenance of both temperature and concentration
equilibrium are accomplished in a variety of ways. The common method is the use of a vapor
lift pump (a Cottrell pump) where the boiling liquid is raised to a position from which it
can flow in a thin film until superheat is dissipated and its true boiling temperature can be
measured. This technique has one disadvantage: the pumping rate depends on the heat input.
This is of particular importance with polymer solutions in which problems due to foaming
occur. To overcome this problem mechanical pumps were sometimes applied. Other
ebulliometer types have been reported that use the methods of surface volatilization, spray
cooling, two-stage heating, or rotating ebulliometer; for more details please see Refs. 33,34,40
Methods of temperature measurement within ebulliometer experiments will not be discussed
here, as they change rapidly with continuing progress of electronics and computerization.
Pressure control is important for single temperature ebulliometers, as the boiling
temperature depends on pressure. It is not so important in differential type ebulliometers,
owing to the simultaneous and compensating change in reference temperatures. Therefore,
direct changes in boiling temperatures present no serious problem if sufficient time is allowed
for calibration. It is usually recommended that the ebulliometer be thoroughly
cleaned and dried between experiments. Small amounts of polymer adsorbed on the surface
must be avoided.
(vi) Vapor-pressure osmometry (VPO)
Vapor-pressure osmometry is, from its name, compared with membrane osmometry by
considering the vapor phase to act like the semipermeable membrane, however, from its
principles it is based on vapor pressure lowering or boiling temperature elevation. Since the
direct measure of vapor pressure lowering of dilute polymer solutions is impractical because
of the extreme sensitivity that is required, VPO is in widespread use for oligomer solutions
(M n less than 20,000 g/mol) by employing the thermoelectric method as developed
by Hill in 1930. 124 In the thermoelectric method, two matched temperature-sensitive thermistors
are placed in a chamber that is thermostated to the measuring temperature and where
the atmosphere is saturated with solvent vapor. If drops of pure solvent are placed on both
thermistors, the thermistors will be at the same temperature (zero point calibration). If a solution
drop is placed on one thermistor, a temperature difference ΔT occurs which is caused
by condensation of solvent vapor onto the solution drop. From equilibrium thermodynamics
follows that this temperature increase has its theoretical limit when the vapor pressure of