n - PATh :.: Process and Product Applied Thermodynamics research ...

Experimental Methods, Results and Discussion
0.05K is immersed. The cloud points were determined by visual observation by heating the
samples until a homogeneous phase is obtained followed by slow cooling of the mixtures
until phase separation is observed.
The accurate detection of cloud points at higher pressures was performed with a
He-Ne laser light scattering cell. The apparatus, as well as the methodology used for the
determination of phase transitions, have been described in detail elsewhere (Sousa and
Rebelo, 2000). The apparatus is easy, fast and safe to operate since both temperature and
pressure are computer controlled and it is fully automated (including data acquisition and
treatment).
The solution is contained in a high-pressure glass capillary (or sapphire capillary)
and the phase transition can be induced either by changing the cell temperature (at constant
pressure) or by changing pressure (at constant temperature), or by any other combination
of these changes. The temperature accuracy is typically
± 0.01 K (240 < T/K < 520). The pressure accuracy is ± 0.01 MPa for pressures ranging
from (0 to 7) MPa (glass cell) or 35 MPa (sapphire cell), although the final pressure
accuracy of each cloud point determination is somewhat poorer.
During an experiment, the cell containing the solution of a known composition is
immersed in the high precision oil (or water) bath at a given temperature and pressure, in
the homogeneous one-phase region. The top cap of the capillary cell is fitted to an HPLC
high-pressure valve, which allows the cell to be filled with solvent using syringes. Stirring
is accomplished by using a magnetic stirrer, which is turned off during the data acquisition
in order to avoid shear effects. By changing temperature and/or pressure the phase
transition is induced. Light from a He-Ne LASER passes across the solution contained in
the cell. Scattered and transmitted light intensities are converted into electrical signals by
two photo-detectors. These signals are measured with a digital multimeter equipped with
an automatic channel scanner. Pressure is generated by a high pressure screw injector,
coupled to a stepper motor, and controlled by a control box and a personal computer.
Pressure is measured by a digital manometer and by the multimeter. Temperature is
measured with a platinum resistance thermometer linked to a 6 1/2 digits multimeter. Both
multimeters have IEEE-488 interfaces, which allow the computer data aquision.
Cloud-points are detected when there is a sudden increase in scattered light from
the He-Ne LASER accompanied by a sudden decrease in transmitted light exiting the cell.
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