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

Table 2.20. (continued)
x1 = 0.0828
T / K P / MPa
344.34 0.16
344.68 0.84
345.34 1.85
345.78 2.50
346.46 3.48
347.12 4.41
Experimental Methods, Results and Discussion
C8F18 + C9H20
It was observed that temperature versus composition diagrams for these mixtures
are more symmetric when represented in terms of volume fraction than mole fraction, as
can be observed in Figure II.18 a and b. Compositions in terms of volume fractions ( ) are
calculated using the relation
x
φ =
(II.28)
x + K(
1−
x)
where K = ρ1.M2/ρ2.M1, being ρ and M the densities and the molecular weights of
components perfluoro-n-octane (1) and alkane (2).
The experimental data measured were correlated using relations derived from
renormalization group (RG) theory, developed to describe systems near the critical point
(Nagarajan et al., 1980). In the vicinity of the critical point, the corresponding
thermodynamic functions are not analytical because of the flat slopes of the two branches
of the coexistence curve in that region. The RG theory is used to express the composition
for the critical point.
According to Sengers et al. (1976) the following relation is verified at the critical
point
( ) β
τ
Δ M = B
(II.29)
where ΔM is the difference in order parameter between the coexisting phases. The order
parameter is some quantity, mole fractions, volume fractions, densities, etc, chosen as a
measure of the different between the two coexisting phases. In the non-asymptotic region
Equation II.29 is modified by the presence of corrections to scaling (Wegner, 1972)
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