B. SEDUNOV
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properties of pure fluids. The principal source of initial
data is the database for thermophysical pr operties of f luid
systems [8]. Due to advanced computer processing of
raw experimental data from different sources, this da-
tabase contains the generalized data with an improved
resulting precision. The data improvement process is de-
veloped by the Thermodynamics Research Center (TRC)
of the US National Institute of Standards and Techn olog y
[9] and is named as the critical evaluation of experimen-
tal data. Its main idea sounds as: “critical evaluation is a
process of analyzing all available experimental data for a
given property to arrive at recommended values together
with estimates of uncertainty, providing a highly useful
form of thermodynamic data for our customers. The
analysis is based on intercomparisons, interpolation, ex-
trapolation, and correlation of the original experimental
data collected at TRC. Data are evaluated for thermody-
namic consistency using fundamental thermodynamic
principles, including consistency checks between data
and correlations for related properties. While automated
as much as possible, this process is overseen by experts
with a great deal of experience in the field of thermody-
namic data” [9].
The principal goal of this paper is to analyze deeply
the structural changes [7] of the supercritical fluid near
the extension of the saturation curve to the supercritical
region. More practical goals are to correlate the pressure
driven steep solubility growth in SFs [1,10] with the
structural transition from the gas-like to the liquid-like
supercritical fluid and to correlate the high diffusivity [1]
of molecules in supercritical fluids with the monomer
fraction density [11].
2. The Nature of the Gas-To-Liquid
Transition in the Supercritical Zone
In our approach the transition from the gas-like to the
liquid-like supercritical fluid is named as the Soft Struc-
tural Transition [7,12,13 ]. It differs from the well known
Structural Transitions in solids [14]. The solids have long
range correlations of their elementary cells’ structures.
But the structure of a supercritical fluid is not homoge-
neous in space and dynamically changes. It can have
only short r ange structural correlations with a correlation
radius smaller than the dimensions of clusters and pores.
And it differs also from the first order liquid-vapor phase
transition that is based on the collective behavior of
molecules in a liquid state with a radius for correlations
of potential energy much larger than the dimensions of
pores. But the correlation radius for potential energy in
supercritical fluids is smaller than the dimensions of
clusters in the gas-like fluid or pores in the liquid-like
fluid.
2.1. The Structure of the Supercritical Ridge
Zone
The lack of the long range structural and energy correla-
tions in supercritical fluids may be explained by large
density fluctuations discovered by the group of research-
ers headed by Dr. Keiko Nishikawa [15]. This group has
investigated by the X-ray diffraction and Raman spec-
troscopy the density fluctuations in supercritical fluids.
They discovered the peak line Pr (T) of density fluctua-
tions on the (T, P) diagram and named it as the ridge. A
similar sort of the ridge line Pr (T) may be seen on the
diagrams of many thermophysical properties, such as
heat capacities, compressibility factors, etc. [7,16].
The saturation curve Ps (T), marking the first order
phase transition between liquid and vapor phases on the
PT diagram, terminates at the critical point (Pc, Tc). The
ridge line Pr (T) is the extension of the saturation line Ps
(T) to the supercritical zone, but depends on the nature of
the property selected to build this lin e [7,16]. So, there is
a plurality of the extension lines, forming together the
zone of the soft structural transition. The zone widens
with a growth of the T – Tc difference, Figure 1, [17].
The data are from [8].
The ridge zone is the zone of large but controllable
changes in thermophysical properties and can be recom-
mended as the zone favorable for technological processes.
If the pressure P grows at a constant supercritical tem-
perature T, the fluid passes through the structural transi-
tion from the gas-like structure to the liquid-like one. But
this transition is not accompanied by abrupt changes of
thermophysical properties. In the ridge zone abruptly
changes only topology of the medium: it converts from a
scattered form, filled with clusters, to a condensed one,
filled with pores, [7,6,17]. The scattered clusters of the
gas-like structure over the ridge pressure collect together
into an infinite cluster, filled with pores of the same total
volume, as the total volume of clusters.
The supercritical fluid is a heterogeneous system [18]
that can change only its structure at changing pressure or
temperature. The structure of the supercritical fluid at
pressures lower the ridge pressure Pr (T) is fog-like and
consists of different size clusters flying in an ideal gas of
monomers, but over the ridge pressure the picture is quite
contrary: the structure is foam-like and contains pores of
different sizes spread in the continuous liquid media,
Figure 2. At the ridge pressure the average densities of
both structures are equal and close to the critical density
Dc. At the ridge pressure coexist large regions with the
gas-like structure and large regions with the foam-like
structure, mutually penetrating in each other. It gives rise
to the giant fluctuations of density, directly measured in
experiments of Dr. K. Nishikawa and her group.
In the earlier author’s work [16] the method o f estima-
tion the populations and bond parameters of clusters in
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