chapter one the estimation of physical properties

PURE COMPONENT CONSTANTS
2.20 CHAPTER TWO
T c for all types of organics with somewhat better accuracy than Joback’s method;
Sastri, et al. (1997) treating only V c and obtaining somewhat better accuracy than
Joback’s method; Tobler (1996) correlating V c with a substance’s temperature and
density at the normal boiling point with improved accuracy over Joback’s method,
but also a number of substances for which all methods fail; and Daubert [Jalowka
and Daubert, 1986; Daubert and Bartakovits, 1989] using Benson groups (see Sec.
3.3) and obtaining about the same accuracy as Lydersen (1955) and Ambrose (1979)
for all properties. Within limited classes of systems and properties, these methods
may be more accurate as well as easier to implement than those analyzed here.
As mentioned in Sec. 2.1, there is also a great variety of other estimation methods
for critical properties besides the above group/bond/atom approaches. The
techniques generally fall into two classes. The first is based on factor analysis that
builds correlation equations from data of other measurable, macroscopic properties
such as densities, molecular weight, boiling temperature, etc. Such methods include
those of Klincewicz and Reid (1984) and of Vetere (1995) for many types of substances.
Somayajulu (1991) treats only alkanes but also suggests ways to approach
other homologous series. However, the results of these methods are either reduced
accuracy or extra complexity. The way the parameters depend upon the type of
substance and their need for other input information does not yield a direct or
universal computational method so, for example, the use of spreadsheets would be
much more complicated. We have not given any results for these methods.
The other techniques of estimating critical and other properties are based on
molecular properties, molecular descriptors, which are not normally measurable.
These ‘‘Quantitative Structure-Property Relationships’’ (QSPR) are usually obtained
from on-line computation of the structure of the whole molecule using molecular
mechanics or quantum mechanical methods. Thus, no tabulation of descriptor contributions
is available in the literature even though the weighting factors for the
descriptors are given. Estimates require access to the appropriate computer software
to obtain the molecular structure and properties and then the macroscopic properties
are estimated with the QSPR relations. It is common that different methods use
different computer programs. We have not done such calculations, but do compare
with the data of Appendix A the results reported by two recent methods. We comment
below and in Sec. 2.5 on how they compare with the group/bond/atom methods.
The method of Gregoras is given mainly for illustrative purposes; that of Jurs
shows the current status of molecular descriptor methods.
Method of Grigoras. An early molecular structural approach to physical properties
of pure organic substances was proposed by Grigoras (1990). The concept was
to relate several properties to the molecular surface areas and electrostatics as generated
by combining quantum mechanical results with data to determine the proper
form of the correlation. For example, Grigoras related the critical properties to
molecular properties via relations such as
V 2.217A 93.0 (2-2.13)
c
T 0.633A 1.562A 0.427A 9.914A 263.4 (2-2.14)
c HB
where A is the molecular surface area, A and A are the amounts of negatively
and positively charged surface area on the molecule and A HB is the amount of
charged surface area involved in hydrogen bonding. Examples of values of the
surface area quantities are given in the original reference and comparisons are made
for several properties of 137 compounds covering many different types. This is the
only example where a tabulation of descriptors is available.
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