Adventures in Thermochemistry
James S. Chickos*
Department of Chemistry and Biochemistry
University of Missouri-St. Louis
Louis MO 63121
E-mail: jsc@umsl.edu
10
Confluence of the
Missouri and Mississippi
Rivers
Applications of the The Correlation-Gas Chromatographic Method
Objectives: To go where no one else has gone
1. Evaluation of the vaporization enthalpies of large molecules
2. Application of Correlation-Gas Chromatography to a Tautomeric
Mixture - Acetylacetone
3. The Vaporization Enthalpies of Drugs and Related Substances
4. Evaluation of the Vaporization Enthalpies and Vapor
Pressures of Plasticizers
Dialkyl phthalates and related isomers are important industrial products, and many
have been produced in large quantities for a considerable period of time. Their
importance ranges from their use in polymers as plasticizers to applications in
cosmetics.
Some plastics become brittle with time simply due to the evaporation of the plasticizer
Phthalate esters have been in use for a considerable amount of time. Due to the
nature of their properties, and longevity of use, they are ubiquitous in the
environment. The vapor pressures and vaporization measured repeatedly over
the years. This has led to large discrepancies in their thermodynamic properties.
Dibutyl phthalate and bis (2-ethylhexyl) phthalate have been selected as
reference compounds for vapor pressure measurements by the US EPA.
We decided to examine if it were possible to establish a set of self consistent
experimental values in an area that has been characterized by numerous
discordant values. We are not aware of any other method capable of this.
As an example of the problems associated with these compounds, we purchased
diocyl phthalate and terephthalate from Aldrich only to discover that both
materials were actually the 2-ethylhexyl derivatives. Di-n-octyl phthalate is also
referred to as dioctyl phthalate.
Estimation of compounds containing multiple functional groups
lgH(298 K)/ kJ·mol-1 = 4.69(nC - nQ) + 1.3·nQ + ΣFi·bi + 3.0 + C where the value if Fi
depends on the hybridization and substitution pattern of the carbon to which the functional group
is attached.
b (ester) = 10.5 kJ·mol-1; C(H3)- : F = 1.62, -C(C)(H2)-, F = 1.08, =C(C3)-, F = 0.85,
C(C2)(H)- = 0.6; C = -2 kJ·mol-1/C branch on an sp3 hybridized carbon; average deviation ~ ± 8%
ln(p/po) = A’ – B’/RT
No references to the original work are provided.
Therefore we did not use the vapor pressure values as
standards
(1)
Vapor pressures reported as:
ln (p/po) = (1-To/T)exp[Ao +A1(T/K) +A2(T/K)2]
Cox Eq
Vapor pressures reported as:
(f)
ln(p/po) = a + b(T/K)-1 + c(T/K)-2
Small, P. A.; Small, K. W.; Cowley, P. The Vapor Pressure of Some High
Boiling Esters. Trans. Faraday Soc. 1948, 44, 810-6.
Measurements by Sergey
Verevkin
lgH( 298.15 K) =
(95.0  1.1) kJmol-1
Average value from
Hales et al.; Verevkin,
and this work
2
0
-2
-4
ln(p/po)
-6
-8
-10
-12
-14
-16
-18
-20
250
300
350
400
450
500
550
600
650
T/K
Figure. Bottom curve: A plot of ln(p/po) versus T/K for vapor pressures
reported for dibutyl phthalate by Small et al., (line),17 Hales et al. ( ),18
and this work, transpiration (▲).
This process was repeated at T = 10 K intervals from (298.15 to 550) K and the
resulting vapor pressures fit to the following third order polynomial (r 2 > 0.99)
ln(p/po) = A”(T/K)-3 + B”(T/K)-2 + C”(T/K)-1 + D”
(9)
A duplicate run resulted in the same value for bis (2-ethylhexyl) phthalate. Since
this value is within experimental error of the EPA value, the EPA value was used
of 116.7±0.5 kJ mol-1 was used in subsequent runs
bis 2-ethylhexyl phthalate116.7±0.5 EPA value
Using vapor pressures for:
(9)
vapor pressures were evaluated from correlations between ln(to/ta) and ln(p/po)
as a function of temperature from T = 298.15 to 550 K
Dimethyl terephthalate and isophthalate, and dicyclohexyl phthalate are crystalline
solids at room temperature. Since vapor pressures of the liquid as a function of
temperature, vaporization and fusion enthalpies at and T = (Tfus and 298.15 K) are
available, it is also possivle to calculate both sublimation vapor pressures and
enthalpies.
ln(pcr/Pa) = [crgH (Tfus) + crgCp·T] [ 1/Tfus/K – 1/298.15]/R + ln(p(Tfus/Pa))
where
crgCp·T = [0.75 + 0.15·Cp(cr)][(Tfus /K -298.15)/2]
Visitng Undergraduate Student
Mikhail Kozlovskiy from Moscow
Massiel Mori from South America
High School Student
John Vikman
Undergraduate Students
Jessica Spencer
Christian Koebel
Graduate Student
Chase Gobble
Joe Wilson
Faculty Collaborator
Sergey Verevkin, University
of Rostock, Rostock Germany