Adventures in Thermochemistry

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Adventures in Thermochemistry
James S. Chickos Department of Chemistry and
Biochemistry University of Missouri-St. Louis
Louis MO 63121 E-mail jsc_at_umsl.edu 10
Confluence of the Missouri and Mississippi Rivers
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• 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
• The Vaporization Enthalpies of Drugs and Related
  Substances
• Evaluation of the Vaporization Enthalpies and
  Vapor Pressures of Plasticizers

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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
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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.
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Estimation of compounds containing multiple
functional groups ?lgH(298 K)/ kJ?mol-1
4.69(nC - nQ) 1.3?nQ SFi?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
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ln(p/po) A B/RT (1)
No references to the original work are provided.
Therefore we did not use the vapor pressure
values as standards
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Vapor pressures reported as ln (p/po)
(1-To/T)expAo A1(T/K) A2(T/K)2 Cox Eq
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Vapor pressures reported as ln(p/po) a
b(T/K)-1 c(T/K)-2
(f) Small, P. A. Small, K. W. Cowley, P. The
Vapor Pressure of Some High Boiling Esters.
Trans. Faraday Soc. 1948, 44, 810-6.
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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
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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 (?).
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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 gt 0.99)
ln(p/po) A(T/K)-3 B(T/K)-2 C(T/K)-1
D (9)
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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 116.70.5 kJ? mol-1 was
used in subsequent runs
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bis 2-ethylhexyl phthalate116.70.5 EPA value
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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
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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

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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