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Adventures in Thermochemistry

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  1. 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

  2. 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

  3. 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

  4. 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.

  5. 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%

  6. 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

  7. Vapor pressures reported as: ln (p/po) = (1-To/T)exp[Ao +A1(T/K) +A2(T/K)2] Cox Eq

  8. 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.

  9. 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

  10. 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 (▲).

  11. 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)

  12. 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-1was used in subsequent runs

  13. bis 2-ethylhexyl phthalate116.7±0.5 EPA value

  14. 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

  15. 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]

  16. 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