Temperature dependence of the Kováts retention index. Convex or concave curves

M. Görgényi, Zoltán A. Fekete, Herman Van Langenhove, Jo Dewulf

Research output: Contribution to journalArticle

8 Citations (Scopus)

Abstract

The non-linearity in the temperature dependence of the Kováts index, I (the formation of convex or concave curves) was characterized by the second derivative, d2I/dT2. The expression deduced on a purely mathematical-physicochemical basis is d2I/dT2 = [2TΔS(CH2)dI/dT - 100δΔCp]/TΔG(CH2). The solute-dependent factor for dI/dT, d2I/dT2, and the extreme temperature in the I vs. T relationship is δΔCp, which is the molar solvation heat capacity difference between the solute and a hypothetical n-alkane which elutes at the same time as the given solute, while the solvent-dependent factors are the solvation entropy and free energy of the methylene unit, ΔS(CH2) and ΔG(CH2). Experimentally, convex I vs. T curves with a minimum are formed when δΔCp ≫ 0, while concave ones with a maximum are observed when δΔCp ≪ 0. In the event of a linear temperature dependence, the former equation can be simplified: dI/dT = 100δΔCp/2TΔS(CH2). The deviation from linearity (higher d2I/dT2) increases with increasing δΔCp values. The model equations were tested from the dataset published by the Kováts group on C78 (19,24-dioctadecyldotetracontane), POH (18,23-dioctadecyl-1-untetracontenol), PCN (1-cyano-18,23-dioctadecyluntetracontane) and TMO (1,38-dimethoxy-17,22-bis-(16-methoxyhexadecyl)-octatriacontane) and by present measurements on the Innowax phase.

Original languageEnglish
Pages (from-to)178-185
Number of pages8
JournalJournal of Chromatography A
Volume1206
Issue number2
DOIs
Publication statusPublished - Oct 10 2008

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Solvation
Temperature
Pregnenolone Carbonitrile
Alkanes
Entropy
Free energy
Specific heat
Hot Temperature
Derivatives
octatriacontane
Datasets

Keywords

  • Alcohols
  • Convex or concave curves
  • Kováts retention index
  • Polyethylene glycol
  • Solvation heat capacity
  • Temperature dependence of the retention index

ASJC Scopus subject areas

  • Analytical Chemistry
  • Organic Chemistry
  • Biochemistry

Cite this

Temperature dependence of the Kováts retention index. Convex or concave curves. / Görgényi, M.; Fekete, Zoltán A.; Van Langenhove, Herman; Dewulf, Jo.

In: Journal of Chromatography A, Vol. 1206, No. 2, 10.10.2008, p. 178-185.

Research output: Contribution to journalArticle

Görgényi, M. ; Fekete, Zoltán A. ; Van Langenhove, Herman ; Dewulf, Jo. / Temperature dependence of the Kováts retention index. Convex or concave curves. In: Journal of Chromatography A. 2008 ; Vol. 1206, No. 2. pp. 178-185.
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T1 - Temperature dependence of the Kováts retention index. Convex or concave curves

AU - Görgényi, M.

AU - Fekete, Zoltán A.

AU - Van Langenhove, Herman

AU - Dewulf, Jo

PY - 2008/10/10

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N2 - The non-linearity in the temperature dependence of the Kováts index, I (the formation of convex or concave curves) was characterized by the second derivative, d2I/dT2. The expression deduced on a purely mathematical-physicochemical basis is d2I/dT2 = [2TΔS(CH2)dI/dT - 100δΔCp]/TΔG(CH2). The solute-dependent factor for dI/dT, d2I/dT2, and the extreme temperature in the I vs. T relationship is δΔCp, which is the molar solvation heat capacity difference between the solute and a hypothetical n-alkane which elutes at the same time as the given solute, while the solvent-dependent factors are the solvation entropy and free energy of the methylene unit, ΔS(CH2) and ΔG(CH2). Experimentally, convex I vs. T curves with a minimum are formed when δΔCp ≫ 0, while concave ones with a maximum are observed when δΔCp ≪ 0. In the event of a linear temperature dependence, the former equation can be simplified: dI/dT = 100δΔCp/2TΔS(CH2). The deviation from linearity (higher d2I/dT2) increases with increasing δΔCp values. The model equations were tested from the dataset published by the Kováts group on C78 (19,24-dioctadecyldotetracontane), POH (18,23-dioctadecyl-1-untetracontenol), PCN (1-cyano-18,23-dioctadecyluntetracontane) and TMO (1,38-dimethoxy-17,22-bis-(16-methoxyhexadecyl)-octatriacontane) and by present measurements on the Innowax phase.

AB - The non-linearity in the temperature dependence of the Kováts index, I (the formation of convex or concave curves) was characterized by the second derivative, d2I/dT2. The expression deduced on a purely mathematical-physicochemical basis is d2I/dT2 = [2TΔS(CH2)dI/dT - 100δΔCp]/TΔG(CH2). The solute-dependent factor for dI/dT, d2I/dT2, and the extreme temperature in the I vs. T relationship is δΔCp, which is the molar solvation heat capacity difference between the solute and a hypothetical n-alkane which elutes at the same time as the given solute, while the solvent-dependent factors are the solvation entropy and free energy of the methylene unit, ΔS(CH2) and ΔG(CH2). Experimentally, convex I vs. T curves with a minimum are formed when δΔCp ≫ 0, while concave ones with a maximum are observed when δΔCp ≪ 0. In the event of a linear temperature dependence, the former equation can be simplified: dI/dT = 100δΔCp/2TΔS(CH2). The deviation from linearity (higher d2I/dT2) increases with increasing δΔCp values. The model equations were tested from the dataset published by the Kováts group on C78 (19,24-dioctadecyldotetracontane), POH (18,23-dioctadecyl-1-untetracontenol), PCN (1-cyano-18,23-dioctadecyluntetracontane) and TMO (1,38-dimethoxy-17,22-bis-(16-methoxyhexadecyl)-octatriacontane) and by present measurements on the Innowax phase.

KW - Alcohols

KW - Convex or concave curves

KW - Kováts retention index

KW - Polyethylene glycol

KW - Solvation heat capacity

KW - Temperature dependence of the retention index

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