Thermodynamic aspects of adsorption and separation of the enantiomers of some monoterpenes on a capillary column β-DEX 120

Abstract

The object of the research was capillary column WCOT (β-DEX 120) consisting of
polydiphynildimethylsiloxane (PPMS, 35%-phynil, 65%-methyl) with 20% addictive permethylated β-
cyclodextrin (Me-β-CD) and capillary column DB-35ms with pure PPMS. The aim of the work was studying
the thermodynamic characteristics of sorbtion and “host-guest” complexation in the polymeric stationary
phase of β-DEX 120 for monoterpenes C10H16 ((±)-limonene, (±)-α-pinene, (±)-camphene, (±)-menthol).
Retention factors have been observed for some enantiomers of monoterpens (limonene, camphene,
α-pinene, menthol) for two capillary columns (WCOT) with stationary phases consisting of
polydiphenyldimethylsiloxane with additive permethylated β-cyclodextrin (column β-DEX )120 and no
chiral additive (DB-35ms) at a temperature range of 80-1400 by capillary gas chromatography. Energetic and
entropy contribution in retention, thermodynamic parameters of «host-guest» complexation between
monoterpenes and macromolecular in stationary phase of β-DEX 120 were calculated.
Results of the work: by adding in medium polar polymer PPMS 20% mass Me-β-CD, the retention
of monoterpenes grows, as internal energy is increased because of "host-guest" complexation. The ratio of
this increase changes from -8 kJ (limonenes) to -16kJ (menthols). Under the assumption 1:1 stoichiometry of
complexes, we calculated the constant of complexation that increases in the sequence: limonenes-pinenes-
Евдокимова и др. / Сорбционные и хроматографические процессы. 2015. Т. 15. Вып. 2
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camphenes-menthols. Еnthalpy of the exothermic process (+)-isomers of hydrocarbons С10Н16 and (-)-
isomers of menthol is higher (in absolute) than in the case of enantiomers with the opposite sign of optical
rotation that provides for DEX 120 moderately expressed enantioselective properties.
In this paper, we submited a thermodynamic analysis of reasons that lead to a reduction of
enantioselectivity with increasing temperature and calculated temperature compensation Tcom at which
optical isomers eluted from the column at the same time. Results of this paper have important value for the
development of supramolecular chemistry and the design of analytical methods of separation and
determination of the enantiomers in natural and synthetic mixtures.
Revealed that adding macrocycle in polymer leads to an increase in retention because of exothermic
contribution in heat of sorption that is due to "host-guest" complexation in stationary phase. Thermodynamic
characteristics of complexation for monoterpenes were calculated in PPMS solution. Constant of
complexation and enthalpy of this process increase in the row limonenes-pinenes-camphenes-menthols.
Enthalpy and entropy contribution in enantioselectivity of column DEX 120 in relation to the optical isomers
of monoterpenes Tcom have calculated, at which this two contribution compensate each other.