Influence of the chemical nature of siloxanes on the thermodynamic characteristics of sorption for a series of linear and cyclic saturated hydrocarbons

Abstract

The thermodynamic characteristics of sorption (Henry´s constants of sorption equilibrium K sorp , heats of sorption q, and standard entropies of sorption ∆S°) were studied by gas chroma tography to describe the behavior of linear and cyclic saturated hydrocarbons on phenyl methylsiloxane HP 5, fluoromethylsiloxane OV 210, and methylsiloxane OV 1. The K sorp and q values and, correspondingly, the energy of dispersion interactions decrease in the series HP 5 > OV 1 > OV 210. The difference in structures of adamantane, trans decalin, and decane molecules has almost no effect on the heats of sorption. Henry´s constants increase on going from decane to trans decalin and, further, to adamantane. This effect becomes more pronounced with an increase in the sorbent polarity (OV 1 < HP 5 < OV 210). The entropy factor can play a key role in sorption processes on siloxanes.Siloxanes are used as stationary phases for gas chro matography. 1-4 The thermodynamic characteristics of sorption (TCS) in systems involving siloxanes are studied in few publications only.Linear alkanes are testing compounds commonly used for the quantitative estimation of the energy of dispersion interaction with the stationary phase (both liquid and solid). 1,5 Contributions of the specific interaction to the Gibbs energy and heat of sorption are determined on the basis of the sorption values of n alkanes. 6 Chromato graphic data for n alkanes are necessary to determine the Kovats retention indices and to calculate relative charac teristics of sorption in gas chromatography. 1 The latter is one of the main methods for determining linear and cy clic alkanes in various objects, including oil and petro leum refining products. 7, 8 The physicochemical properties of adamantane (AdH) and its derivatives, including sorption behavior, are in tensely studied due to the wide use of this class of com pounds in medicine, their potential biological activity, 9,10 and, especially, the unique features of the AdH struc ture. 7,11 The number of publications on the relations of the retention parameters of AdH and its derivatives to the stationary phase nature has substantially increased in re cent time. 12-17 Nevertheless, the retention parameters of AdH in a wide temperature range remain virtually un known and, hence, the TCS of these compounds were not determined.In this work, we used gas chromatography to study the influence of the chemical nature of siloxanes (methyl siloxane, phenylmethylsiloxane, and fluoromethylsil oxane) on the TCS (constants, heats, and standard entro pies) of several linear and cyclic alkanes. ExperimentalStationary phases were 5% phenylmethylsiloxane HP 5 -[Si(Ph) 2 -O-] n -[Si(Me) 2 -O-] m -(thickness of the grafted phase 0.25 µm; the density of the grafted phase was assumed to be ρ = 1 g mL -1 ), 2 fluoromethylsiloxane OV 210 -[(CF 3 (CH 2 ) 2 )Si(Me)-O-] n -with the 50% content of trifluoropropyl groups (10% on Chromaton NAW, Russia; particle size 0.16-0.25 mm), and methylsiloxane OV 1 -[Si(Me) 2 -O-] n -(10% on Gas Chrom Q, USA; particle s...

Equilibrium parameters of a liquid—vapor system and thermodynamic characteristics of adsorption of cyclic and cage hydrocarbons in squalane

“…of adamantane derivatives under the GLC conditions are scarce. [26][27][28][29] They are important for understanding of the mechanism of chromatographic retention and for studying the character of intermolecular sorbate-sorbent interactions. In addition, they can help to establish a relationship between the retention parame ters and specific features of the molecular structure and the physicochemical properties of sorbate molecules, which are manifested during adsorption.…”