Elsevier

Fluid Phase Equilibria

Volume 248, Issue 1, 5 October 2006, Pages 78-88
Fluid Phase Equilibria

Infinite dilution activity coefficients, specific retention volumes and solvation thermodynamics of hydrocarbons in C78H158 branched alkane solvent

https://doi.org/10.1016/j.fluid.2006.07.015Get rights and content

Abstract

Kováts's retention indices were determined for 60 hydrocarbons by capillary gas–liquid chromatography at temperatures from 373.15 to 433.15 K using fused silica open tubular column coated with 19,24-dioctadecyldotetracontane (C78H158) as stationary phase. Gas–liquid equilibrium parameters (specific retention volumes, standard Gibbs energy differences and limiting activity coefficients) were calculated for isoalkanes, cycloalkanes, alkenes, alkynes, aromatics on the basis of absolute retention data of n-alkanes determined by GLC using packed column with the same stationary phase. It was concluded that molecular shape and size have an important effect on limiting activity coefficient values. The standard Gibbs free energy differences of solute transfer were correlated by a Kirchhoff-type temperature function and the thermodynamic parameters of the solvation (partial molar standard enthalpy and entropy differences at reference temperature T* = 403.15 K, furthermore heat capacity differences) were determined. The predictive ability of various entropic models was investigated on the new experimental data of saturated hydrocarbons and it is shown that the modified UNIFAC-FV model, Kannan-FV, represents satisfactorily the presented experimental limiting activity coefficient data of alkanes in size-asymmetric solutions.

Introduction

The developments of activity coefficient models and entropic expressions need limiting activity coefficient data for solutes of different shapes and sizes at various temperatures in asymmetric systems. Infinite dilution activity coefficients provide a convenient way of testing the performance of the combinatorial, free volume, rotational and vibrational models in the range of maximum degree of non-ideality. These investigations are necessary, because the separate effects of molecular interactions, size, shape and differences in free volume ratios in liquids are not yet adequately treated by the solution of groups methods [1].

However, there is a gap in experimental data over 373 K for hydrocarbon solutes in branched alkane solvents with carbon numbers of 50–120. The partition gas–liquid chromatography (GLC) utilizes the changes of three important thermodynamic parameters (free energy, entropy and enthalpy) of chemical and physical processes involved in a system, hence has potential applications to many and varied systems and materials. GLC is a traditional method to determine gas–liquid partition coefficients, activity coefficients of solutes at infinite dilution and thermodynamic parameters of solvation [2], [3]. The application of fused-silica open tubular capillary columns can decrease the undesirable effects in packed columns (interfacial adsorption at interfaces, long retention times). Because the measurement of the amount of the stationary phase in the capillaries is difficult, indirect ways are recommended to estimate specific retention volumes from capillary columns, either calculating the mass of the stationary phase from specific retention volumes obtained on packed column [4], [5], or using a standard solute with partition data measured on packed column [6], [7]. It has been recently shown [8] that for solutes having a wide range of retention times, the use of homologous series of n-alkanes as reference solutes are more convenient than the application of a single retention factor and that the Kováts retention index system [9] is useful to obtain reliable absolute retention data from indices measured by capillary GLC. It was demonstrated during a validation procedure that the accuracy of the capillary column method is appropriate and the specific retention volumes of solutes calculated from retention indices determined on capillary columns agree well with those obtained directly on packed column [8], [10].

This stop-gap work presents activity coefficients at infinite dilution and thermodynamic quantities of solvation for 60 hydrocarbon solutes (normal, branched, cyclic alkanes, alkenes, alkynes, aromatics) at temperatures from 373.15 to 433.15 K in a branched chain paraffin solvent C78H158 (19,24-dioctadecyldotetracontane, C78). The specific retention volumes and limiting activity coefficients data given were calculated from retention indices determined by GLC with fused-silica open tubular capillary column coated by C78 as stationary phase, using the specific retention volumes of n-alkanes obtained by gas–liquid chromatography on packed column wetted also with C78. The standard Gibbs energy difference data for transfer of solutes were correlated by a Kirchhoff-type temperature function to determine the thermodynamic parameters of the gas–liquid phase transition: the standard solvation enthalpy, the standard solvation entropy and heat capacity differences at reference temperature T* = 403.15 K.

In order to examine the importance of the combinatorial and free volume (FV) effects, the experimental limiting activity coefficient data of saturated hydrocarbons have been compared with values predicted using various entropic activity coefficient models: the combinatorial part of the original UNIFAC model [35], the improved Flory–Huggins term of the modified UNIFAC model [37], the entropic free volume model of Elbro et al. [40] and the improved UNIFAC-FV model with new free volume contribution proposed by Kannan et al. [41]. We discuss features of the Kannan-FV model that can best represent experimental data of alkane systems.

Section snippets

Experimental

A modified CP 9000 (Chrompack) gas chromatograph equipped with flame ionization detectors (FID) was used for determination of retention times of the solutes. The measuring method and devices were described in detail previously [10]. The temperature in the oven was measured with a platinum sensor (100 Ω, DIN 43710) and a model S 1221 measuring device from Systemteknik to better than ±0.05 K. The helium carrier gas flow (6.0, Messer Griesheim) was controlled by a precision pressure regulator (Model

Data reduction, results and discussion

The retention index for a solute, Ij, was evaluated by Eq. (1), using its adjusted retention time (tR,j) and the adjusted retention times (tR,z,tR,z+1) of n-alkanes with carbon numbers z and z + 1, which eluated before and after the solute on the capillary column under isothermal condition:Ij=100(logtR,jlogtR,z)logtR,z+1logtR,z+100z(tR,ztR,jtR,z+1)Based on the retention index obtained on capillary column wetted by C78, the specific retention volume of the solute (Vw,j) was

Acknowledgements

We are indebted to Prof. Z. Juvancz for the capillary column preparation. Support of the Foundation “Pro Arte Chimica Helveto-Pannonica” (Veszprém, Hungary) and of the Hungarian Scientific Research Foundation (OTKA 35220) is gratefully acknowledged.

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