Prediction of Activity Coefficients in Low-Molecular-Weight Paraffins from Gas Chromatographic Data

Abstract

It is shown that gas-liquid partition coefficients measured by gas chromatography on nonvolatile paraffin stationary phases at high temperatures allow estimation of gasliquid partition coefficients of solutes at ambient temperatures in volatile alkanes as solvents. Extrapolation to low temperatures was successful by use of the Kirchhoff equation for the description of the temperature dependence of the standard chemical potential of the solute related to its molal Henry coefficient. A new equation is proposed for the prediction of partition data in lowmolecular-weight paraffins. It was derived by using the Flory-Huggins model as a guide, and it combines theory with experiment. It is proposed to accept a combinatorial entropy term determined by experiment as a solute property. The necessary experimental information consists of a set of gas chromatographic data measured at at least four temperatures on two pure, high-molecularweight paraffins. Molal Henry coefficients extrapolated to low-molecular-weight solvents allowed calculation of activity coefficients. Predicted and measured data agreed within ( 20%. The reproducibility of activity coefficients by classical experimental methods is of the same order of magnitude.Properly designed gas liquid chromatographic experiments permit a rapid collection of accurate gas/liquid partition data at ideal dilution. 1,2 In the chromatographic column the high-molecular-weight solvent (the stationary liquid) is disposed of as a thin film on a support. Therefore, its specific surface area is high, typically of the order of some 5 m 2 g -1 (corresponding to a film thickness of 0.2 µm). Hence, already weak adsorption at the liquid/solid (liquid/support) and the gas/liquid interfaces shall influence the measurement and will result in wrong partition data. By the use of an "inactive support", adsorption at the liquid/ support interface can be eliminated. Adsorption at the liquid/gas interface is roughly proportional to the surface tension of the solvent. 3-5 It cannot be eliminated but can be reduced to a minimum by the adequate choice of solvent and by the use of highly loaded stationary phases, i.e., by reducing the specific surface area of the solvent.Partition data of a solute, j, in a given solvent, sv, as a function of temperature are best presented by calculating the corresponding standard chemical potential, ∆µ j,sv (i.e., the partial molar standard Gibbs free energy). 6Here, κ j,sv , is a partial pressure (fugacity) characterizing partition, R is the universal gas constant, T is the thermodynamic temperature, and the symbols ∆H j,sv (T) and ∆S j,sv (T) are for the partial molar enthalpy and entropy differences of the solute between the ideal gas phase and the ideal dilute solution. The relationship of eq 1 shows that the standard chemical potential difference depends both on specific enthalpic interactions between solute and solvent (attraction due to polarity, hydrogen bonding, etc.) and on the entropy. The latter also includes the so-called combinatorial entropy: contributi...

Solvation enthalpies and heat capacities ofn-alkanes in four polymer phases by capillary gas chromatography

“…This method is consistent with that of Kovµts et al, who suggested the determination of thermodynamic quantities from the molal Henry's law coefficient [6,14,15].…”

Field desorption mass spectrometry of large multiply branched saturated Hydrocarbons

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