ORGANIC SOLUTES IN PARAFFIN SOLVENTS - INFLUENCE OF THE SIZE OF THE SOLVENT MOLECULE ON SOLUTION DATA

Solution properties at ideal dilution were measured for 124 solute probes by gas chromatography on five pure paraffins of well defined molecular weight at temperatures of 110-190°C. Standard chemical potential differences, related to the molal Henry coefficient, were calculated from retention data. Their temperature dependence allowed the calculation of partial molar solution enthalpy, entropy and heat capacity differences between the ideal gas phase and the ideal dilute solution. It was found that the standard chemical potential difference depended linearly on the variable ζ = 1000/Ml, where Ml is the molecular weight of the paraffin stationary phase. The linear dependence was best explained by assuming that it is due to the linear dependence of the partial molar solution entropy of the solute on the variable ζ. Consequently, on a molal basis, a solute forms equithermal solutions in any of the liquid paraffins. Comparison with literature data suggested that these results have general validity. Approximate relationships are given to calculate the coefficient of variation of the retention index with temperature and with the variable ζ from thermodynamic data. © 1990.