This Week ’ s

In gas chromatography, the logarithm of the net retention volume of the members of the homologous seriesof the n-alkanes, C~H,~÷ 2 , gives a nearly linear plot asa function of the carbon number z. By designating the logarithmic retention of the n-alkanes as 100 z, the position of a peak can easily be characterized by linear interpolation between two adjacent n-paraffins eluted before and after the compound. It is very easy to show that the resulting “retention index” is a measure of the standard chemical potential difference of the compound in question. The fixed points are the standard chemical potential differences ofthe n-alkanes that are nearly equidistant with the carbon number. This function compares the chemical potential of a substance in an ideal dilute solution with that in the ideal gas state. In the gaseous state, the molecules of the solute are not attracted by the force field of neighboring molecules. In the ideal dilute solution, they are in intimate contact with the stationary phase, the solvent. In such an equilibrium system, the force of intermolecular attraction is relatively weak, and, in first approximation, the entropy introduced by this force is proportional to it. inother words,the molar entropy difference is proportional to the molar enthalpy. The first consequence of this proportionality is that the retention index is practically independent of temperature. The second consequence is that one can treat retention indices as if they are a measure of interaction forces between the solute and the force field ofthe solvent molecules, in a first approximation, dispersion forces and polar forces are additive. Therefore, retention indices can be predicted by additivity rules on the basis of the structure of the solute. Rules could be also given for the calculation of the additional retention of a compound, il, on a polar stationary phase compared to the retention on a nonpolar solvent, Let me mention some early applications. L Rohrschneider has shown that, with the help of five il-values of judiciously chosen molecular probes, the polarityof a solvent could be characterized and that with the help of this “property vector” retention could be predicted.’ B.L Karger, LR. Snyder, and C. Eon deduced a set of “solubility parameters”by extending the concept of j.H. Hildebrand and R.L Scott. 2 Also, the il-values were seen to correlate with olfactory quality of substances. 3 Obviously, il-values characterize the polarity of a solute and ofa solvent in some detail. They can be related to other polarity scales. 4