Estimation of the Molecular Weight of Ions by Isotachophoresis. IV. Comparison of the Correlations of Mobility with Molecular Volume and with Molecular Weight

The determination of the molecular volumes of organic substances for which values of specific gravity (d) are not available was investigated. The values of van der Waals volume (VA:Å3/molecule) for aliphatic saturated monocarboxylic acids are closer to those of molecular weight per specific gravity (M/d: cm3/mol) than those of Vw (cm3/mol). The slope of the correlative equation between VA and M/d for organic anionic substances is 1.03, but that between Vw and M/d is 0.62. The values of VA for organic anionic substances show a good correlation with the absolute ionic mobility (mo) as well as M/d. The values of VA for amino acids and dipeptides also showed a better correlation with mo than the molecular weight (M). Thus, the values of mobility could be better estimated from the molecular volume than the molecular weight.