Determination of activity coefficients in gas chromatographic systems in the presence of mixed retention mechanisms: alkanols (C3–C5) at infinite dilution in squalane and in n-octadecane

Abstract

Symmetrical peaks, with sample size independent retention times, are obtained when alkanols are chromatographed in columns packed with alkane stationary phases coated on a deactivated solid obtained by coating Chromosorb W with Carbowax 20M, followed by thermal treatment in inert atmosphere and exhaustive extraction with methanol. Adsorption on the solid support/alkane interface is precluded by this deactivation method, but adsorption on the gas/alkane interface persists as a non-negligible contribution to solute retention. Retention volumes of ten alkanols with three to five carbon atoms were measured at five temperatures within the 30–50°C interval in columns containing between 2 and 12 wt.% of squalane or of n-octadecane on the deactivated support. Partition and adsorption coefficients were obtained from the dependence of retention volumes on wt.% of stationary phase. Alkanols infinite dilution activity coefficients were calculated from partition coefficients; it is demonstrated that important errors are introduced on neglecting adsorption contributions. An indirect proof of consistency between calculated adsorption and partition parameters is given by comparing results obtained with both stationary phases. It is furthermore demonstrated that non-combinatorial contributions to the activity coefficients are independent of the alkane solvent.