Determination of activity coefficients in gas chromatographic systems in the presence of mixed retention mechanisms: alkanols (C-3-C-5) at infinite dilution in squalane and in n-octadecane

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