Effect of several salts on the elution behaviour of heavy hydrocarbon liquids during size exclusion chromatography with 1-methyl-2-pyrrolidinone as eluent

The addition of salts has been reported to shift chromatograms of coal-derived liquids to longer elution times (i.e. smaller molecular masses). The observation has been attributed to the disagglomeration of sample molecules. The aim of this work is to investigate whether size exclusion chromatograms (SEC) obtained, using NMP (1-methyl-2-pyrrolidinone) as eluent, are free from sample agglomeration. The direct addition of salts to the eluent does not allow distinguishing between (a) the effect of these salts on the large mass material and (b) distortions to the size exclusion mechanism introduced by the effect of the salt itself. The present work, attempts to resolve the two effects. Lithium bromide and trichloroacetic acid were added to solvents before use during the planar chromatographic (PC) fractionation of the samples. Any presumed disaggregation of the sample molecules would be expected to have taken place during this fractionation. The fractions thus separated from three coal-derived samples were examined by SEC. The apparently large molecular mass material observed in SEC does not disaggregate following PC elution in salty solvents. Indeed, the action of salty solvents was to displace from the silica of the PC-plates even larger mass materials than the solvent alone. The action of salts is to increase the polarity of the solvent and enhance its ability to displace larger and more polar molecules from the silica on the PC-plates. However, when the salts are added directly to the SEC eluent, they promote surface interactions between sample and packing. The observations thus obtained appear to reflect the distortion of the size exclusion mechanism rather than show disaggregation.