Effect of the solid support on the chromatographic properties of a liquid crystal stationary phase

Abstract

The gas—liquid chromatographic (GLC) retention characteristics of the liquid crystal stationary phase N,N′-bis( p-methoxybenzylidene)-α,α′-bi p-toluidine (BMBT) are significantly affected by the type of solid support used and the amount of liquid crystal deposited on the support. The solid examined were Chromosorb W HP, porous silica galss beads, Porasil C and F. The per cent loading above which the influence of the support is no longer observed varies with the type of support material, being highest for the silica beads of the Porasil C type. The retention volumes of the solute probes, phenanthrene and anthracene, were measured at four temperatures in the nematic range of BMBT coated on Chromosorb W HP and Porasil C. With Chromosorb W HP, the specific retention volume, V o g of a given probe at a given temperature decreases as the loading increases up to 2.5%, beyond which which V o g is practically constant. With Porasil C, V o g continuously decreases as the loading increases up to 30%. Differential scanning calorimetry in combination with GLC revealed the presence of two types of configuration of BMBT molecules on coated solid supports; namely, a liquid crystall film on the surface and a bulk liquid region in the capillaries of the support. With low per cent loadings only surface films are observed, while at higher loadings the support capillaries are also filled with bulk liquid. With surface-active support material the GLC working temperature range is extended to lower temperatures, however this advantage is offset by the drastically lower selectivity offered by thin films as compared to the bulk selectivity achieved with high loadings on inert solid supports.