This work involves the preparation of two liquid crystalline compounds, 4,4´-bis-(4-propoxybenzylidene amino)biphenyl (2b), and 2,5-bis-[4-(4´-propoxybenzylideneamino)phenyl]1,3,4-oxadiazole (1f).
The prepared compounds were characterized qualitatively by using FTIR, 1HNMR, and elemental analysis (EA). The liquid crystalline properties of the synthesized compounds were verified using hot-stage polarizing microscope and differential scanning calorimetry (DSC). The prepared liquid crystal compounds have a wide mesophase range that gives useful properties to these compounds as stationary phases in Gas Chromatography (GC). Compounds (2b) and (1f) were prepared to be used as stationary phases by loading them separately on chromosorb W/HP 100 - 120 mesh size, as solid support with different loading ratios (1%, 3%, and 20%). The prepared stationary phases were packed through the stainless steal columns, and tested for separation of poly aromatic hydrocarbons PAHs (naphthalene, acenaphthene, acenaphthylene, phenanthrene, anthracene, and pyrene), alcohols (methanol, ethanol, 2-propanol, and 1- propanol), and positional isomers (o, m, and p-xylene). A chromatographic study of the interaction and elution characteristics of the studied analytes through the prepared columns was carried out at different column temperatures of, 140-320 ºC for 4,4´-bis-(4-propoxybenzylideneamino)biphenyl (2b) and 170-280 ºC for 2,5-bis-[4-(4´-propoxybenzylideneamino) phenyl]1,3,4-oxadiazole (1f). The best chromatographic conditions, efficiency, and selectivity of the columns for separation of PAHs, alcohols, and positional isomers were characterized by measuring the effective plate number of column (Neff.), resolution (Rs), and separation factor (?). It was found that the supercooling phenomenon has occurred with 2b column, which made a decrease in solid-nematic transition temperature. The best separation of alcohols was obtained at 180 ºC through 20% 2b column. PAHs could be separated through 3% and 1% 2b column at 240 ºC. The best chromatographic performance for separation of PAHs using 1f column was achieved at 220 ºC. This was related to the fact that the higher order of this liquid crystal occurred at these temperatures, which made the analytes interact more with the stationary phase. Specific retention volumes (Vg°) were calculated to study the thermodynamic behaviors of the analytes (solutes) on the liquid crystal stationary phases LCSPs (solution). From the plots of ln Vg° (mL.g-1) versus 1/T (K-1), the enthalpy (?H) and entropy (?S) of the solutions were measured. Gibbs free energy (?G) of the separated analytes at different temperatures was calculated. These values showed that the dissolution of the analytes on LCSPs were spontaneous. The study also included measurements of the activity coefficients at infinite dilution (??). These values were < 1, which indicates a negative deviation of the solutions from ideality.