Studies of Adsorption Characteristics in Supercritical Fluids by FTIR and Spectroscopic Measurements

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The present work has been intended to investigate the adsorption characteristics in supercritical fluids by using two different methods, Infrared spectroscopy and Chromatography. In spectroscopic measurements, IR spectra were obtained for adsorption systems composed of an organic solvent diluted in supercritical CO_2 on a large-pore Silica Gel (SG800) from atmospheric pressure to 15 MPa at 313.2 K.The organic solvents we measured are methanol, acetone, triethylamine, acetonitrile, tetrahydrofuran, and diethyl ether. A most important result to be noted is that all the intensities of hydrogen-bonded OH bands of sylanol, except for a system of triethylamine, showed a maximum at about 20-50 MPa and then they dcreased with increasing pressure. The characteristic curves of the IR observations are almost the same as the adsorption isotherms calculated by Monte Carlo simulations for a system of benzene diluted in supercritical CO_2 in slitpores of graphitic walls. The molecular simulations hav … More e already suggested two factors for adsorption characteristics in supercritical fluids : that is, an increase in solubility into a supercritical fluid and an increase in competitive adsorption of the solvent (CO_2) molecules with increasing pressure.A supercritical fluid chromatograph, newly constructed in the present work by use of commercial pumps, has been used for measuring the adsorption isotherms of acetone and benzene on SG800 in supercritical CO_2. The elution curves obtained showed a large tailing forming a common envelope for different amount of injections, which indicated that (a) the ECP (Elution by Characteristic Point) method was applicable for obtaining the adsorption isotherms of solutes and (b) the SG800 has energetically heterogeneous surfaces that should be taken into consideration for a theoretical analysis.The results obtained in the present work may be summarized as follows. (1) The isotherm curves of acetone obtained from the chromatographic measurements qualitatively coincide with the isotherms obtained from the IR spectroscopy. (2) The adsorption isotherms of the chromatographic measurements decrease more in the high-pressure region, which may be attributable to the less contribution of the van der Waals force on adsorption in the region of high competitive-adsorption. (3) The adsorption isotherms of benzene in CO_2 resemble those of acetone though benzene molecules do not show hydrogen-bonding with OH groups on silica gel while acetone molecules show an IR spectroscopic evidence of hydrogen-bonding. Less