HIGH-PRESSURE PHASE DISTRIBUTION ISOTHERMS FOR SUPERCRITICAL FLUID CHROMATOGRAPHIC SYSTEMS .2. BINARY ISOTHERMS OF CARBON-DIOXIDE AND METHANOL

The binary isotherms of mixtures of carbon dioxide and 2% methanol on silica and octadecyl-bonded silica (ODS) were measured at three temperatures over a range of pressures up to 150 bar. Binary isotherms of pure CO2 and mixtures of 2-4% methanol in CO2 on the same adsorbents were determined at 50-degrees-C. The effects of temperature, pressure, and composition on the adsorption of both methanol and CO2 were determined over a range of conditions with supercritical fluid mobile phases. The amounts of CO2 and methanol adsorbed increased with decreasing pressure and increased dramatically at pressures close to the critical pressure of the binary mobile phase. Under the conditions normally employed for packed-column SFC, significant amounts of CO2 and modifier both were adsorbed on the solid adsorbent and functioned as components of the stationary phase. Capacity factors were also measured for two probe solutes, n-hexane and benzene, concurrently with the adsorption measurements. The capacity factors decreased with increasing density or increasing temperature but increased with concentration of methanol in the mobile phase. The partial molar enthalpies for the phase-transfer process at constant density with 2% methanol in the fluid phase were in the range 4.7-5.7 kcal/mol, indicating a partition-like retention mechanism for the nonpolar solutes, rather than an adsorption mechanism.