Abstract
Gas-liquid, gas-solid, and gas-liquid-solid equilibrium isotherms were determined for acetone and butane on graphitized carbon black (Carbopack C) and liquid-modified graphitized carbon black (Carbopack C plus 0.2% Carbowax 1500). The measurements were carried out over the range of temperatures from 30 to 75°C by mass spectrometric tracer pulse chromatography with stable isotopic solutes. Studies of the gas-liquid systems show that the bulk liquid undergoes a phase transition (wax → liquid) at 40°C and that the equilibrium isotherms vary significantly with temperature for the wax form. The temperature of the phase transition is shifted to 50-55°C for the same liquid coated on Carbopack C as a thin film. The adsorption isotherms of acetone on the coated and uncoated Carbopacks were interpreted by means of a model of simultaneous competitive and cooperative adsorption effects. The Carbowax-modified adsorbent generally adsorbed as much, if not more, acetone at any given pressure than the uncoated adsorbent. This effect was attributed to specific lateral interactions between the solute and liquid or wax modifier. The magnitude of the enhanced capacity was much greater than could be accounted for by bulk solubility of acetone in the Carbowax. The effect of preadsorbed acetone on the adsorption and retention of other solutes was also investigated. The specific retention volumes of small samples of butane were measured as a function of the surface coverage by acetone or acetone plus Carbowax 1500. Small amounts of preadsorbed acetone had little or no effect on the retention volume of butane, and the retention volumes were greater on the uncoated adsorbent. However, at surface coverages close to a monolayer, the retention volume of butane decreased dramatically with increased acetone adsorption. Also, at higher surface coverages, the retention volumes were the same for the coated and uncoated adsorbent at any fixed amount of acetone adsorbed. These “interference plots” were also explained in terms of the model previously discussed.
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