SOLUBILITY OF CHOLESTEROL IN SUPERCRITICAL ETHANE AND BINARY GAS-MIXTURES CONTAINING ETHANE

The solubility of cholesterol in supercritical ethane at 313.1, 323.1, and 333.1 K and 70-190 bar is presented as well as the solubility in ethane with 3.5 and 14 mol % propane and ethane with 3.5, 14, 50, and 96.5 mol % carbon dioxide at temperatures from 308.1 to 338.1 K and 85 -220 bar. The results are compared with previous measurements in pure carbon dioxide and are discussed in terms of the effect of mixture den sity and properties of the solute. Cholesterol appears to behave as a predominantly hydrocarbon molecule despite the presence of an OH group and a dipole moment of 1.9 D. The cosolvent effects observed with pro pane and with CO2 can be explained by the density behavior of the solv ent mixtures. The data are correlated with the Peng-Robinson and Patel -Teja equations of state. It is shown that estimated critical properti es for cholesterol are unsatisfactory for phase equilibrium calculatio ns, and a modified Peng-Robinson equation is introduced in which the s olute parameters a2 and b2 are made adjustable. The resulting equation of state has three adjustable parameters for an entire binary solid-s upercritical fluid data set instead of one parameter per isotherm. Wit h the modified Peng-Robinson equation, the solubility of cholesterol i n the ethane-CO2 mixtures was predicted with an average absolute relat ive deviation of 21 % using only parameters from the binary systems.