Phase equilibrium measurements of long chain acids in supercritical carbon dioxide

Data on the phase behaviour of long chain fatty acids (octanoic, decanoic, undecanoic, dodecanoic, tetradecanoic, hexadecanoic and octadecanoic) in supercritical carbon dioxide is presented at temperatures between 308 and 358 K and pressures up to 27 MPa. No three-phase regions were observed and at constant composition, an increase in temperature leads to an increase in phase transition pressure. An increase in hydrocarbon backbone length also leads to an increase in phase transition pressure. Comparison of the measured data with literature data of n-alkanes, 1-alcohols, methyl esters and ethyl esters of the same hydrocarbon backbone length shows that carbon dioxide is able to easily distinguish between acids and n-alkanes, methyl esters or ethyl ester and, with selection of the correct conditions, carbon dioxide is also able to distinguish between acids and 1-alcohols. However, unlike for propane, the phase behaviour of an acid in carbon dioxide does not mimic that of an alkane with double the number of carbon atoms, most probably due to the effect of the quadrupole moment of carbon dioxide.