Measurement of the phase behaviour of the binary systems {carbon dioxide (CO2) + non-ionic surfactants (CiEOj)}

Solubility experiments of ethoxylates surfactants denoted as CiEOj (where Ci = hydrocarbon tail, EOj = oxyethylene groups, i = 6 to 8 and j = 3 to 5) in sub- and supercritical carbon dioxide were carried out at different temperatures, pressures, and concentrations in a Cailletet apparatus as a representative model for dry cleaning system. For a variety of compositions, results are reported for binary systems within temperature and pressure ranges of (260 to 310) K and (2.0 to 10) MPa respectively. In each experiment, the surfactants reach equilibrium with carbon dioxide at different concentrations. The data show that with all the surfactants upon increasing concentration, the liquid–liquid (L–L) curve shifts to lower temperatures. Therefore, the one-phase solution gap is reduced in pressure and temperature at higher concentrations. When the length of the hydrocarbon tail remains constant and the ethoxylated chain is increased from three to five groups, the L–L curve once again shifts to lower temperatures and the two phases start earlier. This effect can be attributed to the higher surfactant’s polarity due to the increase in j. On the other hand, when the number of ethoxylated groups j remains constant and the length of the hydrocarbon tail is increased from six to eight carbon atoms, no significant shift in the L–L curve is observed. That signals the fact that the appearance of two phases is directly related to the number of ethoxylated group which determine the polarity of the molecule.

► We established the solubility of CiEOj surfactants in carbon dioxide.
► The Cailletet apparatus enables the construction of detailed phase diagrams.
► Changing the length of the hydrocarbon tail of the surfactant has no significant effect on the phase diagram.
► A change in the number of ethylene oxide groups of the surfactant shifts the phase transition to lower temperatures.
► Liophobic/liophilic balance limits surfactant miscibility.