Effect of the ethoxy groups distribution on the phase behaviour of the binary systems carbon dioxide CO2 with industrial non-ionic surfactants (CiEOj)

Solubility experiments of industrial ethoxylated surfactants denoted as CiEOj (where Ci = hydrocarbon tail, EOj = oxyethylene groups, i = 6–8 and j = 3–5) 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 the study, industrial surfactants with an ethoxy group distribution were used to replicate the actual dry-cleaning process. For a variety of compositions, results are reported for binary systems within temperature and pressure ranges of 260–310 K and 2.0–10 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 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 LL 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 group’s j remains constant and the length of the hydrocarbon tail is increased from six to eight carbon atoms, there is also a significant shift in the LL curve. That signals the fact that the appearance of two phases is also related with the length of the hydrocarbon chain and therefore the surfactant structure itself. These results differ from the trend presented for the pure surfactants in CO2 and can be attributed to the ethoxy group distribution of the industrial type surfactants.

► We established the solubility of industrial CiEOi surfactants in carbon dioxide.
► The Cailletet apparatus enables the construction of phase behaviour diagrams.
► Increasing the number of ethoxy groups shifts the LL curve to lower temperatures.
► Increasing the length of the hydrocarbon tail shifts the LL curve to lower temperatures.
► Ethoxy group distribution of industrial surfactants has a significant effect on the solubility.