Solubilities of novel ethylene oxide diphosphate-based chelating agents in supercritical carbon dioxide

• Highly CO2-soluble ethylene oxide diphosphates were designed and synthesized.
• Solubility data for these compounds were determined and correlated by Bartle model.
• Structure effects on solubility were discussed through COSMO-RS method.
• Branched side chain played an important role in the enhancement of solubility.

The solubilities of newly synthesized chelating agents, i.e. diethylene glycol bis (2,4,4-trimethyl-1-pentyl) dimethyl diphosphate (EG2TP), diethylene glycol bis (3,5,5-trimethyl-1-hexyl) dimethyl diphosphate (EG2TH), and diethylene glycol dioctyl dimethyl diphosphate (EG2Oct) in supercritical carbon dioxide (scCO2) were determined at temperatures ranging from 313 to 333 K and pressures ranging from 9 to 17 MPa. Solubility increases in the order of EG2Oct (MW = 518.56) < EG2TH (MW = 546.61) < EG2TP (MW = 518.56). Solubility parameter and connectivity index of these compounds were calculated and discussed, which indicated that branched side chains of the ligands play an important part in increasing solubility in scCO2. A semi empirical model proposed by Bartle was used to correlate the experimental data and AARD values were calculated to be 5.2%, 4.4%, and 3.4% for EG2TP, EG2TH, and EG2Oct, respectively. Also the quantum chemical calculation and COSMO-RS method were introduced to estimate the structure effect on the interaction with CO2 by computing the charge density distribution on the surface of these compounds. As expected, better masking effect of the side chains result in higher solubility.