Phase behaviour of tri-n-butylmethylammonium chloride hydrates in the presence of carbon dioxide

Abstract : The phase behaviour of the system water–tri-n-butylmethylammonium chloride (TBMAC)–CO2 was investigated by pressure-controlled differential scanning calorimetry in the range 0–10 mol% TBMAC in water and at CO2 pressures ranging from 0 to 1.5 MPa. In the absence of CO2, an incongruent melting hydrate, which estimated composition corresponds to TBMAC·30H2O, crystallizes at temperatures below −13.6 °C and forms with ice a peritectic phase at approximately 3.9 mol% TBMAC. In the presence of CO2 at pressures as low as 0.5 MPa, curves evidenced the presence of an additional phase exhibiting congruent melting at temperatures that are strongly pressure dependent and significantly higher than those of hydrates obtained without CO2. This new phase, whose enthalpy of dissociation and CO2 content increase slightly with CO2 pressure, was identified as a mixed semi-clathrate hydrate of TBMAC and CO2 of general formula: (TBMAC + xCO2)·30H2O.