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

The phase behaviour of the system water-tri-n-butylmethylammonium chloride (TBMAC)-CO ₂ was investigated by pressure-controlled differential scanning calorimetry in the range 0-10 mol% TBMAC in water and at CO ₂ pressures ranging from 0 to 1.5 MPa. In the absence of CO ₂, an incongruent melting hydrate, which estimated composition corresponds to TBMAC•30H ₂O, crystallizes at temperatures below -13.6 °C and forms with ice a peritectic phase at approximately 3.9 mol% TBMAC. In the presence of CO ₂ 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 CO ₂. This new phase, whose enthalpy of dissociation and CO ₂ content increase slightly with CO ₂ pressure, was identified as a mixed semi-clathrate hydrate of TBMAC and CO ₂ of general formula: (TBMAC + xCO ₂)•30H ₂O.