Dissociation behaviour of (tetra-n-butylammonium bromide plus tetra-n-butylammonium chloride) mixed semiclathrate hydrate systems

The thermal properties of {tetra-n-butylammonium bromide + tetra-n-butylammonium chloride (TBAB + TBAC)} mixed semiclathrate hydrates prepared from aqueous solutions were investigated by dissociation temperature measurements and differential scanning calorimetry (DSC). The maximum dissociation temperature of the mixed hydrate crystals at 0.1 MPa is 288.5 K for chi(TBAB) = 0.2 {mole fraction of TBAB to (TBAB + TBAC)}, which is higher than that of the pure hydrates {T = (285.5 and 288.2) K for TBAB and TBAC hydrates, respectively}. In addition, the dissociation enthalpies of the mixed hydrates are higher than those of the pure hydrates {(5.55 +/- 0.06) kJ . mol (1) H2O for pure TBAB hydrate and (5.30 +/- 0.05) kJ . mol (1) H2O for pure TBAC hydrate}, with a maximum of (5.95 +/- 0.12) kJ (.) mol (1) H2O recorded at approximately chi(TBAB) = 0.4. It was therefore suggested that the crystal distortion in (TBAB + TBAC) mixed hydrates, caused by replacing water molecules by both bromide and chloride anions, was smaller than that observed for each pure hydrate. Consequently, the hydration numbers in the mixed hydrates were hypothesized to be slightly higher than those of the pure hydrates. (C) 2015 Elsevier Ltd. All rights reserved.