Solvent-free synthesis, crystal structure and thermal stability of ionic liquid l-hexadecyl-3-methylimidazolium bromide

Ionic liquid l-hexadecyl-3-methylimidazolium bromide [C₁₆mim]Br was synthesized by solvent-free alkylation of N-methylimidzole with hexadecyl bromide. A large transparent single crystal of l-hexadecyl-3-methylimidazolium bromide monohydrate ([C₁₆mim]BrH₂O), 4 mm in length, was firstly obtained in the water-trichloromethane-toluene growth system (V wateri: Vtrichiomethane:Vtoluene = 0.1:1:2). The crystal structure was determined by single-crystal X-ray diffraction method. It crystallizes in the triclinic system, space group P1̄, with a = 5.4962(15), b = 7.839(2), c = 27.279(8) Å, a = 94.375, β = 91.500, y = 101.999°, Z = 2, V= 1145.2(5) ų, C₂₀H₄₁BrN₂0, M_r = 405.46, D_c = 1.176 Mg/m³, μ = 1.804 mm^-1, F(000) = 436, the final R = 0.0523 and wR = 0.1345. The 3D supramolecular structure is constructed through weak interactions between imidazolium cations, Br" anions and lattice water molecules. The long alkyl chain to the imidazolium ring and lattice water molecules play an important role in the self-assembly process. Moreover, it is proposed that [C₁₆mim]Br in water has aggregation behavior and the possible self-assembled structure is the interdigitated pattern. Finally, thermal stability of [C₁₆mim][Br] H₂O was also studied by DSC and TGA analysis.