Cubic mesophase in an unsymmetrical alkyl ammonium salt. Synthesis and structural model.

N,N,N-butylethylpentylpropylammonium iodide 4 and related molecules have been selectively synthesised from commercially available aldehydes, amines and alkyl iodides using a reductive alkylation procedure. The crystalline texture of 4 obtained on cooling is optically isotropic between crossed polarisers, indicating a cubic structure. Differential scanning calorimetry (DSC, +10 K min-1) reveals a glass phase transition at -59 degrees C and a melting point at 192 degrees C. The melting entropy (23.9 J mol-1 K-1) indicates a first-order transition between a highly disordered mesophase and the isotropic liquid. Powder X-ray diffraction patterns were indexed in the cubic system (a = 14.08A; Pm3n space group). In this cell, the molecular packing with Z = 6 corresponds to a rather low compactness of 65%. Iodine and tetraalkylammonium ions occupy positions with a 4m2 site symmetry. These highly symmetrical states may be generated by stepwise rotation of the ammonium cation. The same structural model for orientationally disordered crystal (ODIC) phases can be applied to a series of tetraalkylammonium bromides and iodides.