The crystalline and liquid crystalline structures of benzyl-tri-octadecylammonium bromide complete the puzzle: how do Group VA halide salts with one-four long n-alkyl chains pack?

The first single crystal structure of a Group VA halide salt with three equ ivalent long n-alkyl chains, benzyltrioctadecylammonium bromide (BzN18Br), is reported. It consists of alternating interdigitated and non-interdigitat ed regions of alkyl chains separated by ionic planes. Two chains per molecu le are paired and extend to one side in a non-interdigitated region. The th ird chain is on the opposite side of the ionic plane and pairs intermolecul arly to form an adjacent, interdigitated region. The thickness of two nearl y extended molecules defines the bilayer unit-two ionic planes Banked by a region with intramolecularly paired chains and separated by an interdigitat ed chain region. Powder X-ray diffraction and optical microscopy data of li quid crystalline BzN18Br are consistent with an enantiotropic smectic A(2) (SmA(2)) phase: the three n-alkyl chains of each molecule are projected fro m one side of an ionic plane, and head groups of neighbouring molecules are oriented head-to-head, in a non-interdigitated bilayer assembly. The struc ture of BzN18Br fills an important gap in our knowledge about the crystal p acking of ammonium and phosphonium salts with one-four equivalent long n-al kyl chains. A comparison of their packing arrangements is made and the tran sitional nature of the BzN18Br structures is demonstrated. Although salts w ith one, two, or three long n-alkyl chains form SmA(2) phases, each is dist inctive in its molecular packing. A large molecular reorganization accompan ies the crystal-to-liquid crystal transition of BzN18Br.