Smectic liquid-crystalline phases of quaternary group VA (especially phosphonium) salts with three equivalent long n-alkyl chains. How do layered assemblies form in liquid-crystalline and crystalline phases?

The neat phase behavior of 28 phosphonium salts (nPmA), each with three equ ivalent long n-alkyl chains containing m carbon atoms and one shorter chain containing n carbon atoms or a benzyl group, have been examined. Eighteen form a liquid-crystalline smectic Az (SmA(2)) phase, where the Lipophilic c hains assemble in bilayers that sandwich an "ionic plane" consisting of an array of anions and positively charged phosphorus atoms. Several analogous ammonium salts are examined as well. These are the first structures reporte d for liquid-crystalline amphiphiles containing three equivalent long n-alk yl chains and a single atom headgroup. In addition, the nPmA demonstrate ho w the steric interactions of lipophilic chains and electrostatic interactio ns among the ionic parts must be balanced in layered assemblies when the cr oss-sectional area of the chains is much larger than that of the headgroups . A model is proposed to explain the dependence of layer thicknesses in the liquid-crystalline phases of the nPmA on m, n, and the size of the anion. A comparison is made of the packing modes among analogous Group VA salts (N .B., N or P) with 1-4 long n-allcyl chains (and the remainder as methyl gro ups) in their neat crystalline and liquid-crystalline phases.