The neat phase behavior of 28 phosphonium salts (nPmA), each with three equivalent 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 A2 (SmA2) phase, where the lipophilic chains 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 reported for liquid-crystalline amphiphiles containing three equivalent long n-alkyl chains and a single atom headgroup. In addition, the nPmA demonstrate how the steric interactions of lipophilic chains and electrostatic interactions among the ionic parts must be balanced in layered assemblies when the cross-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-alkyl chains (and the remainder as methyl groups) in their neat crystalline and liquid-crystalline phases.
Journal of the American Chemical Society 122(13), pp.3053-3062