Lyotropic liquid-crystalline phase behavior of some alkyltrimethylphosphonium bromides

The lyotropic liquid crystal (LLC) behavior of a homologous series of alkyl trimethylphosphonium bromide salts is described. While alkyltrimethylammoni um salts are common LLC mesogens that have served in mesoporous materials, phase-transfer catalysis, and polymer-surfactant assemblies, the LLC behavi or of the analogous phosphonium salts has been relatively unexplored. Dodec yl- (C12TMPB), tetradecyl-(C14TMPB), and hexadecyltrimethylphosphonium brom ide (C16TMPB) were found to exhibit hexagonal, lamellar, and cubic mesophas es similar to their ammonium counterparts; however, there is a key differen ce: Changing to a phosphorus-based headgroup appears to stabilize the cubic phase of C12TMPB and C14TMPB such that the phases extend down to room temp erature. Some trends in the lattice parameters of the resulting LLC phases of the alkyltrimethylphosphonium bromides with respect to phase composition and temperature are also discussed. Finally, preliminary use of P-31 NMR s pectroscopy, as a method of structural characterization of the phases, show ed variation in the peak widths for the different phases but not the expect ed chemical shift anisotropy seen in the more commonly studied phosphate-ba sed systems; an explanation for this difference is proposed.