Formation of micellar nematic phases from surfactants and aqueous orthophosphoric acid

Abstract

It is shown that aqueous orthophosphoric acid can be used as a solvent for cationic surfactants to form micellar nematics which align spontaneously in magnetic fields. The cationic surfactants investigated were decylammonium chloride (DAC1) and decylammonium dihydrogen phosphate (DAPH₂). The percentage of pure H₃PO₄ in the nematic phases of DAC1 could be varied from 10 to 70 per cent while DAPH₂ gave a nematic phase in a narrow range with 60 per cent H₃PO₄. As the percentage of pure H₃PO₄ in the DAC1 nematic phase increased, the ²³Na quadrupolar splitting decreased; this was attributed to an alteration in the binding sites of these systems. ³¹P N.M.R. of H₃PO₄ in DAC1 and DAPH₂ nematic phases gave a symmetric singlet indicating that the chemical shift of H₃PO₄ in these systems is isotropic. A mixture of the anionic surfactants, sodium decylsulphate (SDS) with 10 per cent H₃PO₄ gave a micellar nematic phase as a result of a replacement reaction in which orthophosphoric acid monodecylester (PDE) was produced. The director of the nematic phases of the DAC1, DAPH₂ and SDS systems aligned perpendicular to the magnetic field direction (negative diamagnetic anisotropy, Δχ < 0). The ³¹P N.M.R. spectrum of PDE in the SDS nematic phase gave an asymmetric singlet which is assumed to result from the ³¹P chemical shift anisotropy. Solutions of n-decanol and 85 per cent H₃PO₄ gave viscous birefringent phases which contained an isotropic phase and a lamellar phase.