Formation of pyrrolidinium fatty acid soap and its lyotropic liquid crystalline phase behavior

Abstract

The hydrogen bond induced acid–soap like complex has been prepared from a nitrogen-containing heterocyclic compound, pyrrolidine, and the myristic acid (MA). The complex composition and the intermolecular actions between two component molecules have been characterized with techniques of nuclear magnetic resonance (NMR), X-ray diffraction (XRD) and Fourier transformation infra-red (FTIR) spectroscopy. The molar ratio between the soap (pyrrolidium myristate, PM) and the acid (MA) is identified as 1:1. As an important driving force of assembly, the existence of hydrogen bond is further confirmed by a molecular mechanics calculation. It is interesting that such an acid–soap complex (PM–MA) exhibits amphiphilicity and its lyotropic liquid crystal phase in water has been observed over a wide concentration range. A lamellar phase structure is established using the polarized optical microscopy (POM) and small-angle X-ray scattering (SAXS) measurements. The change of lattice spacing indicates a typical one-dimensional swelling scheme for the uptake of water by this lamellar phase. The frequency-independent storage and loss module could be found for these lamellar samples by rheological measurements, reflecting a highly viscoelastic nature. The obtained results should add new insights to better understand the structure and aqueous behavior of long-chain fatty acid–soaps with a large size nitrogen-heterocyclic counterion.