NMR studies of the viscoelastic surfactant solutions formed by cetyltrimethylphosphonium bromide

The use of cetyltrimethylphosphonium bromide (CTPB) as an alternative to the popular cetyltrimethylammonium bromide (CTAB) was explored. The preparation and characterization of the isotropic-micellar and viscoelastic phases of CTPB with sodium p-toluate (NaTol) were examined with a variety of NMR techniques, as well as electrical conductivity, dynamic light scattering, and viscosity. It was determined that CTPB forms systems nearly identical with CTAB. A model for the binding and geometry of the CTPTol ion pair in the micelle was proposed from chemical shift and dipolar relaxation measurements. The relaxation mechanism of 31P was found to be a combination of dipole-dipole and chemical shift anisotropy relaxation. The timescales of rotational motion extracted from the frequency dependent-relaxation data of 31P and 13C were interpreted in terms of the surfactant motion and the micellar structure. The procedure for calibrating, acquiring, and processing diffusion experiments on a Bruker DRX500 is outlined. Recently, a multiple quantum diffusion sequence known as the BPP-MAXY-D was introduced, but has received little attention in the literature. In order to determine the practical utility of this pulse sequence for measuring the small diffusion coefficients of large micelles, a comparison was made with the BPP-LED pulse sequence. While the BPP-MAXY-D experiment is capable of accurately measuring diffusion coefficients, does allow selectivity in overlapped spectra, and eddy current effects are reduced, it was unable to extend the detection limit of the diffusion coefficient. The short T2 associated with slowly rotating particles limits the accessible diffusion delays available to the BPP-MAXY-D experiment. The anticipated loss of sensitivity and consequent increase in experiment time for 1H-13C multiple-quantum experiments was observed