Characterization of dynamics and internal structure of a mixed-surfactant wormlike micellar system using NMR and rheometry

Abstract

We use complementary experiments-proton NMR diffusometry and relaxometry, deuterium NMR lineshapes, and rheometry-to construct a comprehensive picture of the microscopic structure of a mixed-surfactant wormlike micellar system composed of a zwitterionic surfactant and an anionic surfactant in brine. In this system, at some surfactant concentrations, the time for micellar breaking and recombination s b is not small compared with the micellar reptation time s R , weakening the condition to obtain a stress relaxation function with just one relaxation time at long times. From NMR relaxometry, we determine the overlap concentration. Deuterium NMR spectral lineshapes indicate the presence of a wide angular distribution in the orientational order. NMR diffusometry and rheology probe different timescales and yield complementary information indicating polymer-like behaviour at the corresponding lengthscales. Via NMR, surfactant diffusion coefficients are seen to decrease with increasing diffusion time, consistent with restricted diffusion within a reptating micelle. At the same time, comparison of measurements with protonated and deuterated surfactants strongly suggests that the measured short and long time diffusion coefficients correspond to intra-micellar and micellar diffusion, respectively. Fitting the diffusion results to a simple model, the average end-to-end micellar distance is estimated to be in the 1 mm range and only weakly dependent on concentration. The water diffusion measurements, on the other hand, imply a high degree of water structuring at the micellar surface. We also find that the wormlike micelles obeyed simple polymer-like scaling behaviors, with a crossover from Zimm-like (diffusion) to Rouse-like (rheology) exponents.

Thermodynamics, size, and dynamics of zwitterionic dodecylphosphocholine and anionic sodium dodecyl sulfate mixed micelles

“…3S, Supplementary Material). This results from free bulk diffusion (unrestricted system) with the diffusion coefficient independent of the diffusion time [45,46].…”

The use of solvent relaxation NMR to study colloidal suspensions

“…The majority of NMR measurements on surfactants in solution concentrate on the diffusion or relaxation of protons within the surfactant molecules themselves, allowing for more direct information on phase changes. [66][67][68][69] Söderman et al highlighted the effect of the size of a spherical micelle upon the relaxation rate. The R 2 can be linked to the rotational tumbling of the micelle, with a correlation time of s r and the diffusion of the surfactant over the curved micellar surface, s d .…”

Paramagnetic Relaxation Enhancement Experiments: A Valuable Tool for the Characterization of Micellar Nanodevices

“…When trying to appreciate the full potential of this approach, information on the localization and orientation of molecular receptors within the micelle is of paramount importance. Nuclear magnetic resonance spectroscopy (NMR), through the monitoring of changes in the chemical shifts and relaxation times of surfactant protons or via intermolecular surfactant–receptor NOEs, has proved to be efficient in this context yielding information on the loci of incorporated molecules in micelles. − …”