Issue 26, 2012

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 τb is not small compared with the micellar reptation time τR, 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 μm 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.

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

Article information

Article type
Paper
Submitted
31 Jan 2012
Accepted
26 Apr 2012
First published
25 May 2012

Soft Matter, 2012,8, 6950-6957

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

S. Barhoum, R. Castillo and A. Yethiraj, Soft Matter, 2012, 8, 6950 DOI: 10.1039/C2SM25237F

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