Paper Number
PG8 

Session
Polyelectrolytes, Self-assembling Systems & Gels

Title
From well-entangled to partially-entangled wormlike micelles: A characterization by rheological modeling, diffusing wave spectroscopy, and small-angle neutron scattering

Presentation Date and Time
October 16, 2018 (Tuesday) 5:00

Track / Room
Track 3 / Bellaire

Authors (Click on name to view author profile)

1. Zou, Weizhong (University of Michigan)
2. Tan, Grace (University of Michigan)
3. Jiang, Hanqiu (University of Cincinnati)
4. Vogtt, Karsten (University of Cincinnati)
5. Weaver, Michael R. (Procter & Gamble)
6. Koenig, Peter (Procter & Gamble)
7. Beaucage, Gregory (University of Cincinnati)
8. Larson, Ronald G. (University of Michigan, Department of Chemical Engineering)

Author and Affiliation Lines (in printed abstract book)
Weizhong Zou¹, Grace Tan¹, Hanqiu Jiang², Karsten Vogtt², Michael R. Weaver³, Peter Koenig³, Gregory Beaucage², and Ronald G. Larson^1
1Department of Chemical Engineering, University of Michigan, Ann Arbor, MI; ²University of Cincinnati, Cincinnati, OH; ³Procter & Gamble, Mason, OH

Speaker / Presenter 
Tan, Grace

Text of Abstract
We use mechanical rheometry and diffusing wave spectroscopy (DWS) along with small angle neutron scattering (SANS), combined with a mesoscopic simulation model, the “pointer algorithm,” to obtain characteristic lengths and time constants for wormlike micelle (WLM) solutions over a range of surfactant and salt concentrations encompassing the transition from unentangled to entangled solutions. The solutions contain both sodium lauryl one ether sulfate (SLE1S) and cocoamidopropyl betaine (CAPB), and a simple salt (NaCl). The pointer algorithm is extended to allow fast relaxation of unentangled micelles; thereby micelle length, breakage rate and entanglement and persistence lengths can be extracted from linear rheological measurements of partially entangled WLM solutions. DWS provides the high frequency data needed to determine micelle persistence length. By fitting G’ and G” to the predictions of the pointer algorithm, a rapid change in micellar length from 1 to 7 μm is observed over a change in salt ion concentration from [Na+] = 0.6-0.8 M, as the solution enters the well-entangled regime. The micelle lengths from SANS at a 7.5-fold smaller surfactant concentration are 0.4 to 1 μm over a similar range of salt concentrations, roughly consistent with the values derived from rheology once the effect of the lower surfactant concentration is accounted for. The micellar persistence length from rheology (40 nm) is found to be roughly consistent with that from SANS (30 nm) at a lower surfactant concentration. This is, to our knowledge, the first time that quantitative comparisons of structural features including micelle length are made between rheology and SANS. Finally, the scaling laws for micelle diffusion and recombination times indicate that micelle kinetics are reaction controlled leading to mean-field recombination with surrounding micelles over the concentration range of interest except at very low and high surfactant concentrations where either short micelles or branched micelle clusters are dominant.