Diffusion-Ordered NMR Spectroscopy to Characterize a Continuum of Brij-35 Self-Assembly

Brij-35 is a nonionic surfactant with diverse uses in protein solubilization, biochemical assays, cosmetics, drug formulations, and advanced material applications that require self-assembly. The critical micelle concentration (cmc) of Brij-35 has reported values ranging from 0.046 to 0.21 mg/mL depending on the method used. These values span nearly an order of magnitude and suggest that Brij-35 has more complex self-assembly behavior than may have been investigated. Diffusion-ordered NMR spectroscopy was used to measure the diffusion coefficient (D) of Brij-35 over nearly 5 orders of magnitude of concentration, and we believe our results indicate five distinct stages in Brij-35 assembly that can explain the disparate cmc's measured by other methods. At 0.01 mg/mL Brij-35 in D₂O, distinct diffusion coefficients are observed for the dodecyl and PEG chains, which suggests that the regions are moving differently in a dilute regime. At 0.02 mg/mL, the diffusion coefficients of the two regions overlap, suggesting partial aggregation of the molecules that then dominates diffusion. Micelle formation at a cmc is indicated at 0.11 mg/mL by the first significant decrease in diffusion coefficient. A diffusion coefficient plateau between 5 and 20 mg/mL and a subsequent decrease could indicate a cmc-(II). Gelation was first observed at a concentration of 273 mg/mL, which is just above the predicted overlap concentration of the approximately 1000 g/mol polyethylene glycol in Brij-35. We also believe that our results highlight the sensitivities of other measurement techniques to different stages in our proposed assembly scheme.