Dissimilar effects of solubilized p-toluidine on the shape of micelles of differently charged surfactants

In the present study, pH controlled solubilization of p-toluidine (PTD) in micelles of surfactants having dissimilar head group charge is examined. A connection is revealed between viscosity enhancement and locus of solubilized PTD on the micelles of different surfactants viz. dodecyl trimethylammonium bromide (DTAB, cationic), sodium dodecyl sulfate (SDS, anionic) and dodecyl-N,N-dimethyl-3-amino-1-propane sulfonate (DDMPS, zwitterionic), dodecyl-dimethyl amine oxide (DMDAO, cationic–nonionic) and p-tert-octylphenoxy polyethylene (9.5) ether, Triton X-100 (nonionic) was examined by viscosity, dynamic light scattering and 2D NOESY studies. PTD works as cationic hydrotrope in acidic pH and remains neutral in basic pH. DTAB and DDMPS did not show any significant growth with solubilized p-toluidine, while pH dependant micellar growth/transitions were observed for SDS, DMDAO and TX-100. Dissimilar effects were observed for different surfactants; DMDAO micelles showed maximum growth at pH ∼5.0–6.0 with almost no effect in more acidic pH, SDS micelles grew only in acidic conditions, while Triton X-100 micelles grew more in basic media. The effect of temperature on DMDAO and Triton X-100 micelles in the presence of PTD was also examined. The location of PTD in micelle was obtained from two-dimensional nuclear Overhauser enhancement spectroscopy (2D-NOESY).