Surface charge modulation of sulfobetaine micelles by interaction with different anions: A dynamic light scattering study

This work reports a Dynamic Light Scattering study on aqueous micelles formed by tetradecyl dialkylammonium propanesulfonate surfactants (sulfobetaines; with alkyl = methyl, ethyl, n-propyl and n-butyl) within a range of surfactant concentrations (0.01-0.40 M) both in pure water and in the presence of various concentrations of NaBr, NaOH and NaClO4 (0.02-0.50 M NaBr; 0.10-1.00 M NaOH; 0.005-0.50 M NaClO4). From values of diffusion coefficients, D, we obtained micellar hydrodynamic radii, R-h, by application of the Stokes-Einstein relation. Plots ofD vs. sulfobetaine concentrations can be qualitatively explained with a model based on a linear interaction theory, which allowed to separate thermodynamic and hydrodynamic perturbations to D. Results show that: i) formally neutral sulfobetaine micelles become negatively charged by preferential interaction with strongly interacting, soft anions; ii) the surface negative charge increases with the hydrophobicity of the anions; iii) bulkier alkyl substituents on the sulfobetaine head groups lead to less charged, less hydrated aggregates, which result in opposite perturbations to D; (iv) highly hydrated, high charge density hydroxide ions lead to an increase of micellar sizes through a disc-like growth pattern. (C) 2019 Elsevier B.V. All rights reserved.