Characterization of the Surface Layer of Micelles: Advanced Investigation by Salt Incorporation

Abstract

The surface effects and surface layer of micelles of 3-(dimethyldodecylammonio)-propanesulfonate and cetyltrimethylammonium bromide, CTAB, are characterized in terms of the electrical double layer upon additions of 0.01 and 0.4 mol/L NaBr, NaN₃, sodium salicylate, and potassium benzoate. Nitrophenol violet and myristic acid were used as probes. Molecular dynamics simulation, laser Doppler electrophoresis, UV–vis spectroscopy, acid–base equilibrium approach, and chemical kinetic method were applied to investigate probe localization, zeta potential, solvatochromism, surface potential, and ion-exchange, respectively. Experimental findings include deceleration of nucleophilic addition, complete deactivation of the kinetic micellar effect, substrate desolubilization upon salt addition, micelle overcharging, deviation from the lyotropic series, and nonindifferent ions affecting cationic and zwitterionic micelles. These results are crucial when charge interactions determine the dynamics of binding, desorption, and fusion. The CTAB-based nanospecies with surface potentials close to zero are of particular interest because of their unique behavior, which resembles that of so-called “living” polymers. Desolubilization of the sample in a salt solution may be relevant when it is necessary to release the substrate from the nanocarrier into the target cells. The charge inversion in medical electrophoresis (purification techniques, crossing membrane barriers) is crucial as initially positively charged particles start migrating toward the anode instead of their natural direction. Moreover, the results offer valuable insights into designing surfactant-functionalized surfaces, including nanotubes enveloped in surfactant bilayers and related nanostructures.