In search of an optimal acid-base indicator for examining surfactant micelles: Spectrophotometric studies and molecular dynamics simulations

We report on combined experimental and theoretical investigations of the water/micelle interface of cationic, anionic, zwitterionic, and non-ionic surfactants using a new hydrophobic acid-base indicator 2,6-dinitro-4-n-dodecylphenol. The indices of the so-called apparent ionization constant, pK_a ^app, of the indicator fixed in the micellar pseudophase are determined by the spectrophotometric method. The data allows estimating the Stern layer's electrostatic potential of the ionic micelles Ψ. Molecular Dynamics modeling was used to locate the dye molecule and, in particular, its ionizing group OH→O^– within the micelles of the studied surfactants. The comparison of the Ψ values estimated using 2,6-dinitro-4-n-dodecylphenol with both our computer simulation and literature experimental results reveals obstacles in monitoring electrical interfacial potentials. In particular, the Ψ values of the surfactant micelles with alkylammonium groups determined via 2,6-dinitro-4-n-dodecylphenol are overestimated. The reason is specific interactions of the indicator anion with the surfactant head groups. For anionic surfactants, however, this indicator is quite suitable, which is confirmed by the location of HA and A^– equilibrium forms in the pseudophase.