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

Abstract

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 2,6dinitro-4-n-dodecylphenol. The indices of the so-called apparent ionization constant, app a pK , 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 . Molecular Dynamics modeling was used to locate the dye molecule and, in particular, its ionizing group OH  Owithin the micelles of the studied surfactants. The comparison of the  values estimated using 2,6-dinitro-4-ndodecylphenol with both our computer simulation and literature experimental results reveals obstacles in monitoring electrical interfacial potentials. In particular, the  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 Aequilibrium forms in the pseudophase.

Molecular dynamics study of an acid-base indicator dye in Triton X-100 non-ionic micelles

“…In this connection, it is of critical importance to know the location of these molecular probes. In our previous publications, we used the molecular dynamics (MD) simulations for sol ving this proble m. The Reichardt 's solvatochromic indicators [1,2] and some acid-base indicators [3,4] were studied in micelles of ionic surfactants and conclusions were made about the location and microenvironments of the indicator dyes. The present paper is aimed to extend the research to non-ionic micelles.…”

“…2,6-dinitro-4-ndodecylphenol (DDP) was taken as an indicator dye. This indicator was previously examined both experimentally and computationally in micelles of ionic surfactants [4]. It is a weak acid dissociating by hydroxyl group forming yellow-colored anion.…”

“…The potential model for DDP was taken from our previous works [4]. It was derived in the basis of the common well-validated OPLS-AA force field.…”

“…2). Then, a cavity was created near the surface of micelle following the protocol, described in detail elsewhere [1][2][3][4]. In the cavity, a dye molecule was placed, and the cavity was collapsed providing a configuration where dye is solubilized in micelle.…”

Protolytic Equilibria in Organized Solutions: Ionization and Tautomerism of Fluorescein Dyes and Related Indicators in Cetyltrimethylammonium Chloride Micellar Solutions at High Ionic Strength of the Bulk Phase

“…It means that the ΨF/2.303RT value is about 0.26 and the difference between pK a a and pK w a is caused mainly by the log(γ B /γ HB ) item. A much higher Ψ value obtained using 2,6-dinitro-4-n-dodecyl phenol was explained by assuming some specific interactions [50].…”