The influence of the micellar pseudophase of the double-chained cationic surfactant di-n-tetradecyldimethylammonium bromide on the absorption spectra and protolytic equilibrium of indicator dyes

Protolytic equilibria of ACE inhibitors in micellar solution of nonionic surfactant Brij 35

The acid-base equilibria of six ACE inhibitors (ACEIs), captopril, cilazapril, enalapril, lisinopril, quinapril, and ramipril, were investigated in the presence of micelles of nonionic surfactant Brij 35. The pKa values were potentiometrically determined at 25 °C and at a constant ionic strength (0.1 M NaCl). The obtained potentiometric data were evaluated in the computer program Hyperquad. On the basis of the shift in the pKa values (ΔpKa) determined in micellar media in relation to the pKa values previously determined in "pure" water, the effect of Brij 35 micelles on ACEIs ionization was estimated. The presence of nonionic Brij 35 micelles caused a shift in the pKa values of all ionizable groups of the investigated ACEIs (ΔpKa from - 3.44 to + 1.9) while shifting the protolytic equilibria of both acidic and basic groups toward the molecular form. The Brij 35 micelles expressed the most pronounced effect on the ionization of captopril among the investigated ACEIs and stronger effect on the ionization of amino than on the ionization of carboxyl groups. The obtained results suggest that ionizable functional groups of ACEIs are involved in interactions with palisade layer of nonionic Brij 35 micelles, which potentially can be considered in physiological conditions. Distribution diagrams of the investigated ACEIs equilibrium forms as a function of pH indicate that the change in distribution is most strongly expressed in pH range 4-8, which includes biopharmaceutically important pH values.

Graphical abstract

Micelle and Surface Tension of Double-Chain Cationic Surfactants

Since the early 20th century, the slightly disparate measurements of a surfactant's critical micelle concentration, via either surface tension or electrical conductivity, have been assumed one and the same. As a consequence, the possibility that micelles can adsorb at the air/water surface has been disregarded and has led to some abnormalities in the literature that remain as yet unresolved. In this paper, we closely examined the two critical concentrations for a double-chain cationic surfactant. We confirmed that the two concentrations represent two different physical phenomena. Furthermore, the results verified the existence of surface micelles, which are different from the bulk micelles. The formation of the surface micelles can be explained by the structural changes of the adsorption layer, which was also corroborated by molecular simulations. The findings open new challenges to examine the surface adsorption, which  offers new insights into the molecular levels.

Character of Localization and Microenvironment of Solvatochromic Reichardt's Betaine Dye in Sodium n-Dodecyl Sulfate and Cetyltrimethylammonium Bromide Micelles: Molecular Dynamics Simulation Study

The problem of using surfactant micellar aqueous solutions as reaction media centers on estimating the polarity of the micellar pseudophase. The most popular approach is the utilization of solvatochromic dyes. Among the last, the strongest ones are the dipolar pyridinium N-phenolate dyes. The complication of such approach, however, consists in the nonuniform character of the environment of the indicator fixed in the micellar pseudophase. The aim of this study is to reveal the character of localization and orientation of the standard solvatochromic pyridinium N-phenolate dye, 4-(2,4,6-triphenylpyridinium-1-yl)-2,6-diphenylphenolate, the so-called Reichardt's dye, within the micellar pseudophase of an anionic (sodium n-dodecyl sulfate, SDS) and cationic (cetyltrimethylammonium bromide, CTAB) surfactants using MD simulations. The locus and hydration of the dye are found to be dependent on the surfactant nature. New approaches are proposed to quantitatively describe the state of the dye within the pseudophase. The results confirm the experimental data, which indicate the higher polarity of the interfacial region in the case of the SDS micelles. Because this dye is also used as an interfacial acid-base probe, the corresponding study is simultaneously performed for its protonated, i.e., cationic form. The neutral and protonated forms of the dye are found to be localized and hydrated in a different way in both SDS and CTAB micelles. This should be taken into account when using the Reichardt's dye as an acid-base indicator for estimating the electrical surface potential of micelles. The presented approach may be recommended to shed light upon the locus of other solvatochromic and acid-base indicators in micelles and micellar-like aggregates.