In search of an optimal acid-base indicator for examining surfactant micelles: Spectrophotometric studies and molecular dynamics simulations (original) (raw)
Related papers
2013
In this paper, the different methods of estimation of the electrostatic potential, , of surfactant micelles via acid-base indicators are compared. All the methods are based on the determination of the indices of the so-called "apparent" ionization constants, app a pK. The approach developed in this Laboratory and based on using the indicator N,N /-din -octadecylrhodamine is utilized for determination of the value in the Stern layer of cetyltrimethylammonium-based micelles in the presence of tosylate ion.
Journal of Physical Organic Chemistry, 2007
The peculiarities of the structure of the fluorescent dye N,N'-di-n-octadecylrhodamine advantage its using as an interfacial acid-base probe in aqueous micellar solution of colloidal surfactants. Two long hydrocarbon tails of the dye provide similar orientation of both cation and zwitterion on the micelle/water interface, with the ionizing group COOH exposed to the Stern region in all the systems studied. Further, the charge type of the acid-base couple, A þ B AE , ensures similar values of the 'intrinsic' contribution, pK i a , to the 'apparent' pK a a value in micelles of different surfactants. This makes the indicator suitable for determination of electrical surface potentials, C. The pK a a s have been obtained in cationic, anionic, zwitterionic, and nonionic surfactant systems, at various salt background. In total 17 systems were studied. At bulk counterion concentration of ca. 0.05 M, the pK a a values vary from 2.14 AE 0.07 in nÀC 18 H 37 NðCH 3 Þ þ 3 Cl À micelles to 5.48 AE 0.06 in nÀC 16 H 33 OSO À 3 Na þ micelles. The C values, corresponding to the Stern region of micelles, have been evaluated as C¼ 59:16 ðpK i a À pK a a Þ for T ¼ 298.15 K. The pK i a parameter was equated to the average value of 4.23 in nonionic surfactants (4.12-4.32, depending on the surfactant type). For cetyltrimethylammonium bromide and sodium n-dodecylsulfate micelles, the C values (AE(7-11) mV) appeared to be þ118 mVand at bulk Br À concentration 0.019 M and À76 mVat bulk Na þ concentration 0.020 M, respectively. This satisfactorily agrees with the theoretical values þ111 and À84 mV, estimated using the Oshima, Healy, and White equation for these well-defined colloidal systems. Finally, not only absorption, but also fluorescence spectra display the same response to changes in bulk pH.
Journal of Molecular Liquids, 2018
Examining micellar pseudophases and related systems by using acid-base indicators is presently a very popular method. Normally, the acid-base couple of an appropriate indicator was expected to be located in a definite place, i.e., the microenvironment of both acidic and basic dye species is equal. This assumption allowed modeling the solvation properties of the pseudophase by water-organic mixed solvents or by some other media. This vision of the indicator locus should be considered using some independent approaches. In the present paper, the molecular dynamics modeling was used for such purpose. As indicator, an azo dye was used, which was first utilized for estimating electrical potential of micelles by Hartley and Roe in their pioneering study (1940). The modeling was processed in micelles of the same surfactants which were used by Hartley and Roe, namely cetylpyridinium bromide and cetyl sulfonate salt. The results support the idea of indicator location within the interfacial region (Stern layer). However, the orientation of the dye molecule and the hydration of the ionizing group, OH O-, are quite different for the acidic (HA) and basic (A-) forms either in cationic or anionic micelles. This is a precaution against the simple concept of uniform location of the equilibrium forms of indicators. Hence, further molecular dynamics study of other indicators, often used nowadays for examining micelles and other molecular assemblies, seems to be pertinent and timely.
Specific interactions within micelle microenvironment in different charged dye/surfactant systems
Arabian Journal of Chemistry, 2015
The interactions of two ionic dyes, Crystal Violet and Methyl Orange, with different charged surfactants and also with a nonionic surfactant were investigated using surface tension measurements and visible spectroscopy in pre-micellar and post-micellar regions. It was found that for the water dominant phase systems the dye was localized between the polar heads, at the exterior of the direct micelle shells for all the systems. For the oil dominant phase systems, in case of the same charged dye/surfactant couples, the dye was localized in the micelle shell between the hydrocarbon chain of the surfactant nearby the hydrophilic head groups while for nonionic surfactant and oppositely charged dye/surfactant, localization of dye was between the oxyethylenic head groups towards the interior of the micelle core. Mixed aggregates of the dye and surfactant (below the critical micellar concentration of cationic surfactant), dye-surfactant ion pair and surfactantmicelles were present. The values of equilibrium constants (for TX-114/MO and TX-114/CV systems were 0.97 and 0.98, respectively), partition coefficients between the micellar and bulk water phases and standard free energy (for the nonionic systems were À12.59 kJ/mol for MO and
Molecular dynamics study of an acid-base indicator dye in Triton X-100 non-ionic micelles
Voprosy himii i himičeskoj tehnologii, 2020
Despite wide laboratory and industrial use, the structure of non-ionic surfactant micelles and the state of molecular probes adsorbed by them are relatively poorly studied by computational methods in comparison to the ionic surfactants. The state of an acid-base indicator dye, namely, 2,6-dinitro-4-n-dodecylphenol, solubilized by micelles of Triton X-100 non-ionic surfactant, was revealed by means of molecular dynamics simulations. Both neutral and ionized protolytic forms were considered. The average positioning of the forms with respect to micelle, composition of their local environment and number of hydrogen bonds with water molecules were obtained and compared with those in ionic micelles. The dye was found to be located on the surface of hydrocarbon core and be significantly less hydrated than in ionic micelles due to covering by polyoxyethylene chains. Both forms are localized similarly, while the anionic form is hydrated stronger than the neutral one, which resembles the situation in ionic micelles. The convergence of simulation results in such micelles is found to be slower than in the ionic micelles, especially for the neutral form.
Journal of Colloid and Interface Science, 1991
The competition between bromide and chloride for the binding at the surface of mixed micelles composed of tetradecyltrimethylammonium and Brij 35 surfactants has been studied by fluorescence quenching techniques. The results obtained conform to the pseudophase ion-exchange formalism over a wide range of micelle compositions and allow for the determination of the selectivity coefficients. It was found that the competitive binding is well described by a unique K(Br/C1) = 6.3 _+ 1 selectivity coefficient, irrespective of the micelle's composition within the range considered (mole fraction of the ionic component in the micelle from 1 to 0.4). The results indicate that the factors involved in determining the selective binding are almost totally related to the nature of the exchanging counterions and very little dependent on the characteristics of the micelle surface and superficial charge density.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2018
In this paper, the locus and hydration character of three solvatochromic pyridinium-N-phenolate dyes in ionic surfactant micelles were examined using molecular dynamics modeling. These dyes, also called 'betaine dyes', are of various hydrophobicity due to different substituents in 2,6-positions of the phenolate moiety. They are as follows: 4-(2,4,6-triphenylpyridinium-1-yl)phenolate, 2,6-dichloro-4-(2,4,6-triphenylpyridinium-1-yl)phenolate, and 2,6-di-tert-butyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate; the abbreviations are BD-H, BD-Cl, and BD-tBu, respectively. The results were compared with those obtained previously with the so-called Reichardt's standard betaine dye, 4-(2,4,6-triphenylpyridinium-1-yl)-2,6-diphenylphenolate, or BD-Ph. The aggregates of widely used ionic surfactants, sodium dodecyl sulfate, SDS, and cetyltrimethylammonium bromide, CTAB, were selected as the most typical micellar media. The results of modeling shed some light on the state of the solvatochromic dipoles within the micellar pseudophase. Though the location of the dyes is rather similar, with the phenolate oxygen atom directed toward the Stern layer, the more hydrophobic dyes, BD-tBu and BD-Ph, appeared to be immersed somewhat deeper as compared with BD-H and BD-Cl. The average compositions of the local microenvironments of the dye molecules and the hydration of the oxygen atom were estimated. The last finding confirms the higher values of the E T parameter of the interfacial micellar region of SDS as against those of CTAB, in line with experimental data. The versatile information concerning the dye locus was compared with the NMR data available in the literature. Also, the same studies were performed with the protonated colorless forms of the betaine dyes, keeping in mind that these compounds are also often used as acid-base indicators in micellar media. Noteworthy, for all of the dyes under study, and in both micelles, the protonated colorless forms are oriented somewhat different as compared with the corresponding colored dipolar molecules. This should be taken into account when utilizing these compounds as interfacial pH-probes, in particular, for estimating the surface electrical potential of micelles and other colloidal species.
Electrostatic surface potential and critical micelle concentration relationship for ionic micelles
Langmuir, 1990
The relationship between the micellar interfacial electrostatic potential (\kpro as measured by reliable acid-base indicator techniques, and the critical micelle concentration (cm$: or dodecyltrimethylammonium bromide/NaBr, dodecyltrimethylammonium chloride/NaCl, sodium dodecyl sulfate/NaCl, sodium dodecyl sulfate/NaClO,, and sodium decyl sulfate/NaClO, systems has been examined. For these dilute micellar systems, it has been found that ~A\kpro,,~/unit change in log cmc is circa 59 mV at 25 "C. A thermodynamic treatment, appropriate for the objective of interpreting the relationship between \Ilprobe and the cmc, demonstrates that this form of behavior is consistent with \k being equal to the mean electrostatic potential at the micellar surface, i.e., the thermodynamic or &%it interfacial potential (\ko) of the micelle. Furthermore, it is consistent to regard the aqueous surfactant monomer species as the potential-determining ion in the charged micellar systems.