Micellar Pseudophase Research Articles

Parallel (competitive) reactions of micelle formation and cyclodextrin–surfactant inclusion complex formation in aqueous solution: A consequence of the Gibbs–Duhem equation—invariance of the mass action law and the possibility of determining the equilibrium constant of the inclusion complex

The Gibbs–Duhem equation results in the independence between the standard Gibbs free energy of micelle formation and the standard Gibbs free energy of monocomponent surfactant’s and cyclodextrin’s (CD’s) inclusion complex formation in aqueous solution. The experimental data show that the surfactant’s critical micellar concentration increases in the presence of CD. At a constant temperature, the standard Gibbs free energy of micelle formation remains constant, irrespective of the presence of parallel competitive processes such as inclusion complex formation. This indicates that the critical micellar concentration remains unchanged at the corresponding concentration scale. As a result, the equilibrium constant for surfactant–CD inclusion complex formation can be determined by changing the critical micellar concentration in the presence and absence of CD. A graphical method for determining the equilibrium constant of the inclusion complex is also presented. An analytical approach for analyzing the influence of the formation of the inclusion complex with CD on the formation of the binary mixed micellar pseudophase is presented. Suppose that different values exist for the equilibrium constants of the formation of the inclusion complex for two surfactants from their binary mixture. In this case, there is a change in the initial mole fraction from the initial mixture of surfactants, which should be considered when applying the regular solution theory protocol when determining the parameters of a binary mixed micelle in the presence of CD.

Ternary Mixed Micelle Hexadecyltrimethylammonium Bromide-Dodecyltrimethylammonium Bromide-Sodium Deoxycholate: Gibbs Free Energy of Mixing and Excess Gibbs Energy of Mixing.

Pharmaceutical, food, and cosmetic formulations often contain binary or ternary surfactant mixtures with synergistic interactions amongst micellar building blocks. Here, a ternary mixture of the surfactants hexadecyltrimethylammonium bromide, dodecyltrimethylammonium bromide, and sodium deoxycholate is examined to see if the molar fractions of the surfactants in the ternary mixed micellar pseudophase are determined by the interaction coefficients between various pairs of the surfactants or by their propensity to self-associate. Critical micelle concentrations (CMC) of the analyzed ternary mixtures are determined experimentally (spectrofluorimetrically using pyrene as the probe molecule). Thermodynamic parameters of ternary mixtures are calculated from CMC values using the Regular Solution protocol. The tendency for monocomponent surfactants to self-associate (lower value of CMC) determines the molar fractions of surfactant in the mixed micelle if there is no issue with the packing of the micelle building units of the ternary mixed micelle. If a more hydrophobic surfactant is incorporated into the mixed micelle, the system (an aqueous solution of surfactants) is then the most thermodynamically stabilized.

Solvatochromic and Acid–Base Molecular Probes in Surfactant Micelles: Comparison of Molecular Dynamics Simulation with the Experiment

This article summarizes a series of seventeen publications by the authors devoted to molecular dynamics modeling of various indicator dyes (molecular probes) enclosed in surfactant micelles. These dyes serve as generally recognized tools for studying various types of organized solutions, among which surfactant micelles in water are the simplest and most explored. The modeling procedure involves altogether 50 to 95 surfactant molecules, 16 to 28 thousand water molecules, and a single dye molecule. The presentation of the simulation results was preceded by a brief review of the state of experimental studies. This article consists of three parts. First, despite numerous literature data devoted to modeling the micelles itself, we decided to revisit this issue. The structure and hydration of the surface of micelles of surfactants, first of all of sodium n-dodecylsulfate, SDS, and cetyltrimethylammonium bromide, CTAB, were studied. The values of the electrical potential, Ψ, were estimated as functions of the ionic strength and distance from the surface. The decrease in the Ψ value with distance is gradual. Attempts to consider both DS− and CTA+ micelles in water without counterions result in a decay into two smaller aggregates. Obviously, the hydrophobic interaction (association) of the hydrocarbon tails balances the repulsion of the charged headgroups of these small “bare” micelles. The second part is devoted to the study of seven pyridinium N-phenolates, known as Reichardt’s dyes, in ionic micelles. These most powerful solvatochromic indicators are now used for examining various colloidal systems. The localization and orientation of both zwitterionic and (colorless) cationic forms are generally consistent with intuitive ideas about the hydrophobicity of substituents. Hydration has been quantitatively described for both the dye molecule as a whole and the oxygen atom. A number of markers, including the visible absorption spectra of Reichardt’s dyes, enable assuming a better hydration of the micellar surface of SDS than that of CTAB. However, our data show that it is more correct to speak about the more pronounced hydrogen-bonding ability of water molecules in anionic micelles than about better hydration of the SDS micelles as compared to CTAB ones. Finally, a set of acid–base indicators firmly fixed in the micellar pseudophase were studied by molecular dynamics. They are instruments for estimating electrostatic potentials of micelles and related aggregates as Ψ= 2.303RTF−1 (pKai − pKaapp), where pKai and pKaapp are indices of so-called intrinsic and apparent dissociation constants. In this case, in addition to the location, orientation, and hydration, the differences between values of pKaapp and indices of the dissociation constants in water were estimated. Only a semi-quantitative agreement with the experimental data was obtained. However, the differences between pKaapp of a given indicator in two micellar solutions do much better agree with the experimental data. Accordingly, the experimental Ψ values of ionic micelles, as determined using the pKaapp in nonionic micelles as pKai, are reproduced with reasonable accuracy for the corresponding indicator. However, following the experimental data, a scatter of the Ψ values obtained with different indicators for given micelles is observed. This problem may be the subject of further research.

Interfacial catalytic influence of sodium lauryl sulphate in the oxidation of cyclic alcohols by Cr(VI)

In the present work effort has been made to investigate the electron transfer mechanism for the oxidation of cyclopentanol of importance be Cr(VI) in presence and absence of surfactant (Sodium lauryl sulphate, SDS) and develop a kinetic model. The progress of the reaction was monitored spectrophotometrically by measuring the fall in absorbance of Cr(VI) at its λmax (wave-length of maximum absorbance). Under the conditions of [cyclopentanol] >≫ [Cr(VI)], the ln(Absorbance) vs time plots are obtained having constant slope value indicating the reaction exhibited a first order dependence with respect to Cr(VI). Unit order dependence of rate on [cyclopentanol], complex dependence on acid strength, nil effect of added salt (K2SO4) is what we observed while monitoring the progress of the reaction. The thermodynamic parameters like activation energy, enthalpy, entropy is computed by executing the reaction at 25, 30 and 35 0C. In presence of varying [SDS] it is observed that the rate marginally increases with increase in [SDS] until an optimum [SDS] which is around cmc and there after the rate decreases exponentially. In SDS medium partitioning of both the cationic Cr(IV) species and the substrate between water and micellar pseudo-phase is considered and accordingly Berezin et al. model is extended to explain the kinetic data. Based on all experimental findings a suitable kinetic model has been proposed for both aqueous and micellar phase reaction.

Regular solution theory regarding sodium cholate and hexadecyltrimethylammonium bromide or dodecyltrimethylammonium bromide binary mixed micelles

The formation of binary mixed micelles of sodium cholate (SC) – hexadecyltrimethylammonium bromide (HDTAB) and sodium cholate (SC) – dodecyltrimethylammonium bromide (DTAB) is investigated in an aqueous NaCl solution (0.300 mol kg−1). According to the 2D ROESY spectrum, SC steroid skeleton is in contact via its convex surface with the hydrophobic core of the micelle. Existing cross peaks indicate the spatial proximity of the SC carboxylate group and HDTAB or DTAB cationic heads, i.e., there are attractive electrostatic interactions. In the HDTAB-SC 2D ROESY spectrum there are cross peaks that may belong to HDTAB surfactants with hydrocarbon chains that have globular conformations. In both mixed micelles, there are synergistic interactions between structurally different surfactants. Dependence of the molar excess Gibbs free energy (gE) on the mole fraction of the surfactant i (xi) from the binary micellar pseudophase in both HDTAB-SC and DTAB-SC systems is described by the symmetric first-order Margules function. From theoretical considerations, it follows that the first-order Margules function that describes the dependence gE=f(xi) can be obtained if the number of microstates in binary mixed micellar pseudophases approaches the number corresponding to the number of microstates of random mixing of structurally different surfactants (HDTAB-SC), or there exists a compensation of molar excess configurational enthropy (a departure from random mixing of different particles at the atomic level) with molar excess conformational enthropy (DTAB-SC).

Estimation of Nanoparticle’s Surface Electrostatic Potential in Solution Using Acid–Base Molecular Probes II: Insight from Atomistic Simulations of Micelles

Exploiting acid-base indicators as molecular probes is one of the most popular methods for determining the surface electrostatic potential Ψ in hydrophilic colloids like micellar surfactant solutions and related systems. Specifically, the indicator's apparent acidity constant index is measured in the colloid solution of interest and, as a rule, in a nonionic surfactant solution; the difference between the two is proportional to Ψ. Despite the widespread use of this approach, a major problem remains unresolved, namely, the dissimilarity of Ψ values obtained with different indicators for the same system. The common point of view recognizes the effect of several factors (the choice of the nonionic surfactant, the probe's localization, and the degree of hydration of micellar pseudophase) but does not allow to quantitatively assess their impact and decide which indicator reports the most correct Ψ value. Here, based on the ability to predict the reported Ψ values in silico, we examined the role of these factors using molecular dynamics simulations for five probes and two surfactants. The probe's hydration in the Stern layer was found responsible for approximately half of the dissimilarity range. The probe's localization is found important but hard to quantify because of the irregular structure of the Stern layer. The most accurate indicators among the examined set were identified. Supplementing experiments on measuring Ψ with molecular dynamics simulation is proposed as a way of improving the efficacy of the indicator method: the simulations can guide the choice of the most suitable probe and nonionic surfactant for the given nanoparticles.

α-нуклеофилы – основа супернуклеофильных организованных микрогетерогенных систем для разложения фосфорорганических соединений

Peroxyhydrolysis of 4-nitrophenyl esters of phosphoric and phosphonic acids has been studied in organized microheterogeneous systems based on dimeric cationic imidazole-containing surfactants (AlkIm+-(CH2)m-Im+Alk ∙ 2Br- , m = 2,3,4, Alk = C12H25, C14H29). Micellar effects of the surfactants (at pH = const and [surfactant]0 = const) reach ~ 10–100 times. Physicochemical parameters of the peroxyhydrolysis process (such as substrate binding constants, hydroperoxide anion nucleophilicity in micellar pseudophase) are described in terms of the pseudophase distribution model. Observed rate enhancement mainly depends on reagent concentration increasing directly connected to hydrophobicity of the substrate and surfactant. Changes of HOO- nucleophilicity under the displacement of the peroxyhydrolysis from water to surfactant micelles also contributed to the micellar effects. Comparative analysis of the regularities of peroxyhydrolysis and alkaline hydrolysis testifies for generality of such features in OMS for nucleophilic substitutions. In this regard, the α-effect as rate constants ratio of peroxyhydrolysis (𝑘𝑘2,𝐻𝐻𝐻𝐻𝑂𝑂− 𝑚𝑚 ) to base hydrolysis (𝑘𝑘2,𝐻𝐻𝑂𝑂− 𝑚𝑚 ) in micellar pseudophase 𝑘𝑘2,𝐻𝐻𝐻𝐻𝑂𝑂− 𝑚𝑚 /𝑘𝑘2,𝐻𝐻𝑂𝑂− 𝑚𝑚 ) reaches ~ 80–100 times. Therefore, the α-effect appears not only in water, but in OMS, too.

Процессы «turnover» в реакциях разложения ацилсодержащих субстратов в организованных микрогетерогенных системах

Design of organized microheterogeneous systems for fast disrupting (disintegration) of acylcontaining substrates (including organophosphorus compounds) is based on the use of highly reactive compounds. Reagents (catalysts) ability to provide “turnover” is of great impotence. Kinetic regularities for 4- nitrophenylacetate (PNPA) deacylation with 1-methyl-3-(2-hydroximinoethyl)-imidazolium chloride (oxime), chloral, and their mixtures were studied in water and cetyltrimethylammonium bromide (CTAB) solutions, under pH = const and [CTAB] = 10-2 M. Rate constants dependences upon nucleophile (catalyst) and detergent concentration are typical for reactions in water and micellar pseudophase. Transferring of PNPA deacylation from water into surfactant micelles leads to three orders rate enhancement. Kinetic experiments at different substrate concentrations indicate that a) first order rate constant in the chloral-CTAB system does not change up to tenfold excess of PNPA; b) first order rate constant in the oxime-CTAB system decreases for approx. 40% at PNPA excess; c) all advantages of “turnover” are achievable in the system chloral-oximeCTAB, where chloral provides first disintegration of acylated oxime with generation of highly reactive oximate-ion. Results obtained pave the way to modifying organized microheterogeneous systems, first of all, by surfactant structure changes.

Role of ionic surfactants on the activation of K2S2O8 for the advanced oxidation processes

Effects of cetyltrimethylammonium bromide (CTAB) and sodium dodecylsulfate were studied to elucidate the persulfate based in-situ chemical oxidation of subsurface contamination. Advanced oxidation processes (AOP) utilizing Fenton’s like reagent was investigated for the degradation of acid orange 7 (AO7) with and without ionic surfactants. The new findings are that the CTAB and SDS retarding the S2O82− activation to some extent. The kinetics of S2O82− assisted decomposition of AO7 were performed under different reaction parameters such as [S2O82−], [Fe2+], [AO7], pH, and temperature. The rate constants (kψ) were decreased markedly in presence of both surfactants. CTAB showed the most potential deactivation than that of SDS. The stability of both surfactants were determined in presence of S2O82−, and no drastic changes was observed in the critical micellar concentration (CMC). The columbic attraction and repulsion between the reactants (S2O82−, and AO7) with ionic head groups of CTAB and SDS are responsible to deactivating the S2O82− decomposition. AO7 and S2O82−, respectively, solubilized and transferred from bulk phase to micellar surface through electrostatic interactions, which may contribute to the inhibitory role of CTAB micelles in the AOP of AO7. Finally the role of surfactants in the Fenton’s like oxidation of AO7 based on the micellar-pseudo phase model was proposed and discussed.

Symmetry (Asymmetry) of the Molar Excess Gibbs Free Energy Function of the Binary Mixed Micelles of Bile Acid Anion and Classical Cationic Surfactant: Influence of Sterically Shielded and Sterically Unshielded Polar Groups of the Steroid Skeleton

Binary mixtures of surfactants build a binary mixed micelle in which the ratio of surfactants usually differs from the initial ratio of surfactants in their binary mixture. The thermodynamic stabilization of the binary mixed micellar pseudophase about the hypothetical ideal state (intermolecular interactions between the different particles and the conformational states of the particles are identical to those of monocomponent states) is described by the molar excess Gibbs free energy (gE). The dependence of gE on the molar fraction of surfactant i (xi) from the binary mixed micelle can be described by a symmetric function (symmetry is described to the line parallel to the y-axis and passes through xi = 0.5) or by an asymmetric function. Theoretical analysis (canonical partition function, conformational analysis) examines how the presence of different polar functional groups, some of which are sterically shielded from the steroid skeleton of bile salts (surfactant), affect the symmetry of the function gE of the binary mixed micelle of the cholic acid anion (bile salts) and classic cationic surfactant (hydrophobic tail and polar head). Suppose the steroid skeleton of the bile salt contains non-sterically shielded polar groups (or the temperature is relatively high). In that case, gE is a symmetric function. At the same time, if the steroid skeleton also contains sterically shielded polar groups, then the gE function is asymmetric.

Thermodynamics of trimethyltetradecylammonium bromide – Sodium deoxycholate binary mixed micelle formation in aqueous solution: Regular solution theory with mutual compensation of excess configurational and excess conformational entropy

A binary mixture of trimethyl(tetradecyl)ammonium bromide = TB-SD = sodium deoxycholate surfactants forms non-ideal binary mixed micelles that have molar excess Gibbs free energy described with the Margules function of the first order. Strong synergistic interactions exist between TB and SD particles (interaction coefficient with a magnitude of 10). Based on the temperature dependence of interaction coefficient and temperature dependence of TB and SD micellar molar fractions product logarithm, it follows that the molar excess entropy is zero. Therefore, although the surfactants TB and SD are structurally very different, their binary mixed micelles can still be described with regular solution theory (random mixing of TB and SD). However, the 2D ROESY spectrum indicates that the binary micellar pseudophase is not random in terms of the spatial distribution of TB and SD particles. Namely, intermolecular interactions between deoxycholic acid anions are missing in the binary mixed micelle. This results in the appearance of excess configuration entropy. However, a zero value of the total molar excess entropy and the existence of the first order Margules function for the molar excess Gibbs free energy means that the molar excess configurational entropy is compensated with the molar excess conformational entropy and that there are correlation effects between the excess configurational enthalpy and entropy as well as between the excess conformational enthalpy and entropy.

Significance of dimeric surfactant on kinetic study of organophosphorus compounds

AbstractDimeric surfactants have shown significant role on hydrolysis of organo‐phosphorus compounds. In this study, we have studied the kinetic hydrolysis of p‐nitrophenyl acetate (PNPA), p‐nitrophenyl benzoate (PNPB), and p‐dinitrophenyl diphenyl phosphate (PNPDPP) in presence of novel dimeric surfactants 12‐4‐12, 2Br– (butanediyl‐1,4‐bis(dimethyldodecylammonium bromide), 12–4(OH)‐12, 2Br– (2‐butanol‐1,4‐bis(dimethyldodecylammonium bromide) with conventional nucleophiles, benzohydroxamic acid (BHA), and salicylhydroxamic acid (SHA) by spectrophotometry at 27°C. Kinetic study on effect of micelles on reaction rates have been investigated and rationalization effects of micelles determination using kinetic model. The experimental kinetic data were fitted with micellar pseudophase model for determination micellar substrate binding and parameters. The second order rate constant is processed for comparing the reactivities in aqueous and micellar pseudophase.

Humic Acids Aggregates as Microheterogeneous Reaction Media: Alkaline Hydrolysis Reactions

The influence of humic aggregates in a water solution upon the chemical stability under basic conditions of different substrates was reviewed. The kinetic behavior of each substrate was modeled in terms of a micellar pseudophase model.

The ternary mixed micelle of tween 20- sodium deoxycholate- sodium cholate: The molar excess thermodynamic potencials

Micellisation of the ternary mixture Tween-20, sodium deoxycholate and sodium cholate is examined in the temperature interval T = (278.15–318.15) K. Experimentally are determined: critical micelle concentration (cmc) of ternary mixtures of examined surfactants and critical micelle concentrations of binary mixtures and monocomponent surfactants. Based on the Regular solution theory, the parameters of the ternary mixed micelle are calculated from the measured cmc values. The non-ionic surfactant dominates in the amount of ternary mixed micelle. Both bile acid anions in the binary mixed micelle with Tween 20 have synergistic interactions, i.e. have negative interaction coefficients of similar magnitudes. However, sodium deoxycholate is a bile salt which, regardless of the composition of the initial ternary mixture of surfactants, represents the main amount of bile salt in the ternary mixed micelle. The entry of bile acid anions into the ternary micellar pseudophase determines which bile acid anion can reduce the total Gibbs energy of the aqueous surfactant solution, rather than their contribution to the thermodynamic stabilization of real ternary mixed micelles, relative to the hypothetical ideal state of the same micelle.

Development of novel experimental and modelled low density polyethylene (LDPE)-water partition coefficients for a range of hydrophobic organic compounds

Knowledge about partitioning constants of hydrophobic organic compounds (HOCs) between the polymer and aqueous phases is critical for assessing chemical environmental fate and transport. The conventional experimental method is characterized by large discrepancies in the measured values due to the limited water solubility of HOCs and other associated issues. In the current work, a novel three-phase partitioning system was evaluated to determine accurate low-density polyethylene (LDPE)-water partition coefficients (KPE-w). By adding sufficient surfactant (Brij 30) to form the micellar pseudo-phase within the polymer/water system, the KPE-w values were obtained from a combination of two experimentally measured values, that is, the micelle-water partition coefficient (Kmic-w) and the LDPE-micelle partition coefficient (KPE-mic). The method presented here is capable of shortening the equilibration time to half a month, and avoiding defects of the traditional method with respect to directly measured aqueous phase concentrations. Herein, the KPE-w values were determined for HOCs with little errors. Meanwhile, based on the 120 experimental KPE-w data, several in silico models were also developed as valid extrapolation tools to estimate missing or uncertain values. Analysis of the underlying solubility interactions in the nonionic surfactant micelles were investigated, providing additional support for the reliability of the proposed method.

Role of ionic surfactants on the nucleation and growth of silver nanoparticles

Sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) were used as stabilizers for the preparation of silver nanoparticles (AgNPs) by the reduction of silver nitrate with proanthocyanidines (Pas). UV–visible spectra shows that the surface Plasmon resonance (SPR) intensity of AgNPs increases and decreases with increasing SDS and became constant at higher concentration. CTAB has no effect on the nucleation of AgNPs due to the insolubility of Pas. The position of SPR band (λmax = from 438 to 476 nm) depends on the concentration of Pas and a red shift of 38 nm was observed in absence of externally added SDS. The micellar pseudo phase proposed by Menger and Portnoy, modified by Bunton was used to determine the rate constants in micellar media. The Ag+ ions were formed complex with negative head group (OSO3−) of SDS electrostatically. Raman scattering spectroscopy, transmission electron microscopy, and scanning electron microscopy were used to discuss the capping action of SDS. Zeta potential of SDS-capped AgNPs was negative and resulting sols were stable for many months.

Conformationally rigid and flexible surfactant binary (pseudoternary) mixed micelle with mutual synergistic interaction

Molar excess Gibbs energy of a binary mixed micelle with a conformationally flexible and rigid surfactant can be represented by first-order Margules function. Excess entropy comes from the conformational entropy rather than the configurational entropy. Such a binary mixed micelle with the synergistic interactions between micellar building blocks can be considered a pseudoternary mixed micelle from the conformationally rigid surfactant, twisted (globular) conformation of the conformationally flexible surfactant, and elongated conformation of the same surfactant. Excess entropy of such mixed micelles can be approximated with a microcanonical ensemble of the conformationally flexible surfactants. Particles of these surfactants are distributed between the average lower energy (elongated conformation) and the average higher energy (twisted conformation). A new structural parameter of mixed micelles was introduced - the parameter of disorder, which represents the molar fraction of the conformation with an average higher energy (twisted conformation) of the conformationally flexible surfactant in the mixed micellar pseudophase. Influence of the new structural parameter on the conformational rigid surfactant's excess chemical potential is discussed.

Role of ionic surfactants on the plasmonic oxidative dissolution of silver nanoparticles by ferric ions

Effects of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) on the optical sensing of AgNPs toward ferric ions were investigated spectrophotometrically in an aqueous media. Surface Plasmon resonance (SPR) intensity of AgNPs was decreases after addition of ferric ions at room temperature. CTAB and SDS, respectively, catalyzed and inhibited the oxidative dissolution rates of AgNPs. The redox reaction between AgNPs and Fe3+ ions proceeds through the complex formation. The reaction follows first-, and fractional-order kinetics with AgNPs and Fe3+ ions, respectively. CTAB catalytic role was explained with micellar pseudo-phase model, and associated parameters were estimated. The activation parameters (Ea, ΔH#, and ΔS#) were found to be 23.7 kJ/mol, 16.6 kJ/mol, −226 J/K/mol, and 16.5 kJ/mol, 13.5 kJ/mol, −238 J/K/mol for aqueous and CTAB assisted dissolution of AgNPs. SDS coordinated with Fe3+ through electrostatic interactions in pre-, and post-micellar region, which in turn reduces the electron gaining tendency of Fe3+, and overall rates of sensing reaction decreases.

Reactivity of Inorganic α-Nucleophiles in Acyl Transfer Processes in Water and Surfactant Micelles: III. Systems Based on Dimeric Cationic Imidazolium Surfactants in Alkaline Hydrolysis of Ethyl 4-Nitrophenyl Ethylphosphonate

The kinetics of alkaline hydrolysis of ethyl 4-nitrophenyl ethylphosphonate in micelles formed by cationic dimeric imidazolium surfactants [AlkIm+(CH2)mIm+Alk]·2Br– (where Alk = C16H33–C10H21, m = 2, 3, 4) have been studied. The rate of the reaction in the micellar pseudophase depends on the efficiency of solubilization of the reactants, and the effect of reactant concentrating increases in parallel with the length of the alkyl “tail.” Micellar microenvironment is an important factor responsible for the nucleophilicity of hydroxide ion. The observed acceleration of alkaline hydrolysis of ethyl 4-nitrophenyl ethylphosphonate (micellar catalysis) is primarily related to these two factors. An obvious superiority of dimeric surfactants to monomeric ones is that equal reaction rates are attained at 1–2 orders of magnitude lower concentrations of the former.

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

Ionic equilibrium of 22 hydroxyxanthenes, including halogen and nitro derivatives of fluorescein, and their thio- and aza analogues, were studied spectrophotometrically in micellar solutions of cetyltrimethylammonium chloride at ionic strength of the bulk phase 4.0 M KCl. This micellar pseudophase is characterized by the electrostatic surface potential of +(15–16) mV and the ETN value of 0.623. In the case of dyes bearing the COOH group, colorless lactone is the predominant tautomer of the molecular form H2R. A new classification of fluoresceins is developed. The dyes were divided into four groups based on the nature of tautomerism of the anions. In the case of the fluorescein type, the monoanions HR− exist predominantly as “carboxylate” tautomers, with ionized carboxylic and non–ionized hydroxylic group. For the dyes of the eosin type, the situation is opposite, while for the intervening type of compounds, the concentrations of the two tautomers are comparable. Dyes capable of forming lactone anions HR− were classified as the fourth type. For some of them, even the dianion R2− exists as a lactone. The relationship between the stepwise ionization constants, Ka1/Ka2, varies from 1.3 to 1.07 × 105 and is determined by the state of tautomeric equilibrium of molecules and ions.