Estimation of Nanoparticle's Surface Electrostatic Potential in Solution Using Acid-Base Molecular Probes I: In Silico Implementation for Surfactant Micelles.

Surface electrostatic potential Ψ is a key characteristic of colloid particles. Since the surface of the particles adsorbs various compounds and facilitates chemical reactions between them, Ψ largely affects the properties of adsorbed reactants and governs the flow of chemical reactions occurring between them. One of the most popular methods for estimating Ψ in hydrophilic colloids, such as micellar surfactant solutions and related systems, is the application of molecular probes, predominantly acid-base indicator dyes. The Ψ value is calculated from the difference of the probe's indices of the apparent acidity constant between the examined colloid solution and, usually, some other colloid solution with noncharged particles. Here, we show how to implement this method in silico using alchemical free energy calculations within the framework of molecular dynamics simulations. The proposed implementation is tested on surfactant micelles and is shown to predict experimental Ψ values with quantitative accuracy depending on the kind of surfactant. The sources of errors in the method are discussed, and recommendations for its application are given.

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.

Estimation of Nanoparticle's Surface Electrostatic Potential in Solution Using Acid-Base Molecular Probes. III. Experimental Hydrophobicity/Hydrophilicity and Charge Distribution of MS2 Virus Surface.

MS2 bacteriophage is often used as a model for evaluating pathogenic viruses' behavior in aqueous solution. However, the questions of the virus surface's hydrophilic/hydrophobic balance, the charge distribution, and the binding mechanism are open. Using the dynamic light scattering method and laser Doppler electrophoresis, the hydrodynamic diameter and the ζ-potential of the virus particles were measured at their concentration of 5 × 1011 particles per mL and ionic strength 0.03 M. The values were found to be 30 nm and -29 or -34 mV (by Smoluchowski or Ohshima approximations), respectively. The MS2 bacteriophage surface was also investigated using a series of acid-base indicator dyes of various charge type, size, and structure. Their spectral and acid-base properties (pKa) are very sensitive to the microenvironment in aqueous solution, including containing nanoparticles. The electrostatic potential of the surface Ψ was estimated using the common formula: Ψ = 59 × (pKai - pKa) in mV at 25 °C. The Ψ values were -50 and +10 mV, respectively, which indicate the "mosaic" way of the charge distribution on the surface. These data are in good agreement with the obtained ζ-potential values and provide even more information about the virus surface. It was found that the surface of the MS2 virus is hydrophilic in solution in contrast to the commonly accepted hypothesis of the hydrophobicity of virus particles. No hydrophobic interactions between various molecular probes and the capsid were observed.

Unexpected Colloidal Stability of Fullerenes in Dimethyl Sulfoxide and Related Systems.

It is known that fullerenes are poorly soluble in polar solvents, but readily form colloidal solutions in such media. These solutions are typically solvophobic (hydrophobic when prepared in water), that is, thermodynamically unstable colloidal systems with negatively charged particles. To understand the stability factors of a colloidal system, the thresholds for coagulation of a sol or suspension by electrolytes are of key importance. While hydrosols and aqueous suspensions coagulate at concentrations of 1:1 inorganic electrolytes about 0.1-0.2 M, in acetonitrile and methanol, the corresponding critical concentrations of coagulation are ca. 3 orders of magnitude lower. Given the wide variety of properties of organic solvents, it seemed important to complete the picture to study solvents with more basic properties. This is all the more reasonable since electrophilic fullerenes are in fact Lewis acids. Our choice was dimethyl sulfoxide, DMSO, and related solvent systems. The colloidal solutions of fullerenes C60 and C70 in DMSO and N,N-dimethyl formamide, DMF, are unexpectedly easy to prepare by mechanical methods, and addition of water leads to formation of relatively stable organo-hydrosols. UV-visible spectra and dynamic light scattering were used to characterize the solutions of C60 and C70 in DMSO, benzene-DMSO, acetonitrile-DMSO, and benzene-acetonitrile-DMSO systems, as well as in DMF. Our present study demonstrated that, in contrast to organosols in methanol and acetonitrile, colloids of C70 and C60 fullerenes in DMSO and DMF are surprisingly as stable with respect to electrolytes as the corresponding hydrosols are. Such high stability is caused by the non-DLVO interactions, or, in terms proposed by Churaev and Derjaguin, by the so-called structural effect. These results shed light on the nature of the solvation of colloidal fullerene particles in solvents of various chemical natures.

Formation, Stability, and Coagulation of Fullerene Organosols: C70 in Acetonitrile-Toluene Solutions and Related Systems.

This paper is aimed at better understanding the nature of C70 aggregates in organic solvents. As liquid media, acetonitrile-toluene mixed solvents were chosen. At a high content of CH3CN, e.g., 90 vol %, colloidal particles with a size of ca. 225 ± 10 nm are formed with a negative ζ-potential of -(55 ± 5) mV and are stable over time. The interaction with electrolytes containing single-, double-, and triple-charged cations was examined using dynamic light scattering and UV-visible spectra. Additional experiments were carried out with methanol and benzene instead of acetonitrile and toluene, respectively. For comparison, data were obtained with C60 organosols. It was found that coagulation obeys the classical Schulze-Hardy rule. The specificity of the coagulating power of various single-charged cations was explained by their different abilities to adsorb on negatively charged C70 aggregates. The overcharging effect is expressed not only for Ca2+ and La3+ ions but even for Li+ and is caused by poor solvation of such cations in a cationophobic solvent, acetonitrile. After introduction of the cryptand [2.2.2], a substantial increase in the critical concentrations of coagulation for Na+, Li+, and Ca2+ was observed owing to conversion of "bare" metal cations into their cryptates. The application of the Derjaguin-Landau-Verwey-Overbeek theory allowed for evaluation of the Hamaker constant of the C70-C70 interaction in vacuum, AFF, which lies in the interval of 5.8-16.6 × 10-20 J. Such an estimate, close to that made previously for C60 organosols, was received after withdrawing electrolytic systems where the hetero- and mutual coagulation were highly likely. However, it is impossible to completely exclude the interfering influence of the latter phenomena. Based on the obtained AFF values, two approaches to understanding the behavior of fullerenes in water were proposed.

Computing pKa Shifts Using Traditional Molecular Dynamics: Example of Acid-Base Indicator Dyes in Organized Solutions.

A compound's acidity constant (Ka) in a given medium determines its protonation state and, thus, its behavior and physicochemical properties. Therefore, it is among the key characteristics considered during the design of new compounds for the needs of advanced technology, medicine, and biological research, a notable example being pH sensors. The computational prediction of Ka for weak acids and bases in homogeneous solvents is presently rather well developed. However, it is not the case for more complex media, such as microheterogeneous solutions. The constant-pH molecular dynamics (MD) method is a notable contribution to the solution of the problem, but it is not commonly used. Here, we develop an approach for predicting Ka changes of weak small-molecule acids upon transfer from water to colloid solutions by means of traditional classical molecular dynamics. The approach is based on free energy (ΔG) computations and requires limited experiment data input during calibration. It was successfully tested on a series of pH-sensitive acid-base indicator dyes in micellar solutions of surfactants. The difficulty of finite-size effects affecting ΔG computation between states with different total charges is taken into account by evaluating relevant corrections; their impact on the results is discussed, and it is found non-negligible (0.1-0.4 pKa units). A marked bias is found in the ΔG values of acid deprotonation, as computed from MD, which is apparently caused by force-field issues. It is hypothesized to affect the constant-pH MD and reaction ensemble MD methods as well. Consequently, for these methods, a preliminary calibration is suggested.

Aminofluoresceins Versus Fluorescein: Ascertained New Unusual Features of Tautomerism and Dissociation of Hydroxyxanthene Dyes in Solution.

Within the course of this spectroscopic research, we revealed novel features of the protolytic behavior, which extend the knowledge of the chemistry of xanthene dyes and rationalize the utilization of these compounds. In addition to the well-known tautomerism of the molecular form, H2R, of fluorescein dyes, new aspects of tautomeric transformation of anions are disclosed. First, for the dyes bearing the substituents in the phthalic acid residue, 4'- and 5'-aminofluoresceins and 4'-fluorescein isothiocyanate, the monoanion HR- exists in non-hydrogen-bond donor solvents not only as a tautomer with the ionized carboxylic and nonionized OH group but also as a "phenolate" ion with a nonionized COOH group. Such state of HR- ions is typical for dyes bearing halogen atoms or NO2 groups in the xanthene moiety but was not observed until now in the case of substitution in the phthalic residue. Second, the possibility of the existence of the HR- species in DMSO in the form of colorless lactone is deduced for the 5'-aminofluorescein using the visible and infrared spectra. This results in a dramatic difference in medium effects. For instance, whereas for fluorescein in DMSO, the inversion of the stepwise ionization constants takes place and the Ka1/Ka2 value equals 0.08, the same ratio for 5'-aminofluorescein is as high as ∼800. In addition, the pKa values of sulfonefluorescein, erythrosin, methyl ether of fluorescein, and phenol red were obtained to verify the acidity scale in DMSO and to support the detailed scheme of protolytic equilibria of fluorescein dyes.

Aminofluoresceins Versus Fluorescein: Peculiarity of Fluorescence.

In this article, we examined the fluorescent properties of 4'- and 5'-aminofluorescein, unsubstituted fluorescein, and its 4'-nitro derivative in a set of solvent systems. Fluorescence lifetimes, quantum yields, time-resolved fluorescence spectra, and quantum chemical calculations allowed clarifying the reasons of the emitting properties in this dye series. In water, the dianions R2- of aminofluoresceins are practically nonfluorescent; in alcohols, the quantum yields are low. In dimethylsulfoxide (DMSO), acetonitrile, and other non-hydrogen bond donor solvents, the bright fluorescence of R2- ions is quenched either on adding small amounts of water which hydrate the carboxylate group or under conditions of protonation of this group (COO- → COOH). The last observation is possible owing to the peculiarities of the tautomerism of the 5'-aminofluorescein monoanion, HR-, which exists in DMSO as an equilibrium mixture of a colorless lactone and colored "phenolate" tautomer with an ionized xanthene moiety and unionized carboxylic group. In contrast, the R2- anion of 4'-nitrofluorescein demonstrates spectral behavior different from that of the amino derivatives. It practically does not emit in aprotic solvents; however, in alcohols or water media, its quantum yield increases to some extent. Such changing spectral properties are explained in terms of the excited-state interfragmental charge transfer.

Interaction of Polymethine Dyes with Detonation Nanodiamonds.

Among cationic, anionic, and merocyanine polymethine dyes, the binding to detonation nanodiamond (DND) colloid particles in hydrosol occurs only for negatively charged dye species. This, in view of the positive ζ-potential of the DND used in this study, suggests the predominance of electrostatic interactions over other intermolecular forces in such systems. Indeed, after decorating the merocyanine and the cationic dye by one and two negatively charged sulfopropyl groups, respectively, so that the net charge of their colored species becomes negative, the compounds also demonstrate affinity to the DND particles. In all cases, the binding of dyes to DND is accompanied by a decrease in fluorescence intensity and a bathochromic shift of their absorption and fluorescence bands. A quantitative study of the dyes adsorption on the DND nanoparticles as performed using the Küster-Freundlich and Langmuir equations reveals some peculiarities of their attaching to the DND aggregates and allows estimating the specific area of the DND particles at a concentration of 0.0212 wt/vol %.

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.

Absorption, fluorescence, and acid-base equilibria of rhodamines in micellar media of sodium dodecyl sulfate.

Rhodamine dyes are widely used as molecular probes in different fields of science. The aim of this paper was to ascertain to what extent the structural peculiarities of the compounds influence their absorption, emission, and acid-base properties under unified conditions. The acid-base dissociation (HR(+)⇄R+H(+)) of a series of rhodamine dyes was studied in sodium n-dodecylsulfate micellar solutions. In this media, the form R exists as a zwitterion R(±). The indices of apparent ionization constants of fifteen rhodamine cations HR(+) with different substituents in the xanthene moiety vary within the range of pKa(app)=5.04 to 5.53. The distinct dependence of emission of rhodamines bound to micelles on pH of bulk water opens the possibility of using them as fluorescent interfacial acid-base indicators.

Interactions of Nanosized Aggregates of Fullerene C60 with Electrolytes in Methanol: Coagulation and Overcharging of Particles.

Contrary to numerous studies on the stability of fullerene aqueous colloidal solutions in the presence of electrolytes, the corresponding issue for the organosols was until recently almost unexplored. In this article, the state of C60 in methyl alcohol and the regularities of the coagulation of colloidal solution in this solvent were examined in the presence of electrolytes. Alcosols with a fullerene concentration of 4 × 10-6 M were prepared by the dilution of the C60 saturated solution in toluene by methanol. The ca. 300 nm-sized aggregates possess a negative electrokinetic potential value, ζ = -37 ± 8 mV. To determine the critical coagulation concentrations, CCC, the size increase of the species was followed up using the dynamic light scattering method. The analysis of the coagulation in terms of the Fuchs function, W, was accompanied by zeta potential monitoring. The consideration of the data for 1:1 electrolytes NaClO4 and N(n-C4H9)4ClO4 allows a rough estimate of the Hamaker constant of fullerene-fullerene attraction. Whereas in the case of these two electrolytes the colloidal species are negatively charged at the CCC, expressed overcharging of up to ζ = +36 mV by H+, Ca2+, Ba2+, and La3+ ions was observed. The action of HClO4 should be attributed to the interfacial acid-base reaction, whereas the excessive attraction of metal cations is caused by poor solvation in methanol; the negative charge is restored when the metal cations are shielded by a macrocyclic ligand.

Towards better understanding of C60 organosols.

It is of common knowledge that fullerenes form colloids in polar solvents. However, the coagulation via electrolytes and the origin of the negative charge of species are still unexplored. Using a 'radical scavenger' and electrospray ionization spectroscopy (ESI), we proved the formation of ion-radical C60˙(-) and its (probable) transformation into C60(2-) or (C60)2(2-). The coagulation of C60 organosols by NaClO4 and other perchlorates and nitrates in acetonitrile and its mixture with benzene obeys the Schulze-Hardy rule. At higher Ca(ClO4)2 and La(ClO4)3 concentrations, instead of coagulation, stable re-charged colloidal particles appeared, up to a zeta-potential of +(20-42) mV, as compared with -(33-35) mV of the initial organosols. The influence of both HClO4 and CF3SO3H was similar. This phenomenon is attributed to poor solvation of inorganic cations in cationo- and protophobic acetonitrile, which was proven using [2.2.2] cryptand. Further increasing the concentration of Ca(ClO4)2 led again to coagulation, thus demonstrating a novel type of 'coagulation zones'.

Fluorescence of aminofluoresceins as an indicative process allowing one to distinguish between micelles of cationic surfactants and micelle-like aggregates.

Among the vast set of fluorescein derivatives, the double charged R2- anions of aminofluoresceins are known to exhibit only low quantum yields of fluorescence, [Formula: see text]. The [Formula: see text] value becomes as high as that of the fluorescein dianion when the lone electron pair of the amino group is involved in a covalent bond. According to Munkholm et al (1990 J. Am. Chem. Soc. 112 2608-12), a much smaller increase in the emission intensity can be observed in the presence of surfactant micelles. However, all these observations refer to aqueous or alcoholic solvents. In this paper, we show that in the non-hydrogen bond donor (or 'aprotic') solvents DMSO and acetone, the quantum yields, φ, of the 4'- (or 5')-aminofluorescein R2- species amount to 61-67% and approach that of fluorescein (φ = 87%), whereas in water φ is only 0.6-0.8%. In glycerol, a solvent with an extremely high viscosity, the φ value is only 6-10%. We report on the enhancement of the fluorescence of the aminofluorescein dianions as an indicative process, which allows us to distinguish between the micelle-like aggregates of cationic dendrimers of low generation, common spherical surfactant micelles, and surfactant bilayers. Some of these colloidal aggregates partly restore the fluorescence of aminofluoresceins in aqueous media. By contrast, other positively charged micellar-like aggregates do not enhance the quantum yield of aminofluorescein R2- species. Results for several related systems, such as CTAB-coated SiO2 particles and reverse microemulsions, are briefly described, and the possible reasons for the observed phenomena are discussed.

Ionization and tautomerism of methyl fluorescein and related dyes.

The protolytic equilibrium of methyl ether of fluorescein is studied in water, aqueous ethanol, and in other solvents. The constants of the two-step dissociation are determined by spectrophotometry. In water, the fractions of the zwitterionic, quinonoid, and lactonic tautomes are correspondingly 11%, 6%, and 83%, as deduced from the UV-visible spectra. Corresponding study of the ionization of the methyl ether ester of fluorescein, fluorescein ethyl ester, and sulfonefluorescein allows testing the correction of the attribution of the microscopic dissociation constants of methoxy fluorescein. The results of nuclear magnetic resonance and infrared spectroscopy, as well as the X-ray analysis confirm the predomination of the lactonic structure of the molecular species in solid state and in DMSO. Contrary to it, the spectroscopic studies in both hydrogen-donor bond (HDB) and non-HBD solvents confirm that the presence of lactonic monoanion is atypical for the dye under study and, with high probability, also for the mother compound fluorescein.

The influence of beta-cyclodextrin on acid-base and tautomeric equilibrium of fluorescein dyes in aqueous solution.

The protolytic equilibrium of fluorescein in aqueous solutions was studied in the presence of cycloheptaamylose (beta-cyclodextrin, or beta-CD). The constants of stepwise ionization of the dye (H(3)R(+)left arrow over right arrowH(2)Rleft arrow over right arrow HR(-)left arrow over right arrowR(2-)), K(a0), K(a1), and K(a2) were determined using vis-spectroscopy at ionic strength 0.05 M (NaCl+buffer) and 25 degrees C. In the presence of 0.0086 M beta-CD, the indices of ionization constants are as follows: pK(a0)=1.21+/-0.12, pK(a1)=5.08+/-0.03, pK(a2)=6.35+/-0.02. The changes in these pK(a)s, as compared with the values determined without cyclodextrin, are unequal. Namely, the pK(a0) value decreases by 1.0, while the pK(a1) value increases by 0.7. Thus, the introduction of beta-CD allows to govern the ratios K(a0)/K(a1) and K(a1)/K(a2), which are equal to, respectively, 141 and 151 in water, and 7.4 x 10(3) and 18.6 with cyclodextrin added. Rationalization of the observed phenomenon is possible taking into account the detailed scheme of protolytic equilibrium. Conclusions concerning tautomerism of dye molecules were deduced from absorption spectra; the fractions of tautomers, tautomerization constants, and microscopic ionization constants were evaluated. These data allow concluding that the main reason for the aforementioned pK(a) alterations is the binding of H(2)R by the cyclodextrin cavity accompanied by turning these neutral species into the colorless lactone. The host-guest interaction of neutral species of fluorescein isothiocyanate, 2,7-dichlorofluorescein, and 3',4',5',6'-tetrachlorofluorescein also results in the cyclodextrin-assisted shift of tautomeric equilibrium. Such nature of interactions is proved by the addition of competing agents, camphor-4-carboxylic acid and sodium n-nonylsulfonate, which results in the removing of neutral dye species from the cycloheptaamylose cavity.

Medium effects on the prototropic equilibria of fluorescein fluoro derivatives in true and organized solution.

The stepwise ionization (H(3)R(+) <==> H(2)R <==> HR(-) <==> R(2-)) of four fluorescein fluoro derivatives was studied by visible spectroscopy. The pK(a) values were determined in water, in 50 mass % aqueous ethanol, in oil-in-water microemulsions (benzene + CTAB + pentanol-1 in water with 1.0 M KCl; CTAB = cetyltrimethylammonium bromide), and in reversed ones (water + AOT in n-octane; AOT = bis-2-ethylhexylsulphosuccinate or Aerosol OT). The medium effects, DeltapK(a), i.e., changes in pK(a) of these dyes on going from water to some other solvent systems, were rationalized by considering the tautomerism, the values of microscopic ionization constants, and the charge types of the acid-base couples. An expressed shift of the tautomeric equilibria of H(2)R toward colorless lactone was registered on going from water to both aqueous ethanol and organized solutions. While the monoanions HR(-) of 3',4',5',6'-tetrafluoro- and 2,7,3',4',5',6'-hexafluorofluorescein exist in all the systems studied as a tautomer with ionized carboxylic and nonionized hydroxy groups, in the case of 2,4,5,7-tetrafluorofluorescein, the prevalence of another tautomer was observed (COOH and O(-) groups). For 2,7-difluorofluorescein (Oregon Green 488), the partial shift of the tautomeric equilibrium of HR(-) was registered from (COO(-) and OH) in water to (COOH and O(-)) in other solvent systems. The data for the dyes located in an AOT-based pseudophase indicate that the interior of the latter exerts essential differentiation of the acid strength of the dyes, probably caused by the peculiarity of dye species location in water pools. While the state of tautomeric equilibria resembles that in nonaqueous media, the absorption maxima of R(2-) species are close to those in water. Such nonuniform influence displayed by AOT-based water droplets should be taken into account when examining them by using different molecular probes.

Nature of cationic poly(propylenimine) dendrimers in aqueous solutions as studied using versatile indicator dyes.

Aqueous solutions of four cationic poly(propylenimine) low-generation dendrimers of different architecture and hydrophobicity have been examined as media for acid-base reactions of indicator dyes. The cationic dendrimers in solution can be considered as oligomers of cationic polyelectrolytes, or surfactant-like species, able to form micelles through self-association or sometimes even as unimolecular micelles. The dendrimers influence the ionization constants, tautomeric equilibria, and absorption/emission/excitation spectra of indicator dyes. The p K a values of the majority of the indicator dyes decrease in dendrimer solutions, often by 1-2 p K a units, similar to effects registered in micellar solutions of cationic surfactants. Analogously, the shifts of absorption band maxima indicate that the microenvironments of the dyes bound to the dendrimers are less polar than in water. However, some spectral effects denote the specificity of the dendrimers. The greatest difference between the dendrimers and spherical surfactant micelles is revealed by kinetic processes, especially of bromophenol blue alkaline fading in a dendrimer solution but not in a micellar surfactant solution. Within the dendrimer series, the most significant differences were observed for substances possessing n-dodecyl tails on the one hand and those without such hydrophobic portions on the other. For the last-named, the decrease in p K a's of indicators, band shifts of their anions, and in particular displacement of tautomeric equilibria compared with aqueous solutions are much smaller than for more hydrophobic dendrimers.

Counterion-induced transformations of cationic surfactant micelles studied by using the displacing effect of solvatochromic pyridinium N-phenolate betaine dyes.

In this paper, we demonstrate that the behavior of a set of eight large-sized negatively solvatochromic pyridinium N-phenolate betaine dyes reflects the principle transformations, occurring in aqueous micellar solutions of three cationic surfactants. As surfactants, cetyltrimethylammonium bromide (CTAB), n-octadecyltrimethylammonium chloride (OTAC), and N-cetylpyridinium bromide (CPB) were used. Normally, for such probes coupled with micelles, a red shift of the vis absorption band is expected as a result of a hydrophobization ("drying") of the micellar interface. However, under addition of electrolytes with anions such as tosylate, salicylate, and some n-alkanesulfonates or n-alkanecarboxylates to the micellar solutions, an unexpected effect was observed. Instead of a red shift, a blue shift of the vis absorption band of some of the dissolved betaine dyes was registered, as compared with the spectrum measured in pure aqueous micellar solutions of CTAB, OTAC, or CPB (Deltalambda(max) up to ca. 80 nm). This blue shift, indicating an increase in the polarity of the dye microenvironment, is explained by displacing the large dye dipoles from the thinned micelles toward the aqueous phase. The effect is well expressed at concentrations of C(betaine dye) approximately 10(-5) M, C(cationic surfactant) approximately 0.001 M, and C(organic anion) approximately 0.01 M. Transmission electron microscopy of dried samples confirms the distinct changes occurring in the studied micellar systems upon the addition of organic anions. The excess of inorganic salts [C(NaBr, KBr, or KCl) = 0.5-4.0 M] restored the position of the vis absorption band or even shifted it toward the red. Moreover, some of the betaine dyes studied (i.e., the more hydrophobic ones) stay in the micellar pseudophase or precipitate under the aforementioned concentration conditions. The peculiarities of the behavior of these betaine dyes are in agreement with their molecular structure.