Micellar effects upon the reaction of hydroxide ion with 2-phenoxyquinoxaline

Cationic micelles of alkyltrimethylammonium chloride and bromide (alkyl = n? C₁₂H₂₅, n? C₁₄H₂₉, and n? C₁₆H₃₃) catalyze and anionic micelles of sodium dodecyl sulfate inhibit the reaction of hydroxide ion with 2-phenoxyquinoxaline (1). Inert anions such as chloride, nitrate, mesylate, and n-butanosulfonate inhibit the reaction in CTABr by competing with OH^? at the micellar surface. The overall micellar effects on rate in cationic micelles and dilute electrolyte can be treated quantitatively in terms of the pseudo-phase ion-exchange model. The determined second-order rate constants in the micellar pseudo-phase are smaller than the second-order constants in water. © 1994 John Wiley & Sons, Inc.     

Micellar Catalysis of Composite Reactions II. The Effect of CTAB Micelles on the Alkaline Fading of Malachite Green

The alkaline fading of malachite green, which is interpreted as parallel first order and second order reactions, has been studied in cetyltrimethylammonium bromide (CTAB) micellar solution at 25°C using spectrophotometry. A micellar catalytic model is proposed in this paper for constant concentration of hydroxideion. For this model, the first order and the second order rate constants in CTAB micellar phase,k_1m and k_2m have been obtained.

The experimental results indicate that the first order reaction of malachite green cation with water is catalysed by CTAB micelles while the second order reaction of malachite green cation with hydroxide ion is inhibited by CTAB micelles. The first order rate constant in CTAB micellar phase, k_1m , is 210 times of that in the bulk phase, but the second order rate constant in CTAB micellar phase, k_2m , is 0. 166 time of that in the bulk phase. The results are interpreted mostly in relation to the micellar micropolarity and electrostatic interaction. @Keywords: Micelle, Micellar catalysis, Parallel first order and seond order reactions, Malachite green     

Influence of N-hexadecyl-N,N,N-trimethylammonium nitrate on the dehydrobromination reaction of 2-phenylethyl derivatives

The basis dehydrobromination of p-substituted 2-phenylethyl bromides promoted by hydroxide ion has been studied in micelles of N-hexadecyl-N,N,N-trimethylammonium nitrate (CTANO₃). The pseudophase ion exchange model can be adapted to explain the kinetic results at low hydroxide ion concentration, i.e., less than 0.075 M of NaOH. At high hydroxide ion concentration in solution the applied model does not describe the kinetic results unless the empirical variations are applied for the mol ratio of reactive ion to micellar head group (β) and the binding constant of the organic reagent to the micelle (K_s).     

Catalytic action of CTAB/KBr/C9OH micellar media on the basic hydrolysis of crystal violet

The kinetics of basic hydrolysis of crystal violet (CV) in CTAB/KBr/C₉OH micellar media was investigated under pseudo-first-order conditions. The reaction was monitored spectrophotometrically by measuring the decrease in absorbance of CV at 590?nm. It was observed that the pseudo-first-order rate constant increases with increase in C₀. The enhancement of reaction rate with C₀ is explained on the basis of dependence of reaction rate on micellar morphology. Further, the viscosity and DLS analysis supports nonanol-induced morphological transitions. Fluorescence spectroscopy has been used to understand dye–micelles interactions. The enhancement of fluorescence intensity of CV with C₀ suggests an increase in dye–micelles interaction with C_0. The concentration of surfactant and salt had a marked effect on reaction rate. The inhibition of reaction rate at high concentration of surfactant and salt is due to the ionic competition of OH^? and Br^? ions for the reaction center. The influence of [OH^?] on CV hydrolysis was also investigated. The results show that the pseudo-first-order rate constant, k’, increases linearly with hydroxide ion concentration, indicating first-order dependence on [OH^?].     

Kinetics of the alkaline hydrolysis of fenuron in aqueous and micellar media

The kinetics of the hydrolysis of fenuron in sodium hydroxide has been investigated spectrometrically in an aqueous medium and in cationic micelles of cetyltrimethylammonium bromide (CTAB) medium. The reaction follows first‐order kinetics with respect to [fenuron] in both the aqueous and micellar media. The rate of hydrolysis increases with the increase in [NaOH] in the lower concentration range but shows a leveling behavior at higher concentrations. The reaction followed the rate equation, 1/k_obs = 1/k + 1/(kK[OH^?]), where k_obs is the observed rate constant, k is rate constant in aqueous medium, and k is the equilibrium constant for the formation of hydroxide addition product. The cationic CTAB micelles enhanced the rate of hydrolytic reaction. In both aqueous and micellar pseudophases, the hydrolysis of fenuron presumably occurs via an addition–elimination mechanism in which an intermediate hydroxide addition complex is formed. The added salts decrease the rate of reaction. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 638–644, 2007     

Investigation on the basic hydrolysis of crystal violet in mixed reverse micelles formed with AOT and nonionic surfactants

 The kinetics and thermodynamics of the basic hydrolysis of crystal violet (CV) in mixed reverse micelles formed with anionic surfactant AOT and nonionic surfactants have been investigated. It was found that the mixed reverse micelles had inhibitory effects on CV hydrolysis compared with the normal aqueous solution, and the equilibrium constant K of the reaction in mixed reverse micellar systems is smaller than that in pure water. The influence of water content and surfactant composition in reverse micelles on the second-order rate constant k ₁ of the positive reaction, on the first-order rate constant k _-1 of the reverse reaction, as well as on the equilibrium constant K of the reaction has been studied, and the results obtained were interpreted in terms of the nature of surfactants and the properties of microenvironment where the reaction took place. Received: 24 October 1997 Accepted: 18 March 1998     

Kinetics of alkaline fading of methyl violet in micellar solutions of surfactants: Comparing Piszkiewicz's,Berezin's,and pseudophase ion-exchange models

The micellar effect of surfactants of various types on the rate of the reaction between methyl violet and hydroxide ion is studied. The absorption spectra show that the cation of methyl violet is bound by micelles of all types at proper concentrations of surfactants. The observed rate constant in micellar systems containing nonionic Brij-35, zwitterionic 3-(dimethyldodecylammonio)-propanesulfonate, cationic cetyltrimethylammonium bromide and hydroxide surfactants is higher, whereas in solutions of the anionic surfactant sodium dodecylsulfate is lower than that one in the surfactant-free system. Piszkiewicz's, Berezin's, and pseudophase ion-exchange models of the kinetic micellar effect are used for the treatment of the dependences of the above-mentioned constants on the surfactant concentration. The values of the corresponding kinetic parameters are compared and discussed. The influence of nonionic, zwitterionic, and anionic micelles on the reaction rate is discussed on the basis of medium and concentration kinetic effects. The character of the cationic micelles effect is somewhat paradoxical. Although the observed pseudo–first-order reaction rate constant substantially increases in the presence of such micelles, the second order-rate constant in these micelles is lower than the corresponding value in surfactant-free aqueous solution. As a possible explanation, the decrease in the reactivity of the HO^– ions is proposed, owing to their electrostatic association with the cationic headgroups (“diverting effect”).     

阳离子表面活性剂溴代十六烷基吡啶胶束对2,4-二硝基氯苯水解反应的影响

阳离子表面活性剂溴代十六烷基吡啶(CPB)胶束对水中OH^-和2, 4-二硝基氯苯的反应有催化作用.随CPB浓度的增大, 反应速率常数开始时迅速增大, 然后趋于常数.OH^-浓度的增大使胶束催化水解的二级速率常数减小.CPB胶束对此反应的速率提高作用比十六烷基三甲基溴化扶胶束更有效.对这两种胶束催化作用的差异进行了讨论.     

Kinetics and mechanism of intramolecular general base-catalyzed alkanolysis of phenyl salicylate in the presence of anionic micelles

The alkanolysis of ionized phenyl salicylate, PS^?, has been studied in the presence and absence of micelles of sodium dodecyl sulphate, SDS, at 0.05 M NaOH, 30 or 32°C and within the alkanol, ROH, (ROH = HOCH₂CH₂OH and CH₃OH) contents of 15–74 or 92%, v/v. The alkanolysis of PS^? involves intramolecular general base catalysis. At a constant concentration of SDS, [SDS]_T, the observed pseudo first-order rate constants, k_obs, for the reactions of ROH with PS^? obtained at different concentration of ROH, [ROH]_T, obey the relationship: k_obs = k[ROH]_T/(1 + K_A[ROH]_T) where k is the apparent second-order rate constant and K_A is the association constant for dimerization of ROH molecules. Both k and K_A decrease with increase in [SDS]_T. At a constant [ROH]_T, the rate constants, k_obs, show a decrease of nearly 2-fold with increase in [SDS]_T from 0.0–0.3M. These results are explained in terms of pseudo-phase model of micelle. The rate constants for alkanolysis of PS^? in micellar pseudophase are insignificant compared with the corresponding rate constants in aqueous-alkanol pseudophase. This is attributed largely to considerably low value of [ROH] in the specific micellar environment where micellar bound PS^? molecules exist. The increase in [ROH]_T decrease the value of the binding constant of PS^? with SDS micelle. The effects of anionic micelles on the rates of alkanolysis of PS^? are explained in terms of the porous cluster micellar structure.     

Solubilization of styrene in the catanionic system dodecyltrimethylammonium hydroxide–n′-dodecanephosphonic acid

 The solubilization of styrene in micelles of the catanionic surfactant dodecyltrimethylammonium hydroxide (DTAOH)–n-dodecane-phosphonic acid (DPA) was studied by UV–Vis. spectrometry, as a function of the DTAOH:DPA proportion in the surfactant mixture. The styrene molecules are adsorbed at the surface of the micelles, with the vinyl group closer to the hydrocarbon core than the aromatic ring, which is oriented to the water. In micelles with an excess of DTAOH, the dielectric constant of the water surrounding the micelles was strongly affected by the non-neutralized –N(CH₃)⁺ ₃ groups at the Stem layer. In micelles with an excess of DPA, the –PO₃H₂ groups which are not neutralized by –N(CH₃)⁺ ₃, remain almost unionized and hydrogen-bonded. The effect of the micellar surface on the surrounding water dielectric constant dropped sharply. The dielectric constant in the hydrogen-bonded polar layer is ∼65, rising to the value of pure water very close to the micellar surface. Received: 2 September 1997 Accepted: 20 October 1997     

Micellar effects upon electron transfer from ferrocenes

The oxidations of ferrocene (FcH) and n-butylferrocene (FcBu) by ferric salts (nitrate or bromide) are strongly inhibited by aqueous cetyltrimethylammonium bromide and nitrate (CTABr and CTANO₃, respectively). The kinetics of inhibition fit a model in which the substrates are distributed between water, and the micelles and binding constants K_s to the micelle can be estimated. The oxidations are strongly catalyzed by micelles of sodium lauryl sulfate (NaLS), and the kinetics can be fitted to a model in which the reaction rate depends upon the concentration of both reactants in the micellar pseudophase and the rate constants in that pseudophase, which for both substrates are very similar to those in water. Some added salts reduce the micellar catalysis by excluding ferric ions from the micelle. The oxidations of FcH and FcBu by ferricyanide ions are too fast to be followed in water, but they are inhibited by anionic micelles of NaLS. By analyzing the rate surfactant profiles using independently measured values of K_s the second-order rate constants in water have been estimated.     

Water-induced micellar structure change in Pluronic P103/water/o-xylene ternary system

Both laser light scattering (LLS) and small-angle X-ray scattering (SAXS) were used to study the water-induced formation and structure of micelles and supramolecules of Pluronic P103 [(EO)₁₇(PO)₆₀(EO)₁₇] in o-xylene, a selective solvent for the long middle block. In pure o-xylene, P103 molecules exist as unimer coils with an equivalent hard-sphere radius of 1.6 nm even at fairly high concentrations. Micelles with a PEO/water core and a PPO dominated corona were formed in the presence of water when the P103 concentration ≥0.046 g/mL. The size and structure of micelles have been studied as a function of solubilized water content Z (the molar ratio of water to EO units) in micelles. The micelles change from a somewhat open structure with some EO units either dangling out of the micellar core or being incorporated into neighboring micellar cores at low Z values to a flower-like structure with relatively sharp interface at high Z values. At low Z values (< about 2.9), micelles tend to have a structure with part of the poorly solvated PEO blocks present in the corona. With more water added to the core, the PEO blocks in the corona gradually entered into the core, and the PPO blocks backfolded to form loops. With increasing Z, the micellar core radius, R_c, and the hard-sphere volume fraction, ϕ, of micelles increased; the aggregation number, N, kept nearly a constant; but the hydrodynamic radius, 〈R_h〉₀, and the corona thickness, R_s, decreased. At high Z values (> about 2.9), micelles have a flower-like structure with the two end PEO blocks belonging to the small micellar core. With increasing Z, the values of R_c, ϕ, and N increased, while R_s kept nearly a constant. In the concentrated regime (C > 0.30 g/mL), a stiff polymer network at a critical ϕ value of 0.49 was formed. The supramolecular structures with a face-center cubic packing, and a possible hexagonal packing at higher polymer concentrations (i.e. > 0.55 g/mL), were observed, respectively. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 889–900, 1998     

Kinetic micellar effects in tetradecyltrimethylammonium bromide-pentanol micellar solutions

The reactions 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane + OH(-) and 2-(p-nitrophenyl)ethyl bromide + OH(-) were studied in tetradecyltrimethylamonium bromide, TTAB, and TTAB-pentanol micellar solutions. The influence of changes in the surfactant concentration as well as changes in the hydroxide ion concentration on the observed rate constant was investigated. If changes in the cmc and ionization degree provoked by the presence of the different amounts of n-pentanol in the micellar solutions are taken into account, the experimental kinetic data can be rationalized quantitatively by using the PIE model. Assuming that the ion-exchange equilibrium constant, K(OH(-)/Br(-)), for the competition between the bromide and the hydroxide ions in all TTAB and in TTAB-pentanol micellar solutions studied is the same, a good agreement between the theoretical and the experimental kinetic data was found in all the micellar media for the two processes studied. This assumption was checked by experimentally determining the ion-exchange equilibrium constant K(OH(-)/Br(-)) in TTAB and TTAB-pentanol micellar solutions through a spectroscopic method, results showing that the presence of n-pentanol does not affect substantially the value of K(OH(-)/Br(-)). The second-order rate constants obtained from the fittings decrease slightly when the amount of pentanol increases, being greater than that in aqueous solution. This acceleration can be explained considering that micelles accelerate the reactions in which the charge is delocalized in the transition state.     

Kinetics of the autoxidation of adrenaline and [copper(II)(adrenaline)]2+ in alkaline aqueous and micellar media

The kinetics of autoxidation of adrenaline and [Cu(adrenaline)]²⁺ complex by dissolved oxygen in alkaline aqueous and micellar media has been studied. The reaction is initiated by the removal of amino-H⁺ protons of adrenaline by hydroxide ion, followed by cyclization. The values of (1/k _obs) for the autoxidation of both species were found to be linearly dependent upon 1/[OH^?]. The reaction follows a consecutive pathway in which the intermediate adrenochrome remains stable for few minutes, and then undergoes further reactions to yield adrenolutin and other products. The [Cu(adrenochrome)]⁺ complex is stable for a few hours. Studies on the effects of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) on the reactivity of both species revealed different behaviors. The micelles of CTAB catalyzed the rates of autoxidation for both species, whereas SDS micelles inhibited the autoxidation of adrenaline but catalyzed the rate of autoxidation of [Cu(adrenaline)]²⁺. Addition of the reactive counterion surfactant, cetyltrimethylammonium hydroxide (CTAOH) initially increased the rate constant with the increasing [CTAOH], until it reached a plateau for k _ψ ?[CTAOH]. Salts such as NaCl, NaBr, tetramethyl ammonium bromide, and tetraethyl ammonium bromide increased the rate when added at lower concentrations, but had negligible effect at higher concentrations. The results obtained in micellar media were treated according to Berezin’s Pseudophase Model. The various kinetic parameters for the reactions occurring in aqueous and in micellar media are reported.     

Effect of cationic and anionic surfactants on the addition–elimination-type interaction between o-toluidine and d-glucose

The kinetics of the o-toluidine–d-glucose reaction has been studied as a function of [o-toluidine], [d-glucose], [acetic acid], and temperature by UV–visible spectrophotometry at 630 nm in the absence and presence of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). The reaction follows second-order kinetics, being unity in each of the reactants in both media. The effect of added surfactants has also been investigated. The model of micellar catalysis, such as the Menger–Portony model modified by Bunton, is applied to explain the catalytic role of CTAB and SDS micelles. The association/incorporation constants (K _s and K _n), the rate constant in micellar media (k _m), and the activation parameters of this system have been calculated and discussed. The value of the rate constant is found to be higher in SDS than in CTAB. Hydrophobic and electrostatic interactions are responsible for higher reaction rates in SDS. From all observed facts, a reaction mechanism involving a nucleophilic addition–elimination path has been suggested.     

Kinetics of the alkaline hydrolysis of isoproturon in CTAB and NaLS micelles

Kinetics of the alkaline hydrolysis of isoproturon has been studied in the absence and presence of cetyltrimethylammonium bromide (CTAB) and sodium lauryl sulfate (NaLS) micelles. CTAB micelles were found to enhance the rate of reaction, while NaLS micelles inhibited the reaction rate. The reaction obeyed first‐order kinetics in [isoproturon] and was linearly dependent on [NaOH] at lower concentration. The rate of reaction became independent at higher [NaOH]. At lower [NaOH] the reaction proceeded via formation of hydroxide ion addition complex, while at higher [NaOH] the reaction occurred via deprotonation of ? NH? , leading to the formation of isocyanate. The values of k_w, k_m, and K_s were determined by considering the pseudophase ion exchange model. The activation parameters have also been reported. The effect of added salts (NaCl and KNO₃) on the reaction rate has also been studied. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 39–45, 2007     

Study of the Reaction 2-(p-Nitrophenyl)Ethyl Bromide + OH(-) in Sulfobetaine Aqueous Micellar Solutions in the Presence and Absence of Added Salts

The reaction of dehydrobromination of 2-(p-nitrophenyl)ethyl bromide with hydroxide ions has been studied in aqueous micellar solutions of N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, SB3-14. The kinetic effects of added salts (NaF, NaCl, NaBr, and NaNO(3)) on the reaction rate in SB3-14 aqueous micellar solutions have also been studied. They were rationalized by considering the binding of the anions, which come from the salt, to the sulfobetaine micelles and their competition with the reactive hydroxide ions for the micellar surface. The equilibrium binding constant of the 2-(p-nitrophenyl)ethyl bromide to the sulfobetaine micelles was estimated by recording the changes in the spectra of the organic substrate when the SB3-14 concentration in the micellar medium changed. This value was in agreement with that obtained from fitting of kinetic data. The second-order rate constant in the micellar pseudophase revealed that the reaction is faster in SB3-14 micelles than in water. This acceleration seems independent of the presence of added salts and can be explained by considering that SB3-14 micelles favor reactions in which charge is delocalized in the transition state. Copyright 2001 Academic Press.     

Surfactant effects on the reaction of 2‐(4‐cyanophenoxy)‐quinoxaline with hydroxide ion

A reaction of 2‐(4‐cyanophenoxy)quinoxaline 1 with hydroxide ion is accelerated by supramolecular aggregates of cetyltrialkylammonium chlorides (alkyl = Me, n‐Pr, and n‐Bu). In diluted surfactant solutions, with relatively high substrate concentration (7.0 × 10^?5 M), rate constants go through double rate maxima with increase in the surfactant concentration. The first rate maximum is ascribed to a reaction occurring in premicellar aggregates and the second to reaction in micelles. At low substrate concentration (7 × 10^?6 M), second‐order rate constants in the micellar pseudophase are dependent on the surfactant headgroup size, which is related to charge dispersion in the transition state. Nonmicellizing tri‐n‐octylmethylammonium ions (TOAMs) increase the reaction of 1 with hydroxide ion. The observed rate enhancements may be due to the formation of small, hydrophobic aggregates which bind the substrate and promote the nucleophilic substitution reaction. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 510–515, 2006     

Effects of alcohols on micellization and on the reaction methyl 4‐nitrobenzenesulfonate + Br− in cetyltrimethylammonium bromide aqueous micellar solutions

The effects of n‐hexanol, n‐pentanol, and n‐butanol on the critical micelle concentration (cmc), on the micellar ionization degree (α), and on the rate of the reaction methyl 4‐nitrobenzenesulfonate + Br^? have been investigated in cetyltrimethylammonium bromide (CTAB) aqueous solutions. An increase in the alcohol concentration present in the solution produces a decrease in the cmc and an increase in the micellar ionization degree. Kinetic data show that the observed rate constant decreases as alcohol concentration increases. This result was rationalized by considering variations in the equilibrium binding constant of the methyl 4‐nitrobenzenesulfonate molecules to the micelles, variations in the interfacial bromide ion concentration, and variations in the characteristics of the water–alcohol bulk phase provoked by the presence of alcohols. When these operative factors are considered, kinetic data in this and other works show that the second‐order rate constants in the micellar pseudophases of water–alcohol micellar solutions are quite similar to those estimated in the absence of alcohols. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 634–641, 2004     

Catalytic effect of CTAB reverse micelles on the kinetics of dissociation of bis(2,4,6–tripyridyl-s-triazine) iron(II)

The kinetics of dissociation of bis(2,4,6–tripyridyl-s-triazine) iron(II), ([Fe(TPTZ)₂]²⁺) has been studied in CTAB/chloroform/hexane reverse micellar medium. In the absence of acid, the reaction is immeasurably slow and does not go to completion in conventional aqueous medium but is markedly accelerated and takes place with a rate constant equal to 55.3 × 10^?3 s^?1 and goes to completion in reverse micelles. The significant increase in rate is attributed to the special properties of the water pool in the reverse micelles like low dielectric constant, nucleophilic effect of Br^- ion, and favorable partitioning of TPTZ in the organic phase. The rate of the reaction decreases with increase in W (=[H₂O]/[CTAB]) at constant CTAB concentration and remains constant with increase in CTAB at fixed W. The results are compared with other closely related systems.