Aqueous Cationic Micelles Research Articles

Micellar, salt, and organic solvent effects on the rate of [Cu(II)–Gly-l-Ala]+ complex–ninhydrin reaction

The kinetics of the title reaction has been investigated both in the aqueous and aqueous cationic micelles of tetradecyltrimethylammonium bromide (TTABr) solutions using UV–vis spectrophotometry at 343K and pH5.0. Ninhydrin reacts with [Cu(II)–Gly-l-Ala]+ complex in the stoichiometric ratio of 1:1 in both media. To explain the dependence of the reaction rate on [TTABr], pseudo-phase model proposed by Berezin et al. [27] and developed by Bunton [28] and Vera and Rodenas [29] was used. The reaction follows first- and fractional-order kinetics with respect to [Cu(II)–Gly-l-Ala]T+ and [ninhydrin]T, respectively, in aqueous and TTABr micelles. The values of km, kw, KA, KNin, Ea, ΔH#, ΔS#, and ΔG# have been calculated. The effect of inorganic (NaNO3, Na2SO4, Na3PO4)/organic (NaBenz, NaSal) salts and organic solvents on the reaction rate has also been seen. A mechanism consistent with observed kinetic data has been proposed.

Kinetic and Mechanistic Studies on [Zn(II)-Gly-Phe]+–Ninhydrin Reaction in Aqueous and Cationic CTAB Surfactant Micelles

The rates of reaction between metal-dipeptide complex ([Zn(II)-Gly-Phe]+) and ninhydrin have been determined in aqueous and aqueous–cationic micelles of cetyltrimethylammonium bromide (CTAB) at 70°C and pH 5.0. The rate data indicate that the reaction follows the template reaction mechanism in both the media. The reaction followed a first-order and fractional-order kinetics with respect to [Zn(II)-Gly-Phe]+ and [ninhydrin], respectively, in the excess of ninhydrin over [Zn(II)-Gly-Phe]+. The rate constant is affected by [CTAB] changes and maximum rate enhancement is approximately three-fold. CTAB micelles decrease the activation enthalpy and make the activation entropy less negative. Quantitative kinetic analysis of rate constant (k ψ)–[CTAB] data was performed on the basis of pseudophase model of the micelles (proposed by Menger and Portnoy and developed by Bunton). The values of binding constants K S for [Zn(II)-Gly-Phe]+ and K N for ninhydrin with micelles are calculated with the help of observed kinetic data. The results obtained in micellar medium are treated quantitatively on the basis of pseudophase model.

Determination of an Ion Exchange Constant by the Use of a Kinetic Probe: A New Semiempirical Kinetic Approach Involving the Effects of 3-F- and 4-F-Substituted Benzoates on the Rate of Piperidinolysis of Anionic Phenyl Salicylate in Aqueous Cationic Micelles

Pseudo-first-order rate constants (k(obs)) for the nucleophilic substitution reaction of piperidine (Pip) with ionized phenyl salicylate (PS(-)), obtained at a constant [Pip](T) (= 0.1 M), [PS(-)](T) (= 2 x 10(-4) M), [CTABr](T) (cetyltrimethylammonium bromide), < or = 0.06 M NaOH, and a varying concentration of MX (= 3-FC(6)H(4)CO(2)Na, 3-FBzNa and 4-FC(6)H(4)CO(2)Na, 4-FBzNa), follow the kinetic relationship k(obs) = (k(0) + thetaK(X/S)[MX])/(1 + K(X/S)[MX]) which is derived by the use of the pseudophase micellar (PM) model coupled with an empirical equation. The empirical equation explains the effects of [MX] on CTABr micellar binding constant (K(S)) of PS(-) that occur through X(-)/PS(-) ion exchange. Empirical constants theta and K(X/S) give the parameters F(X/S) and K(X/S), respectively. The magnitude of F(X/S) gives the measure of the fraction of micellized PS(-) transferred to the aqueous phase by the limiting concentration of X(-) through X(-)/PS(-) ion exchange. The values of F(X/S) and K(X/S) have been used to determine the usual thermodynamic ion exchange constant (K(X)(Y)) for ion exchange process X(-)/Y(-) on the CTABr micellar surface. The values of K(X)(Br) (where Br = Y) have been calculated for X = 3-FBzNa and 4-FBzNa. The mean values of K(X)(Br) are 12.8 +/- 0.9 and 13.4 +/- 0.6 for X(-) = 3-FBz(-) and 4-FBz(-), respectively. Nearly 3-fold-larger values of K(X)(Br) for X = 3-FBz(-) and 4-FBz(-) than those for X = Bz(-), 2-ClBz(-), 2-CH(3)Bz(-), and the 2,6-dichlorobenzoate ion (2,6-Cl(2)Bz(-)) are attributed to the presence of wormlike micelles in the presence of > 50 mM 3-FBz(-) and 4-FBz(-) in the [CTABr](T) range of 5-15 mM. Rheological properties such as shear thinning behavior of plots of shear viscosity versus the shear rate at a constant [3-FBz(-)] or [4-FBz(-)] as well as shear viscosity (at a constant shear rate) maxima as a function of the concentrations of 3-FBz(-) and 4-FBz(-) support the conclusion, derived from the values of K(X)(Br), for the probable presence of wormlike/viscoelastic micellar solutions under the conditions of the present study.

Micellar and Solvent Effects on the Rate of Reaction Between L-Tyrosine and Ninhydrin

Kinetics of the reaction of L-tyrosine and ninhydrin has been studied spectrophotometrically at pH = 5.0 and temperature 80°C in aqueous, aqueous-organic and aqueous cationic micelles of cetyltrimethylammonium bromide (CTAB). The product formed in all the media remains the same. Also, the reaction is catalyzed in both organic solvent and CTAB micellar media. The results obtained in micellar media are treated quantitatively in terms of pseudo-phase model. The rate constants and binding constants with the micelles have been evaluated.

Kinetics and mechanism of interaction of dipeptide (glycyl–glycine) with ninhydrin in aqueous micellar media

AbstractThe rates of reaction between ninhydrin and dipeptide glycyl–glycine (Gly–Gly) have been determined by studying the reaction spectrophotometrically at 70°C and pH 5.0 in aqueous and in aqueous cationic micelles of cetyltrimethylammonium bromide (CTAB). The reaction follows first‐ and fractional‐order kinetics, respectively, in [Gly–Gly] and [ninhydrin]. The observed rate constant is affected by [CTAB] changes and the maximum rate enhancement is ca. three‐fold. As the kψ − [CTAB] profile shape is characteristic of bimolecular reactions catalyzed by micelles, the catalysis is explained in terms of the pseudo‐phase model of the micelles (proposed by Menger and Portnoy and developed by Bunton and Romsted). The presence of inorganic salts (NaCl, NaBr, Na2SO4) does not reveal any regular effect but the data with organic salts (NaBenz, NaSal) show an increase in the rate followed by a decrease. The kinetic data have been used to calculate the micellar binding constants KS for Gly–Gly and KN for ninhydrin and the respective values are 317 and 69 mol−1 dm3. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 643–650, 2006

New Application of 1,4‐Dihydropyridine System: Michael Reactions Mediated by 1,4‐Dihydropyridine‐Enolate Adduct in Micellar Medium.

AbstractFor Abstract see ChemInform Abstract in Full Text.

New Application of 1,4-Dihydropyridine System: Michael Reactions Mediated by 1,4-Dihydropyridine–Enolate Adduct in Micellar Medium

Abstract 1,4-dihydropyridine-acetophenone enolate adduct, in catalytic amount effects Michael reactions in aqueous cationic micelles of cetyltrimethylammonium bromide. The enolate, generated by dissociation of the adduct abstracts a proton from readily enolizable substrates to bring about the Michael reaction under mild conditions in fair to good yields without side products.

Concentration of Urea in Interfacial Regions of Aqueous Cationic, Anionic, and Zwitterionic Micelles Determined by Chemical Trapping

A molecular level description of the effect of urea on the stability of proteins or supramolecular aggregates such as micelles requires information about its interfacial concentration. Estimating interfacial urea concentrations has proved refractory because urea's association with interfaces is difficult to observe spectrometrically. Here we used a chemical trapping method based on product yields from spontaneous dediazoniation of micelle-bound 2,6-dimethyl-4-hexadecylbenzenediazonium ion to estimate the interfacial concentration of urea in micelles of cationic cetyltrimethylammonium halide (CTAX, X = Cl, Br), anionic sodium dodecyl sulfate, SDS, and sodium dodecyl sulfonate, SDSu, and zwitterionic N-hexadecyl-N,N-dimethylammonium propanesulfonate, HPS, surfactants. Up to 6−8 M added urea, the interfacial urea concentration is about 70% (HPS), 80% (CTABr), and 90% (CTACl, SDS, and SDSu) of its stoichiometric concentration in solution, that is, not much less than its concentration in bulk solution. These r...

Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate in aqueous cationic micelles: kinetic evidence of microinterface property changes

We studied decarboxylation of 6-nitrobenzisoxazole-3-carboxylate, 1, as a kinetic probe to investigate microinterface properties of aqueous micelles formed by cationic surfactants of increasing head group bulk, i.e., cetyltrialkylammonium bromide, with alkyl = Me (CTABr), Et (CTEABr), n-Pr (CTPABr), n-Bu (CTBABr) and p-octyloxybenzyltrialkylammonium bromide surfactants with alkyl = Me (pOOTABr), n-Pr (pOOTPABr), and n-Bu (pOOTBABr), and the longer p-dodecyloxybenzyltrimethylammonium bromide (pDoTABr) at concentrations higher than 0.05 M. The pseudophase kinetic treatment fails to fit the data that show anomalies with abrupt increases in k obs for CTPABr and CTBABr (but not for CTEABr) and with smooth and continuos increase of k obs for all p-alkyloxybenzyltrilakylammonium bromides. Abrupt and successive modifications of the micellar interface properties, undergoing only when the polar head or the alkyl chain have some covalent structure, account for the observed kinetic behavior.

Effect of Ethanol on Micellization and on Decarboxylation of 6-Nitrobenzisoxazole-3-carboxylate in Aqueous Cationic Micelles

The effects of ethanol on the critical micellar concentration (cmc) and the rates of decarboxylation of 6-nitrobenzisoxazole-3-carboxylate (6-NBIC) have been investigated in aqueous cationic surfactants of the cetyltrialkylammonium family with bromide [CT(R)ABr], chloride [CT(R)ACl], and nitrate [CT(R)ANO3] counterions, and methyl (CTAX), n-propyl (CTPAX), and n-butyl (CTBAX) as the head group alkyl moieties. Effects upon cmc and reactivity are similar, featuring a break at the ethanol mole fraction, xEtOH, of ca. 0.055; these effects have been related to changes in solvent structure, with formation of a clathrate at xEtOH=0.055. Rate data in CTBABr were further investigated and fitting of raw kinetic data to the pseudophase model is possible up to xEtOH=0.1, showing an unexpected decrease of rate constant values in the micellar pseudophase, k′M, as ethanol content increases: a significant variation of micellar ionization degree could account for this kinetic effect.

Hydrolyses of Dinitroalkoxyphenyl Phosphates in Aqueous Cationic Micelles. Acceleration by Premicelles

Hydrolyses of dianionic 2,4-dinitrophenyl phosphate, substituted at position 5 by Me, OMe, n-OC14H29 (DNMePP2-, DNMeOPP2-, and DNTDOPP2- respectively) are accelerated by cationic micelles of cetyl trialkylammonium bromide (CTABr, CTPABr, and CTBABr, with alkyl = Me, n-Pr, and n-Bu, respectively). There are well-defined rate maxima in dilute surfactant, below the critical micelle concentration, for reaction of DNTDOPP2- in the sequence CTBABr > CTPABr > CTABr, because of acceleration by premicelles which is not observed with the methoxy and unsubstituted derivatives. The rate maxima disappear as these species “dissolve” in micelles at higher surfactant concentrations.

Are the Interfacial Basicities of Aqueous Cationic Micelles and Cationic Reverse Microemulsions Different by Orders of Magnitude?

Formation of 5-hexadecyl-7-methylindazole (16-Ind) from 2,6-dimethyl-4-hexadecylbenzenediazonium tetrafluoroborate (16-ArN2+) bound to aqueous cationic micelles (prepared in aqueous buffers of pH 7.0) has been shown to originate from the remarkably high interfacial basicity of aqueous cationic micelles. The rationale behind our conclusion is that formation of indazole from o-methylarenediazonium salts has never been observed if the medium of the diazonium salts is not basic. On the same ground, the absence of any 16-Ind product from 16-ArN2+ in cationic reverse microemulsions is consistent with the interfacial basicities of cationic reverse microemulsions being significantly lower than that of cationic micelles.

Microscopic Polarities of Interfacial Regions of Aqueous Cationic Micelles: Effects of Structures of the Solvatochromic Probe and the Surfactant

We report on the use of solvatochromic probes to determine the microscopic polarity of water at their (average) solubilization sites in cationic micelles. The following probes were employed: 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-1-phenolate (RB), 1-methyl-8-oxyquinolinium betaine (QB), sodium 1-methyl-8-oxyquinolinium betaine-5-sulfonate (QBS), and 1-methyl-3-oxypyridinium betaine (PB). The cationic surfactants studied were cetyltrimethylammonium bromide, cetyldimethylbenzylammonium bromide, dodecyltrimethylammonium bromide, and dodecyldimethylbenzylammonium bromide. The microscopic polarity, ET, kcal/mol, depends on structures of both probe and surfactant. PB, the most hydrophilic probe, does not partition into the micellar pseudophase, whereas RB, the most hydrophobic one, samples a much lower microscopic polarity than QB and QBS because of its deeper penetration into the cationic micelle. Polarities measured by (zwitterionic) QB and (anionic) QBS differ because the latter probe exchanges for th...

Hydrolysis of 2,4-dinitrophenyl phosphate in normal and reverse micelles

The rate of spontaneous hydrolysis of the 2,4-dinitrophenyl phosphate dianion (DNPP2–) is strongly increased by aqueous cationic micelles of cetyltrialkylammonium bromides (alkyl = Me, Pr, Bu) and tetradecylquinuclidinium bromide. First order rate constants reach limiting values in dilute surfactant where DNPP2– is fully micellar bound. Rate enhancements increase with increasing bulk of the head group and are associated with decreases in activation enthalpies. Betaine sulfonate micelles (C14H29N+R2[CH2]3SO3–, R = Me, Et, Pr, Bu) also speed hydrolysis, but first order rate constants continue to increase even up to 0.2 mol dm–3 surfactant, because DNPP2– is not strongly bound. Reverse micelles of cetyltrialkylammonium bromide (alkyl = Me, Bu) in dichloromethane give rapid hydrolysis, but rate constants decrease on addition of water approximately to the value observed with aqueous cationic micelles as ‘water pool’ reverse micelles of CTABr are formed. N,N-Dimethylhydroxylamine increases the rate of hydrolysis in water, but not in reverse micelles, and a (hypothetical) phosphoramidate is not trapped by F–.

Multicomponent solubilization in aqueous micelles of dodecyl- and tetradecyltrimethylammonium bromide: Solubilization equilibria

Solubilization equilibria for hexane and a series of polycyclic aromatic compounds (dyes) in aqueous cationic micelles of dodecyltrimethylammonium bromide (DTAB) and tetradecyltrimethylammonium bromide (TTAB) were studied experimentally. Solubilizates were compared first individually, for effects of molecular shape and structure, and then as mixtures, in a search for interactive effects. Hexane solutions of solid dyes, in binary mixtures of relative mole fraction 0 to 1 for each dye, were partitioned into the aqueous micellar phase, which was analyzed by ultraviolet-visible spectrophotometry for the dyes and gas chromatography for hexane. Solubilization ratios (SRs), expressed as the ratio of solubilizate to surfactant molecules in a micelle, were obtained from the experimental data. In our experiments, the micelles contained more solubilized hexane than dye. For cases of three-component solubilization (two dyes and hexane), compounds of similar molecular shape and chemical structure performed alike as solubilizates, possibly occupying sites in the micelle interchangeably. However, when a large molecule with three or four fused rings and a smaller two-ring molecule were contained in the micelle, solubilization of the smaller molecule was always enhanced.

Specific micellar rate effects on unimolecular decarboxylation and cyclization

AbstractDecarboxylation of 6‐nitrobenzisoxazole‐3‐carboxylate ion and cyclization of o‐3‐halopropyloxyloxyphenoxide ion (PhY7) (Y = Br, I) are accelerated by aqueous cationic and zwitterionic micelles. For cationic micelles the rate enhancements (kM′kW′) increase with increasing bulk of the surfactant head groups but are largest for decarboxylation. There are good linear free energy relationships between micellar effects on these reactions and the plots of log kM′ for cyclization against log kM′ for decarboxylation have slopes of 0.46 and 0.64 for Y = Br and I, respectively. Surfactants that have twin tail or tri‐N‐alkylbenzyloxy head groups do not fit the relationship. Despite mechanistic differences between decarboxylation and cyclization (an intramolecular SN2 reaction), similar factors control micellar effects on these reactions. In the absence of surfactant there is only a qualitative relationship between free energies of activation of cyclization and decarboxylation in different solvents.

Dephosphorylation by peroxyanions in micelles and microemulsions

AbstractDephosphorylation of p‐nitrophenyl diphenylphosphate (pNPDPP) by OH− in aqueous cationic micelles of cetyltrimethylammonium hydroxide (CTAOH) is inhibited by tert‐butyl hydroperoxide (t‐BuO2H), which reduces the binding of OH−, and whose anion is a relatively ineffective nucleophile. Reaction is faster in microemulsions of CTACl, n‐octane and OH−, with t‐BuO2H as cosurfactant, than in otherwise similar microemulsions with t‐butyl alcohol as cosurfactant. Dephosphorylation by m‐chloroperoxybenzoate ion is slower in microemulsions of cetyltrimethylammonium mesylate or benzenesulfonate, with n‐octane and tert‐butyl alcohol, than in cationic micelles, but a wide range of concentrations can be examined. The reaction rates qualitatively follow the concentration of peroxy acid relative to surfactant, and inert anions slow the reaction. Dephosphorylation by peroxyphthalate dianion in micelles of CTACl fits a pseudophase kinetic model, except in very dilute surfactant where reaction is faster than predicted.

Anomalous salt effects on a micellar‐mediated reaction of bromide ion

AbstractReaction of bromide ion with α‐picryl‐p‐bromoacetophenone (1) is speeded by aqueous cationic micelles of cetyltrimethylammonium bromide (CTABr; C16H33NMe3Br) and dodecyltrimethylammonium bromide (DoTABr; C12H25NMe3Br) and rate constants reach limiting values when 1 is fully bound to micelles of CTABr. Limiting values are not reached in DoTABr, but the data can be fitted to a simple model for the distribution of reactants between water and micelles. Estimated second‐order rate constants at the micellar surface are similar to values in water, but this model cannot explain the observed rate enhancements on addition of NaBr to CTABr. Inert anions such as nitrate, mesylate, n‐butanesulfonate, phenylmethanesulfonate and camphor‐10‐sulfonate inhibit reaction in CTABr by competing with Br− at the micellar surface. Other n‐alkanesulfonate ions (RSO, R = n‐C5H11, n‐C6H13, n‐C7H15, n‐C8H17) and arenesulfonate ions (benzene‐, toluene‐, naphthalene‐1‐ and naphthalene‐2‐sulfonate) behave anomalously. These ions expel Br− from the micelle, as shown electrochemically, but there are maxima in plots of rate constant against mole fraction of Br−. These rate extrema are apparently due to perturbation of the micellar surface structure that overcomes the inhibition due to competition with Br−. These results show that the simple pseudo‐phase, ion‐exchange model can be applied only in dilute electrolyte and in the absence of hydrophobic anions.

Micellar inhibition of S N1 reactions of sterically hindered compounds

Aqueous cationic micelles of cetyltrimethylammonium surfactants (CTAX; X = Br, Cl, or 0.5SO4) and dodecyltrimethylammonium bromide, and anionic micelles of sodium lauryl sulphate (NaLS) inhibit SN1 hydrolyses of sterically hindered arenesulphonates and chlorides. The substrates were 2-adamantylp-bromobenzenesulphonate (brosylate) and p-nitrobenzenesulphonate,1,2,2-trimethylpropyl (pinacolyl) tosylate, brosylate, benzenesulphonate, and p-methoxy- and p-nitrobenzenesulphonate, and 2,2-dimethyl-1-phenylpropyl chloride and tosylate. Micellar inhibition increases with increasing substrate hydrophobicity but is always larger with cationic than with anionic micelles by factors of between 2 and 5 for the arenesulphonate, and 10 for 2,2-dimethyl-1-phenylpropyl chloride. The difference is much larger for hydrolysis of diphenylmethyl chloride and bromide. For hydrolysis of 1-benzylbutyl tosylate, with nucleophilic participation by water, micellar inhibition is smaller than with the hindered substrates but greater with cationic than with anionic micelles. These micellar inhibitions are compared with those on deacylations which are dominated by bond-making and where reaction is faster in cationic than in anionic micelles. Micellar medium effects are related to the mechanisms of these spontaneous hydrolyses and to substrate structure. Substituent effects were examined for SN reactions in water; ρ≈ 1.5 for hydrolyses of pinacolyl arenesulphonates, and is similar for hydrolyses in micelles.

Partition coefficients of 20 polar molecules between water and trimethyldodecylammonium bromide micelles: Correlation with the water + octanol system

The partition coefficient P between trimethyldodecylammonium bromide micelles and water of 20 nonaromatic polar molecules was determined at 298.15°K by a combination of salting constant and critical micelle concentration determinations; included are amines, alcohols, ethers, esters, ketones, and some high dipole moment molecules. The partition coefficients obtained for the alcohols are about one-half their value in aqueous micellar sodium dodecylsulfate solutions. It is shown that for straight, branched, and cyclic amines, alcohols, esters, and ketones there is an excellent correlation between partition coefficients in the aqueous cationic micelles and in the classical octanol + water system. Some ethers and halogenated alkanes do not follow this correlation; the P values in these cases in the micellar solutions are about one-third of the values predicted from the correlation. The same observation can be made for halogenated alkanes in aqueous sodium dodecylsulfate solutions. The behavior of simple polar solutes in the micellar systems studied resembles that found with dimyristoyl lecithin liposomes. The average solubilization site of the nonaromatic solutes onto the cationic micelles may be looked upon as having the properties of a solvent such as isobutanol saturated with water.