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The bimolecular reactions methyl naphthalene-2-sulfonate + Br − , MNS + Br − , and 2-(4-nitrophenyl) ethyl bromide + OH − , PEB + OH − , were studied in micellar solutions of cationic monomeric and dimeric surfactants with phenyl and cyclohexyl rings in the head group. Morphological transitions occur in these micellar solutions when surfactant concentration increases. A kinetic equation based on the pseudophase model could not quantitatively explain the kinetic micellar effects on the process MNS + Br − . Micellar growth brings about a decrease in the micellar ionization degree which has to be considered in order to explain the experimental kinetic data. Besides, the formation of elongated micelles is followed by changes in the characteristics of the interfacial region which can affect the equilibrium binding constant as well as the second order rate constant at the micellar pseudophase. These medium micellar effects are larger the stronger the tendency of the surfactant to micellar growth is. The kinetic micellar effects on the reaction PEB + OH − were quantitatively rationalized by considering a kinetic equation based on the ion-exchange pseudophase model. This could be taken as indicative that the equilibrium ion-exchange constant controlling the exchange between the surfactant counterion, Br − , and OH − at the micellar surface does not substantially change with micellar growth. The effects of the number of alkyl chains and of the nature of the ring present in the head group on reactivity were explained by considering their influence on the polarity of the interfacial region and on the tendency of the micelles to grow.

The dehydrobromination reaction 2-(p-nitrophenyl)ethyl bromide + OH− was investigated in several alkanediyl---bis(dodecyldimethylammonium) bromide, 12-s- 12,2Br− (with s = 2, 3, 4, 5, 6, 8, 10, 12) micellar solutions, in the presence of NaOH 5 × 10−3 M. The kinetic data were quantitatively rationalized within the whole surfactant concentration range by using an equation based on the pseudophase ion-exchange model and taking the variations in the micellar ionization degree caused by the morphological transitions into account. The agreement between the theoretical and the experimental data was good in all the dimeric micellar media studied, except for the 12-2-12,2Br− micellar solutions. In this case, the strong tendency to micellar growth shown by the 12-2-12,2Br− micelles could be responsible for the lack of accordance. Results showed that the dimeric micelles accelerate the reaction more than two orders of magnitude as compared to water Consejería de Innovación, Ciencia y Empresa de la Junta de Andalucía FQM-274and P07-FQM-03056 DGCYT grant BQU2009-07478

The SN2 substitution reactions between methyl naphthalene-2-sulfonate and methyl 4-nitrobenzenesulfonate with bromide ions were studied in alkanediyl-α,ω-bis(dimethyldodecylammonium) bromide, 12-s-12,2Br− (s = 3, 4, 6), micellar solutions in the absence and in the presence of alcohols. The observed rate constant for the two processes was found to be sensitive to changes in micellar characteristics caused by the presence of alcohols. At high surfactant concentrations, kinetic data made evident the sphere-to-rod transition accompanying the increase in the dimeric surfactant concentration in all the micellar reaction media investigated. Kinetic results were quantitatively rationalized within the whole surfactant concentration range, below and above the second CMC, by using a new treatment based on the pseudophase kinetic model.

The reaction methyl naphthalene-2-sulfonate + Br(-) was investigated in several alkanediyl-α-ω-bis(dodecyldimethylammonium) bromide, 12-s-12,2Br(-) (with s = 2, 3, 4, 5, 6, 8, 10, 12), micellar solutions in the absence and in the presence of various additives. The additives were 1,2-propylene glycol, which remains in the bulk phase, N-decyl N-methylglucamide, MEGA10, which forms mixed micelles with the dimeric surfactants, and 1-butanol, which distributes between the aqueous and micellar phases. Information about the micellar reaction media was obtained by using conductivity and fluorescence measurements. In all cases, with the exception of water-1,2-prop 12-5-12,2Br(-) micellar solutions, with 30% weight percentage of the organic solvent, a sphere-to-rod transition takes place upon increasing surfactant concentration. In order to quantitatively explain the experimental data within the whole surfactant concentration range, a kinetic equation based on the pseudophase kinetic model was considered, together with the decrease in the micellar ionization degree accompanying micellar growth. However, theoretical predictions did not agree with the experimental kinetic data for surfactant concentrations above the morphological transition. An empirical kinetic equation was proposed in order to explain the data. It contains a parameter b which is assumed to account for the medium micellar kinetic effects caused by the morphological transition. The use of this empirical equation permits the quantitative rationalization of the kinetic micellar effects in the whole surfactant concentration range.

Micellization and micellar growth of cationic dimeric surfactants of the alkanedyil-α,ω-bis(dimethyldodecylammonium) bromide type, 12-s-12,2Br− (s = 3, 4, 6), in the presence of various amounts of 1-butanol, 1-pentanol, and 1-hexanol have been investigated. The influence of the nature and concentration of alcohol on the cmc, on the micellar ionization degree, on the average micellar aggregation number, and on the polarity of the micellar interfacial region was investigated by using conductivity and fluorescence measurements. Subsequently, effects of alcohol addition on the surfactant concentration range where sphere to rod transitions occur were examined and information about changes in the micropolarity and in the microviscosity accompanying the morphological transition was obtained. The experimental results were explained by considering the variations in the different contributions to the Gibbs energy of micellization caused by the presence of alcohols. The study of the reaction methyl naphthalene-2-sulfonate + Br− in some water–alcohol 12-6-12,2Br− micellar solutions provided information about the characteristics of the dimeric micelles as microreactors and show the complexity of the microheterogeneous systems studied.

Aggregation of tetradecyltrimethylammonium bromide, TTAB, in mixed solvent systems containing ethylene glycol, EG, 1,2-propylene glycol, 1,2-PROP, 1,3-propylene glycol, 1,3-PROP, and tetraethylene glycol, TEG, has been investigated by employing conductivity and fluorescence methods. Gibbs energies of micellization were determined in order to evaluate the effects of the co-solvent on the aggregation process. Information about the influence of the organic solvent on the surfactant adsorption at the air-solution interface, on the micellar size and on the polarity of the micellar interfacial region was obtained by means of surface tension and fluorescence measurements. The study of the reaction methyl naphthalene-2-sulfonate + Br - in the water–glycol TTAB micellar solutions provided information about the characteristics of TTAB micelles as microreactors in the water–solvent binary mixtures.

Effects of ethylene glycol (EG) addition on the micellization and on the micellar growth in two aqueous didodecyl dicationic dibromide surfactant, 12-s-12,2Br- (s=2, 6) solutions, with the weight percentage of EG up to 50%, have been investigated. An increment in the amount of EG makes the aggregation process less spontaneous due to the water-EG mixtures being better solvents for the cationic dimeric surfactant molecules than pure water (solvophobic effect). Results show that C*, the surfactant concentration where the sphere-to-rod transition occurs, increases when EG content in the bulk phase increases. The amount of the organic solvent influences C* principally through the decrease in the hydrocarbon/bulk phase interfacial tension (air/bulk phase surface tension) caused by its presence. Changes in the aggregation number, in the micropolarity, in the microviscosity, and in the rheological behavior accompanying micellar growth were studied in the water-EG micellar solutions. Kinetic studies provide information about the characteristics of the dimeric micelles as microreactors. Kinetic data also show that an increase in the surfactant concentration leads to micellar growth.

The reaction between methyl 4-nitrobenzenesulfonate and bromide ions has been studied in mixed single-chain-gemini micellar solutions of n-dodecyltrimethylammonium bromide, DTAB, and dodecyl tricosaoxyethylene glycol ether, Brij35, with alkanediyl-α-ω-bis(dodecyldimethylammonium) bromide, 12-s-12,2Br− ( s = 3 , 4 , 5 ). Kinetic micellar effects show that an increase in the solution mole fraction of the single-chain surfactant, X single - chain , results in a diminution of the mixed micelles tendency to form spherocylindrical aggregates upon increasing surfactant concentration. The dependence of the surfactant concentration at which the sphere-to-rod transition occurs, C ∗ , on X single - chain showed through kinetic data was in agreement with results obtained by means of fluorescence measurements.

Critical micelle concentrations of the Cm TAB+12- s-12 (s=3, 4, 5 and m=10, 12, 14, 16) binary systems have been determined, through conductivity and fluorescence measurements, at 298 K. Application of different theoretical approaches to explain mixed micellization shows that non-ideality of the binary systems follows the trend C16TAB+12-3-12

Experimental data of amphiphiles aggregation phenomena in water-organic solvent mixtures were considered with the idea of investigating the role of the solvophobic effect on micellization. Changes in the critical micelle concentration, in the micellar ionization degree (for ionic surfactants) and in the aggregation number accompanying variations in the composition of the bulk phase of the micellar solutions were examined with the scope of understanding which properties of the water-organic solvent mixtures are important in the micellization process. Results point out that the cohesive energy density, measured either through the Hildebrand-Hansen solubility parameter or the Gordon parameter, seems to play an important role in determining the contribution of the solvophobic effect on the Gibbs energy of micellization in water-organic solvents mixtures.

The reactions 4-nitrobenzenesulfonate + Br− and methyl naphthalene-2-sulfonate + Br− were studied in various water–ethylene glycol, EG, [C12H25(CH3)2N (CH2)sN(CH3)2C12H25]Br2 micellar solutions (12-s-12,2Br− with s = 3 – 5 methylene groups). Results showed that the observed rate constant of the two reactions varied when a sphere-to-rod transition occurs. This morphological transition is accompanied by changes in the interfacial region water content and in its polarity. The micellar ionization degree is also altered, making the counterion interfacial concentration change. Finally, variations on the molar surfactant volume, V m , also follow the sphere-to-rod transitions. A simple pseudophase kinetic model is inadequate for quantitatively discussing the kinetic micellar effects observed, since the changes accompanying micellar growth affect the second-order rate constant in the micellar pseudophase, the equilibrium binding constant and the surfactant molar volume, neither of them remaining constant in the whole surfactant concentration range. However, this simple model can be helpful in the treatment of kinetic data for surfactant concentrations below the morphological transitions, providing some interesting, although approximate, information.

Micellization of cetyltrialkylammonium bromide (alkyl = methyl, ethyl, propyl, and butyl) surfactants has been studied in various water–ethylene glycol, EG, mixtures. This study provided the opportunity of investigating how, for a given hydrophobic tail length, the head group size determines the effect of the addition of EG on the micellization process. Conductivity, surface tension, spectroscopic and steady-state fluorescence measurements were carried out. The influence of additional methylenes in the headgroups on the Gibbs energy of micellization, Δ G M ° , was found to derive from changes in the solvophobic Gibbs energy contribution and from variations in the electrostatic headgroups interaction contribution.

The reaction methyl 4-nitrobenzenesulfonate + Br- was studied in water-formamide tetradecyltrimethylammonium bromide micellar solutions. A pseudophase kinetic model was used to quantitatively rationalize the kinetic micellar effects observed

The reaction of bromide ions with methyl naphthalene-2-sulfonate (MeNS) has been investigated in water–dimethyl sulfoxide, DMSO, tetradecyltrimethylammonium bromide, TTAB, micellar solutions, with the weight percentage of DMSO up to 50%. In order to quantitatively rationalize the micellar kinetic effects observed, conductivity, surface tension, and steady-state fluorescence measurements were used to get information about the micellar reaction media. Results showed that changes caused by the addition of different amounts of DMSO to TTAB aqueous micellar solutions are made evident from the kinetic micellar effects, these being a helpful tool to obtain information on the micellar reaction media in the presence of the added organic solvent. Copyright © 2006 John Wiley & Sons, Ltd.

The influence of the addition of various amounts of ethylene glycol, EG, up to a weight percent of 50%, on the micellization process in N-hexadecyl, N-tetradecyl, and N-dodecyltrimethylammonium chloride micellar solutions was investigated. Conductivity, fluorescence, and spectroscopic measurements give information about changes in the cmc, in the micellar ionization degree, in the aggregation number and in the polarity of the interfacial region upon changing the percentage by weight of the organic solvent. These changes were compared to those found when ethylene glycol was added to the analogous alkyltrimethylammonium bromide aqueous micellar solutions, results showing that the effects caused by the presence of the organic solvent were practically independent of the counterion nature. This conclusion was in agreement with the micellar kinetic effects observed on the spontaneous hydrolysis of phenyl chloroformate in both water-ethylene glycol alkyltrimethylammonium bromide and chloride micellar solutions.

Micellization in water-ethylene glycol (EG) N-dodecyl, N-tetradecyl, and N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB3-12, SB3-14, and SB3-16, respectively) micellar solutions, with the weight percent of EG changing within the range 0-40, was studied by means of surface tension measurements. Information about the influence of the added EG on the aggregation number of the sulfobetaine micelles and on the polarity of the interfacial region of micelles was obtained through fluorescence and spectroscopic measurements. Surface tension measurements also provide information about the dependence of the surface excess concentration, the minimum area per surfactant molecule, the surface pressure at the cmc, and the standard Gibbs energy of adsorption on the added weight percent of the organic solvent. The Gordon parameter of the water-EG mixtures was also estimated by means of surface tension measurements. The thermodynamic and structural changes originated by the presence of EG control the micellar kinetic effects observed in the reaction methyl 4-nitrobenzenesulfonate + Br(-) occurring in the water-EG sulfobetaine micellar solutions. Information about the distribution of bromide ions between the bulk and micellar pseudophases was obtained through conductivity measurements. The kinetic micellar effects were quantitatively explained by using the pseudophase kinetic model.

The spontaneous hydrolysis of phenyl chloroformate was studied in water-ethylene glycol, EG, cationic, zwitterionic, nonionic, and anionic micellar solutions, the surfactants being tetradecyltrimethylammonium bromide, tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, tricosaoxyethylene glycol ether, and sodium dodecyl sulfate. The dependence of the observed rate constant on surfactant concentration as well as on the percentage by weight of EG, varying from 0 to 50 wt %, was investigated. Information about changes in the critical micelle concentrations, in the micellar ionization degrees (for ionic surfactants), in the aggregation numbers, and in the polarity of the interfacial region of the micelles upon changing the weight percent of EG was obtained through conductivity, surface tension, spectroscopic, and fluorescence measurements. A simple pseudophase model was adequate to rationalize the kinetic data. Micellar medium effects were explained by considering charge-charge interactions and polarity, ionic strength, and water content in the micellar interfacial region. The acceleration of the reaction produced by an increase in the amount of EG present in the mixture was explained on the basis of the substantial decrease in the equilibrium binding constant of phenyl chloroformate molecules to the micelles, resulting in the contribution of the reaction taking place in the bulk water-EG phase being more important. The weight percent of EG did not substantially influence the rate constant in the micellar pseudophase.

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

The reaction methyl-4-nitrobenzenesulfonate + Br(-) was studied in tetradecyltrimethylammonium bromide (TTAB) aqueous micellar solutions in the absence and in the presence of various amounts of n-hexanol, n-pentanol, and n-butanol. Kinetic micellar effects provoked by the addition of the linear alcohols can be rationalized by using simple pseudophase kinetic models. The equilibrium binding constants of the methyl-4-nitrobenzenesulfonate molecules to the cationic micelles decreases when [alcohol] increases. The (k(2)(m)/V(m)) values found are practically the same for the different TTAB-alcohol micellar solutions studied, independent of the nature and concentration of the alcohol present in the reaction medium. This has been explained by considering the balance of two factors operating on reactivity in opposite ways: (1). an increase in the volume of the micellar interfacial region upon increasing alcohol concentration, and (2). a decrease in the polarity of the interfacial region as the amount of alcohol present in the micellar solutions increases.

The spontaneous hydrolysis of phenyl chloroformate was studied in water-ethylene glycol (EG) micellar solutions of hexadecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and dodecyltrimethylammonium bromide, the percentage by weight of EG varying within the 0-50% range. The dependence of the observed rate constant on the surfactant concentration was investigated in all the water-EG mixtures. In addition to the kinetic measurements, conductivity, spectroscopic, and fluorescence measurements were done to obtain information about the micellar solutions used as reaction media. For the three surfactants studied, an increase in the amount of EG present in the mixture resulted in an increase in the critical micelle concentration and in an increase in the micellar ionization degree but in a decrease in the aggregation number of micelles. The polarity of the bulk-micelle interface region seems to increase somewhat by increasing the percentage by weight of EG. A simple pseudophase model was useful in rationalizing the micellar kinetic effects observed. The adjustable parameters obtained from the fittings of the experimental data show that the reaction rate in the micellar pseudophase was similar for all the water-EG cationic micellar solutions investigated. The equilibrium binding constant for the phenyl chloroformate molecules to the micellar aggregates decreases as the amount of EG in the mixture increases, this decrease being responsible for the diminution in the observed rate constant when the amount of EG present in the mixture increases.

The 1-chloro-2,4-dinitrobenzene + OH- reaction was studied in tetradecyltrimethylammonium bromide aqueous micellar solutions. Influence of changes in [surfactant] as well as in [NaOH] on the observed rate constant were rationalized by using pseudophase kinetic models. At high hydroxide ions concentration an additional pathway across the micellar boundary had to be considered.

The reaction methyl 4-nitrobenzenesulfonate + Cl− was studied in hexadecyltrimethylammonium chloride (CTAC) in the absence and presence of 0.1 M NaCl, as well as in mixed CTAC/Triton X-100 (polyoxyethylene(9.5)octylphenyl ether) aqueous micellar solutions with CTAC molar fractions of 0.9, 0.8, 0.7, and 0.6. Conductivity measurements were used to obtain critical micellar concentrations and micellar ionization degrees of the various micellar reaction media. From these data, thermodynamic information on the cationic/nonionic mixed micellar solutions was obtained. Micellar effects on the observed rate constant were explained by pseudophase kinetic models. The estimated second-order rate constants in the micellar pseudophase of the different micellar reaction media showed that pure CTAC and mixed CTAC/Triton X-100 micelles, at the high cationic surfactant molar fractions studied, provide reaction sites of similar characteristics at the interfacial region. This was in agreement with previous structural studies carried out on mixed CTAC/Triton X-100 micellar solutions. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 45–51, 2003

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.

The reaction methyl 4-nitrobenzenesulfonate + Br- was studied in tetradecyltrimethylammonium bromide aqueous solutions in the presence and absence of 0.1 M NaBr, as well as in N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB3-14) micellar solutions in the presence of 0.1 M NaBr. Discussion of kinetic results in cationic micellar solutions was used to show some problems concerning the estimation of the second-order rate constants in the micellar pseudophases. The errors introduced by rationalizing the kinetic micellar effects in SB3-14 micellar solutions through pseudo-first-order rate constants were also shown; the contribution of the reaction occurring in the aqueous phase and the binding of the bromide ions to the sulfobetaine micelles were the cause of such errors. To complete the work, the influence of the cation coming from the added bromides on the micellar effects observed in SB3-14 micellar solutions was investigated. Results give information about how cations influence the bromide assoc...

The spontaneous hydrolysis of phenyl chloroformate was studied in various anionic, nonionic, zwitterionic, and cationic aqueous micellar solutions, as well as in mixed anionic–nonionic micellar solutions. In all cases, an increase in the surfactant concentration results in a decrease in the reaction rate and micellar effects were quantitatively explained in terms of distribution of the substrate between water and micelles and the first-order rate constants in the aqueous and micellar pseudophases. A comparison of the kinetic data in nonionic micellar solutions to those in anionic and zwiterionic micellar solutions makes clear that charge effects of micelles is not the only factor responsible for the variations in the reaction rate. Depletion of water in the interfacial region and its different characteristics as compared to bulk water, the presence of high ionic concentration in the Stern layer of ionic micelles, and differences in the stabilization of the initial state and the transition state by hydrophobic interactions with surfactant tails can also influence reactivity. The different deceleration of the reaction observed in the various micellar solutions studied was discussed by considering these factors. Synergism in mixed-micellar solutions is shown through the kinetic data obtained in these media. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 445–451, 2002

The elimination reaction between 2-(p-nitrophenyl)ethyl bromide and OH- ions was studied in nonionic aqueous micellar solutions of two esters of long chain fatty acids and sorbitan polyethylene glycol, Tween 20 (monolaurate) and Tween 80 (monooleate). The equilibrium binding constants of the 2-(p-nitrophenyl)ethyl bromide molecules to the nonionic micellar aggregates were obtained by spectroscopic measurements. The experimental kinetic data were quantitatively rationalized by the treatment of first-order reactions based on the pseudophase model. Results from this work and from previous studies show that, whereas the presence of cationic and sulfobetaine micelles accelerates the reaction, the presence of nonionic micelles inhibits the reaction.

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.

The reaction Fe(CN)4(bpy)2− + S2O82− has been studied in aqueous micellar solutions of N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, SB3-14. The influence of changes in the surfactant concentration as well as in the peroxodisulfate ions concentration on kobs was investigated. Spectroscopic and conductivity measurements have given information about the distribution of both anionic reagents between the aqueous and micellar pseudophases of the SB3-14 micellar solutions. A discussion about the adequacy of various equations based on the pseudophase model to rationalize kinetic micellar effects for anion-anion reactions in sulfobetaine micellar solutions has been done. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 225–231, 2001

The reaction of dehydrochlorination of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane, DDT, with hydroxide ions has been studied in aqueous micellar solutions of N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, SB3-16, in the presence of high and moderately high concentrations of various background electrolytes. The results show that at these concentrations the nature of the anion which comes from the salt is not the only factor influencing reactivity, but the nature of the cation also affects the kobs value. This was explained by considering that both the anion and the cation of the added salt bind to the surface of SB3-16 micelles.

The oxidation of 1-methoxy-4-(methylthio)benzene by IO4−, rendering the corresponding sulfoxide and IO3−, was studied in zwitterionic (SB3-16) and nonionic (Brij35 , Tween 20, Tween 40, Tween 60 and Tween 80) aqueous micellar solutions. The association equilibrium constants of the sulfide molecules to the micellar aggregates present in the different media were obtained by spectroscopic measurements. In the case of the sulfobetaine micellar solutions, the equilibrium constant for the incorporation of periodate anions in the zwitterionic micellar aggregates was obtained through conductivity measurements. Taking into account literature data on this reaction in cationic and anionic micellar solutions, a comparison of the second order rate constants in the micellar pseudophases of nonionic, anionic, cationic and zwitterionic micellar solutions was undertaken. This comparison, together with kinetic data obtained in water–cosolvent mixtures, as well as in aqueous electrolyte solutions, shows that charge–charge interactions and the low polarity of the micellar pseudophases account for the large differences between the second order rate constants in pure water and those in micellar solutions.

The ligand substitution reactions Fe(CN)(5)(4-(t)bupy)(3-) + 4-CNpy and Fe(CN)(5)(4-(t)bupy)(3-) + pzCO(2)(-) (4-(t)Bupy = 4-tert-butylpyridine; 4-CNpy = 4-cyanopyridine; pzCO(2)(-) = pyrazinecarboxylate) were studied in several aqueous salt and micellar solutions. Kinetic data in aqueous solutions showed that the two processes follow a dissociative mechanism, D, and the dependence of the first-order rate constants on [salt] on electrolyte aqueous solutions allow the estimation of the activation volumes corresponding to both reactions. Under true first-order conditions no kinetic micellar effects were found in anionic (SDS) and nonionic (Triton X-100) aqueous micellar solutions. In cationic micellar solutions (CTAB, CTAC, and TTAB) small kinetic micellar effects were found. These were related to the different ionic concentrations and the different polarity and structure of the Stern layer surrounding the cationic micellar aggregates, where the reactions take place, with respect to pure water. Copyright 2000 Academic Press.

The ligand substitution reaction [Fe(CN)5(4-tbupy)]3- + pyrazine (4-tbupy=4-tertbutylpyridine) was studied in aqueous concentrated electrolyte solutions at 298 K. Plots of ln(k/kw) against (γ-γw), where the subscript w refers to pure water and γ is the surface tension of the appropriate salt solution, gave a common straight line for all the electrolytes studied and permits to estimate the activation volume.

The ligand substitution reaction between Fe(CN)5H2O3- and β-Co(trien)2(pzCO2)2+ (trien = triethylenetetraammine; pzCO2 = pyrazinecarboxylate) was studied in aqueous salt and sodium dodecyl sulfate micellar solutions. In homogeneus aqueous solutions, an increase in the concentration of the background electrolytes resulted in a decrease in the observed rate constant. This was explained by considering the influence of the salts on the activity coefficients of the participants in the reaction using a Guggenheim−Guntleberg-type equation. In sodium dodecyl sulfate micellar solutions, the effect of the variation of the surfactant concentration operated mainly through its influence on the approaching process between the two oppositely charged reactants. When the surfactant concentration is high enough to ensure the total incorporation of the Co(III) complex molecules into the micellar pseudophase, the decrease in the electrical component of the free energy of activation (controlled by changes in the interfacial e...

The dehydrochlorination of 1,1-dichloro-2,2-bis(p-chlorophenyl)ethane, DDD, with hydroxide ion has been studied in several tretradecyltrimethylammonium bromide, TTAB, concentrations by varying [NaOH] from 0.01 mol dm -3 to 1.5 mol dm -3 . The dependence of the first-order rate constant on the hydroxide ion concentration cannot be explained on the basis of the pseudophase model. The kinetic effects of added electrolytes have been studied in TTAB micellar solutions. At low hydroxide ion concentrations the pseudophase model can rationalize the added salt effects. The influence of the electrolyte nature on the kinetic effects observed can be related to the competition between the anion of the salt and the hydroxide ions for the available sites at the surface of the TTAB micelles. At high hydroxide concentrations, a larger decrease in the reaction rate is found than that expected on the basis of the pseudophase model when the added salt concentration increases. The influence of the nature of the added electrolytes, at a given added salt concentration, is the same in the presence of [NaOH] = 0.02 mol dm -3 as in the presence of[NaOH] = 1 mol dm -3 for the 1:1 salts.

The reaction Fe(CN)5(4-CNpy)3− + S2O82− (4-CNpy=4-cyanopyridine) was studied in aqueous salt solutions in the presence of several electrolytes as well as in anionic, cationic, and nonionic surfactant solutions. In aqueous salt solutions the noncoulombic interactions seem to be important in determining the positive salt effects observed. The salting effects are influencing the activity coefficients of any participant in the reaction, including those ion pairs which can be formed between the anionic reagents and the cations which come from the added salts. The changes in surfactant concentration in anionic and nonionic surfactant solutions do not affect the reaction rate, which is similar to that in pure water at the same ionic strength. In cationic micellar solutions an increase in the rate constant compared to that in pure water is found; the reaction rate decreasing when the surfactant concentration increases. The kinetic trends can be explained assuming that the reagents are totally bound to the micelles and, therefore, an increase in the surfactant concentration results in a decrease in the reagent concentrations at the micellar phase and thus in a decrease in the observed rate constant. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet: 31: 229–235, 1999

The reaction of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane, DDT, with hydroxide ions (NaOH) was studied in cationic micellar aqueous solutions of tetradecyltrimethylammonium bromide, TTAB, and hexadecyltrimethylammonium bromide, CTAB, in the presence and in the absence of various added electrolytes (NaCl, NaF, NaBr, NaNO3, and Na2SO4). In the absence of added salt, the influence of the variations of the surfactant concentration as well as of the total hydroxide ions concentration on the observed rate constant values has been investigated. Kinetic data show that for low [NaOH], a phase separation model can explain the experimental results. However, as the hydroxide ions concentration increases, an additional pathway across the micellar boundary becomes important. When added electrolytes are present in the micellar reaction medium, variations in kobs depend on the nature of the anion of the salt, as well as on [salt]added. For a given added electrolyte concentration the observed trend was kobs(NaF) > kob...

The reaction Fe(CN)4 (bpy)2- + S2 O8 2- has been studied in aqueous solutions of hexadecyltrimethylammonium chloride (CTAC) and bromide (CTAB), sodium dodecyl sulfate (SDS), dodecyl tricosaoxyethylene glycol ether (Brij35 ), and octylphenol(ethyleneoxide)9.5 ether (Triton X-100). The influence of changes in surfactant concentration on the reaction rate has been investigated. Experimental kinetic data show that the reaction rate is not affected by the presence of SDS, Brij35 , and Triton X-100 in the reaction medium within the surfactant concentration range studied. In CTAC and CTAB solutions the observed first order rate constant is much higher than that obtained in pure water, which can be explained by considering that the two anionic reactants are localized at the positively charged micellar surface. This is also in agreement with the observed decrease in the reaction rate when [cationic surfactant] increases. An increase in the surfactant concentration would provoke a decrease in the local concentration of the reagent in excess, S2 O8 2- ions, at the micellar surface, that is, at the reaction site, which would result in a decrease of the observed pseudo-first-order rate constant. Temperature was maintained at 298.2 K.

The kinetics of the reactions I- + IrCl62- and Fe(CN)64- + S2O82- have been studied in anionic micelles of sodium dodecylsulfate (SDS) at 298.2 K. The results obtained in this work, as well as those obtained in sodium bis-2-ethylhexyl sulfosuccinate (AOT)/decane/water microemulsions and aqueous sodium nitrate solutions, can be explained by using as a starting point the Bronsted equation.

The ligand substitution reaction Fe(CN)5H2O3− + pyrazine Fe(CN)5 pyrazine3− + H2O has been studied in sodium dodecyl sulfate SDS, hexadecyltrimethylammonium bromide, CTAB, and salt aqueous solutions at 298.2 K. Kinetics were studied in dilute and concentrated salt solutions and in SDS and CTAB solutions at surfactant concentrations below and above the critical micelle concentration. Experimental results show that salt effects can be explained by considering the interaction between the cations present in the working media which come from the background electrolyte, and the Fe(CN)5H2O3− species in the vicinity of the cyanide ligands. This interaction makes the release of the aqua ligand from the inner-coordination shell of the iron(II) complex to the bulk more difficult resulting in a decrease of the reaction rate when the electrolyte concentration increases. Kinetic data in surfactant solutions show that not only micellized surfactants are operative kinetically, but also nonmicellized surfactants are influencing the reactivity. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 377–384, 1997

The electron transfer reactions of the cis-tetraamminediaquocobalt(III) complex with hexacyanoferrate(II) and octacyanomolibdate(IV) in four different water-cosolvent binary mixtures have been investigated. k et and K IP values have been experimentally determined and discussed within the framework of current theories on electron transfer reactions and ionic association. For this purpose, the standard formal potentials of hexacyanoferrate(II) and octacyanomolibdate(IV) were also determined. Comparison of the second order rate constants (k 12 = k et K IP ) to predictions of the Marcus Cross-relation gives some indication of the adiabaticity of the processes investigated.

The electron transfer reaction between [Co(NH3)5(N-cyanopiperidine)]3+, pentaammine(N-cyanopiperidine)cobalt(III), and hexacyanoferrate(II) has been studied in aqueous solutions. Typical saturation behavior was observed at high hexacyanoferrate(II) concentrations, which allows the precursor complex formation equilibrium constant, KIP, and the true electron transfer rate constant, ket, to be obtained. Variations of ket in the presence of various background electrolytes (sodium sulfate and sodium chloride) and surfactants (sodium dodecyl sulfate, SDS, dodecyltricosaethylene glycol ether, Brij35, and octylphenol(ethylene oxide)9.5 ether, Triton X-100) have been investigated at 298.2 K. Rationalization of the experimental data was assisted by the use of the Marcus theory on electron transfer. The conclusion is that micellar effects on the electron transfer rate constant can be explained by considering the micelles as a special background electrolyte with a high charge and a strong power of hydration.

The binuclear complex formation between Fe(CN)5H2O3- and Co(en)2(2-pzCO2)2+ (en = ethylenediammine, pzCO2 = pyrazinecarboxylate) has been studied in several aqueous electrolyte solutions and in sodium dodecyl sulfate, SDS, micellar systems in the presence and absence of various salts at 298.2 K. In dilute salt aqueous solutions the main factor operating on reactivity is the influence of the electrolyte concentration on the approaching process between the two ionic reactants of opposite sign, whereas in concentrated salt solutions the direct interaction cation−iron(II) complex seems to be the determining factor controlling changes in the reaction rate upon changing salt concentration. In the case of the SDS micellar systems, kinetic micellar effects have been rationalized by considering the influence of interfacial electrical potential variations, with changing surfactant (or salt) concentration, on the process studied.

Salt effects on the oxidation of the iron(II) complexes Fe(CN)4(bpy)2−, cis-(CN)2(bpy)2 and Fe(bpy)32+ by S2O82− as well as on the reaction Fe2+ + Co(NH3)5Cl2+ have been studied in concentrated electrolyte solutions at 298.2 K. We have gone from anion–anion to cation–cation reaction with the intermediate cases of anion–neutral and cation–anion reactions. Results show that the main cause of the kinetic salt effects observed is the interaction between supporting electrolytes and the solvent. © 1994 John Wiley & Sons, Inc.

The micellization and micellar growth in the mixtures of N,N-dimethyl, N-phenyl,N-dodecylammonium bromide, PH12, N,N-dimethyl,N-ciclohexylmethyl,N-dodecylammonium bromide, CH12, and their two dimeric counterparts m-dimethylphenyl-α-ω-bis(dodecyldimethylammonium) bromide, 12PH12, and m-dimethylciclohexyl-α-ω-bis(dodecyldimethylammonium) bromide, 12CH12, with dodecyltrimethylammoniumbromide, DTAB, and with N-decanoyl N-methylglucamide, MEGA10, were investigated at 303 K. Circular dichroism, CD, experiments showed the formation of mixed micelles. Two-dimensional, 2D, rotating frame nuclear Overhauser effect spectroscopy (ROESY) experiments indicated that the arrangement of the rings in the pure and mixed micelles is similar, with the rings bent into the micelle interior avoiding contact with water. Application of different theoretical approaches shows that PH12 and CH12 mixtures with DTAB and with MEGA10 behave almost ideally. The binary systems of 12PH12 and 12CH12 with DTAB and with MEGA10 show a non-ideal behavior. An increment in the solution mole fraction of MEGA10 and DTAB diminishes the tendency of the micellar aggregates to grow.

In this work the two monomeric surfactants N -benzyl- N , N -dimethyl- N -(1-dodecyl)ammonium bromide (PH12,Br − ) and N -cyclohexylmethyl- N , N -dimethyl- N -(1-dodecyl)ammonium bromide (CH12,Br − ) and their two dimeric counterparts N , N ′-(1,3-phenylenebis(methylene))bis( N , N- dimethyl- N -(1-dodecyl)ammonium dibromide (12PH12,2Br − ) and N , N ′-(cyclohexane-1,3-diylbis(methylene))bis( N , N- dimethyl- N -(1-dodecyl)ammonium dibromide (12CH12,2Br − ) were prepared and characterized. The critical micelle concentration, the micellar ionization degree and the average aggregation number were obtained by using different techniques. The results are discussed with the help of 1 H NMR two-dimensional, 2D, rotating frame nuclear Overhauser effect spectroscopy measurements, ROESY, which seem to indicate that the phenyl and cyclohexyl rings present in the head groups of the surfactants are bent towards the micellar interior in order to avoid contact with water. The surface activity of the surfactants was studied by means of surface tension measurements. The occurrence of morphological transitions upon increasing surfactant concentration and the variations in the polarity and in the microviscosity of the interfacial region accompanying this sphere-to-rod transition were investigated. It was shown that the formation of spherocylindrical micelles also causes changes in the 1 H NMR spectra of the surfactant solutions.

The formation of the binuclear complex between aquopentacyanoferrato(II) and tetraamminepyrazinecarboxyiatocobalt(III) ions has been studied in various concentrated electrolyte solutions at 298.2 K. In all cases the reaction rate decreases with the increase in ionic strength of the medium. The observed salt effects depend on the nature of the supporting electrolyte. Results have been rationalized by considering the electrostatic interactions between the different cations (which come from the salt) and the Fe(CN)5H2O3− complex, which make the release of the water molecule from the inner coordination sphere of the iron(II) center to the bulk more difficult.

The micellization and micellar growth in pure aqueous alkanediyl-alpha-omega-bis(dodecyldimethylammonium) bromide, 12-s-12,2Br(-) (with s = 2,5,6,8,10,12), and N-decanoyl-N-methylglucamide MEGA10 solutions and their mixtures are investigated at 303 K. Application of different theoretical approaches to the binary mixtures shows a nonideal behavior. It also shows that the spacer length does not play an important role in the attractive interactions shown by the mixed systems. The sphere-to-rod morphological transition in the pure dimeric micellar solutions is studied at 303 K. From comparison of these results with those at 298 K the key role played by the spacer in the micellar growth is shown. The spacer length controls not only the surfactant concentration at which the morphological transition happens but also the sign of the enthalpy change accompanying the sphere-to-rod equilibrium. Spacers with an even number of methylenes show smaller C* values than those with an odd number of -CH(2)- units. An endothermic enthalpy change is found for even spacers whereas an exothermic enthalpy change is found for odd spacers. To the authors knowledge, this is the first time this experimental trend has been shown. Addition of MEGA10 diminishes the tendency of the aggregates to grow. An increment in the solution mole fraction of MEGA10 makes the formation of elongated micelles difficult. Microviscosity measurements provide additional information about the influence of the MEGA10 content on the sphere-to-rod transition.

The kinetics of replacement of 4-cyanopyridine (4-CNpy) by CN− in [Fe(CN)5(4-CNpy)]3− have been studied in different concentrations of electrolyte. Plots of ln (k/kw)versus (γ−γw), where the subscript w refers to pure water and γ is the surface tension of the appropriate salt solution, gave a common straight line for all the electrolytes studied. This result seems to confirm theD character of the process studied and permits an estimate of the activation volume to be made from kinetic data.

The oxidation of MnEDTA2−. (EDTA=ethylenediaminetetra-acetate) by hexachloroiridate(IV) has been studied in concentrated electrolyte solutions at 298 K. The estimation of the activity coefficients of the species from the Stokes-Robinson hydration theory indicates that the observed salt effects have their origin in a greater destabilization of the initial state with respect to the transition state when increasing salt concentration.