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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 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.

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 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.

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 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 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 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 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 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 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.