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The micellization of four dimeric cationic surfactants (“gemini surfactants”) derived from N-dodecyl-N,N,N-trimethylammonium chloride was studied in pure water and in water–ionic liquid (IL) solutions by a wide range of techniques. The dimeric surfactants are distinguished by their rigid spacer groups separating the two surfactant motifs, which range from C3 to C5 in length. In order to minimize organic ion pairing effects as well as the role of the ionic liquids as potential co-surfactants, ILs with inorganic hydrophilic anions and organic cations of limited hydrophobicity were chosen, namely ethyl, butyl, and hexyl-3-imidazolium chlorides. 1H NMR two-dimensional, 2D, rotating frame nuclear Overhauser effect spectroscopy measurements, ROESY, supported this premise. The spacer nature hardly affects the micellization process, neither in water nor in water–IL solutions. However, it does influence the tendency of the dimeric surfactants to form elongated micelles when surfactant concentration increases. In order to have a better understanding of the ternary water–IL surfactant systems, the micellization of the surfactants was also studied in aqueous NaCl solutions, in water–ethylene glycol and in water–formamide binary mixtures. The combined results show that the ionic liquids play a double role in the mixed systems, operating simultaneously as background electrolytes and as polar organic solvents. The IL role as organic co-solvent becomes more dominant when its concentration increases, and when the IL alkyl chain length augments.

Micellization in the aqueous binary mixtures of n-decanoyl N-methylglucamide, MEGA10, and alkanediyl-α,ω-bis(dodecyldimethylammonium) bromide, 12-s-12 (with s = 7, 9, 11), was investigated at 303 K and the data compared to those previously found in MEGA10-12-s-12 mixed solutions, with s = 2, 3, 4, 5, 6, 8, 10, and 12. Application of different theoretical approaches to the binary mixtures showed a non ideal behavior, the spacer length not affecting substantially the Rubingh interaction parameter. However, circular dichroism experiments show and odd–even effect in the interactions of the dimeric surfactants with MEGA10 molecules within the mixed micelles. Micellar growth was also studied in the mixed micellar solutions, the addition of MEGA10 diminishing the tendency of the dimeric aggregates to form elongated micelles upon increasing surfactant concentration.

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.

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.