Amalia Rodríguez

Effects of Addition of Polar Organic Solvents on Micellization

Micellization of several surfactants in water-organic solvent mixtures has been investigated. Only solvents localized mainly in the bulk phase of the micellar solutions (they do not incorporate into the micelles) were studied, with either higher or lower permittivity than that of pure water. Results show that the influence of organic solvent addition on the aggregation process can be approximately accounted for by considering the changes in the bulk phase cohesive energy density, described by the Gordon parameter, G. To our knowledge, this is the first time that, for a given surfactant, the Gibbs energies of micellization, Delta G M degrees , obtained in several water-organic solvent mixtures have been fitted together. It is worth noting that data from different research groups have been considered. The Delta G M degrees versus G correlation will permit the estimation of the variations in the Gibbs energy of micellization upon addition of known quantities of a given polar organic solvent. Speaking in a general way, organic solvent addition results in the bulk phase becoming a better solvent for the surfactant molecules. This would make the hydrophobic tail transfer from the bulk phase into the micelles less favorable and, as a consequence, Delta G M degrees increases (becomes less negative), making the aggregation process less spontaneous.

Water−Ethylene Glycol Cationic Dimeric Micellar Solutions: Aggregation, Micellar Growth, and Characteristics As Reaction Media

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.

Mixtures of monomeric and dimeric surfactants: hydrophobic chain length and spacer group length effects on non ideality.

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<C14TAB+12-3-12<C12TAB+12-3-12<C10TAB+12-3-12, and C12TAB+12-5-12 approximately C12TAB+12-4-12<C12TAB+12-3-12. Literature data corresponding to the C12TAB+12-2-12, C12TAB+ m-s-m (s=2, 4, 6 and m=12, 14, 16) and TritonX-100+12-s-12 (with s=3, 6, 12) mixtures were considered in order to investigate the hydrophobic chain length and the spacer group length roles on the observed non ideal behavior. It was found that the capacity of the mixture components to form micelles of similar or different shapes plays a major role in non-ideality.

Micellization and micellar growth of alkanediyl-α,ω-bis(dimethyldodecylammonium bromide) surfactants in the presence of medium-chain linear alcohols

Micellization and micellar growth of cationic dimeric surfactants of the alkanedyil-alpha,omega-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.

Substitution reactions at pentacyanoferrate(II) complexes: linear free-energy relationships in mixed solvents

The kinetics of replacement of 4-phenylpyridine (4-phpy) by cyanide in Fe(CN)5(4-phpy)3– have been studied in various isodielectric water–cosolvent mixtures at 298 K. The experimental data are well correlated through the following equation: δΔG#exp=aδAm+bδBm+cGexc where Am, Bm and Gexc are the acidity parameter, the basicity parameter and the excess Gibbs free energy of the mixture, respectively. This multiparameter regression was also applied to data, taken from the literature, of substitution reactions at different pentacyanoferrate(II) complexes in order to show its general validity for explaining the effect of the medium on these types of processes in binary aqueous mixtures.

Oxidation of Fe(CN)4–6 by S2O2–8 in AOT–oil–water microemulsions

The Oxidation of hexacyanoferrat (II) by peroxodisulfate has been studied in sodium bis(2-ethylhexyl) sulfosuccinate (aerosol-OT or AOT)–oil–water microemulsions at 298.2 K. The rate of this reaction is appreciably higher in a microemulsion than when it takes place in a conventional aqueous solution. The dependence of the rate constants on the surfactant concentration, on the nature of the oil phase and on the molar ratio w=[H2O]/[AOT] has been investigated. Results show that when the AOT concentration remains constant, the reaction rate increases by decreasing the molar ratio, w. For a given [H2O]/[AOT] value, the reaction rate shows no dependence on the surfactant concentration or on the nature of the oil phase.

Study of the Micellization and Micellar Growth in Pure Alkanediyl-α-ω-Bis(dodecyldimethylammonium) Bromide and MEGA10 Surfactant Solutions and Their Mixtures. Influence of the Spacer on the Enthalpy Change Accompanying Sphere-to-Rod Transitions

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.

Concentration and medium micellar kinetic effects caused by morphological transitions.

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.

Study of the reaction between methyl 4-nitrobenzenesulfonate and bromide ions in mixed single-chain-gemini micellar solutions: kinetic evidence for morphological transitions.

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, Brij(35), with alkanediyl-alpha-omega-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.

Synthesis and physicochemical characterization of alkanedyil-α-ω-bis(dimethyldodecylammonium) bromide, 12-s-12,2Br-, surfactants with s=7, 9, 11 in aqueous medium.

In this work, three didodecyl dicationic dibromide dimeric surfactants 12-s-12,2Br(-), with different methylene spacer lengths (s=7, 9, and 11) were prepared and characterized and their properties compared to those of 12-s-12,2Br(-) surfactants with s=2, 3, 4, 5, 6, 8, 10, and 12. Information about the critical micelle concentration, the micellar ionization degree, the average aggregation number and the polarity of the interfacial region, and microviscosity of the micellar interior was obtained by using different techniques. Their surface activity was investigated by means of surface tension measurements. Micellization was also studied by using (1)H NMR and diffusion NMR (DOSY) spectroscopy as well as isothermal titration calorimetry. The values of the thermodynamic parameters show that the dimeric surfactants micellization is exothermic and driven by entropy. The occurrence of morphological transitions upon increasing surfactant concentration was studied, and the results indicate that the spacer length, s, plays a key role in the micellar growth of 12-s-12,2Br(-) aggregates. The value of s not only control the magnitude of C(*), the surfactant concentration above which the morphological transition from spherical micelles into elongated ones occurs, but also the sign of the enthalpy change accompanying the sphere-to-rod transition.