María del Mar Graciani

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.

Study of the ligand substitution reaction Fe (CN)5H2O3? + pyrazine in micellar solutions

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.

Kinetics of the oxidation of iodide by persulphate in AOT–oil–water microemulsions

The oxidation of iodide by persulphate has been studied in different sodium bis-2-ethylhexylsulphosuccinate (aerosol-OT or AOT)–oil–water microemulsions at 298 K. The rate of this reaction is appreciably higher in a microemulsion than when it takes place in a conventional aqueous medium. The dependence of the rate constant on the surfactant concentration as well as on the molar ratio R(=[H2O]/[AOT]) has been examined. The results are interpreted on the basis of the anionic nature of the reactants and the surfactant itself, and the high concentrations of sodium cations inside the aqueous cores. The dependence of the rate constant on the nature of the organic phase present in the system has also been investigated.

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.

Salt effects on the kinetics of dissociation of the pentacyano-4-cyanopyridineferrate(II) anion

SummaryThe 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.