Gaspar Fernández

Effects of glycols on the thermodynamic and micellar properties of TTAB in water.

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.

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 1-methoxy-4-(methylthio)benzene + IO4−: importance of micellar medium effects

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.

Study of the reaction Fe(CN)5(4-CNpy)3− + S2O82− in aqueous salt and micellar solutions

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.

Study of the Ligand Substitution Reaction [Fe(CN)5(4-tbupy)]3- + Pyrazine in Concentrated Aqueous Electrolyte Solutions: Estimation of the Activation Volume

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.

Chapter 12 – STUDY OF DEHYDROCHLORINATION REACTIONS IN MICELLAR SOLUTIONS

The chapter discusses the research that involves in the study of micellar effects on inorganic electron transfer and ligand substitution reactions—most of the processes studied involve hydrophilic charged species. As a consequence, some difficulties arise: (1) frequent solubility problems were found, sometimes precluding experiments needed for the rationalization of the experimental kinetic data; (2) no kinetic micellar effects were observed in nonionic micellar solutions, (3) the equations describing the dependence of the observed rate constant values on the nature and the concentration of surfactant and additives are complicated. The dehydrochlorination reactions of various halogenated pesticides in basic media seemed good candidates. The equation presented in the chapter describes that the dependence of the observed rate constant values on the surfactant and the hydroxide ion concentrations in micellar solutions are manageable. In addition, because in nonionic micellar solutions the pesticides are located in the micellar pseudophase—where the processes take place—kinetic micellar effects are expected in these polydisperse media.