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The 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.

A simple working method for the estimation of the aggregation numbers and core radii from weight measurements of sodium bis-2-ethylhexylsulphosuccinate (aerosol-OT orAOT)/decane/water microemulsions of several compositions is proposed. Starting out from the hypotheses that (i) the density of the water inside the droplets is the same as that of free water, (ii) all the surfactant is localized as interface, and (iii) the penetration of the organic phase into the interface is negligible, values for the aggregation number and core radii comparable to those determined using more sophisticated methods were obtained. The results/methodological simplicity ratio may be of great advantage in kinetic work on reaction mechanisms in microemulsions.

The kinetics and mechanism of the reaction of Ni(II) with heptane-3,5-dione (Hhptd) have been investigated in concentrated sodium perchlorate aqueous solutions at 298 K. The kinetic data in the different media are consistent with a mechanism in which Ni(II) reacts with the enol form of the ligand. The observed positive salt effect on the rate of the reaction of formation of the complex Nihptd + is rationalized taking into account the increase of the rate of water exchange from the first solvation sphere of the metal ion to the bulk solvent when increasing salt concentration

The wavelengths of absorption maxima are reported for two ligand field bands of hexathiocyanatochromate(III), $ {⤪ Cr(NCS)_{6}^{3-}} $, over a range of water-cosolvent mixtures and different electrolyte concentrations. Correlations of the shifts of ligand field band energies as a function of dielectric constant and polarizability of the medium have been investigated. The data for binary aqueous mixtures indicate preferential solvation of the complex by the organic solvent.

The oxidation of MnEDTA2−. (EDTA=ethylenediaminetetra-acetate) by hexachloroiridate(IV) has been studied in concentrated electrolyte solutions at 298 K. The estimation of the activity coefficients of the species from the Stokes-Robinson hydration theory indicates that the observed salt effects have their origin in a greater destabilization of the initial state with respect to the transition state when increasing salt concentration.

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.

The formation of adducts of Cr(bpy)33+, Cr (phen)33+, Ru(bpy)32+, and Pt(bpy)(NH3)22+ (bpy = 2, 2′-bipyridine; phen = 1, 10-phenanthroline) with the lasalocid A anion (LAS-) in CH2Cl2 solution has been investigated by means of electronic absorption and emission spectroscopy. Cr(bpy)33+ forms a 1:1 adduct with LAS−. In this adduct, the fluorescence of LAS− is quenched, whereas the phosphorescence intensity and lifetime of Cr(bpy)33+ increase. In the case of Cr(phen)33+, only a dynamic quenching of the Cr(phen)33+ phosphorescence by LAS− is observed. For Ru(bpy)32+, either no adduct is formed or the interaction is too weak to cause changes in the absorption and emission properties. For Pt(bpy)(NH3)22+, evidence has been obtained for formation of a 1:2 Pt(bpy)(NH3)22+ / LAS− adduct, where the LAS− fluorescence is statically quenched.

Rate constants are reported for the oxidation of iodide by hexachloroiridate(IV) in various concentrated electrolyte solutions containing up to 6 mol dm–3 salt at 298.2 K. The conclusion is that the salt effects are mainly due to ion–solvent interactions. This is supported by considering the activity coefficients of the reaction participants, estimated by using the Stokes–Robinson hydration treatment. The study of kinetics at 333.2 K shows that the distinct behaviour of potassium nitrate seems related not only to the different structural character of this salt with respect to the rest of the electrolytes studied, but also to specific interactions which are operative favouring the reaction.

A kinetic study of the reactions I− + S2O8= and CV+ + OH− (CV = 4, 4′, 4″-tris(dimethylamino)triphenylmethyl chloride or crystal violet) in different water in oil microemulsions is reported. The dependence of the rate constants on the nature of the surfactant as well as on the molar ratio R = [H2O]/[Surfactant] has been investigated. The results are interpreted on the basis of the electrostatic interactions between the reactants and the surfactant polar heads and considering the water properties inside the aqueous core of the droplets.

Charge-transfer transitions of Fe(CN) 5 (3-pzCO 2 ) 4− (pzCO 2 =pyrazinecarboxylate) and Fe(CN) 6 3− are studied in different electrolyte solutions. The applicability of the dielectric continuum theory has been investigated. Results indicate that the electrolyte present in the medium may affect the optical transition energies in two different ways: indirectly, changing the polarity and polarizability of the medium, and directly through ion-pair formation.

The kinetics of replacement of 4,4′-bipyridine, (4,4′-bipy), and 4-cyanopyridine, (4-CNpy), by cyanide in [Fe(CN)5-(4,4′-bipy)]3-. at 298 K have been studied in binary aqueous mixtures containing different amounts of t-butanol, methanol, glycerol, ethyleneglycol and sucrose. The plots of logarithms of the limiting rate constantsversus the mole fraction of water are linear over the entire composition range studied, showing the importance of solvation phenomena. A different straight line of log (kL/s−1)versus XH2O is obtained for each mixture, indicating the influence of other solvent parameters on the reaction rate. A multiparameter regression of δΔG exp ≠ with A (acidity vector), B (basicity vector) and GE is used for both reactions; plots of δΔG calc ≠ versus ΔΔG exp ≠ are linear with slopes of near unity.

Charge-transfer transitions of Ru(bipy) 3 2+ , Fe(bipy) 3 2+ , Fe(CN) 6 3- , and free bipy (bipy=2,2′-bipyridine) are solvent dependent. Evidence is presented that dielectric continuum theory provides a reasonable basis for interpreting medium effects on the electronic transition energies in binary solvent mixtures as well as in pure solvents.

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.

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.

The micellization and micellar growth of cationic dimeric surfactant propanediyl-alpha-omega-bis(dodecyldimethylammonium) bromide, 12-3-12,2Br-, have been studied in several water-organic solvent mixtures. The organic solvents were ethylene glycol, glycerol, 1,2-propylene glycol, 1,3-propylene glycol, acetonitrile, dioxane, formamide, and N,N-dimethylformamide. Results showed that the aggregation process was less favored in the binary mixtures than in pure water, which was explained by considering the influence of the solvophobic effect on micellization. The addition of organic solvents was accompanied by a diminution in the average aggregation number, Nagg, of the dimeric micelles. This diminution was due to the decrease in the interfacial Gibbs energy contribution, Delta G0interfacial, to the Gibbs energy of micellization caused by the decrease in the hydrocarbon/bulk-phase interfacial tension. As a result of the micelle size diminution, the concentration at which the sphere-to-rod transition occurred, C*, was higher in the mixtures than in pure water. Micelle size reduction is accompanied by a decrease in the ionic interactions and in the extra packing contribution to the deformation of the surfactants tails, making the formation of cylindrical micelles less favorable.

Micellization of three didodecyl dicationic dibromide gemini surfactants with different methylene spacer lengths, 12-s-12,2Br- where s = 3-5 methylene groups, has been investigated in water-ethylene glycol, EG, mixtures with weight percentages of EG up to 50%. Subsequently, effects of the addition of the organic solvent on the micellar growth of these surfactants and on the surfactant concentration range where sphere-to-rod transitions occur were studied by means of steady-state and time-resolved fluorescence quenching and spectroscopic measurements. Results show that an increase in the weight percentage of ethylene glycol added to aqueous 12-s-12,2Br- (s = 3-5) micellar solutions causes the sphere-to-rod transition to occur at higher surfactant concentrations than in pure water. The diminution in the average aggregation number, N(agg), when wt % EG increases, provoked by the decrease in the interfacial Gibbs energy contribution to DeltaG degrees M, is the main factor responsible for this observation. The decrease in N(agg) is accompanied by a decrease in the ionic interactions and the extra packing contribution to the deformation of the surfactants tails, making formation of cylindrical micelles less favorable. Besides, an increase in the solvent content and polarity of the interfacial region does not favor formation of direct ion pairs, decreasing the tendency of micelles to grow.

Various amounts of N,N-dimethylformamide (DMF) with the weight percentage of DMF varying within the range 0-20, were added to aqueous micellar solutions of hexadecyl-, tetradecyl-, and dodecyltrimethylammonium bromides (CTAB, TTAB, and DTAB, respectively). Information about changes in the critical micelle concentrations, in the micellar ionization degrees, in the aggregation numbers, and in the polarity of the interfacial region of micelles upon changing the weight percent of DMF was obtained through conductivity and fluorescence measurements. Surface tension measurements permitted the estimation of the Gordon parameter of the water-DMF mixtures. The thermodynamic and structural changes provoked by the addition of DMF to the cationic micellar solutions were evidenced through the micellar kinetic effects observed in the reaction methyl 4-nitrobenzenesulfonate + Br-, investigated in the water-DMF cationic micellar solutions. The pseudophase kinetic model was adequate to quantitatively rationalize the dependence of the observed rate constant on surfactant concentration as well as on the weight percent of DMF.

Micellization in sodium dodecyl sulfate (SDS)-N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate and SDS-polyoxyethylenesorbitan monolaurate binary surfactant solutions was studied by means of conductivity and surface tension measurements. These studies showed that two types of micellar aggregates are present in the mixed micellar solutions. Two reactions were investigated in these micellar media, the oxidation of 1-methoxy-4-(methylthio)benzene by IO(4)(-) and the spontaneous hydrolysis of phenyl chloroformate. Information on the distribution of reagents in the micellar reaction media was obtained through conductivity and spectroscopic measurements. Discussion of the kinetic data showed that the redox reaction takes place mainly in the aqueous phase of the mixed solutions, whereas hydrolysis occurs in the aqueous as well as in the micellar pseudophase. Variations in the observed rate constants of the two processes studied are gradual within the whole surfactant concentration range investigated, revealing little information about the mixed micellar medium.

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.

The reaction of methyl 4-nitrobenzenesulfonate (MNB) and Br− has been studied in water–glycerol (GLY) alkyltrimethylammonium bromide micellar solutions, with the weight percentage of glycerol up to 50%. A pseudophase kinetic model was used for quantitatively discussing the kinetic data. Results showed that the equilibrium-binding constant for the organic substrate molecules to the cationic micelles decreases upon increasing the amount of glycerol present in the micellar reaction media. The second-order rate constant of the reaction in the micellar pseudophase is practically independent of wt% of GLY. Similar results were found in other water–organic solvent alkyltrimethylammonium bromide micellar solutions for the same process. However, the dependence of the reaction rate, for a given surfactant concentration, on the wt% of organic solvent is weaker for glycerol than for the other organic solvents. This was explained by considering that the cationic micellar ionization degree is nearly independent of wt% GLY. As a consequence, bromide ions concentration in the interfacial region (the reaction site) does not change by varying wt% of GLY. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 845–582, 2008

Rate constants are reported for the oxidation by peroxodisulphate ions of octacyanomolybdate(IV) in concentrated aqueous salt solutions containing up to 6 mol dm−3 salt including mixed salt solutions. The dependence of the logarthm of rate constant on salt concentration is discussed in terms of ionic hydration enthalpies and B-viscosity coefficients. The analysis confirms that ion-pairs play a key role in determining reactivities in these systems.

The rate of the reaction between permanganate and iodide ions had been measured in several concentrated salt solutions. There is a salt effect, which is dependent on the hydration heat of the salt present in the reaction medium.

A series of three experiments to familiarize students with microemulsions, their basic structural parameters, and theoretical and practical aspects of their influences on reaction kinetics.

Micellar and salt effects on the reactions Ru(NH 3 ) 5 (pz) 2+ + S 2 O 8 2- and Ru(NH 3 ) 5 (pz) 2+ + Co(ox) 3 3- have been studied. It is shown that the well-known pseudophase model, an extension of the Olson-Simonson treatment to the micellar case, is equivalent to the Bronsted relation in regard to rationalization of the micellar effects. It is also shown that micellar systems behave as low-concentration/high-ionic-strength systems.

The kinetics of the oxidation of dicyanobis(2,2′-bipyridine) iron(II) [Fe(CN)2(bpy)2] by peroxodisulfate (S2O82-) has been studied in different water–cosolvent mixtures. The cosolvents used were methanol, tert-butyl alcohol, ethylene glycol and glycerol. The results are explained assuming an additional component of the reorganization free energy of the solvent in the mixtures, caused by a translation of the solvent molecules, as a consequence of the changes in the composition of the (at least) innermost solvation shell. A quantitative estimation of this component is attempted.