Clifford A. Bunton

Organic Reactivity in Aqueous Micelles and Similar Assemblies

Publisher Summary This chapter discusses thermal reactions in aqueous colloidal systems. It aims is to analyze the reactions from a mechanistic point of view, and therefore discussion is focused on organic reactions in aqueous micelles. The structure of micelles or other colloidal droplets is considered only to the extent needed to understand reactivity. A key feature of micelles and similar colloidal aggregates is that they can incorporate solutes. For example an ionic micelle can bind a non-ionic solute and also, by virtue of its charge, attract counterions. It may affect reaction rates and equilibria by bringing reactants together or keeping them apart, but because the micelle can exert a medium effect it is necessary to separate the “medium” and “concentration” effects of the micelle. It reviews that the overall subject of reaction in submicroscopic aggregates has expanded so rapidly and in so many different directions that interest on specific areas has to be focused. The general principles, which govern the effects of normal, aqueous, micelles on reaction rates and equilibria are considered and some specific reactions and the relation of micellar effects to mechanism are discussed.

Reactivity in aqueous association colloids. Descriptive utility of the pseudophase model

Abstract The ability of aqueous micelles and similar association colloids to control reaction rates is treated in terms of pseudophase models in which micelles and water are regarded as distinct reaction media, with consideration of reactant concentrations and rate constants in the two regions. An alternative model, based on transition state theory, considers the transfer free energy of the activated complex between water and micelles. Both models fit data for reactions in solutions of cetyltrimethylammonium hydroxide, for example. Their values in relating rate effects to reaction mechanism and structures of reactants and surfactants are discussed.

Micellar catalysis, a useful misnomer

Aggregates of amphiphiles often accelerate or ‘catalyze’ thermal reactions, but they also inhibit reactions and current pseudophase models treat both phenomena. Recent advances refine current models, offer new ones, treat solute and ion binding with more sophistication, enhance reactivity and selectivity with special catalysts, and apply the pseuodophase approach to more complex mixtures and novel applications.

Kinetics and products of reactions of MTBE with ozone and ozone/hydrogen peroxide in water

Methyl-t-butyl-ether (MTBE) has become a prevalent groundwater pollutant due to its high volume use as a nationwide gasoline additive. Given its physicochemical properties, it requires new treatment approaches. Both aqueous O(3) and a combination of O(3)/H(2)O(2), which gives *OH, can remove MTBE from water, making use of O(3) a viable technology for remediation of groundwater from fuel contaminated sites. Rate constants and temperature dependencies for reactions of MTBE with O(3) or with *OH at pH 7.2, in a range of 21-45 degrees C (294-318K) were measured. The second-order rate constant for reaction of MTBE with O(3) is 1.4 x 10(18)exp(-95.4/RT) (M(-1)s(-1)), and for reaction of MTBE with *OH produced by the combination of O(3)/H(2)O(2) is 8.0 x 10(9)exp(-4.6/RT) (M(-1)s(-1)), with the activation energy (kJ mol(-1)) in both cases. At 25 degrees C, this corresponds to a rate constant of 27 M(-1)s(-1) for ozone alone, and 1.2 x 10(9) M(-1)s(-1) for O(3)/H(2)O(2). The concentration of *OH was determined using benzene trapping. Products of reactions of O(3) and O(3)/H(2)O(2) with MTBE, including t-butyl-formate (TBF), t-butyl alcohol (TBA), methyl acetate, and acetone, were determined after oxidant depletion. A reaction pathway for mineralization of MTBE was also explored. Under continuously stirred flow reactor (CSTR) conditions, addition of H(2)O(2) markedly increases the rate and degree of degradation of MTBE by O(3).

Synthesis of a new zwitterionic surfactant containing an imidazolium ring. Evaluating the chameleon-like behavior of zwitterionic micelles.

Synthesis of a new zwitterionic surfactant containing the imidazolium ring 3-(1-tetradecyl-3-imidazolio)propanesulfonate (ImS3-14) is described. The solubility of ImS3-14 is very low but increases on addition of a salt which helps to stabilize the micellized surfactant. Fluorescence quenching and electrophoretic evidence for ImS3-14 shows that the micellar aggregation number is only slightly sensitive to added salts, as is the critical micelle concentration, but NaClO(4) markedly increases zeta potentials of ImS3-14 in a similar way as in N-tetradecyl-N,N-dimethylammonio-1-propanesulfonate (SB3-14) micelles. The rate of specific hydrogen ion catalyzed hydrolysis of 2-(p-heptoxyphenyl)-1,3-dioxolane and equilibrium protonation of 1-hydroxy-2-naphthoate ion in zwitterionic micelles of ImS3-14 and SB3-14 are increased markedly by NaClO(4) which induces anionoid character and uptake of H(3)O(+), but NaCl is much less effective in this respect. Comparison of ImS3-14 with SB3-14 is based on experimental evidence, and computational calculations indicate similarities and differences in structures of both compounds.

Nucleophilic catalysis of hydrolyses of phosphate and carboxylate esters by metallomicelles: Facts and misconceptions

Metallomicelles of Zn(II) and Cu(II) provide impressive accelerations of hydrolyses of triarylphosphates and p-nitrophenyl acetate, which are often ascribed to some special feature due to incorporation in an association colloid. If the rate data are analyzed in terms of local reactant concentrations in the micellar pseudo-phase, the second-order rate constants are very similar to those of reactions of monomeric complexes in water, i.e. the rate increases are due largely to concentration of reactants in the small volume at the micelle–water interface. Therefore the special micellar effects which are postulated to provide impressive accelerations of reactions are chimera which disappear when transfer equilibria between the aqueous and micellar pseudo-phases are taken into account.

The binding of phenols and phenoxide ions to cationic micelles

Abstract The proton NMR spectra of phenol, p -cresol, and p -ethylphenol bound to micelles of cetyltrimethylammonium bromide (CTABr) support binding adjacent to the cationic head groups. Anions of these phenols and of p-n -propylphenol and p -t-amylphenol also bind adjacent to the head groups. Changes in the chemical shifts of the hydrogen atoms of the surfactant due to interactions with the ring current of the solute are consistent with its inserting into the micelle rather than binding tangentially.

Suicide Nucleophilic Attack: Reactions of Benzohydroxamate Anion with Bis(2,4-dinitrophenyl) Phosphate

The reaction between the benzohydroxamate anion (BHO(-)) and bis(2,4-dinitrophenyl)phosphate (BDNPP) has been examined kinetically, and the products were characterized by mass and NMR spectroscopy. The nucleophilic attack of BHO(-) follows two reaction paths: (i) at phosphorus, giving an unstable intermediate that undergoes a Lossen rearrangement to phenyl isocyanate, aniline, diphenylurea, and O-phenylcarbamyl benzohydroxamate; and (ii) on the aromatic carbon, giving an intermediate that was detected but slowly decomposes to aniline and 2,4-dinitrophenol. Thus, the benzohydroxamate anion can be considered a self-destructive molecular scissor since it reacts and loses its nucleophilic ability.

Diffusion of interacting micelles: A general treatment at low salt concentrations

The micellar diffusivity, measured by quasielastic light scattering, is described by a simple phenomenological expression for a variety of surfactants over a range of surfactant concentrations and temperatures at low salt concentration where electrostatic interactions dominate. This universal expression contains the low order perturbations to the micellar diffusivity in the absence of interactions. The dominant correction scales with the ratio of surfactant to salt concentrations and results from repulsive electrostatic interactions, screened by the added salt ions. By fitting experimental data to this expression, we obtain the Stokes–Einstein micellar radius and a relative measure of the micellar ionization fraction.

Determination of acid dissociation constants of anomers of amino sugars by 1H NMR spectroscopy

Abstract Acid dissociation constants of α- and β- d -glucos-, mannos-, and galactos-ammonium ions have been determined from 1H NMR chemical shifts of the individual anomers in D2O. Values of pKa(D) for the α- and β-ammonium ions are, respectively: glucosamine, 8.12 and 7.87, mannosamine, 7.78 and 8.50, galactosamine, 8.49 and 8.02. The differences are ascribed largely to differences in the hydration requirements of ammonium and amino groups in the axial and equatorial positions and hydration at upper and lower faces of the sugars. Acid dissociation constants of the 1-hydroxyl group of nonionic d -glucosamine and d -glucose are higher for the β than the α anomer.