Pseudophase Micellar Model Research Articles

Growth Kinetics of Nanosize Silica in a Nonionic Water-in-Oil Microemulsion: A Reverse Micellar Pseudophase Reaction Model

The growth kinetics of silica nanoparticles synthesized by the microemulsion-mediated alkoxide hydrolysis method was investigated with tetraethoxysilane (TEOS) as the silica precursor and polyoxyethylene (5) nonylphenylether (NP-5)/cyclohexane/ammonium hydroxide as the water-in-oil microemulsion system. The time evolution of the mean diameter of the silica particles was determined for different values of the water-to-surfactant molar ratio (R). Particle growth was found to be a slow process, where under typical experimental conditions at room temperature ([TEOS] = 0.024 M, 29.6 wt% NH3, water-to-TEOS molar ratio (h) = 7.8) the particles achieved their terminal size after several days. During the early stages of the reaction, particle growth followed first-order kinetics, and the observed first-order growth rate constants decreased with increase in R. A reverse micellar pseudophase model (which considered the partition of reactants between the reverse micellar pseudophase and the bulk oil phase) was developed to analyze the growth kinetics under the hypothesis that TEOS hydrolysis was rate controlling. The pseudophase model predicted an inverse relationship between the observed growth rate and R, in agreement with experiment. The roles of steric effects and the bound state of water molecules, in retarding the hydrolysis rate, were highlighted by examining the effect of R on the TEOS hydrolysis rate constant in the reverse micellar pseudophase.

Micellar kinetics of acyl transfer from n-nonanoyloxybenzenesulfonate and phenyl nonanoate bleach activators to hydrogen peroxide and pernonanoic acid: effect of charge on the surfactant and activator

The effect of surfactants on the kinetics of the title reactions is analysed using a combined multiple micellar pseudophase model and transition state pseudoequilibrium constant approach which leads to the micellar association constants of the reactants and the apparent (virtual) micellar association constants of the transition states. These association constants reflect the stabilisation of the reactants and transition states, respectively, by the micelle compared with their stability in the bulk aqueous phase. For the reaction of n-nonanoyloxybenzenesulfonate with pernonanoate in the presence of sodium dodecyl sulfate (SDS) the apparent micellar association constant of the transition state is larger than the micellar association constant of the activator; this is consistent with micellar catalysis, whilst micellar inhibition occurs with the anion of hydrogen peroxide. The micellar association constant of phenyl nonanoate with SDS is four orders of magnitude greater than that of n-nonanoyloxybenzenesulfonate due to the absence of the negatively charged sulfonate group, whilst the apparent micellar association constant of the transition state for its reaction with pernonanoate is more than an order of magnitude less. In the non-ionic surfactant Brij-35, the micellar association constant of n-nonanoyloxybenzenesulfonate and the corresponding apparent quantity for the transition state for the reaction with the hydroperoxide anion are, respectively, about one and two orders of magnitude greater than in SDS.

Kinetic treatment of the reaction of m-chloroperbenzoic acid and iodide in mixed anionic/non-ionic micelles

Catalysis and inhibition of the title reaction in mixed micelles of Brij-35 and SDS is described. The kinetics are treated using a combined multiple micellar pseudophase model and transition state pseudoequilibrium constant approach that was previously used for the catalysis and inhibition of the reaction of peracids and iodide in non-ionic and anionic micelles, respectively (D. M. Davies, N. D. Gillitt and P. M. Paradis, J. Chem. Soc., Perkin Trans. 2, 1996, 659). In the present mixed micellar system the following factors are taken into account: (i) the mole ratio of the surfactants in the mixed micelle, which is the relevant quantity for the micellar kinetics and is different from the stoichiometric mole ratio of the surfactants; (ii) non-ideal mixing of the surfactants, which also influences the composition of the mixed micelle; (iii) the ideal behaviour of the apparent molar volume of micellized surfactant in mixed micelles; and (iv) deviations from ideality of the partitioning of reactants and transition state between mixed micelles and the bulk aqueous phase. Logarithms of micellar association constants of the reactants and transition state in the two surfactants have been obtained, together with parameters closely related to the excess Gibbs function associated with the deviations from ideality described by (iv). The relationship between the parameters that describe non-ideal mixing of the surfactants in (ii) and the deviations from ideality in (iv) is discussed.

Catalysis and inhibition of the iodide reduction of peracids by surfactants: partitioning of reactants, product and transition state between aqueous and micellar pseudophases

A multiple micellar pseudophase model of kinetics in aqueous surfactant solutions is described. The model has been developed using the transition state pseudoequilibrium constant approach. The advantage of this approach is that no assumptions are made about the nature of the micelle pseudophases. The kinetics of the reduction of pernonanoic and 3-chloroperbenzoic acids by iodide in SDS, Brij-35 and Triton X-100 are reported. The results are used to obtain the micellar association constants of the peracids and also the apparent (virtual) micellar association constants of the transition states. These are compared with the micellar association constants of the parent acids, obtained by pH titration. The association constants are discussed in terms of micellar catalysis and inhibition. Ratios of association constants indicate the relationship between initial, transition and final states.

Effects of urea and a nonionic surfactant on the micellization and counterion binding properties of cetyltrimethyl ammonium bromide and sodium dodecyl sulfate

Micellization characteristics and counterion binding properties of cetyltrimethyl ammonium bromide (CTAB) in presence of urea and a nonionic surfactant polyoxyethylene sorbitan monolaurate (PSML), and of sodium dodecyl sulphate (SDS) in presence of urea as well as of several mixtures of CTAB with a bile salt, sodium cholate (NaC), and sodium chloride have been studied. Both urea and PSML have increased the critical micelle concentration (CMC) of the surfactants, the former being more effective than the latter. The analysis of the results supports the pseudophase micellar model to hold over the mass action model. Pure CTAB micelles bind more counterions (96 %) than pure SDS micelles (87 %), and the decreasing effect of urea on the binding is less in case of the former than the latter. A 4∶1 mixture of CTAB and sodium cholate (NaC) can micellize and the micelles bind 87 % bromide ion, whereas 2∶1 and 1∶1 mixtures do not micellize. Micelles of 1∶1 mixture of CTAB and NaCl can bind counter bromide ions to the extent of 92 %. The limiting concentrations of urea required to effect counterion binding by CTAB and SDS micelles are 0.15 mol dm−3 and 0.25 mol dm−3, respectively. Such effect is shown by PSML on CTAB at a ratio 0.28∶1. The activation energy of conduction of SDS has increased in the presence of urea up to a concentration of 4 mol dm−3, at higher concentrations the activation energy has decreased, the effect being more for surfactant concentration above CMC than below.

Effect of micelle formation on the absorption spectra of a functionalized detergent with the anthraquinone moiety

Measurements of the absorption spectra of aqueous solutions of (2-anthraquinonylmethyl)dodecyldimethylammonium (AQD+) bromide have been performed. From the concentration dependence of the molar absorption coefficient at λmax= 330 nm (εapp), the c.m.c. and the molar absorption coefficient of AQD+ participating in its micelle formation were calculated by assuming a micellar pseudo-phase model. The change in εapp above the c.m.c. was found to be due to interactions of the anthraquinone moieties, by studying the effect of the addition of hexadecyltrimethylammonium bromide (CTAB) to the AQD+ micellar solution. The effect of the addition of CTAB to the AQD+ monomer solution was also examined, but the spectra of the AQD+ monomer were little affected by the addition of CTAB. Measurements of the absorption spectra of aqueous solutions of the monomeric AQD+ and sodium dodecylsulphate (SDS) point to the formation of a stable ion-pair complex between them. On further addition of SDS, the complex disappeared above the c.m.c. of SDS. This amphiphatic anthraquinone derivative serves as a probe for determining the c.m.c. of SDS in the absence and presence of electrolyte because the molar absorption coefficient at 290 nm changes by a factor of 7 above and below the c.m.c. of the host surfactant.

Studies in bile salt solutions. VI. Micellar catalysis of ester hydrolysis by bile-salt-stimulated human milk lipase

The reactivity of 4-nitrophenylacetate has been examined in the presence of human milk lipase stimulated by solutions of sodium cholate, deoxycholate, and chenodeoxycholate, and their taurine and glycine conjugates, in 0.01 mole dm −3 bistris propane buffer at pH 6.50, T = 310.5°K. The reactivity of phenylsalicylate (salol) has also been examined in the presence of sodium cholate. The rate constant-bile salt concentration profiles have been analyzed according to the pseudophase micellar model and the values of the binding constant, K HML, between the enzyme and the various bile salts, the kinetic values of the critical micelle concentrations, CMC, and the values of the micelle-catalyzed rate constants, k M , have been identified.