Laurence S. Romsted

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.

Quantitative determination of α-tocopherol distribution in a tributyrin/Brij 30/water model food emulsion

Until recently, determining the distribution of antioxidants, AOs, between the oil, interfacial and aqueous regions of opaque emulsions has not worked well because the concentrations of AOs in interfacial regions cannot be determined separately from their concentrations in the oil and water phases. However, our novel kinetic method based on the reaction between an arenediazonium ion and vitamin E, or alpha-tocopherol, provides the first good estimates for the two partition constants that describe alpha-tocopherol distribution between the oil/interfacial and water/interfacial regions of tributyrin/Brij 30/water emulsions without physical isolation of any phase. The reaction is monitored by a new derivatization method based on trapping unreacted arenediazonium ion as an azo dye and confirmed by linear sweep voltammetry, LSV. The results by both derivatization and LSV methods are in good agreement and show that alpha-tocopherol distributes strongly in favor of the interfacial region when the oil is tributyrin, e.g., ca. 90% when the surfactant volume fraction is Phi I=0.01. The second-order rate constant for reaction in the interfacial region is also obtained from the results. Our kinetic method provides a robust approach for determining antioxidant distributions in emulsions and should help develop a quantitative interpretation of antioxidant efficiency in emulsions.

A new method for estimating counter-ion selectivity of a cationic association colloid: Trapping of interfacial chloride and bromide counter-ions by reaction with micellar bound aryldiazonium salts

Abstract Yields of aryl halides and phenolic products from dediazoniation of a hydrophobic aryldiazonium salt bound to cetyltrimethylammonium halide, CTAX, micelles in 0.1 to 0.2 M HX with added NaX(XCl − and Br − ) are used to estimate, simultaneously, the quantity of halide counter-ions and water at the micelle surface. The interfacial counter-ion concentration increases, both in solutions of CTABr with added NaBr and CTACl with added NaCl with a proportionate decrease in interfacial water. The selectivity of CTAX micelles toward Br − and Cl − in solutions containing mixtures of the two ions is obtained directly, unlike all previous methods which require an estimate of the fraction of bound counter-ions. Counter-ion exchange constants, K Br Cl , estimated from aryl bromide/aryl chloride yield ratios at high NaX, are consistent with published values. Our results show, for the first time, that K Br Cl is insensitive to ionic strength up to 3.0 M NaX when [Br − ]/[Cl − ]=1.0, but has modest dependence on the mole fraction of Br − at constant total NaX. K Br Cl is also independent of surfactant concentration, after correcting for the fraction of bound counter-ions. In principle, product ratios from reaction of hydrophobic aryldiazonium salts can be used to estimate the relative concentrations of all nucleophiles at the surfaces of association colloids, provided stable aryl substituted products are formed via rate determining loss of N 2 .

To Model Chemical Reactivity in Heterogeneous Emulsions, Think Homogeneous Microemulsions.

Two important and unsolved problems in the food industry and also fundamental questions in colloid chemistry are how to measure molecular distributions, especially antioxidants (AOs), and how to model chemical reactivity, including AO efficiency in opaque emulsions. The key to understanding reactivity in organized surfactant media is that reaction mechanisms are consistent with a discrete structures-separate continuous regions duality. Aggregate structures in emulsions are determined by highly cooperative but weak organizing forces that allow reactants to diffuse at rates approaching their diffusion-controlled limit. Reactant distributions for slow thermal bimolecular reactions are in dynamic equilibrium, and their distributions are proportional to their relative solubilities in the oil, interfacial, and aqueous regions. Our chemical kinetic method is grounded in thermodynamics and combines a pseudophase model with methods for monitoring the reactions of AOs with a hydrophobic arenediazonium ion probe in opaque emulsions. We introduce (a) the logic and basic assumptions of the pseudophase model used to define the distributions of AOs among the oil, interfacial, and aqueous regions in microemulsions and emulsions and (b) the dye derivatization and linear sweep voltammetry methods for monitoring the rates of reaction in opaque emulsions. Our results show that this approach provides a unique, versatile, and robust method for obtaining quantitative estimates of AO partition coefficients or partition constants and distributions and interfacial rate constants in emulsions. The examples provided illustrate the effects of various emulsion properties on AO distributions such as oil hydrophobicity, emulsifier structure and HLB, temperature, droplet size, surfactant charge, and acidity on reactant distributions. Finally, we show that the chemical kinetic method provides a natural explanation for the cut-off effect, a maximum followed by a sharp reduction in AO efficiency with increasing alkyl chain length of a particular AO. We conclude with perspectives and prospects.

Temperature and emulsifier concentration effects on gallic acid distribution in a model food emulsion.

We determined the effects of emulsifier concentration and temperature on the distribution of gallic acid (GA) in a food-grade emulsion composed of 1:9 vol:vol stripped corn oil, acidic water and Tween 20. The distribution of GA can be defined by the partition constant between the aqueous and the interfacial regions, P(W)(I), which was determined by using a kinetic method and the pseudophase kinetic model. Once P(W)(I) is known, determining the distribution of GA is straightforward. Our results show that at least 40% of the total GA is located in the interfacial region of the emulsion at 0.005 volume fraction of Tween 20, and this percentage increases to ca. 85% of the total GA at 0.04 volume fraction of Tween 20. The variation of P(W)(I) with the temperature was used to estimate the thermodynamic parameters for the GA transfer from the aqueous to the interfacial region of the emulsion and the activation parameters for the reaction between 16-ArN(2)(+) and GA in the interfacial region. The free energy of transfer from the aqueous to the interfacial region, ΔG(T)(0,W→I), is negative, the enthalpy of transfer is small and negative, but the entropy of transfer is large and positive. Our results demonstrate that the partitioning of GA in acidic emulsions between aqueous and interfacial regions depends primarily on droplet concentration and is only slightly dependent on temperature.

Rates and pH-dependent product distributions of the CuCl2-catalyzed dediazoniation of p-nitrobenzenediazonium ­tetrafluoroborate in aqueous acid

The rates of formation and yields of products from the dediazoniation of p-nitrobenzenediazonium tetrafluoroborate (PNBD) in aqueous solutions over a range of HCl, NaCl and CuCl 2 concentrations at 60 °C were examined. Two main products were observed: p-nitrophenol (ArOH) and p-nitrochlorobenzene (ArCl). Trace amounts of nitrobenzene (ArH) and p-nitrofluorobenzene (ArF) were detected. Added CuCl 2 speeds the reaction and both the rate of dediazoniation and ArOH yield (unlike ArCl) are very sensitive to pH. The results are completely consistent with the heterolytic dediazoniation mechanism, i.e. rate-determining formation of a highly reactive aryl cation followed by competitive formation of dediazoniation products. PNBD kinetics are first order (with respect to PNBD) in the absence of and presence of CuCl2, except at low acidity and in the presence of low to moderate CuCl2 concentrations. The non-first-order kinetics are attributed to a competing reaction between PNBD and the ArOH product. The results suggest a simple method for preparing halobenzenes in high yield. Copyright

Using the pseudophase kinetic model to interpret chemical reactivity in ionic emulsions: determining antioxidant partition constants and interfacial rate constants.

Kinetic results obtained in cationic and anionic emulsions show for the first time that pseudophase kinetic models give reasonable estimates of the partition constants of reactants, here t-butylhydroquinone (TBHQ) between the oil and interfacial region, P(O)(I), and the water and interfacial region, P(W)(I), and of the interfacial rate constant, k(I), for the reaction with an arenediazonium ion in emulsions containing a 1:1 volume ratio of a medium chain length triglyceride, MCT, and aqueous acid or buffer. The results provide: (a) an explanation for the large difference in pH, >4 pH units, required to run the reaction in CTAB (pH 1.54, added HBr) and SDS (pH 5.71, acetate buffer) emulsions; (b) reasonable estimates of PO(I) and k(I) in the CTAB emulsions; (c) a sensible interpretation of added counterion effects based on ion exchange in SDS emulsions (Na(+)/H3O(+) ion exchange in the interfacial region) and Donnan equilibrium in CTAB emulsions (Br(-) increasing the interfacial H3O(+)); and (d) the significance of the effect of the much greater solubility of TBHQ in MCT versus octane, 1000/1, as the oil. These results should aid in interpreting the effects of ionic surfactants on chemical reactivity in emulsions in general and in selecting the most efficient antioxidant for particular food applications.

Interfacial compositions of cationic and mixed non-ionic micelles by chemical trapping: a new method for characterizing the properties of amphiphilic aggregates

Abstract A specially synthesized arenediazonium ion bound to amphiphilic aggregates decomposes spontaneously via rate determining loss of N2 to give a highly reactive, unselective, aryl cation intermediate. This intermediate is trapped competitively by weakly basic nucleophiles in the interfacial region of aggregates such as micelles and other association colloids. Product yields, analyzed by HPLC with UV detection, are used to estimate, simultaneously, the interfacial concentrations of a number of different nucleophiles, including water, that are commonly found at the surfaces of biomembranes and in many commercial products. Two applications of the method are discussed. First, we show that the interfacial concentrations of X− (X=Br, Cl) increase steadily with increasing cetyltrimethylammonium halide (CTAX) and tetramethylammonium halide (TMAX) concentrations and that the interfacial concentrations of these counterions increase continuously with their aqueous phase concentrations at a constant degree of micelle ionization. Interfacial Br− and Cl− concentrations also show marked increases at their respective sphere-to-rod transitions. This steady increase in interfacial counterion concentration with increasing aqueous counterion concentration contradicts a basic assumption of the pseudophase ion exchange (PIE) model of chemical reactivity in aggregates, i.e. that the total concentrations of ions at aggregate interfaces is constant and independent of the amphiphile and salt concentrations. The consequences for the PIE model are discussed. Second, the chemical trapping reaction is used to estimate: (a) distributions of terminal OH groups of non-ionic amphiphiles in mixed non-ionic micelles composed of amphiphiles with different lengths of oligoethylene oxide chains and (b) hydration numbers of the inner layers of interfacial region next to the hydrocarbon core in these mixed micelles. Terminal OH groups distributions are well fitted by a radial one-dimensional random walk model. The average hydration number for the inner layers at 40°C is about 3, in agreement with estimates from NMR water (D2O) self-diffusion measurements and with the hydration number of 3 for aqueous solutions of polyethylene oxide. The results suggest that the hydration states of the ethylene oxide (EO) units near the micellar core are near their minimum value. Recent and potential applications of the chemical trapping method are briefly discussed.

Arenediazonium salts : New probes of the interfacial compositions of association colloids. 5.1-4 Determination of hydration numbers and radial distributions of terminal hydroxyl groups in mixed nonionic CmEn micelles by chemical trapping

An aggregate-bound long tail arenediazonium ion, 4-hexadecyl-2,6-dimethylbenzenediazonium ion, 16-ArN2+, was used as a chemical trapping reagent to estimate the hydration state and terminal hydroxyl, OH, group distributions within the interfacial region of sets of binary mixed nonionic micelles of oligooxyethylene alkyl monoether, CmEn, surfactants:  C10E4/C12E6, C10E4/C16E8, C10E5/C16E8, C12E6/C16E8, and C10E5/C12E5. 16-ArN2+ decomposes spontaneously to generate an aryl cation that is trapped by water and by the terminal OH groups of the two nonionic surfactants to give phenol and alkyl aryl ethers as products. Quantitative analyses of product yields by HPLC were used to estimate the hydration numbers and relative number densities of the terminal OH groups of the two surfactants in the two inner layers of the interfacial region adjacent to the micellar core. The thickness of one layer is set equal to the length of one ethylene oxide unit. The radial distributions of terminal OH groups and ethylene oxide ...

Interpreting ion-specific effects on the reduction of an arenediazonium ion by t-butylhydroquinone (TBHQ) using the pseudophase kinetic model in emulsions prepared with a zwitterionic sulfobetaine surfactant.

Specific salt effects on the reduction of an amphiphilic arenediazonium ion, 16-ArN2(+), by TBHQ in opaque, stirred, and kinetically stable emulsions prepared with a zwitterionic sulfobetaine surfactant are consistent with the chameleon effect: selective anion binding/induced cation binding in the interfacial region of the emulsions. Added NaX salts with different anions decrease the observed first-order rate constant, k(obs), for the reduction in the order X(-) = ClO4(-) > Br(-) ≈ CCl3CO2(-) > Cl(-) > MeSO3(-). Added MCln salts of increasing cation valence at constant total Cl(-) concentration increase kobs in the order M(n+) = Cs(+) < Ca(2+) < Al(3+) in the same emulsions. These results, combined with recent results for nonionic and ionic emulsions, demonstrate that pseudophase kinetic models provide general, coherent explanations of chemical reactivity in homogeneous micelles, microemulsions, vesicles, and now biphasic emulsions and with all types of basic surfactant structures: nonionic, cationic, anionic, and now zwitterionic.