[Show abstract][Hide abstract] ABSTRACT: direct correlation between the antioxidant efficiencies of caffeic acid and its alkyl esters and their concentrations in the interfacial region of olive oil emulsions. The pseudophase model interpretation of the ''cut-off'' effect a b s t r a c t Recently published results for a series of homologous antioxidants, AOs, of increasing alkyl chain length show a maximum in AO efficiency followed by a significant decrease for the more hydrophobic AOs, typically called the ''cut-off'' effect. Here we demonstrate that in olive oil emulsions both antioxidant efficiencies and partition constants for distributions of AOs between the oil and interfacial regions, P I O , show a maximum at the C 8 ester. A reaction between caffeic acid, CA, and its specially synthesised C 1 –C 16 alkyl esters, and a chemical probe is used to estimate partition constants for AO distributions and interfacial rate constants, k I , in intact emulsions based on the pseudophase kinetic model. The model provides a natural interpretation for both the maximum and the ''cut-off'' effect. More than 70% of the CA esters are in the interfacial region even at low surfactant volume fraction, U I = 0.005.
[Show abstract][Hide abstract] ABSTRACT: The oxidative stability of emulsions depends on acidity because pH may affect, among other things, the concentration of antioxidants (AOs) at the reaction site. Here we investigated the effects of pH on the partition constant, PWI, and the distribution of two phenolic acid AOs, gallic (GA) and caffeic (CA), between the aqueous (W) and interfacial (I) regions, in emulsions of 1:9 (vol:vol) corn oil/acidic water and Tween 20. PWI values are independent of emulsifier concentration, but change substantially with pH following sigmoidal curves with upper limits of PWI ≈280 (GA) and PWI ≈590 (CA) at high acidity. The distributions of GA and CA between the aqueous and interfacial regions depend strongly on surfactant volume fraction, ΦI, so that at pH ∼4.0 and ΦI=0.005, %GAI ≈20 and %CAI ≈50. These values increase to ca. %GAI=60 and %CAI=90 at ΦI=0.05. Increasing the acidity produces substantial changes in %AOI. At ΦI=0.005, a decrease in pH from ∼4 to ∼3 leads to an increase in %GAI from ∼20 to ∼60 and in %CAI from ∼50 to ∼75, respectively.
No preview · Article · Apr 2015 · European Journal of Lipid Science and Technology
[Show abstract][Hide abstract] ABSTRACT: 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. 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 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 between 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 AO partition coefficients or partition constants, and distributions and interfacial rate constants in emulsions. 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.
[Show abstract][Hide abstract] ABSTRACT: We investigated the effects of increasing temperature and emulsifier volume fraction (ΦI) on the distribution of catechin (CAT) in stripped corn oil-in-water emulsions. We also estimated relevant thermodynamic parameters for the transfer of CAT from the aqueous to the interfacial region because CAT is sparingly soluble in corn oil and mainly distributes between the aqueous and interfacial regions of emulsions. The distribution of CAT was assessed in the intact emulsions by employing a well-established kinetic method based on the reaction between a hydrophobic arenediazonium ion and CAT. Results are interpreted on the basis of the pseudophase kinetic model, which provides estimates of the second order interfacial rate constant, k
I, and the partition constant P
WI between the aqueous and interfacial region of the emulsion from k
obs versus ΦI profiles. The P
WI values are quite high, increasing from 188 (T = 15 °C) to 368 (T = 25 °C). This change in P
WI reflects the dependence of the percentage of CAT in the interfacial region, %CATI, on temperature. At T = 15 °C and ΦI = 0.005, %CATI ≈ 60. This percentage increases upon increasing ΦI to %CATI ≈ 88 at ΦI = 0.04. An increase in T from 15 to 25 °C promotes incorporation of CAT into the interfacial region so that at ΦI = 0.005, %CATI increases from ~60 to ~85. The thermodynamic parameters for the transfer from the aqueous to the interfacial region (ΔG
0,W → I, (ΔH
0,W → I and (ΔS
0,W → I) were obtained from the P
WI values at a series of temperatures by the van’t Hoff method and the Gibbs equation, respectively. ΔG
0,W → I is negative at any temperature, indicating that the transfer of CAT from the aqueous to the interfacial region is an spontaneous process.
[Show abstract][Hide abstract] ABSTRACT: Antioxidant (AO) efficiencies are reported to go through maxima with increasing chain length (hydrophobicity) in emulsions. The so-called "cutoff" after the maxima indicating a decrease in efficiency remains unexplained. Here we show, for gallic acid, GA, and propyl, octyl and lauryl gallates (PG, OG and LG, respectively), that at any given volume fraction of emulsifier, the concentrations of antioxidants in the interfacial region of stripped corn oil emulsions and their efficiency order follow: PG > GA > OG > LG. These results provide clear evidence that an AO's efficiency correlates with its fraction in the interfacial region. AO distributions were obtained in intact emulsions by using the pseudophase kinetic model to interpret changes in observed rate constants of the AOs with a chemical probe and their efficiencies were measured by employing the Schaal oven test. The model provides a natural explanation for the maxima with increasing AO hydrophobicity.
No preview · Article · May 2013 · Journal of Agricultural and Food Chemistry
[Show abstract][Hide abstract] ABSTRACT: 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 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, kobs, 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: Mn+ = Cs+ < Ca2+ < Al3+ in the same emulsions. These results, combined with recent results in nonionic and ionic emulsions demonstrate that pseudophase kinetic models provide general, coherent explanations for 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.
[Show abstract][Hide abstract] ABSTRACT: 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, PO(I), and the water and interfacial region, PW(I), and of the interfacial rate constant, kI, 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 kI 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.
Full-text · Article · Mar 2013 · Journal of Colloid and Interface Science
[Show abstract][Hide abstract] ABSTRACT: Until relatively recently, modeling chemical reactivity in emulsions has proved refractory. The problem lies in developing good methods for determining the distributions of reactants because classical separation of the phases by physical methods do not work, which prevents measurement of the distributions of reactants and components between the oil, interfacial and aqueous regions in emulsions. Without this understanding, they cannot be used efficiently as reaction media. We are using a physical-organic chemistry approach grounded in thermodynamics and inspired by the prior success of pseudophase kinetic models developed for association colloids such as micelles microemulsions, and vesicles. In emulsions, as in association colloids, the observed rate constant depends on the concentrations of reactants in each region and on medium effects. The medium effects reflect the solvent properties of a reaction region and the distributions of reactants depend on their solubilities in each region. Here we introduce: (a) the current concepts and basic assumptions employed to interpret chemical reactivity in nonionic and ionic emulsions; (b) several approaches for estimating the partition constants for substrates between the oil-interfacial, POI, and water-interfacial, PWI, regions of the emulsions; and (c) methods for determining the rate constant in the interfacial region, kI. The results demonstrate that pseudophase kinetic analyses provide a unique, versatile, and robust solution to interpreting chemical reactivity in emulsions. The approach permits identifying the relative importance of various emulsion properties such as oil hydrophobicity, emulsifier structure and HLB, temperature, and acidity on reactant distributions. Representative results for antioxidants are included. The approach offers a new route for identifying most efficient antioxidant for a particular food application.
Full-text · Article · Feb 2013 · Current Opinion in Colloid & Interface Science
[Show abstract][Hide abstract] ABSTRACT: Chemical trapping is a powerful approach for obtaining experimental estimates of interfacial molarities of weakly basic nucleophiles in the interfacial regions of amphiphile aggregates. Here we demonstrate that the chemical probe, 4-hexadecyl-2,6-dimethylbenzenediazonium ion, 16-ArN2+, reacts competitively with interfacial water, with the amide carbonyl followed by cleavage of the headgroups from the tail at the amide oxygen, and with the terminal carboxylate groups in micelles of two N-acyl amino acid amphiphiles, sodium N-lauroylsarcosinate, SLS, and sodium N-lauroylglycinate, SLG, simple peptide bond model amphiphiles. Interfacial molarities, moles/liter of interfacial volume, of these three groups were obtained from product yields and by assuming that selectivity toward a particular nucleophile compared to water is the same in an aqueous reference solution and in the interfacial region. Interfacial carboxylate group molarities are ~1.5 M in both SLS and SLG micelles, but the concentration of the amide carbonyl for SLS micelles is ~4.6-5 times less (ca. 0.7 M) than that of SLG micelles (~3 M). The proton on the secondary N of SLG helps solubilize the amide bond in the aqueous region, but the methyl on the tertiary N of SLS helps solubilize the amide bond in the micellar core, reducing its reaction with 16-ArN2+. Application of chemical trapping to proteins in membrane mimetic interfaces should provide insight into the topology of the protein within the interface because trapping of the amide carbonyl and cleavage at the C-N bond occurs only within the interface, and fragment characterization marks those peptide bonds located within the interface.
[Show abstract][Hide abstract] ABSTRACT: Current models and concepts are introduced and used to characterize the physical properties of homogeneous, small molecule, surfactant solutions such as the critical micelle concentration (cmc), aggregation number, degree of ionization of ionic aggregates and the interactions that determine the size and shape of aggregates, various mesophases such as spherical and rodlike micelles, lamellar, vesicular, bicontinuous, and reverse micelles, and the chemical reactivity in surfactant solutions. Surfactant aggregation is a spontaneous process and a delicate balance of forces governs the size and shapes of the structures at equilibrium. Aggregation is driven by the hydrophobic effect, the minimization of surfactant tail-water contact by transfer of the tail of the surfactant unimer from water to the aggregate core with the concomitant release of water of hydration, and an increase in the entropy of the system. Balance is provided by a combination of electrostatic, ion-specific, and hydration interactions in the interfacial region of the aggregates. Two models are generally used to interpret surfactant aggregation and solution properties, the more generally applied pseudophase model and the mass action model. For example, pseudophase models provide a comprehensive interpretation of micellar effects on chemical reactivity. A major unsolved problem is developing a general interpretation of ion-specific effects, as exemplified by the Hofmeister series on surfactant aggregate properties. The ion-pair hydration model represents one potential solution.Keywords:surfactant;unimer;micelle;microemulsion;vesicle;cmc;balance-of-forces;ion-specific effects;aggregation number;hydrophobic effect;hydration;packing parameter;catalysis;cooperativity, Hofmeister
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · Mar 2012 · Journal of Colloid and Interface Science
[Show abstract][Hide abstract] ABSTRACT: We have determined the distribution of the antioxidant tert-butylhydroquinone, TBHQ, between the oil and interfacial regions
of an emulsion composed of stripped corn oil, acidic water and hexaethyleneglycol monododecyl ether, C12E6, by employing a kinetic method based on the reaction between the hydrophobic 16-hexadecylbenzenediazonium ions, 16-ArN2
+, and TBHQ. The kinetic data are interpreted under the light of the pseudophase kinetic model and provide estimates of the
partition constant, and hence the distribution, of TBHQ between the oil and the interfacial region of the emulsion. Our results
show that more than 80% of TBHQ is located in the interfacial region even at low emulsifier volume fractions, and the fraction
of TBHQ in that region increases upon increasing [C12E6].
[Show abstract][Hide abstract] ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
[Show abstract][Hide abstract] ABSTRACT: The Electroanalytical Techniques applied to get insights the chemical reactivity of arendiazonium ions, ArN2+, is the topic of this research work. It describes the electrochemical reduction of ArN2+ on several electrode materials (mercury, graphite and gold) by different techniques: Differential Pulse Polarography (DPP), Linear Sweep Voltammetry (LSV) and Cycle Voltammetry (CV). For it, one took to end an electrochemical study of the above mentioned ions in aqueous solutions, emulsions, and biomimetic media, giving place to an optimization of the instrumental and experimental parameters, in order to verify the effects that produce on the electrochemical signal of ArN2+ to obtain the best sensibility and selectivity in the analysis. Some examples of arendiazonium´s electrochemistry and its possible use as chemical probes in food chemistry and precursors to design biosensors in biomedical fields are given.
[Show abstract][Hide abstract] ABSTRACT: Symbols Used in TextIntroductionCarboxylic Esters as Probes for Micellar Catalysed ReactionsMicellar StructureThe Simple Kinetic ModelKinetic Models for Micellar Catalysed ReactionsMiscellaneousExperimental ProblemsPreparative AspectsDeacylations in Non-Functional MicellesFunctional Micelles and ComicellesThe Question of Bifunctional CatalysisConclusions, Connections and ConjecturesAcknowledgmentsReferences
[Show abstract][Hide abstract] ABSTRACT: The combined linear sweep voltammetry (LSV)/pseudophase kinetic model method was used to obtain the first estimates of the free energies, enthalpy, and entropies of transfer of alpha-tocopherol (TOC) between the oil and interfacial regions of fluid, opaque, emulsions of n-octane, acidic water, and the nonionic surfactant hexaethyleneglycol mono dodecyl ether (C12E6) from the temperature dependence of TOC's partition constant. Determining structure-reactivity relationships for chemical reactions in emulsions is difficult because traditional methods for monitoring reactions are unsuitable and because the partitioning of reactive components between the oil, interfacial, and aqueous regions of opaque emulsions are difficult to measure. The dependence of the observed rate constant, k(obs), for the reaction of an arenediazonium probe, 16-ArN2+, with TOC was determined as a function of C12E6 volume fraction. The pseudophase kinetic model was used to estimate the interfacial rate constant, k1, and the partition constants of antioxidants between the oil and interfacial, Po(I), regions in the emulsion from k(obs) versus phiI profiles. The thermodynamic parameters of transfer from the oil to the interfacial region at a series of temperatures were respectively obtained from the PoI values (deltaGT0,O-->I), by the van't Hoff method (deltaHT0,O-->I), and from the Gibbs equation (deltaST0,O-->I). The free energy of transfer is spontaneous, and a large positive entropy of transfer dominates a positive enthalpy of transfer, indicating that the TOC headgroup disrupts the structure of the interfacial region in its immediate vicinity upon transfer from n-octane. The methods described here are applicable to any bimolecular reaction in emulsions in which one of the reactants is restricted to the interfacial region and the rate of its reaction with a second component can be monitored electrochemically.
[Show abstract][Hide abstract] ABSTRACT: Understanding ion specific effects on the solution properties of association colloids is a major unsolved problem, and we are studying the chemistry of gemini surfactants in the gas-phase by mass spectrometry and density functional theory (DFT) to probe ion specific effects in the absence of water. Products from gas-phase fragmentation chemistry of dication-monoanion pairs, M2+X(-), of C16H33(CH3)2N+-(CH2)(n-) +N(CH3)2C16H33.2X(-) gemini surfactants were determined by using sequential collision induced dissociation mass spectrometry. The spacer length "n" was systematically varied (n = 2, 3, 4, and 6) for each counterion investigated (X(-) = F(-), Br(-), Cl(-), I(-), NO3(-), CF3CO2(-), and PF6(-)). The M2+X(-) pairs fragment into monocationic products from competing E2 and S N2 pathways that are readily quantified by tandem MS. The dominant reaction pathway depends on dication and anion structure because it switches from E2 to S N2 with decreasing anion basicity and increasing spacer length. For spacer lengths n = 4 and 6, the major S N2 product shifts from attack at methylene to methyl on the quaternary ammonium group. DFT calculations of gemini headgroup model bolaform salts, CH3(CH3)2N+-(CH2)(n-)+N(CH3)2CH3.2X(-) (X(-) = F(-), Cl(-), Br(-), and I(-), n = 2-4), primarily of activation enthalpies, DeltaH, but also of free energies and entropies for the dication-monoanion pairs, M2+X(-), provide qualitative explanations for the MS structure-reactivity patterns. DeltaH values for S N2 reactions are independent of X(-) type and spacer length, while E2 reactions show a significant increase in DeltaH with decreasing anion basicity and a modest increase with spacer length. Comparisons with the DeltaH values of model CH3CH2(CH3)3N+X(-) halides show that the second charge on the dicationic ion pairs does not significantly affect DeltaH and that the change in distance between the nucleophile and leaving group in the ground and transition states structures in S N2 reactions is approximately constant indicating that DeltaH is governed primarily by electrostatic interactions.
Full-text · Article · Dec 2008 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: 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.
Full-text · Article · May 2008 · Journal of Colloid and Interface Science