Pseudophase Kinetic Model Research Articles

Molecular Design of Interfaces of Model Food Nanoemulsions: A Combined Experimental and Theoretical Approach.

The composition and structure of the interfacial region of emulsions frequently determine its functionality and practical applications. In this work, we have integrated theory and experiments to enable a detailed description of the location and orientation of antioxidants in the interfacial region of olive-oil-in-water nanoemulsions (O/W) loaded with the model gallic acid (GA) antioxidant. For the purpose, we determined the distribution of GA in the intact emulsions by employing the well-developed pseudophase kinetic model, as well as their oxidative stability. We also determined, by employing an in silico design, the radial distribution functions of GA to gain insights on its insertion depth and on its orientation in the interfacial region. Both theoretical and experimental methods provide comparable and complementary results, indicating that most GA is located in the interfacial region (~81.2%) with a small fraction in the aqueous (~18.82%). Thus, GA is an effective antioxidant to inhibit lipid oxidation in emulsions not only because of the energy required for its reaction with peroxyl radical is much lower than that between the peroxyl radical and the unsaturated lipid but also because its effective concentration in the interfacial region is much higher than the stoichiometric concentration. The results demonstrate that the hybrid approach of experiments and simulations constitutes a complementary and useful pathway to design new, tailored, functionalized emulsions to minimize lipid oxidation.

Distributions of α- and δ-TOCopherol in Intact Olive and Soybean Oil-in-Water Emulsions at Various Acidities: A Test of the Sensitivity of the Pseudophase Kinetic Model.

During the last years, the formalism of the pseudophase kinetic model (PKM) has been successfully applied to determine the distributions of antioxidants and their effective interfacial concentrations, and to assess the relative importance of emulsion and antioxidant properties (oil and surfactant nature, temperature, acidity, chemical structure, hydrophilic-liphophilic balance (HLB), etc.) on their efficiency in intact lipid-based emulsions. The PKM permits separating the contributions of the medium and of the concentration to the overall rate of the reaction. In this paper, we report the results of a specifically designed experiment to further test the suitability of the PKM to evaluate the distributions of antioxidants among the various regions of intact lipid-based emulsions and provide insights into their chemical reactivity in multiphasic systems. For this purpose, we employed the antioxidants α- and δ-TOCopherol (α- and δ-TOC, respectively) and determined, at different acidities well below their pKa, the interfacial rate constants kI for the reaction between 16-ArN2+ and α- and δ-TOC, and the antioxidant distributions in intact emulsions prepared with olive and soybean oils. Results show that the effective interfacial concentration of δ-TOC is higher than that of α-TOC in 1:9 (v/v) soybean and 1:9 olive oil emulsions. The effective interfacial concentrations of tocopherols are much higher (15-96-fold) than the stoichiometric concentrations, as the effective interfacial concentrations of both δ-TOC and α-TOC in soybean oil emulsions are higher (2-fold) than those in olive oil emulsions. Overall, the results demonstrate that the PKM grants an effective separation of the medium and concentration effects, demonstrating that the PKM constitutes a powerful non-destructive tool to determine antioxidant concentrations in intact emulsions and to assess the effects of various factors affecting them.

Unexpected Antioxidant Efficiency of Chlorogenic Acid Phenolipids in Fish Oil-in-Water Nanoemulsions: An Example of How Relatively Low Interfacial Concentrations Can Make Antioxidants to Be Inefficient.

Selecting effective antioxidants is challenging since their efficiency in inhibiting lipid oxidation depends on the rate constants of the chemical reactions involved and their concentration at the reaction site, i.e., at the interfacial region. Accumulation of antioxidants at the interface of emulsions is key to modulate their efficiency in inhibiting lipid oxidation but its control was not well understood, especially in emulsions. It can be optimized by modifying the physicochemical properties of antioxidants or the environmental conditions. In this work, we analyze the effects of surfactant concentration, droplet size, and oil to water ratio on the effective interfacial concentration of a set of chlorogenic acid (CGA) esters in fish oil-in-water (O/W) emulsions and nanoemulsions and on their antioxidant efficiency. A well-established pseudophase kinetic model is used to determine in the intact emulsified systems the effective concentrations of the antioxidants (AOs). The relative oxidative stability of the emulsions is assessed by monitoring the formation of primary oxidation products with time. Results show that the concentration of all AOs at the interfacial region is much higher (20–90 fold) than the stoichiometric one but is much lower than those of other phenolipid series such as caffeic or hydroxytyrosol derivatives. The main parameter controlling the interfacial concentration of antioxidants is the surfactant volume fraction, ΦI, followed by the O/W ratio. Changes in the droplet sizes (emulsions and nanoemulsions) have no influence on the interfacial concentrations. Despite the high radical scavenging capacity of CGA derivatives and their being concentrated at the interfacial region, the investigated AOs do not show a significant effect in inhibiting lipid oxidation in contrast with what is observed using other series of homologous antioxidants with similar reactivity. Results are tentatively interpreted in terms of the relatively low interfacial concentrations of the antioxidants, which may not be high enough to make the rate of the inhibition reaction faster than the rate of radical propagation.

Effects of Surfactant Volume Fraction on the Antioxidant Efficiency and on The Interfacial Concentrations of Octyl and Tetradecyl p-Coumarates in Corn Oil-in-Water Emulsions.

Surfactants have been used for decades in the food industry for the preparation of lipid-based emulsified food stuffs. They play two main roles in the emulsification processes: first they decrease the interfacial tension between the oil and water, facilitating droplet deformation and rupture; second, they reduce droplet coalescence by forming steric barriers. However, addition of surfactants to binary oil-water mixtures also brings up the formation of three-dimensional interfacial layers, surrounding each emulsion droplet, that significantly alter chemical reactivity. This is the case, for instance, in the inhibition reaction between antioxidants and the lipid radicals formed in the course of the spontaneous oxidation reaction of unsaturated lipids, which are commonly employed in the preparation of food-grade emulsions. The rate of the inhibition reaction depends on the effective concentrations of antioxidants, which are mostly controlled by the amount of surfactant employed in the preparation of the emulsion. In this work, we analyze the effects of the surfactant Tween 20 on the oxidative stability and on the effective concentrations of two model antioxidants derived from cinnamic acid, determining their interfacial concentrations in the intact emulsions to avoid disrupting the existing equilibria and biasing results. For this purpose, a recently developed methodology was employed, and experimental results were interpreted on the grounds of a pseudophase kinetic model.

Modeling Chemical Reactivity at the Interfaces of Emulsions: Effects of Partitioning and Temperature.

Bulk phase chemistry is hardly ever a reasonable approximation to interpret chemical reactivity in compartmentalized systems, because multiphasic systems may alter the course of chemical reactions by modifying the local concentrations and orientations of reactants and by modifying their physical properties (acid-base equilibria, redox potentials, etc.), making them—or inducing them—to react in a selective manner. Exploiting multiphasic systems as beneficial reaction media requires an understanding of their effects on chemical reactivity. Chemical reactions in multiphasic systems follow the same laws as in bulk solution, and the measured or observed rate constant of bimolecular reactions can be expressed, under dynamic equilibrium conditions, in terms of the product of the rate constant and of the concentrations of reactants. In emulsions, reactants distribute between the oil, water, and interfacial regions according to their polarity. However, determining the distributions of reactive components in intact emulsions is arduous because it is physically impossible to separate the interfacial region from the oil and aqueous ones without disrupting the existing equilibria and, therefore, need to be determined in the intact emulsions. The challenge is, thus, to develop models to correctly interpret chemical reactivity. Here, we will review the application of the pseudophase kinetic model to emulsions, which allows us to model chemical reactivity under a variety of experimental conditions and, by carrying out an appropriate kinetic analysis, will provide important kineticparameters.

Inhibitive Effect on the Rate of Hydrolysis of Tetracaine by the Surfactant-Coated Magnetic Nanoparticles (Fe3O4)

The influence of the magnetic nanoparticles Fe3O4 and surfactant coated nanoparticles on the rate of alkaline hydrolysis of tetracaine were investigated spectrophotometrically. The Fe3O4 nanoparticles were synthesized by co-precipitation method. These synthesized nanoparticles were characterized by physical techniques. The XRD patterns showed the crystalline nature and the presence of two bands at 635 cm -1 and 585 cm -1 were assigned to the Fe-O bond vibrations in Fe3O4. The SEM and TEM studies confirmed the spherical structure of nanoparticles. The VSM studies show the magnetic nature. The reaction rate increased linearly with increase in [NaOH]. The rate constant values decreased with the increase in the SDS coated Fe3O4 nanoparticles from 4.0 × 10 -4 s -1 to 1.1 × 10 -4 s -1 in the presence of PEG 1500. Similarly, the increase in the amount of CTABr coated Fe3O4 nanoparticles decreased the rate constant values from 4.0 × 10 -4 s -1 to 1.02 × 10 -4 s -1 in the presence of PEG 1500. The binding constant and rate constant were found to be lower in SDS coated nanoparticles than the CTABr coated particles. The observed results for - [surfactant] in the presence of magnetic nanoparticles and surfactant coated nanoparticles were treated using the pseudophase kinetic model.

Interfacial Concentrations of Hydroxytyrosol Derivatives in Fish Oil-in-Water Emulsions and Nanoemulsions and Its Influence on Their Lipid Oxidation: Droplet Size Effects.

Reports on the effect of droplet size on the oxidative stability of emulsions and nanoemulsions are scarce in the literature and frequently contradictory. Here, we have employed a set of hydroxytyrosol (HT) esters of different hydrophobicity and fish oil-in-water emulsified systems containing droplets of different sizes to evaluate the effect of the droplet size, surfactant, (ΦI) and oil (ΦO) volume fractions on their oxidative stability. To quantitatively unravel the observed findings, we employed a well-established pseudophase kinetic model to determine the distribution and interfacial concentrations of the antioxidants (AOs) in the intact emulsions and nanoemulsions. Results show that there is a direct correlation between antioxidant efficiency and the concentration of the AOs in the interfacial region, which is much higher (20–200 fold) than the stoichiometric one. In both emulsified systems, the highest interfacial concentration and the highest antioxidant efficiency was found for hydroxytyrosol octanoate. Results clearly show that the principal parameter controlling the partitioning of antioxidants is the surfactant volume fraction, ΦI, followed by the O/W ratio; meanwhile, the droplet size has no influence on their interfacial concentrations and, therefore, on their antioxidant efficiency. Moreover, no correlation was seen between droplet size and oxidative stability of both emulsions and nanoemulsions.

Capping action of ionic surfactants on the nucleation of lawsone-Ag+ redox system

Cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were used a stabilizing agent to determine the rate of nucleation and growth process for the reduction of Ag+ ions by lawsone. Critical micellar concentration (CMC) were calculated by using surface tension method and found to be 6.3 × 10−3 and 9.2 × 10−4 mol dm−3 for SDS and CTAB, respectively in presence of Ag+ ions. Reaction rates increases with increasing the surfactant concentrations. SDS aggregates formed complex with Ag+ ions due to electrostatic interaction with negative head group as well as solubilized the lawsone into the aqueous micellar phase through hydrophobically. CTAB effect might be due to the solubilization of lawsone into the Stern layer. The rate constants were found to be 3.2 × 10−4 and 8.1 × 10−5 s−1 for CTAB and SDS, respectively. SDS exhibited inhibition effect at post-micellar concentrations. Menger-portnoy model modified by Bunton and his coworkers was used to determine the micelles assisted parameters. For CTAB micellized reaction, binding constant (Ks = 42 mol−1 dm3 and KL = 86 mol−1 dm3), micellar phase rate constant (km = 3.5 × 10−3 s−1) were calculated with pseudo-phase kinetic model. Morphology of AgNPs strongly depends on nature of surfactant head group.

Modulation of interfacial phenolic antioxidant distribution in Pickering emulsions via interactions between zein nanoparticles and gallic acid

The impacts of protein nanoparticles on the interfacial distribution of antioxidants and the oxidative stability in Pickering emulsions are attracting increasing research interests. In the present work, the distribution of gallic acid (GA) in zein nanoparticles-stabilized Pickering emulsions (ZPE) was determined by employing a pseudophase kinetic model. The interfacial distribution of GA was found to be favored in ZPEs with higher zein nanoparticle concentration (Czein). Upon increasing Czein, the interfacial loading of nanoparticles (Γ) dominated the modulation of %GAI via hydrogen bonding between zein nanoparticles and GA. The interfacial percentage of GA (%GAI) increased from 28% to 39% as Γ increased from 0.48 to 1.12 mg/m2. In the presence of GA, a direct correlation between Czein or Γ and oxidation stability was recognized, whereas the oxidative stability showed a non-linear dependence on either Czein or Γ in the absence of GA. By excluding antioxidant effects of zein nanoparticles, we found that the %GAI, which was regulated by Γ, took the leading role over the physical barrier effect on the oxidative stability of emulsions. The present work extends our current knowledge on how protein based nanoparticles manipulate the interfacial distribution of antioxidant and then affect the oxidative stability of emulsions.

Effects of droplet size on the interfacial concentrations of antioxidants in fish and olive oil-in-water emulsions and nanoemulsions and on their oxidative stability

HypothesisOne fundamental and unsolved question in colloid chemistry, and also in the food industry, is whether molecular distributions, specifically the interfacial concentrations of antioxidants (AOI), are independent of the droplet sizes. Pseudophase kinetic models, widely employed to interpret chemical reactivity in colloidal systems and to determine antioxidant distributions, assume that they are independent. ExperimentsTo prove, or discard, the above hypothesis, we prepared and characterized a series of olive and fish oil-in-water nanoemulsions with different droplet sizes, carried out a kinetic study to evaluate their oxidative stability, both in the presence and absence of gallic acid (GA), and determined its interfacial concentrations. FindingsResults indicate that a change in the droplet size (80–1300 nm) does not alter the oxidative stability of the nanoemulsions in the absence of GA. Addition of GA increases their oxidative shelf-life and, at constant surfactant volume fraction, ΦI, the oxidative stability and the antioxidant distribution do not depend on the droplet size. Overall, results suggest that the droplet size does not affect the ratio between the rates of radical production and of inhibition by antioxidants, ratio that defines an “efficient” (or inefficient) antioxidant, providing experimental evidence supporting the operative assumption of pseudophase kinetic models.

Enhancing the fraction of antioxidants at the interfaces of oil-in-water emulsions: A kinetic and thermodynamic analysis of their partitioning

HypothesisThe distribution of antioxidants (AOs) in O/W emulsions depends on two partition constants, that between oil-interfacial (POI), and that between the aqueous-interfacial (PWI) regions, that need to be determined in the unbroken emulsion to prevent disruptions of the present equilibria. Prediction of the effects of temperature on AO partitioning is challenging because may change both POI and PWI in a different extent and the control of their interfacial concentrations is crucial to optimize their antioxidant efficiency. Such effects can be analyzed in the intact emulsions with the aid of a pseudophase kinetic model. ExperimentsHere we estimated the partition constants of the food-grade antioxidants -propyl (PG), octyl (OG) and lauryl (LG) gallates- in intact corn oil-in-water emulsions and their interfacial concentrations by employing a kinetic methods, and carried out a thermodynamic analysis of the transfer parameters controlling their partitioning. FindingsResults show that the Gibbs free energy for the transfer of gallates to the interfacial region is spontaneous and the transfer process is enthalpy driven. An increase in T favors their incorporation into the interfacial region in an extent that depends on AO hydrophobicity. For any of AOs, the effective interfacial concentrations are much higher (15–170 fold) than the stoichiometric concentration. Results are basic to get a deeper knowledge on the driving force that partitions the AOs in lipid-based foods and to select the best AO to minimize the oxidation of lipids.

Using a pseudophase model to determine AO distributions in emulsions: Why dynamic equilibrium matters

For decades, explorations with ground state, thermal reactions combined with pseudophase kinetic models and methods for interpreting the results have provided insights into the properties of the different regions of homogeneous association colloids. More recent successful determination of antioxidant (AO) distributions by this approach is providing new insights into AO efficiency in opaque, well‐mixed two‐phase intact emulsions and eliminating the need to separate the phases. The chemical probe reacts with AOs exclusively in the interfacial region of the emulsion, permitting simplification of the kinetic treatment, and determining its distribution between the oil, interfacial, and aqueous regions. AO distributions are obtained from the two partition constants, and , of the AOs between the oil‐interfacial and aqueous‐interfacial regions, respectively. and values are obtained by fitting the observed rate constant, kobs, versus surfactant concentration profiles with an overall kinetic approach or model we call the “pseudophase chemical kinetic method.” However, because emulsions break up and reform, and reactants and other components diffuse at various time scales within and between the oil, interfacial, and water regions, kobs could also depend on reactant diffusion coefficients. Here we demonstrate that reactant diffusion is generally orders of magnitude faster than most thermal reactions and reactant distributions between the multiple oil, aqueous, and interfacial droplets and regions of emulsions are in dynamic equilibrium throughout the multiphase systems during the time course of the reaction. Thus, kinetic probes are powerful tools for determining structure‐reactivity, for example, the HLB, relationships governing AO distributions and efficiencies in emulsions.Practical applications: The analysis presented here demonstrates that one of the basic assumptions of the pseudophase chemical kinetic model that we have developed has a solid foundation in the properties of emulsions. That is, we can determine the distributions of reactants between oil (O), interfacial (I), and aqueous (W) regions of the emulsions because the diffusivity coefficients of reactants within emulsions are orders of magnitude greater than the rate of the reaction between the antioxidant and the 4‐hexadecylbenzenediazonium probe. Consequently, we can use the same kinetic model in emulsions as we have used in homogeneous microemulsions. The method permits determination of the partition constants of many antioxidants between the O‐I and W‐I regions of the emulsions and from them their distributions. The method provides new insights into the relationships between antioxidant hydrophobic‐lipophilic balance (HLB) and its efficiency in emulsions and a natural explanation for the cut‐off effect observed with increasing antioxidant HLB.Interpreting chemical reactivity in emulsions requires that reactive components A and B be in dynamic equilibrium, that separate second order rate constants, k defined for the oil, interfacial, and aqueous regions, subscripts O, I, and W, and for simplicity, the volume of each region depends on the added volumes of oil, surfactant, and water.

Distributions of phenolic acid antioxidants between the interfacial and aqueous regions of corn oil emulsions: Effects of pH and emulsifier concentration

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.Practical applications: Our results are based on the pseudophase kinetic model originally developed for association colloids and should be of interest to the food industry and food chemists because phenolic acid antioxidants are widely employed to inhibit lipid oxidation. Depending on food emulsion pH, they may be partially ionized, thus altering their concentrations in the reactive region. We expect other phenolic acids to follow the reported trends because the acidity constants of their carboxylic groups are similar. We demonstrated that the efficiency of series of homologous antioxidants (AOs) can be correlated with their percentage in the interfacial region of the emulsions. Thus, application of the pseudophase kinetic model provides a unique approach to a better understanding of the effects of factors controlling AO distributions and efficiencies and a more rational selection of AOs and emulsifiers in food stabilization (e.g., environmental (acidity, T), emulsifiers, AOs HLBs, nature of the oils, etc.)The partition constant values of phenolic acids between the aqueous and interfacial regions of emulsions, PWI, depend on acidity.

Distributions of phenolic acid antioxidants between the interfacial and aqueous regions of corn oil emulsions—a commentary

Being capable of optimizing the usage of antioxidants is a challenge for any industry concerned with formulating food and non‐food products because it will affect its competitiveness and the product's attractiveness in response to consumer demand. This is also a challenge for scientists who remain puzzled by the unpredictable oxidative behavior of real, complex food, and non‐food matrices. Laurence Romsted and Carlos Bravo‐Diaz have developed some years ago a pseudophase kinetic model to determine the partitioning of chain‐breaking antioxidants in micellar systems and emulsions stabilized by surfactants. In this article, with Sonia Lasada‐Barreiro they used the pseudophase model to determine the partitioning of two phenolic antioxidants (gallic acid and caffeic acid) in a model oil‐in‐water emulsion with increasing surfactant concentrations and at different pH values. This commentary underlines the importance of the pseudo‐phase model and the present work to achieve a better use of antioxidants. It is also suggested that the enigma of antioxidant efficiency in complex systems is far from completely solved.

Kinetic Study of the Formation of Ruhemann's Purple in Micellar and Microemulsion Phases

Abstract The reaction kinetics of ninhydrine (Nin) with glycine–glycine dipeptide (Gly) was studied in aqueous solution. The same reaction was studied in micellar solutions of different surfactants such as sodium dioctylsulfosuccinate (AOT) and sodium dodecyl sulfate (SDS). The reaction was also studied in the L2 microemulsion phase of the three systems AOT/heptane/water, SDS/pentanol/heptane/water, CTAB/pentanol/heptane/water. Spectrophotometric measurements were perfomed at 570 nm. The data show that the reaction is first-order with respect to Nin and Gly in all cases. Values of the observed rate constants (k obs) increase with increasing concentration of the surfactant in micellar media. AOT was more promotional than SDS. The reaction rate in the presence of different micelles could be explained using a pseudo-phase kinetic model. Association constants of Nin and Gly with surfactant micelles are reported. Values of k obs for the reaction rates in the microemulsion phases are reported. AOT/heptane L2 microemulsion system has been clearly found as the most promotional among all the other studied systems. Association constants of Nin and Gly with surfactant in microemulsion were obtained. The rate was increased with the temperature in aqueous, micellar and microemulsion phases. The activation parameters ΔH*, ΔS* and ΔG* have also been obtained. The reaction rate in different microemulsion media is slightly increased with the change of oil (heptane > hexane > pentane), while co-surfactants (butanol, pentanol and hexanol) have no effect on k obs values in AOT, CTAB or SDS microemulsion.

A 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

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, POI, show a maximum at the C8 ester. A reaction between caffeic acid, CA, and its specially synthesised C1–C16 alkyl esters, and a chemical probe is used to estimate partition constants for AO distributions and interfacial rate constants, kI, 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, ΦI=0.005.

Interfacial kinetics in octane based emulsions. Effects of surfactant concentration on the reaction between 16-ArN2+ and octyl and lauryl gallates

Here we have applied the formalism of the pseudophase kinetic model to analyze the reactivity of 4-hexadecylbenzenediazonium, 16-ArN2+, ions with the antioxidants octyl gallate (OG) and lauryl gallate (LG) in emulsions composed of octane, acidic water and hexaetyleneglycol monododecyl ether, C12E6. Reactants were chosen so that their effective concentration in the oil and aqueous regions are negligible, i.e., the reactants are located exclusively in the interfacial region of the emulsions and it is not necessary to consider their partitioning between the different regions of the system. The rate of the chemical reaction is not limited by the transport of matter between droplets or between the different regions of the droplets and, therefore, the observed rate constant is given by the rate in the interfacial region. Observed rate constants, kobs, for the reaction between 16-ArN2+ and OG and LG in the emulsions were obtained by employing a derivatization method that leads to the stoichiometric formation of an stable azo dye whose absorbance can be determined upon dilution in EtOH. kobs values decrease asymptotically upon increasing surfactant concentration by a factor of 4–7 on going from ΦI = 0.005 to ΦI = 0.03. Interfacial rate constants kI were obtained by employing the pseudophase kinetic model by assuming that the system is under dynamic equilibrium. Results indicate that kI values are the same independently of the oil to water ratio employed to prepare the emulsions as well as the antioxidant chain length.

Influence of Temperature on the Distribution of Catechin in Corn Oil-in-Water Emulsions and Some Relevant Thermodynamic Parameters

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, kI, and the partition constant PWI between the aqueous and interfacial region of the emulsion from kobs versus ΦI profiles. The PWI values are quite high, increasing from 188 (T = 15 °C) to 368 (T = 25 °C). This change in PWI 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 (ΔG0,W → I, (ΔH0,W → I and (ΔS0,W → I) were obtained from the PWI values at a series of temperatures by the van’t Hoff method and the Gibbs equation, respectively. ΔG0,W → I is negative at any temperature, indicating that the transfer of CAT from the aqueous to the interfacial region is an spontaneous process.

Maxima in Antioxidant Distributions and Efficiencies with Increasing Hydrophobicity of Gallic Acid and Its Alkyl Esters. The Pseudophase Model Interpretation of the “Cutoff Effect”

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. This paper shows, 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.

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.