Effects of interfacial specific cations and water molarities on AOT micelle-to-vesicle transitions by chemical trapping: the specific ion-pair/hydration model.

Salt induced micelle-to-vesicle transitions of ionic surfactants depend on the surfactant chain length, headgroup structure, counterion type and concentration, but the interfacial molarities of counterions and water that balance the hydrophobic effect are difficult to determine. In anionic micelles of twin-tailed sodium bis(2-ethylhexyl)sulfosuccinate (AOT), the chemical trapping (CT) method provides estimates of the interfacial molarities of anionic headgroups (RSO3-m) and neutral (H2Om) nucleophiles during salt induced transitions of AOT micelles to vesicles. Product yields were measured by HPLC from the competitive dediazoniation reaction using a specially designed hydrophobic probe, 4-hexadecyl-2,6-dimethylbenzenediazonium cation, 16-ArN2+.

Evidence of coexisting microemulsion droplets in oil-in-water emulsions revealed by 2D DOSY H NMR.

Optimizing the macroscopic properties, shelf-life and stability of emulsion products requires a better understanding of the microstructural characteristics such as the type (nano, micro and macro) and the relative distribution of components (i.e., oil and surfactant) within the emulsion droplets.

A novel combined chemical kinetic and trapping method for probing the relationships between chemical reactivity and interfacial HO, Br and H ion molarities in CTAB/CE mixed micelles.

A delicate balance-of-forces governs the interactions responsible for surfactant self-assembly and chemical reactivity within them. Chemical reactions in micellar media generally occur in the interfacial region of micelles that is a complex mixture of: water, headgroups, counterions, co-ions, acids or bases, organic solvents, and the reactants themselves. We have carried out a detailed study of a complex chemical reaction in mixed CTAB/CE micelles by using the chemical kinetic (CK) and chemical trapping (CT) methods.

Interfacial Concentrations of Hydroxytyrosol and Its Lipophilic Esters in Intact Olive Oil-in-Water Emulsions: Effects of Antioxidant Hydrophobicity, Surfactant Concentration, and the Oil-to-Water Ratio on the Oxidative Stability of the Emulsions.

We determined the interfacial molarities of the antioxidants, AOs, hydroxytyrosol (HT), and HT fatty acid esters with chain lengths of 1 to 16 carbons in intact olive oil/water/Tween 20 emulsions. The results were compared with chain length effects on the oxidative stability of the same emulsions, and a direct correlation was established. Both (AOI) molarities (varying 50-250 times greater than the stoichiometric 3.

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.

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, PO(I), 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.

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.

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.

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.

Simultaneous determination of interfacial molarities of amide bonds, carboxylate groups, and water by chemical trapping in micelles of amphiphiles containing peptide bond models.

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-ArN(2)(+)) 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 (in moles per liter of interfacial volume) of these three groups were obtained from product yields, assuming that selectivity toward a particular nucleophile compared to water is the same in an aqueous reference solution and in the interfacial region.

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.

Effects of temperature and emulsifier concentration on alpha-tocopherol distribution in a stirred, fluid, emulsion. Thermodynamics of alpha-tocopherol transfer between the oil and interfacial regions.

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.

Competing gas-phase substitution and elimination reactions of gemini surfactants with anionic counterions by mass spectrometry. Density functional theory correlations with their bolaform halide salt models.

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.

Quantitative determination of alpha-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.

Structural, infrared, and density functional theory studies of N,N,N',N'-Tetramethylimidazolidinium dichloride: a model for cation-anion association of headgroups and counterions in the interfacial regions of gemini micelles.

N,N,N',N'-Tetramethylimidazolidinium dichloride (1-Im-1 2Cl) has been studied as a model system for cation-anion interactions in the interfacial regions of gemini micelles by X-ray crystallography, density functional theory (DFT) calculations, and infrared spectroscopy. Single crystals of 1-Im-1 2Cl contain 1-Im-1 dications, whose five-membered rings adopt a distorted envelope conformation. Eight chloride anions surround each dication, two of which are cradled above and below the five-membered ring (apical) and six of which are dispersed about the periphery of the ring (equatorial).

Micellar induced regioselectivity in the two-step consecutive reaction of SO3(2-) with Br-(CH2CH2)n-Br (n=2-5).

High field (800 MHz) (1)H NMR was used to monitor the two-step consecutive reaction of excess SO(3)(2-) with symmetrical bifunctional alpha,omega-dibromoalkanes with butane (DBB), hexane (DBH), octane (DBO), and decane (DBD) chains in CTAB micelles at 25 degrees C. The first-order rate constant for the first substitution step for DBB and DBH is about 5 times faster than for the second, but the kinetics for DBO and DBD were not cleanly first-order. After 40 min, the solution contained about 80% of the intermediate bromoalkanesulfonate from DBB and DBH and the remainder is alkanedisulfonate and unreacted starting material.

Do amphiphile aggregate morphologies and interfacial compositions depend primarily on interfacial hydration and ion-specific interactions? The evidence from chemical trapping.

Surface-active amphiphiles aggregate spontaneously in water to form association colloids such as micelles, microemulsions, and vesicles. The hydrophobic effect drives aggregation, but the opposing forces that provide balance and determine equilibrium morphologies are not understood, in particular, how specific ion effects, which often follow a Hofmeister series, affect the properties of association colloids. We have harnessed the competitive trapping of arenediazonium ions by weakly basic nucleophiles such as halide counterions, anionic headgroups, alcohols, urea, and water, to estimate their concentrations in the interfacial regions of association colloids from reaction product yields.

Determining alpha-tocopherol distributions between the oil, water, and interfacial regions of macroemulsions: novel applications of electroanalytical chemistry and the pseudophase kinetic model.

The assumptions of the pseudophase model for chemical reactivity in homogeneous microemulsions are used to determine the distribution of alpha-tocopherol (TOC) in macroemulsions from changes in the observed rate constant (k(obs)) for reaction between 4-hexadecylarenediazonium ion (16-ArN2+) probe and TOC with increasing surfactant concentration. Two partition constants are needed to describe the distribution of TOC or other antioxidant (AO) or polar uncharged molecule between the oil and interfacial (P(O)(I)) and the water and interfacial (P(W)(I)) regions of stirred fluid emulsions. The observed rate constants are measured electrochemically.

Specific ion pairing and interfacial hydration as controlling factors in gemini micelle morphology. Chemical trapping studies.

Results from chemical trapping experiments in micellar solutions containing 1.5-5 mM aqueous solutions of three didodecyl dicationic dibromide gemini surfactants with different methylene spacer lengths (12-n-12 2Br where n = 2-4 CH(2) groups) gave quantitative estimates of the molarities of interfacial bromide (Br(m)) and water (H(2)O(m)), the fractions of free and paired headgroups and counterions, and the net headgroup charge. These results are one of the most detailed compositional studies of an association colloid interface to date.

Ion pair formation in water. association constants of bolaform, bisquaternary ammonium, electrolytes by chemical trapping.

The first and second association constants, K1 and K2, for ion pair formation in aqueous 0.02-3.5 M solutions of bis(trimethyl)-alpha,omega-alkanediammonium halides with variable spacer lengths, 1-n-1 2X (n = 2-4, X = Cl, Br) and bolaform salts and for tetramethylammonium halides (TMAX, X = Cl, Br), K(TMAX), were determined by the chemical trapping method.

Determining partition constants of polar organic molecules between the oil/interfacial and water/interfacial regions in emulsions: a combined electrochemical and spectrometric method.

We have developed a new approach for estimating distributions of polar additives in opaque, surfactant based, macroemulsions based on the pseudophase model for homogeneous micellar and microemulsion solutions. The distribution of a polar additive, such as an antioxidant, AO, within emulsions is expressed in terms of two partition constants, one between the oil and interfacial regions, P(O)I, and the other between the water and interfacial regions, P(W)I. To estimate values for P(O)I and P(W)I requires fitting two independent data sets with two independent mathematical relations and solving equations simultaneously for the two parameters.

Origin of the sphere-to-rod transition in cationic micelles with aromatic counterions: specific ion hydration in the interfacial region matters.

Sphere-to-rod transitions of cetyltrimethylammonium (CTA+) micelles with dichlorobenzoate counterions are remarkably substituent dependent. Simultaneous estimates of the interfacial molarities of H2O, MeOH, and Cl- and 2,6- and 3,5-dichlorobenzoate (2,6OBz and 3,5OBz) counterions were obtained by the chemical trapping method in mixed micelles of CTACl/CTA3,5OBz and CTACl/CTA2,6OBz without added salt. Increasing the CTA3,5OBz mole fraction produces a marked concurrent increase in interfacial 3,5OBz- and a decrease in interfacial H2O concentrations through the sphere-to-rod transition.

Effect of alkyl group size on the mechanism of acid hydrolyses of benzaldehyde acetals.

Hydrolyses of benzaldehyde acetals, PhCH(OR)(2), are specific hydrogen-ion catalyzed when R = methyl, n-butyl, but with secondary and tertiary alkyl derivatives, R = i-propyl, s-butyl, t-butyl, t-amyl, hydrolyses are general-acid catalyzed. The Brønsted alpha values for both secondary and tertiary alkyl groups are in the range: alpha = 0.57-0.

Kinetic method for determining antioxidant distributions in model food emulsions: distribution constants of t-butylhydroquinone in mixtures of octane, water, and a nonionic emulsifier.

The absence of reliable estimates of distributions of antioxidants in food emulsions hinders the development of a useful method for comparing the efficiencies of antioxidants. Here we describe the application of a pseudophase kinetic model, originally developed for homogeneous microemulsions, to the determination of distribution constants of tert-butylhydroquinone, TBHQ, in a fluid, opaque, model food emulsion composed of the nonionic emulsifier C(12)E(6), octane, and water. This kinetic method should be applicable to a wide variety of charged and uncharged antioxidants in emulsions composed of charged and uncharged emulsifiers.