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Experimental density measurements are reported along with the derived thermodynamic properties (isothermal compressibility, isobaric expansivity, and thermal pressure coefficient) for trihexyltetradecylphosphonium-based ionic liquids— chloride, bromide, bis(trifluoromethylsulfonyl)imide, dicyanamide and methyl sulfonate—in the pressure range (0.10 to 45.00 MPa) and temperature range (283.15 to 333.15) K. The effect of the anion of the ionic liquid on the properties under study was evaluated. Experimental densities were correlated using the Tait equation, the modified cell model equation of state, and the Sanchez-Lacombe equation of state, and compared against the predictive method proposed by Gardas and Coutinho. It is shown that the three correlations describe well all the ILs studied, with the Tait equation providing the lowest average relative deviation (less than 0.004 %) and the Sanchez-Lacombe equation of state the highest (inferior to 0.5 %), and that the predicted densities estimated by Gardas and Coutinho method are in good agreement with the experimental densities determined.

Abstract______________________________________________

Dediazoniation of o-methylbenzenediazonium tetrafluoroborate was investigated in SDS/BuOH/H2O (SDS)sodium dodecyl sulfate) reverse micelles, RMs, and, for comparison, in binary BuOH/H2Omixtures by employing a combination of spectrophotometric and chromatographic techniques.RMswere characterized by steady-state fluorescence; the data indicate that the aggregation number of the RMs increase upon increasing [SDS], while the radius of the water pool is mainly controlled by the amount of water in the system, and that the thickness of the interfacial region increases upon increasing the amount of BuOH in the system, in agreement with literature reports. Experimental evidence suggests that dediazoniation mainly takes place in the interfacial region of the RMs. Kinetic data show that a turnover from the heterolytic to the homolytic mechanism takes place about pH ) 5; the variation of the observed rate constants, kobs, with pH following an S-shaped curve. At pH  2, kobs values are insensitive to solvent composition both in RMs and in the binary mixture; however, kobs values in RMs are slightly lower than those inBuOH/H2O, probably due to the presence of SDS. High-performance liquid chromatography analyses of the reaction mixture indicate, in both RMs and in binary mixtures, the main dediazoniation products are the heterolytic ArOH and ArOBu, their yields depending on the composition of the system, and only small (<10%) amounts of the reduction ArH product were detected. The data at low pH are interpreted in terms of a DN + AN dediazoniation mechanism, i.e., a rate-limiting formation of an extremely reactive aryl cation that further reacts with available nucleophiles in the solvation shell.

Abstract____________________________________________ __
The assumptions of the pseudophase model for chemical reactivity in homogeneous microemulsions are used to determine the distribution of α- tocopherol (TOC) in macroemulsions from changes in the observed rate constant (kobs) 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 (PO I ) and the water and interfacial (PW I ) regions of stirred fluid emulsions. The observed rate constants are measured electrochemically. Here we report values of PO I and PW I for the distribution of TOC in octane/acidic water/C12E6 (hexaethylene glycol monododecyl ether) and octane/acidic water/C12E4 (Brij 30, tetraethylene glycol dodecyl ether) emulsions obtained by fitting two kinetic data sets with an equation based on the pseudophase model and solving two equations in two unknowns. The partition constants were used to estimate the %TOC in each region of the emulsions. In 1:1 oil:water C12E6 emulsions, at 2% volume fraction of C12E6, 73% of TOC is in the interfacial region, 26% in the octane and about 1% in the water. The distributions of TOC in C12E4 emulsions are similar. The combined electrochemicalpseudophase model approach is applicable to any AO or other compound that reacts with 16-ArN2 +. The second-order rate constant, kI, for reaction in the interfacial region of the emulsions is also estimated from the kinetic data and is about the same for both surfactants (kI≈0.1−0.2 M−1s−1) showing that the medium properties of the interfacial regions of C12E6 and C12E4 emulsions are similar. Comparison of these rate constants for a variety of AOs may provide a scale of AO efficiency that is independent of AO distribution between the oil, interfacial and aqueous regions of emulsions.

Abstract______________________________________________

The kinetics and mechanism of the reaction between hydrophobic 4-hexadecylarenediazonium ions, 16-ArNR2 and vitamin C, VC, in a model emulsion prepared by mixing octane, acidic (HCl) water and the non-ionic surfactant hexaethyleneglycol monododecyl ether, C12E6, were investigated. Because emulsions are opaque, linear sweep voltammetry, LSV, was employed to monitor the reaction. Voltammograms of 16-ArNR2 in emulsions show two reduction peaks as in aqueous systems. The half-life for the spontaneous decomposition of 16-ArNR2 in the emulsion was estimated as t1/2¼14.5 h at T¼25 -C. Upon addition of VC to the system, the first reduction peak of 16-ArNR2 disappears almost immediately and a new reduction peak is detected at Ep¼S0.25 V. Electrochemical titration of 16-ArNR2 shows that the new peak corresponds to the formation of a 1:1 adduct. The iP(Ep¼S0.25 V) values can be linearly correlated with [16-ArNR2] and the observed rate constants, kobs, were determined by fitting the (ip, t) data to the integrated first order equation. The variation of kobs with [VC] follows a saturation kinetics profile, consistent with the formation of an intermediate in a pre-equilibrium step. All the evidence is consistent with a reaction mechanism comprising two competitive pathways, the spontaneous DNRAN mechanism and the unimolecular decomposition of a transient diazo ether (DE) formed in a pre-equilibrium step. The data allowed estimations of the interfacial rate constant for the reaction between 16-ArNR2 and VCS but did not allow the determination of the equilibrium constant for the DE formation.

Abstract______________________________________________

The effect of sodium dodecyl sulfate (SDS) micelles on the reaction between the 3-methylbenzenediazonium (3MBD) ion and either the hydrophilic antioxidant gallic acid (GA) or the hydrophobic analogue octyl gallate (OG) have been investigated as a function of pH. Titration of GA in the absence and presence of SDS micelles showed that the micelles do not alter the first ionization equilibriumof GA. Analysis of the dependence of the observed rate constant (kobs) with pH shows that the reactive species are GA2 and OG. Kinetics results in the absence and presence of SDS micelles suggest that SDS aggregates do not alter the expected reaction pathway. SDS Micelles inhibit the spontaneous decomposition of 3MBD as well as the reaction between 3MBD and either GA or OG, and upon increasing the SDS concentration, with kobs approaching the value for the thermal decomposition of 3MBD in the presence of SDS. Our results are consistent with the prediction of the pseudophase model and show that the origin of the inhibition for the reaction withGAis different to that for the reaction with OG; in the former case, the observed inhibition can be rationalized in terms of the micelle-induced electrostatic separation of reactants in the micellar Stern layer, whereas the observed inhibition in the reaction with OG is a consequence of the dilution effect caused by increasing SDS concentration, decreasing the local OG concentration in the Stern layer.

Abstract______________________________________________

We have measured the rates and product yields of dediazoniation of 1- naphthalenediazonium (1ND) tetrafluoroborate in the presence and absence of sodium dodecyl sulfate (SDS) micellar aggregates by employing a combination of UV–vis spectroscopy and high-performance liquid chromatography (HPLC) measurements. Kinetic data were obtained by a derivatization procedure with product yields were determined by HPLC. HPLC chromatograms show that in aqueous acid and in micellar solutions only one dediazoniation product is formed in significant quantities, 1-naphthol (NOH), and the observed rate constants (kobs) are the same when 1ND loss is monitored spectrometrically and when NOH formation is monitored by HPLC. Activation parameters were obtained both in the presence and absence of SDS micellar aggregates. In both the systems, the enthalpies of activation are high and the entropies of activation are positive. The enthalpy of activation in the absence of SDS is very similar to that in the presence of SDS micelles, but the entropy of activation is lower by a factor of 4. As a consequence, SDSmicelles speed up the thermal decomposition of 1ND and increase kobs by a factor of 1.5 when [SDS] = 0.02 M. In contrast, results obtained in the presence of complexing systems such as crown ethers and polyethers show significant stabilization of the parent arenediazonium ions. Kinetic and HPLC data are consistent with the heterolytic DN + AN mechanism that involves the rate-determining fragmentation of the arenediazonium ion into a very reactive phenyl cation that reacts competitively with available nucleophiles.

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 α-tocopherol, provides the first good estimates for the two partition constants that describe α-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 α-tocopherol distributes strongly in favor of the interfacial region when the oil is tributyrin, e.g., ca. 90% when the surfactant volume fraction is ΦI = 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.

Abstract______________________________________________

We have determined the product distributions, the rate constants of product formation and substrate loss, and the activation parameters for the butanolysis of 2-methylbenzenediazonium, 2MBD, tetrafluoroborate in aqueous 1-Butanol (BuOH) solutions by combining UV–VIS spectroscopy, high performance liquid chromatography (HPLC), and a derivatization protocol that traps unreacted 2MBD as a stable azo dye. BuOH/H2O solutions are miscible over a narrow composition range, but in reverse micelles composed of sodium dodecyl sulfate, SDS, BuOH, and water, are miscible between 45–80%. Two major and two minor dediazoniation products are observed, 2-cresol, ArOH, 2-butyl-tolyl-ether, ArOBu, and small amounts of 2-chlorobenzene, ArCl (from HCl added to control solution acidity) and toluene, ArH (a reduction product). Product yields depend on experimental conditions, but quantitative conversion to products is achieved over the entire composition ranges investigated. The observed rate constants, kobs, obtained by monitoring 2MBD loss or by monitoring ArOH or ArOBu formation, are the same and they are only modestly affected by changes in the solution composition. The activation parameters obtained from the effect of temperature on kobs show that the enthalpy of activation is relatively high compared to those found in bimolecular reactions and the entropy of activation is small but positive. The results suggest that 2MBD is mainly sampling in the BuOH-H2O rich interfacial region of the reverse micelle and are consistent with 2MBD decomposing through a DNRAN mechanism, i.e., a rate limiting formation of an aryl cation that reacts immediately with nucleophiles.

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 R-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, kobs, 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, kI, and the partition constants of antioxidants between the oil and interfacial, PO I , regions in the emulsion from kobs versus ΦI profiles. The thermodynamic parameters of transfer from the oil to the interfacial region at a series of temperatures were respectively obtained from the PO I values (ΔGT
0,OfI), by the van’t Hoff method (ΔHT 0,OfI), and from the Gibbs equation (ΔST
0,OfI). 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.

Abstract______________________________________________

We investigated the effects of solvent composition, acidity and temperature on the switch between heterolytic and homolytic mechanisms in the course of the butanolysis of 4-methylbenzenediazonium (4MBD) ions in binary BuOH/H2O mixtures and in reverse micelles, RMs, composed of n-BuOH, H2O and sodium dodecyl sulfate, SDS, by employing a combination of spectrometric (UV/vis) and chromatographic (HPLC) techniques. In reaction mixtures with high n-BuOH percentages, S-shaped variations of kobs with acidity, defined hereafter as -log([HCl]), are obtained with rate enhancements of up to ~370-fold on going from -log([HCl]) = 2 to 6, with inflection points at -log[HCl] ~ 4. HPLC analyses of the reaction mixtures show that the substitution product 4-cresol, ArOH and the reduction product toluene, ArH, are formed competitively. The variation of their yields with acidity is also S-shaped, so that at high acidities (-log[HCl] < 3) only traces of ArH are detected but on lowering the acidity, the reduction product ArH becomes predominant The largest variations of kobs and of the product yields with acidity are found in the -log[HCl] = 3–5 range, suggesting that a turnover in the dediazoniation mechanism takes place under acidic conditions. The results can be interpreted in terms of two competitive reaction pathways, one heterolytic, involving a rate-determining formation of an extremely reactive aryl cation that traps the nucleophiles available in its solvation shell leading to the formation of substitution products (DN + AN mechanism) and a second route where the BuOH reacts with 4MBD to yield an unstable O-adduct of the type Ar–N N–O–R (diazo ether) in a rapid pre-equilibrium step that initiates a radical process leading to the formation of the reduction product ArH (O-coupling mechanism). The results illustrate how the heterolytic and homolytic mechanisms can be switched by just changing the acidity of the solution. Kinetic analyses of the variations of kobs with acidity at different temperatures allowed us to separate kobs into the components for the heterolytic pathway, kHET, and that for the homolytic one, kHOM, to determine relevant thermodynamic parameters for both reaction pathways and for the equilibrium constant K for the formation of the O-adduct Ar–N N–O–R.

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].

Abstract______________________________________________

A steady-state fluorescence quenching study was undertaken to estimate aggregation numbers, N, and radii for the cell, rc, and for the water pool, rwp, of sodium dodecyl sulfate, SDS, BuOH and water reverse micelles at a fixed [H2O]/[SDS] = 42.7 ratio, which allows variations in the percentage of BuOH in the system ranging 50–90%. For the purpose, pyrene was employed as fluorescent probe and N-cetylpiridinium chloride as static quencher. The results obtained show that N values, which range 8–33, show a linear variation with the amount of surfactant present in the system; meanwhile rwp values are mainly determined by the water content. The minimal water content and the minimal rwp value for reverse micelle formation were also estimated.