Effects of Temperature and Emulsifier Concentration on α-Tocopherol Distribution in a Stirred, Fluid, Emulsion. Thermodynamics of α-Tocopherol Transfer between the Oil and Interfacial Regions

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.

Effects of emulsifier hydrophile–lipophile balance and emulsifier concentration on the distributions of gallic acid, propyl gallate, and α-tocopherol in corn oil emulsions

“…Kinetic data were obtained by employing our derivatization method (azo dye formation) as described in detail elsewhere [33]. This methodology exploits the rapid reaction of ArN þ 2 ions with a suitable coupling agent such as N-(1-naphthyl)ethylenediamine, NED, to yielding an stable azo dye, Scheme 4, that is diluted with a 50:50 (v:v) BuOH/EtOH mixture to yield an optically transparent, homogeneous solution whose absorbance is measured spectrometrically [33][34][35]. Control experiments showed that the absorbance of the azo dye at k ¼ 572nm follows Beeŕs law and is a linear function of the concentration of 16-ArN þ 2 .…”

“…Under these conditions, the observed rate, v, for a bimolecular reaction in an emulsion is the sum of the rates in each region of the macroemulsion [31][32][33][34]. The 16-ArN þ 2 ions are insoluble in the water region because of its hydrophobicity, a 16 carbon tail and an aromatic ring, and also in the oil region because of its cationic headgroup.…”

“…We proceeded in a different way [31][32][33][34][35][36]. Rather than focusing in developing analytical methods to measure analyte concentrations in the different regions of the emulsion, we focused in building up methods to determine the partition constants of the antioxidants in the intact emulsion.…”

“…Because emulsions are opaque, we also developed protocols to monitor the reaction in the intact emulsion to calculate the corresponding observed rate constants k obs . The k obs values depend on both concentration and medium effects, and their relative contribution can be determined by employing the pseudophase kinetic model for chemical reactivity as described elsewhere [32,34,35,37].…”

“…In previous work, we used this methodology to investigate the distribution of a number of antioxidants and to evaluate the relative effects of external parameters (temperature and acidity) on their distribution in the different regions of the food emulsions [31][32][33][34][35][36]. Here, we expand our previous investigations and report on the effects of the emulsifier concentration and of the nature of the emulsifier, characterized by the hydrophile-Lipophile Balance (HLB), on the distribution of three selected antioxidants, gallic acid (GA), propyl gallate (PG), and a-tocoppherol (TOC).…”