molecules Review Modeling Chemical Reactivity at the Interfaces of Emulsions: Effects of Partitioning and Temperature Marlene Costa 1 ,Fátima Paiva-Martins 1 , Sonia Losada-Barreiro 2 and Carlos Bravo-Díaz 2,* 1 REQUIMTE/LAQV, Department of Chemistry and Biochemistry, Faculty of Science, University of Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal; [email protected] (M.C.); [email protected] (F.P.-M.) 2 Departamento de Química—Física, Facultad de Química, Universidade de Vigo, 36310 Vigo, Spain; [email protected] * Correspondence: [email protected] Abstract: Bulk phase chemistry is hardly ever a reasonable approximation to interpret chemical reac- tivity 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 distri- butions 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 Citation: Costa, M.; Paiva-Martins, equilibria and, therefore, need to be determined in the intact emulsions. The challenge is, thus, to F.; Losada-Barreiro, S.; Bravo-Díaz, C. develop models to correctly interpret chemical reactivity. Here, we will review the application of Modeling Chemical Reactivity at the Interfaces of Emulsions: Effects of the pseudophase kinetic model to emulsions, which allows us to model chemical reactivity under a Partitioning and Temperature. variety of experimental conditions and, by carrying out an appropriate kinetic analysis, will provide Molecules 2021, 26, 4703. https:// important kineticparameters. doi.org/10.3390/molecules26154703 Keywords: kinetics; pseudophase model; emulsions; antioxidants Academic Editor: Erich A. Müller Received: 9 June 2021 Accepted: 28 July 2021 1. Introduction Published: 3 August 2021 The three-dimensional interfaces between two immiscible liquids (oil and water) found in emulsions and other colloidal systems have been widely used as models of membrane Publisher’s Note: MDPI stays neutral function to mimic their behavior, as models of lipid-based foods and to model important with regard to jurisdictional claims in (bio)chemical reactions such as the oxidation of lipids [1–4]. A variety of fundamental published maps and institutional affil- processes involved in biocatalysis, such as ion pumping, electron transport, membrane iations. fusion, and photosynthesis, have all been investigated in such interfacial systems for further guidance in developing new biomaterials and medicines and to better understand life science [5–11]. Emulsions are composed of a liquid dispersed as small droplets in a second immis- Copyright: © 2021 by the authors. cible liquid. The droplet interface is encompassed by a narrow region (2–20 nm thick) Licensee MDPI, Basel, Switzerland. surrounding each emulsion droplet, Figure1[ 12]. The interfacial region typically does not This article is an open access article contribute significantly to the total volume of an emulsion unless the droplet sizes are very distributed under the terms and small [13], but has important consequences in the fate of chemical reactions that take place conditions of the Creative Commons in the emulsions. For example, lipid oxidation reactions and those between antioxidants Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ and lipid radicals are largely affected by the presence of interfacial regions, because they are 4.0/). highly anisotropic regions composed of a mélange of oil, water, and surfactants [2,14–16]. Molecules 2021, 26, 4703. https://doi.org/10.3390/molecules26154703 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 26 Molecules 2021, 26, 4703 2 of 25 because they are highly anisotropic regions composed of a mélange of oil, water, and surfactants [2,14–16]. Figure 1. ((Left)) Basic Basic illustration illustration of of an an oil-in-water oil-in-water emulsion emulsion drople droplet.t. The The diameter diameter of ofthe the droplet droplet may may range range from from tens tens of ofnanometers nanometers to tomicrometers, micrometers, and and the the volume volume of ofthe the interfacial interfacial region region is is only only a asm smallall fraction fraction of of the the total total volume of the droplet. (Right) Optical microscope photograph of an O/W emulsion stabilized with Tween 20 and conceptual division of droplet. (Right) Optical microscope photograph of an O/W emulsion stabilized with Tween 20 and conceptual division of the domains with different solvent properties. the domains with different solvent properties. Conceptually, emulsionsemulsions can can be be divided divided into in threeto three regions regions with with distinct distinct solvent solvent prop- properties:erties: the core the ofcore droplets, of droplets, the continuous the continuo phase,us phase, and the and interfacial the interfacial region (Figureregion 1(Figure). The 1).interfacial The interfacial layer surrounding layer surrounding each emulsion each emul dropletsion hasdroplet a crucial has a role crucial in the role applicability in the ap- plicabilityand properties and ofproperties emulsions of because emulsions many be importantcause many chemical, important biological, chemical, physical, biological, and physical,technological and processestechnological take placeprocesses at interfaces take place [16 –at19 ].interfaces Reactive [16–19]. components Reactive can partitioncompo- nentsbetween can the partition different between regions, the reducing different or regi increasingons, reducing the concentrations or increasing of the these concentra- compo- tionsnents of in thethese site components of reaction. Thein the differential site of reaction. solubility The of reactantsdifferential (and solubility of reaction of products)reactants (andin the of various reaction regions products) of the in system the various can alter regions their localof the concentrations, system can alter making their possiblelocal con- to centrations,carry out reactions making at possible rates much to carry higher out (or reac muchtions lower) at rates than much those higher in bulk (or homogeneous much lower) thanphases, those allowing in bulk the homogeneous tuning of reaction phases, rates. allowing It is not the strange, tuning therefore, of reaction that rates. the interfaces It is not strange,between therefore, two immiscible that the liquids interfaces can serve betw aseen convenient two immiscible models liquids for investigating can serve theas con- fate venientof chemical models reactions for investigating in more complex the fate natural of chemical systems reactions such as cellin more membranes complex [20 natural–23]. systemsTo interpretsuch as cell chemical membranes reactivity [20–23]. in emulsions, a kinetic model based on the formalism of theTo pseudo-phase interpret chemical was devised reactivity [19 in,24 emulsi,25]. Inons, this a article,kinetic wemodel will based summarize on the itsformal- main ismfeatures of the and pseudo-phase applications. was We will devised also show[19,24,25] some. illustrativeIn this article, examples we will that summarize highlight theits mainimportance features of and reactant applications. partitioning We andwill ofalso the show main some outcomes illustrative of the examples model, for that example, high- lightdetermining the importance activation of parameters reactant partitioning of reactions and taking of the place main in the outcomes interfacial of regionthe model, of emul- for example,sions. The determining model was originallyactivation developed parameters aiming of reactions at solving taking an important, place in the but interfacial refractory, regionproblem of in emulsions. food science: The how model to quantify was originally the effective developed concentration aiming of antioxidantsat solving an added im- portant,to slow lipidbut refractory, oxidation inproblem the oil, in water, food andscience: interfacial how to regions quantify of foodthe effective grade, lipid-basedconcentra- tionemulsions. of antioxidants added to slow lipid oxidation in the oil, water, and interfacial regions of foodIn grade, the pseudo-phase lipid-based emulsions. model, the whole solution is divided into oil, interfacial, and aqueousIn the regions pseudo-phase (pseudo-phases), model, the with whole the surfactantsolution is located divided in into the boundaryoil, interfacial, between and aqueousthe oil and regions water (pseudo-phases), regions [19,24,26– 30with]. Each the su regionrfactant has located different in solvent the boundary properties between and is thetreated oil and as a water separate regions phase [19,24,26–30]. or pseudo-phase. Each Reactantsregion has (and different other solvent components)