Solvated interfacial region

Girault and Schiffrin [4] proposed an alternative model, which questioned the concept of the ion-free inner layer at the ITIES. They suggested that the interfacial region is not molecularly sharp, but consist of a mixed solvent region with a continuous change in the solvent properties [Fig. 1(b)]. Interfacial solvent mixing should lead to the mixed solvation of ions at the ITIES, which influences the surface excess of water [4]. Existence of the mixed solvent layer has been supported by theoretical calculations for the lattice-gas model of the liquid-liquid interface [23], which suggest that the thickness of this layer depends on the miscibility of the two solvents [23]. However, for solvents of experimental interest, the interfacial thickness approaches the sum of solvent radii, which is comparable with the inner-layer thickness in the MVN model. 

The elementary step of ion transfer is considered to take place between positions x and X2, and therefore the electrical potential drop affecting this transfer is Ao02- The ion transfer involves the renewal of the solvation shell. The change in standard chemical potential Ao f associated with this process takes place over very short distances in the interfacial region [51] and can be assumed to occur between positions X2 and x - Thus, the BV equation for the flux density /, of an ionic species i is [52]... 

Variations of R with A suggest a two-step hydration process solvation and formation of disconnected water clusters centered on polar head groups, followed by the formation of a continuous hydrogen-bond network. At low A, Ri depends logarithmically on co, suggesting bidimensional diffusion of protons in the interfacial region between polymer and water. 

Most treatments of such double-layer effects assume that the microscopic solvation environment of the reacting species within the interfacial region is unaltered from that in the bulk solution. This seems oversimplified even for reaction sites in the vicinity of the o.H.p., especially since there is evidence that the perturbation of the local solvent structure by the metal surface [18] extends well beyond the inner layer of solvent molecules adjacent to the electrode [19]. Such solvent-structural changes can yield considerable influences upon the reactant solvation and hence in the observed kinetics via the work terms wp and wR in eqn. (7a) (Sect. 2.2). While the position of the reaction site for inner-sphere processes will be determined primarily by the stereochemistry of the reactant-electrode bond, such solvation factors can influence greatly the spatial location of the transition state for other processes. 

The solvation term AG olv represents the change in free energy which results from the replacement, on an ion as it adsorbs, of water molecules from the bulk of the solution with water molecules at the interfacial region having a lower dielectric constant. 

Microemulsions have the ability to partition polar species into the aqueous core or nonpolar solutes into the continuous phase (See Fig. 1). They can therefore greatly increase the solvation of polar species in essentially a nonpolar medium. The surfactant interfacial region provides a dramatic transition from the highly polar aqueous core to the nonpolar continuous-phase solvent. This region represents a third type of solvent environment where amphiphilic solutes can reside. Such amphiphilic species will be strongly oriented in the interfacial film so that their polar ends are in the core of the microemulsion droplet and the nonpolar end is pointed towards or dissolved in the continuous phase solvent. When the continuous phase is a near-critical liquid (7)j = r/7 > 0.75) or supercritical fluid, additional parameters such as transport properties, and pressure (or density) manipulation become important aids in applying this technology to chemical processes. 

Thus, micellar solutions consist of three regions of distinctly different solvation properties, a continuous polar aqueous domain, non-polar cores and interfacial regions of intermediate polarity. They are all present in a single homogeneous, thermodynamically stable solution. The totality of the three regions can be treated as separate reaction regions... 

Surface properties can be adjusted by the adsorption of surfactants and polymers. Adsorption itself can essentially be considered to be preferential partitioning of the adsorbate into the into the interfacial region. It is the result of one or more contributing forces arising from electrostatic attraction, chemical reaction, hydrogen bonding, hydro-phobic interactions, and solvation effects. 

The model based on the lattice fluid SCF theory offers a means to calculate fundamental interfacial properties of microemulsions from pure component properties [25]. Because all of the relevant interfacial thermodynamic properties are calculated explicitly and the surfactant and oil molecular architectures are considered, the model is applicable to a wide range of microemulsion systems. The interfacial tension, bending moment, and interaction strength between the droplets can be calculated in a consistent manner and analyzed in terms of the detailed interfacial composition. The mechanism of the density effect on the natural curvature includes both an enthalpic and an entropic component. As density is decreased, the solvation of the surfactant tails is less favorable enthalpically, and the solvent is expelled from the interfacial region. Entropy also contributes to this oil expulsion due to the density difference between the interfacial region and the bulk. The oil expulsion and increased tail-tail interactions decrease the natural curvature. 

The discussions will be mostly centered on ideal systems in which the reactants are considered as particles with given redox properties and solvation energies present at the interfacial region between two dielectric media. In the case of electron transfer (ET), we shall concentrate... 

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