## 1 Introduction  
Reverse micelles (RMs) have been used in diverse applications, including biological, synthetic, and physical chemistry [1, 2, 3]. The interior part of RMs contains an aqueous core where various reactions can be carried out, and hence, the study of reactions in RMs has been of considerable attention. RMs have been employed in enzyme separation [1], pharmaceutical application [2], and protein separation and refolding [3]. Due to the possibility of controlling the volume of water pool within RMs, they have been used as a reaction medium to synthesize nanoparticles [4]. RMs are formed by surfactant droplets dispersed in organic solvents. These surfactant droplets confine the water pool and form ternary systems in RMs. The inner core of the reverse micelle confined by the surfactant molecules of RMs forms a water pool (Figure 1). The water pool is nanosized. Depending on the location in the water pool, properties such as polarity, micro fluidity, bonding, viscosity, and pH, vary from those of bulk water [5]. Water changes states from bound at the interface to free at the center in the water pool [5, 6]. So, the physicochemical properties of water shells inside the water pool also vary, as the distance changes from interfacial molecules' heads in RMs [5, 7]. These properties at the bound state (interface) and free state (center of the shell) also vary with the size of RMs [6, 8]. The RMs size is represented by W\({}_{o}\) and is dependent on the molar ratio of water to surfactant concentration, W\({}_{o}\) = [H\({}_{2}\)O]/[Surfactant]. Several parameters control the size and concentration of reverse micelles [9]. Though different types of surfactants can be used to prepare RMs, dioctyl sodium sulfosuccinate (AOT) is the most used. AOT in isooctane reverse micelles is characterized as spherical in shape in which Na\({}^{+}\) ions remain inside the reverse micelles surrounding the counterions, especially at the interface, as shown in Figure 1b,c. A spherical RM formed by water and NaAOT in isooctane can be divided into three parts [11], including (1) the water pool, A (2) the aqueous interface, B (3) the hydrophobic interface region, C (shown in Figure 1c) [6]. Since each of these three regions differs in their locations in an RM, a solute can experience different physical and chemical environments in these three regions [6,8]. Metal complexes (solutes) can occupy any of these three regions. The location of the metal complexes in RMs affects the rate of electron transfer. The collision of the solvated coordination sphere is required for an outer sphere electron transfer process between transition metal complexes [12]. An outer sphere electron transfer rate depends on the reorganizational energy of the solvent molecules around the reactants and the activated complex [13]. The general rate law for outer sphere electron transfer can be represented by [14]  
\[\text{Rate}=\frac{\text{k}_{\text{et}}\text{-}\text{K}_{\text{IP}}[\text{ reductant}][\text{oxidant}]}{1+\text{K}_{\text{IP}}[\text{ reductant}]} \tag{1}\]  
where k\({}_{\text{et}}\) is the electron transfer rate constant, and K\({}_{\text{IP}}\) is the ion-pairing constant. If one reactant is in excess in experiments of the pseudo-first order, the pseudo-first-order rate constant or the observed rate constant, k\({}_{\text{obs}}\) can be measured, and it depends on the concentration of excess reagent [14]. If the reductant is in excess, the relation follows as  
\[\text{k}_{\text{obs}}=\frac{\text{k}_{\text{et}}\text{K}_{\text{IP}}[\text{ reductant}]}{1+\text{K}_{\text{IP}}[\text{ reductant}]} \tag{2}\]  
When both partners in an electron transfer are substitution inert, and there is no change in the coordination sphere during the reaction, an outer sphere electron transfer reaction takes place. When an outer sphere electron transfer takes place between two reactants, an ion pair is formed by the association of the ions due to electrostatic interactions.