In this review, we deal with the formation and application of biocompatible water-in-oil microemulsions commonly known as reverse micelles (RMs). These RMs are extremely important to facilitate the dissolution of hydrophilic and hydrophobic compounds for biocompatibility in applications in drug delivery, food science, and nanomedicine. The combination of two wisely chosen types of compounds such as biocompatible non-polar solvents and ionic liquids (ILs) with amphiphilic character (surface-active ionic liquids, SAILs) can be used to generate organized systems that perfectly align with the Green Chemistry concepts. Thus, we describe the current state of SAILs (protic and aprotic) to prepare RMs using non-polar but safe solvents such as esters derived from fatty acids, among others. Moreover, the use of the biocompatible solvents as the external phase in RMs and microemulsions/nanoemulsions with the other commonly used biocompatible surfactants is detailed showing the diversity of preparations and important applications. As shown by multiple examples, the properties of the RMs can be modified by changes in the type of surfactant and/or external solvents but a key fact to note is that all these modifications generate novel systems with dissimilar properties. These interesting properties cannot be anticipated or extrapolated, and deep analysis is always required. Finally, the works presented provide valuable information about the use of biocompatible RMs, making them a green and promising alternative toward efficient and sustainable chemistry.

In this work, we have investigated the behavior of 4-aminophthalimide (4-AP) in solvent mixtures of ethyl lactate (EL)– water and EL–n-heptane and in reversed micelles (RMs) media made of EL–water/sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT)/n-heptane. We have used dynamics light scattering (DLS) and absorption, steady-state and time-resolved emission (TRES) techniques. 4-AP is a very interesting and unique molecule used to study preferential solvation in water mixtures since its emission profile changes dramatically when its sphere shell is solvated by water molecules. Thus, in homogeneous media 4-AP is strongly solvated by water in the EL–water mixture and by EL in the EL–n-heptane mixture, results that show the importance of the hydrogen bonding in the 4-AP solvation. We were motivated by this feature of 4-AP and have used it to monitor properties in AOT RMs. Thus, we use 4-AP spectroscopic behavior in conjunction with DLS technique to reveal the location of each polar solvent of the mixture encapsulated within the RMs media. We found that in the EL–water/AOT/n-heptane RMs the results strongly depend on the amount of water dissolved. Below W₀ = [water]/[AOT] = 5, there are no reversed micelles and EL, water, AOT and n-heptane forms a nonstructured mixture. For W₀ values between 5 and 10, the droplet sizes are independent of the EL content because of its strong intermolecular interactions forms an EL polar core and only water is found at the interface. For W₀ values higher than 10, the droplets size increase with the EL content and EL molecules are detected at the AOT RMs interface. We inferred that the RMs sizes will change only if the polar solvent encapsulated interacts with the interface changing the surfactant packing parameter. Then, we can assume that it is possible to create RMs with solvents that do not interact with the interface but can be encapsulated in the polar core. These results, give evidence that expand the knowledge about which are the factors that determine when RMs droplet sizes changes with the polar solvent content, giving insights that will help to control the sizes of the AOT RMs. This will open diverse avenues since RMs are interesting nanoreactors for heterogeneous chemistry, templates for nanoparticles and models for electron transfer reaction that happens in membranes.

C343, a common molecular probe utilized in solvation dynamics experiments, was studied in homogeneous media. Absorption, emission, and ¹HNMR spectroscopies were used to investigate the behavior of C343 in benzene and in benzene/n-heptane mixtures. We demonstrate the implications of the medium polarity, measured as the Kamlet−Taft polarity-polarizability (π*) parameter, in the C343 inter/intramolecular hydrogen bond (H-bond) interactions and the role that this interaction plays in the dimerization process of the dye. In pure benzene, the dimer prevails because the intermolecular H-bond interaction is favored. On the other hand, as the n-heptane content increases the intramolecular H-bond is the strongest and the C343 monomer is favored. As the polarity of the medium decreases, the solvophobic interaction makes that C343 monomer species experiences a more complicated aggregation process beyond the simple monomer dimer equilibrium present in pure benzene. Thus, the addition of n-heptane to the mixture yields a C343 higher-order aggregates species. Thus, our work reveals the importance that the medium has on the behavior of a widespread dye used as chromophore for very different systems such as homogeneous and microheterogenous media. This is very important since the use of chromophores without understanding its chemistry can induce artifacts into the interpretation of solvation dynamics in heterogeneous environments, in particular, those provided by biological systems such as proteins. Considerable care in choosing and characterizing the system is required to analyze the results fully.