Effect of the Constrained Environment on the Interactions between the Surfactant and Different Polar Solvents Encapsulated within AOT Reverse Micelles

Abstract

Herein, we report a study of the interactions between different nonaqueous polar solvents, namely, ethylene glycol (EG), propylene glycol (PG), glycerol (GY), dimethylformamide (DMF), and dimethylacetamide (DMA), and the polar heads of sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT) in nonaqueous AOT/n-heptane reverse micelles. The goal of our study is to gain insights into the unique reverse-micelle microenvironment created upon encapsulation of these polar solvents. For the first time, the study is focused on determining which regions of the AOT molecular structure are involved in the interactions with the polar solvents. We use FTIR spectroscopy--a noninvasive technique--to follow the changes in the AOT C=O band and the symmetric and asymmetric SO(3)(-) vibration modes upon increasing the content of polar solvents in the micelles. The results show that GY interacts through H bonds with the SO(3)(-) group, thereby removing the Na(+) counterions from the interface remaining in the polar core of the micelles. PG and EG interact through H bonds, mainly with the C=O group of AOT, penetrating into the oil side of the interface. Thus, they interact weakly with the Na(+) counterion, which seems to be close to the AOT sulfonate group. Finally, DMF and DMA, encapsulated inside the reverse micelles, interact neither with the C=O nor with the SO(3)(-) groups, but their weakly bulk-associated structure is broken because of the interactions with Na(+). We suggest that DMF and DMA can complex the Na(+) ions through their carbonyl and nitrogen groups. Hence, our results do not only give insights into how the constrained environment affects the bulk properties of polar solvents encapsulated within reverse micelles but--more importantly--they also help us to answer the tricky question about which regions of the AOT moiety are involved in the interactions with the polar solvents. We believe that our results show a clear picture of the interactions present at the nonaqueous reverse-micelle interface; this is important because these media are interesting nanoreactors for heterogeneous chemistry, templates for nanoparticles, and models for membranes.

The effect of water on the microstructure and properties of benzene/[bmim][AOT]/[bmim][BF4] microemulsions

“…[39][40][41][42] Durantini et al correlated changes in the AOT vibrational spectra with the reverse micelle size to find how a, the effective surfactant area, and P, the packing parameter, depend on the nonaqueous polar solvent properties. 43 In contrast with these nonaqueous microemulsions, the case of [ 4 ] and various molecules. Therefore, much still remains to be learned about these intriguing nanoheterogeneous systems both from the perspective of fundamental characterization and application of the reverse microemulsions for novel chemistries.…”

Binding of o-nitroaniline to nonaqueous AOT reverse micelles

“…17 In n-hexane/AOT/water reverse micelles it was found that o-NA binds to the polar heads of AOT through the NH groups in a hydrogen bond interaction and, the water addition decreases the binding constant K b up to W 0 = 10 showing the competition of water for the polar heads of the surfactant. 17 Nevertheless, it is interesting to discover with which part of the AOT polar head, namely the sulfonate or the ester moiety 48 o-NA interacts through hydrogen bond. Thus, we have chosen a set of polar solvents that interact with different parts of the AOT moiety including the Na + counterions.…”

“…Thus, EG interacts weakly with the Na + counterion which seems to be close to the AOT sulfonate group as in the case of W S = 0. 48 In light of the above, we believe that o-NA has to interact by hydrogen bonding with the AOT sulfonate group rather than with the AOT's C=O group because the K b values obtained are lower for GY and water than EG. It seems that the competition for the AOT sulfonate solvation between -NH and water or GY dominates the binding process.…”

“…Thus, the o-NA solubilization must be described in terms of: i) solvent -solute; ii) solvent-AOT headgroups and, iii) solute-AOT headgroup and the corresponding counterions interactions. Since it was already known that the polar solvent structure was dramatically disrupted upon encapsulation because the strong polar solvent -surfactant interaction, 48 we did not consider the solvent -solute interaction in our interpretation. The interactions with the polar head of AOT and the counterions seem to be the main driving force to incorporate o-NA to the reverse micelles.…”

“…[17][18][19] We have shown that water and GY bind through hydrogen bonds to the SO 3 -group of AOT removing the Na + counterions from the interface. 48 Also, the strength of this interaction is greater for GY than water. 35,37 On the other hand, EG penetrates to the lipophilic side of the interface and interacts through hydrogen bond mainly with the AOT's C=O group.…”