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.