Real Structure of Formamide Entrapped by AOT Nonaqueous Reverse Micelles:  FT-IR and ¹H NMR Studies

Abstract

Noninvasive techniques such as FT-IR and (1)H NMR spectroscopy have been employed to investigate the solubilization of formamide, FA, and its aqueous solution, FA-water, by sodium 1,4-bis(2-ethylhexyl)sulfosuccinate, AOT, in heptane or isooctane reverse micelles, respectively. Partially deuterated FA (FADH) was used in the FT-IR experiments and nu(OD), n(ND) were analyzed. Also, the nu(C=O) band of FA was investigated. For AOT, the changes of the SO(3)(-) group's symmetric, nu(s), and asymmetric, nu(a), bands were also studied. The results are showing that FA is interacting strongly with the Na+ counterions of the surfactant through electrostatic interactions maintaining their hydrogen bond network present in the FA bulk. Accordingly, partially deuterated FA is "frozen" inside the aggregates and it is possible to detect, by FT-IR technique, the cis and trans isomers. Curve fitting of the nu(OD) (in the FA-water mixture) band requires use of two peaks because the band is asymmetric, not because the solubilizate molecules are present in layers of different structure. The chemical shifts of the (1)H bound to N and C of FA were studied by (1)H NMR. The comparison of the chemical shift of AOT in reverse micelles with FA and the FA-water mixture in the polar core of the aggregate shows that there is a strong preferential solvation of Na+ by FA (through electrostatic interaction) and the AOT's sulfonate group by water (through hydrogen bond interaction).

Binding of o-nitroaniline to nonaqueous AOT reverse micelles

“…FA is a "special" solvent since we demonstrated that FA encapsulated by AOT reverse micelles, interacts strongly with the Na + counterions and the sulfonate group of the surfactant through electrostatic interactions maintaining their hydrogen bond network present in the FA bulk. 42 Thus, it was expected, and actually observed, that the K b value in this system was similar to the one obtained at W S = 0 because the sulfonate group was involved in an electrostatic interaction in both reverse micelles. On the other hand, an interesting situation was observed when DMF and DMA were sequestrated by the reverse micelles since the K b values obtained were the highest.…”

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

“…In contrast, recent studies have shown that FA inside AOT reverse micelles conserves the bulklike intersolvent H-bond interactions. [25,29] Despite the fact that the AOT molecule has different regions in its moiety, where the different solvents can interact (see 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/nheptane reverse micelles. The goal of our study is to gain insights into the unique reverse-micelle microenvironment created upon encapsulation of these polar solvents.…”

“…To do this, we used different approaches: i) invasive techniques, such as absorption and emission spectroscopy of different molecular probes dissolved in different nonaqueous reverse micelles media, [16,17,19,28,29] and ii) noninvasive techniques, such as FTIR and 1 H NMR spectroscopy, [18,20,25] where the main effort was focused on the interpretation of the spectra of different polar solvents encapsulated inside reverse micelles to gain insights into their structure and the existence or not of a polar solvent pool. Thus, we have followed how the n OD or the n ND bands for EG, GY, PG, and FA change as the W S ratio increases.…”

“…Few reports deal with the investigation of the vibrational modes associated with the sulfonate and the ester moieties of the AOT head group upon water encapsulation, although meaningful information can be obtained from these modes. [25,[31][32][33] On the other hand, there are no reports in the literature about the investigation of the AOT modes for nonaqueous reverse micelles, except for FA. [25] Moreover, we show here that FTIR spectroscopy is a very powerful technique because not only the frequency band shifts can be monitored but also the band intensity.…”

Molecular dynamics simulations of AOT-water/formamide reverse micelles: Structural and dynamical properties

“…16,37,38 ͑iii͒ Laia et al 39 performed light scattering experiments that would indicate that in formamide/AOT micelles the polar head groups ͑PHGs͒ are poorly solvated by formamide, leading to a strong association between the AOT polar heads and Na + counterions, resulting in an overall electrically neutral interfacial region. ͑iv͒ Finally, Correa et al 40 employed Fourier transform infrared and NMR techniques to explore the solubilization of formamide and its aqueous solutions by AOT in reverse micelles. Their results indicate that formamide interacts strongly with the Na + counterions, while preserving the HB network found in the bulk phase.…”