FTIR and ¹H NMR Studies on the Structure of Water Solubilized by Reverse Aggregates of Dodecyltrimethylammonium Bromide; Didodecyldimethylammonium Bromide, and Their Mixtures in Organic Solvents

Abstract

The structure of water solubilized by the reverse aggregates of dodecyltrimethylammonium bromide, DoMe₃ABr in chloroform/n-heptane; di-dodecyldimethylammonium bromide, Do₂Me₂ABr, in n-heptane, and mixture of the two surfactants in the latter solvent has been probed by FTIR and ¹H NMR. The ν _OD band of solubilized HOD (4% D₂O in H₂O) has been recorded as a function of [water]/[surfactant] molar ratio, W/S. Curve fitting of this band showed the presence of a small peak at (2375 ± 12) cm⁻¹ and a major one at (2521 ± 7) cm⁻¹; the latter corresponds to (92.5 ± 1) % of the total peak area. As a function of increasing W/S, ν _OD decreases, its full width at half-height increases and its area linearly increases over the W/S range investigated. Observed ¹H NMR chemical shift, δ _obs, of solubilized water, and the CH ₂–N⁺(CH₃)₃, CH₂–N⁺(CH ₃)₃ groups of DoMe₃ABr change smoothly as a function of increasing W/S. Similar trends have been observed for Do₂Me₂ABr-solubilized water, and for water solubilized by a mixture of DoMe₃ABr plus Do₂Me₂ABr. δ _obs for H₂O-D₂O mixtures solubilized by DoMe₃ABr were measured as a function of the deuterium content of the aqueous nano-droplet. These data were employed to calculate the so called deuterium/protium “fractionation factor”, φ _M, of the reverse aggregate-solubilized water. Plots of a function of δ _obs (for HOD; CH ₂–N⁺(CH₃)₃, and CH₂–N⁺(CH ₃)₃) versus the atom fraction of deuterium in the aqueous nano-droplet were strictly linear, indicating that the value of φ _M is unity. The results of both techniques show that reverse aggregate-solubilized water does not seem to coexist in “layers” of different structures, as suggested, e.g., by the multi-state water solubilization model.