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

摘要

The structure of water solubilized by the reverse aggregates of dodecyltrimethylammonium bromide, DoMe_3ABr in chloroform/n-heptane; didodecyldimethylammonium bromide, Do-2Me_2ABr, in n-heptane, and mixture of the two surfactants in the latter solvent has been probed by FTIR and ~1H NMR. The v_(OD) band of solubilized HOD (4% D_2O in H_2O) has been recorded as a functior 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, v_(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~+(CH_3)_3, CH_2-N~+(CH_3)_3 groups of DoMe_3ABr change smoothly as a function of increasing W/S. Similar trends have been observed for Do_2Me_2ABr-solubilized water, and for water solubilized by a mixture of DoMe_3ABr plus Do_2Me_2ABr. δ_(obs) for H_2O-D_2O mixtures solubilized by DoMe_3ABr 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~+(CH_3)_3, and CH_2-N~+(CH_3)_3) versus the atom fraction of deuterium in the aqueous nano-droplet were strictly linear, indicating that the value of ψ_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.