NMR study of the influence of n-alkanol co-surfactants on reverse micelles in quaternary microemulsions of cetyltrimethylammonium bromide (CTAB).

The effects of different n-alkanol co-surfactants on the size, shape, composition and dynamics of reverse micelles (RMs) in cetyltrimethylammonium bromide (CTAB)/n-alkanol/n-hexane/water and CTAB/n-alkanol/n-pentane/water microemulsions were investigated using T2 relaxation and pulsed gradient stimulated echo nuclear magnetic resonance (NMR) measurements and molecular modelling. NMR T2 relaxation times and diffusion coefficients were determined for the surfactant and co-surfactant in these CTAB quaternary reverse microemulsions, for a range of medium chain length alcohol co-surfactants, from 1-butanol to 1-heptanol. These data revealed a slight RM size dependency on co-surfactant chain length, with RM sizes tending to decrease with increasing alcohol chain length. Molecular modelling of CTAB/n-alkanol/n-hexane/water RMs suggested a variation in RM shape with co-surfactant chain length, where those formed with pentanol were found to be least spherical and those formed with heptanol the most spherical. The NMR data also revealed differences in the behaviour of the micellar structures in the CTAB/n-pentanol/n-hexane/water reverse microemulsion, compared with the other reverse microemulsions in this study, where CTAB was found to be distributed between two environments, which then combined to form larger micelles. The origins of these differences remain unclear. Copyright © 2016 John Wiley & Sons, Ltd.

NMR and molecular dynamics study of the size, shape, and composition of reverse micelles in a cetyltrimethylammonium bromide (CTAB)/n-hexane/pentanol/water microemulsion.

The size, shape, and composition of reverse micelles (RMs) in a cetyltrimethylammonium bromide (CTAB)/pentanol/n-hexane/water microemulsion were investigated using pulsed gradient stimulated echo (PGSTE) nuclear magnetic resonance (NMR) measurements and molecular modeling. PGSTE data were collected at observation times (Δ) of 10, 40, and 450 ms. At long observation times, CTAB and pentanol exhibited single diffusion coefficients. However, at short (Δ ≤ 40 ms) observation times both CTAB and pentanol exhibited slow and fast diffusion coefficients. These NMR data indicate that both CTAB and pentanol molecules reside in different environments within the microemulsion and that there is exchange between regions on the millisecond time scale. Molecular dynamic simulations of the CTAB RM, in a solvent box containing n-hexane and pentanol, produced an ellipsoid shaped RM. Using structural parameters from these simulations and the Stokes-Einstein relation, the structure factor and dimensions of the reverse micelle were determined. Analysis of the composition of the interphase also showed that there was a variation in the ratio of surfactant to cosurfactant molecules depending on the curvature of the interphase.