Structure and dynamics of water in nonionic reverse micelles: A combined time-resolved infrared and small angle x-ray scattering study

Abstract

We study the structure and reorientation dynamics of nanometer-sized water droplets inside nonionic reverse micelles (water/Igepal-CO-520/cyclohexane) with time-resolved mid-infrared pumpprobe spectroscopy and small angle x-ray scattering. In the time-resolved experiments, we probe the vibrational and orientational dynamics of the O-D bonds of dilute HDO:H 2 O mixtures in Igepal reverse micelles as a function of temperature and micelle size. We find that even small micelles contain a large fraction of water that reorients at the same rate as water in the bulk, which indicates that the polyethylene oxide chains of the surfactant do not penetrate into the water volume. We also observe that the confinement affects the reorientation dynamics of only the first hydration layer. From the temperature dependent surfacewater dynamics, we estimate an activation enthalpy for reorientation of 45 ± 9 kJ mol −1 (11 ± 2 kcal mol −1 ), which is close to the activation energy of the reorientation of water molecules in ice.

Structure and dynamics of water in nanoscopic spheres and tubes

“…To investigate if the difference in dielectric response might be due to a difference in the fractions of surface and core water in the two types of reverse micelle, we prepared reverse micelles containing dilute (10%) HDO:H 2 O mixtures, and investigated the orientational relaxation of the OD groups using polarization-resolved vibrational pump-probe spectroscopy, which provides a direct determination of the fractions of surface and core water. This is because the surface-water molecules reorient much more slowly than the core-water molecules, [3][4][5][6][7][8][9][10][11] so that the OD-stretch vibrational anisotropy decays in a bimodal manner, with the amplitudes of the fast and the slow relaxation being proportional to the fractions of the core and surface water, respectively. In Figure 6(a), we show the isotropic transient-absorption changes of HDO molecules in cylindrical and spherical reverse micelles as a function of delay, measured at the maximum of the ODstretch band at 2525 cm −1 .…”

“…The values a and D 0 equal those found previously for reverse micelles in the same ternary system at lower c s . 10 In Figures 1(c) and 1(d), the contributions from the form factor (scattering from a single droplet) and the hard sphere structure factor (droplet interference) are plotted for w 0 = 5 and w 0 = 10 demonstrating the origin of the observed peaks.…”

“…46,47 Nevertheless, our experimental results show that the distribution of reorientation times is approximately bimodal, as has been observed before for a wide range of reverse micellar systems, with both ionic and nonionic surfactants. [3][4][5][6][7][8][9][10][11][48][49][50] The total frequency-dependent complex permittivity (ω) of the reverse micellar system is thus modeled as a sum of the contributions of Igepal, core water, surface water, and cyclohexane…”

“…However, to date, a systematic investigation of the effect of shape on the dynamics of nanoconfined water is lacking. Studies on reverse micelles have focused mainly on the effect of size in the case of spherical confinement, [3][4][5][6][7][8][9][10][11] and investigations on nanoporous materials are restricted to cylindrical geometries (channels). [12][13][14][15][20][21][22][23] Reverse micelles can adopt spherical or cylindrical shapes, depending on the combination of apolar solvent and surfactant.…”

“…[3][4][5][6][7][8][9][10][11][12][13][14][15] The properties of water in nanoconfinement have been studied with model systems like reverse micelles (water-in-oil microemulsions) [3][4][5][6][7][8][9][10][11] and nanoporous materials. [12][13][14][15] Reverse micelles in particular have proven to be convenient model systems because of the ease of sample preparation and the possibility of controlling the water-pool size.…”