The influence of charge on the structure and dynamics of water encapsulated in reverse micelles

Abstract

Hydrogen-bonded structure and relaxation dynamics of water entrapped inside reverse micelles (RMs) composed of surfactants with different charged head groups: sodium bis(2-ethylhexyl) sulfosuccinate (AOT) (anionic), didodecyldimethylammonium bromide (DDAB) (cationic) and Igepal CO-520 (Igepal) (nonionic) in cyclohexane (Cy) have been studied as a function of hydration (defined by ). Sub-diffusive slow (sub-ns) relaxation dynamics of water has been measured by the time resolved fluorescence spectroscopy (TRFS) technique using two fluorophores, namely 8-anilino-1-naphthalenesulfonic acid (ANS) and coumarin-343 (C-343). The hydrogen bonded connectivity network of water confined in these RMs has been investigated by monitoring the hydrogen bond stretching and libration bands of water using far-infrared FTIR spectroscopy. In addition, the ultrafast collective relaxation dynamics of water inside these RMs has been determined by dielectric relaxation in the THz region (0.2-2.0 THz) using THz time domain spectroscopy (THz-TDS). While TRFS measurements establish the retardation of water dynamics for all the RM systems, FTIR and THz-TDS measurements provide with signature of charge specificity.

Protein Stability in Reverse Micelles

“…[10] Protein stability is defined as the modified standard state Gibbs free-energy of unfolding, DG 0 0 u ,w hich equals ÀRTln K u ðÞ ,w here R is the gas constant, T is the absolute temperature,a nd [12b] Dimensions are from the data in Table 1. One reverse micelle contains one protein molecule and about 300 H 2 Omolecules.…”

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 .…”

“…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.…”