Infrared and Dielectric Study of Ca(AOT)₂Reverse Micelles

Abstract

In this paper, we report on IR and dielectric spectroscopy of Ca(AOT) 2 reverse micelles containing water pools of different sizes. We investigate the effect of changing the nature of the counterion (Na + f Ca 2+ ) on the hydration mechanisms and dynamics of AOT microaggregates. The hydration has been studied as a function of the molar ratio W ) [H 2 O]/[AOT] by means of IR absorption of O-H stretching modes in the 3800-3000 cm -1 frequency range. The dynamical properties of the system have been investigated by dielectric permittivity measurements performed in the range 0.02-3 GHz, as a function of W. A relaxation phenomenon has been observed in this region, and a close connection between the behavior of the relaxation time vs W and the progressive hydration of AOT polar headgroups has been found, similar to the case of the NaAOT micellar system. These results could be interpreted in terms of two coexisting diffusion mechanisms: the reorientation of the whole micellar aggregate and the free rotational diffusion of the completely hydrated AOT headgroups.

Inhomogeneous dynamics in confined water nanodroplets

“…In previous studies, two-component models have been proposed to describe both the linear absorption (13) and the vibrational energy relaxation (14,15) of confined water, to reflect the different properties of water located at the surface and in the core of the nanodroplets. As for the structural hydrogen bond rearrangements of liquid water, the effects of nanoconfinement are under much debate.…”

Influence of Confined Water on the Photophysics of Dissolved Solutes in Reverse Micelles

“…These studies, however, do not distinguish whether this slowing down holds for all of the water molecules present in RMs, or only for those present in the region of the surfactant head groups (interface). Recent studies employing techniques like vibrational echo and pump-probe spectroscopy, [4][5][6][7][8] absorption, [9][10][11][12][13][14] fluorescence, [3,[15][16][17][18][19][20][21][22][23][24][25][26] NMR, [27][28][29][30][31][32] calorimetry, [28,33] neutron scattering, [12,34,35] molecular dynamics simulation, [36][37][38][39][40] and so on have helped us in identifying different types of water in RMs, namely, water physically trapped between surfactant head groups, water directly bound (by HThe photophysical parameters of two probes with largely different hydrophobic character, namely, coumarin 1 and coumarin 343, are investigated in sodium bis-(2-ethylhexyl)sulfosuccinate (AOT)/hexane/water reverse micelles at various water/AOT molar ratio w 0 . Correlation of photophysical parameters such as fluorescence quantum yield, fluorescence lifetime, and emission maxima with w 0 indicate distinctly different trends below and above w 0 % 7 for both probes.…”

“…[11,13,22,[41][42][43] A two-component core-shell model has thus been proposed to describe the differential properties of water located at the interface (between nonpolar and polar phase) and in the core of RMs. [6,14,37,44] Although there have also been arguments suggesting inhomogeneities in the reorientational motions of the water molecules throughout the water droplets in RMs, [4,6,37] the above core-shell model has been successfully applied to explain quasielastic neutron scattering results in RMs. [35,39] Spectrally resolved ultrafast mid-IR pump-probe spectroscopy on HDO/H 2 O in RMs has distinctly demonstrated different mobilities for core and interfacial water molecules as well as separation of their relative contributions.…”

Highly viscous wormlike micellar phases formed from the mixed AOT/C14DMAO/H2O system

“…Most studies focused on the reverse structures of AOT with different counterions (Na + , Ag + , Ca 2+ , Ni 2+ , Al 3+ , and In 3+ ) and their use in synthesizing nanomaterials [36][37][38][39][40][41]. However, the results in aqueous solutions are considerably less than those in nonaqueous solutions.…”