Water dynamics in small reverse micelles in two solvents: two-dimensional infrared vibrational echoes with two-dimensional background subtraction

Abstract

Water dynamics as reflected by the spectral diffusion of the water hydroxyl stretch were measured in w(0) = 2 (1.7 nm diameter) Aerosol-OT (AOT)/water reverse micelles in carbon tetrachloride and in isooctane solvents using ultrafast 2D IR vibrational echo spectroscopy. Orientational relaxation and population relaxation are observed for w(0) = 2, 4, and 7.5 in both solvents using IR pump-probe measurements. It is found that the pump-probe observables are sensitive to w(0), but not to the solvent. However, initial analysis of the vibrational echo data from the water nanopool in the reverse micelles in the isooctane solvent seems to yield different dynamics than the CCl(4) system in spite of the fact that the spectra, vibrational lifetimes, and orientational relaxation are the same in the two systems. It is found that there are beat patterns in the interferograms with isooctane as the solvent. The beats are observed from a signal generated by the AOT/isooctane system even when there is no water in the system. A beat subtraction data processing procedure does a reasonable job of removing the distortions in the isooctane data, showing that the reverse micelle dynamics are the same within experimental error regardless of whether isooctane or carbon tetrachloride is used as the organic phase. Two time scales are observed in the vibrational echo data, ~1 and ~10 ps. The slower component contains a significant amount of the total inhomogeneous broadening. Physical arguments indicate that there is a much slower component of spectral diffusion that is too slow to observe within the experimental window, which is limited by the OD stretch vibrational lifetime.

Figure 1

Molecular structure of AOT.

Figure 2

FT-IR absorption spectra for bulk water and water inside AOT∕isooctane and AOT∕CCl₄ reverse micelles. The spectra of reverse micelles of the same size in different solvents are identical. As the micelle size decreases, the spectra systematically shift to the blue.

Figure 3

Population relaxation data for water in the three sizes of small reverse micelles showing the invariance with nonpolar phase.

Figure 4

Orientation relaxation data for water in the three sizes of small reverse micelles, again showing the invariance with the nonpolar phase.

Figure 5

2D IR correlation spectra for w₀ = 2∕CCl₄ at a range of T_w's.

Figure 6

(a) Experimental CLS data for w₀ = 2∕CCl₄ reverse micelles. The line through the data is the normalized frequency–frequency correlation function for w₀ = 2. (b) Experimental center line slope results: w₀ = 2∕CCl₄ (solid dots), w₀ = 2∕isooctane beat subtracted data showing decent agreement with results in CCl₄ (hollow dots), and nonbeat subtracted w₀ = 2∕isooctane data showing extremely fast decay (triangles). The line through the top curve is the normalized FFCF for that sample, while the bottom two lines are the biexponential fits from Table V.

Figure 7

Interferogram data. (a) Resonant interferogram in AOT∕isooctane for w₀ = 2 at T_w = 1 ps and 2580 cm⁻¹, showing the strange beat behavior. (b) AOT∕CCl₄ interferogram for w₀ = 2 at T_w = 1 ps and 2580 cm⁻¹, showing normal echo behavior.

Figure 8

(a) Interferogram for nonresonant pure H₂O in AOT∕isooctane, showing strange beat behavior. (b) The red and blue interferograms respectively show the resonant and nonresonant w₀ = 2 interferograms at T_w = 1 ps and 2580 cm⁻¹ used for beat subtraction. (c) Results of the beat subtraction method in isooctane. The interferogram looks a lot cleaner and similar to the interferogram in Fig. 7(b).

Figure 9

Cartoon illustrating how topography changes through movements of individual sulfonate head groups in the surfactant shell of the reverse micelle can change the environments felt by the water molecules.