Water dynamics in large and small reverse micelles: from two ensembles to collective behavior

Abstract

The dynamics of water in Aerosol-OT reverse micelles are investigated with ultrafast infrared spectroscopy of the hydroxyl stretch. In large reverse micelles, the dynamics of water are separable into two ensembles: slow interfacial water and bulklike core water. As the reverse micelle size decreases, the slowing effect of the interface and the collective nature of water reorientation begin to slow the dynamics of the core water molecules. In the smallest reverse micelles, these effects dominate and all water molecules have the same long time reorientational dynamics. To understand and characterize the transition in the water dynamics from two ensembles to collective reorientation, polarization and frequency selective infrared pump-probe experiments are conducted on the complete range of reverse micelle sizes from a diameter of 1.6-20 nm. The crossover between two ensemble and collective reorientation occurs near a reverse micelle diameter of 4 nm. Below this size, the small number of confined water molecules and structural changes in the reverse micelle interface leads to homogeneous long time reorientation.

Figure 1

The structure of AOT.

Figure 2

Infrared absorption spectra of the OD stretch of dilute HOD in H₂O in bulk water and in AOT reverse micelles. The spectrum shifts to higher frequencies as the reverse micelle size decreases.

Figure 3

Two component fit to the spectrum of the w₀=10 reverse micelle. The core spectrum is the spectrum of bulk water and the interfacial spectrum is the spectrum of the w₀=2 reverse micelle.

Figure 4

Semilog plot of the vibrational population relaxation of the OD stretch at 2548 cm⁻¹ for a range of reverse micelle sizes. As the reverse micelle size decreases, the vibrational relaxation becomes slower. All decays are biexponential, as shown by the deviation from the straight dashed lines for the w₀=16.5 data.

Figure 5

Anisotropy decays at 2578 cm⁻¹ for the large reverse micelles. The solid lines are the results of simultaneous fits of the vibrational population relaxation and anisotropy decays for a range of frequencies for each reverse micelle based on a two component model consisting of a bulk water core and interfacial water.

Figure 6

Vibrational population relaxation of the OD stretch at several frequencies for the w₀=2 reverse micelle. There is a frequency dependence to the vibrational lifetime demonstrating that the HOD molecules experience slightly different environments in spite of the fact that nearly all of them are interacting directly with the interface.

Figure 7

Anisotropy decays for bulk water and three frequencies for the w₀=2 reverse micelle. After an initial fast decay that has a frequency dependent amplitude, the long time anisotropy decay in the w₀=2 reverse micelle is frequency independent, demonstrating that the long time orientational dynamics of all the water in the w₀=2 reverse micelle are the same.

Figure 8

Anisotropy decays at three frequencies for the w₀=5 reverse micelle. After an initial fast decay that has a frequency dependent amplitude, the long time anisotropy decay in the w₀=5 reverse micelle is frequency independent, demonstrating that the long time orientational dynamics of all water in the w₀=5 reverse micelle are the same.

Figure 9

Anisotropy decay at 2558 cm⁻¹ for the w₀=5 and w₀=10 reverse micelles. The w₀=10 has a fast decay to a plateau similar to the larger reverse micelles (see Fig. 5) while the w₀=5 has a fast decay that leads into a slower long time decay. This indicates that the dynamics of water in the w₀=10 reverse micelle are not collective like the w₀=5 dynamics but may be described using a two component model like the larger reverse micelles.

Figure 10

Anisotropy decay at 2599 cm⁻¹ for the w₀=10 reverse micelle. The red solid line is the result of a simultaneous fit of the anisotropy and lifetime decays for a range of frequencies assuming the same time constants as the large reverse micelles. The fit misses the data at both short and long times. The black solid line is the result of a simultaneous fit of the anisotropy and lifetime decays for a range of frequencies allowing the lifetimes and orientational times to vary but restricting them to be slower than bulk water and the interfacial water found in the large reverse micelles.

Figure 11

Surface area per AOT reproduced from Ref. . The surface area increases rapidly up to w₀≈10. The change in area shows that dramatic changes occur in the reverse micelle interface with size for small reverse micelles. The surface area per AOT is essentially constant above w₀≈16.5, showing the similarity of the interface for all of the large reverse micelles.