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. 2007 Nov 21;129(46):14311-8.
doi: 10.1021/ja073977d. Epub 2007 Oct 25.

Confinement or the nature of the interface? Dynamics of nanoscopic water

David E Moilanen  1 , Nancy E Levinger, D B Spry, M D Fayer
Affiliations

Confinement or the nature of the interface? Dynamics of nanoscopic water

David E Moilanen et al. J Am Chem Soc. .

Abstract

The dynamics of water confined in two different types of reverse micelles are studied using ultrafast infrared pump-probe spectroscopy of the hydroxyl OD stretch of HOD in H2O. Reverse micelles of the surfactant Aerosol-OT (ionic head group) in isooctane and the surfactant Igepal CO 520 (nonionic head group) in 50/50 wt % cyclohexane/hexane are prepared to have the same diameter water nanopools. Measurements of the IR spectra and vibrational lifetimes show that the identity of the surfactant head groups affects the local environment experienced by the water molecules inside the reverse micelles. The orientational dynamics (time-dependent anisotropy), which is a measure of the hydrogen bond network rearrangement, are very similar for the confined water in the two types of reverse micelles. The results demonstrate that confinement by an interface to form a nanoscopic water pool is a primary factor governing the dynamics of nanoscopic water rather than the presence of charged groups at the interface.

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Figures

Figure 1
Figure 1
Linear IR spectrum of water in the w0 = 10 AOT reverse micelle (dashed line), the w0 = 7 Igepal reverse micelle (dash-dot line), and bulk water (solid line).
Figure 2
Figure 2
Semilog plot of the vibrational lifetime of water in the w0 = 10 AOT reverse micelle and the w0 = 7 Igepal reverse micelle. The solid lines are the biexponential fits to the experimental data.
Figure 3
Figure 3
a. Orientational relaxation (anisotropy decay) of water in the w0 = 10 AOT reverse micelle (open squares), and the w0 = 7 Igepal reverse micelle (open circles). Solid lines are biexponential fits to the data. b. Orientational relaxation of the two reverse micelles (AOT - open squares, Igepal - closed circles) with bulk water (open diamonds) and neat Igepal (closed triangles) for comparison. Note the different vertical scales for the two graphs.
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