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. 2009 Sep 8;106(36):15243-8.
doi: 10.1073/pnas.0907875106. Epub 2009 Aug 25.

Water dynamics at neutral and ionic interfaces

Emily E Fenn  1 , Daryl B Wong, M D Fayer
Affiliations

Water dynamics at neutral and ionic interfaces

Emily E Fenn et al. Proc Natl Acad Sci U S A. .

Abstract

The orientational dynamics of water at a neutral surfactant reverse micelle interface are measured with ultrafast infrared spectroscopy of the hydroxyl stretch, and the results are compared to orientational relaxation of water interacting with an ionic interface. The comparison provides insights into the influence of a neutral vs. ionic interface on hydrogen bond dynamics. Measurements are made and analyzed for large nonionic surfactant Igepal CO-520reverse micelles (water nanopool with a 9-nm diameter). The results are compared with those from a previous study of reverse micelles of the same size formed with the ionic surfactant Aerosol-OT (AOT). The results demonstrate that the orientational relaxation times for interfacial water molecules in the two types of reverse micelles are very similar (13 ps for Igepal and 18 ps for AOT) and are significantly slower than that of bulk water (2.6 ps). The comparison of water orientational relaxation at neutral and ionic interfaces shows that the presence of an interface plays the dominant role in determining the hydrogen bond dynamics, whereas the chemical nature of the interface plays a secondary role.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Molecular structures for the surfactants Igepal CO-520 (neutral head group containing a hydroxyl) and AOT (ionic sulfonate head group with a sodium counter ion).
Fig. 2.
Fig. 2.
IR absorption spectra of the OD stretch of 5% HOD in H2O for Igepal w0 = 20, AOT w0 = 25, and bulk water.
Fig. 3.
Fig. 3.
Vibrational population relaxation data. (Upper) Data for the OD stretch of HOD in H2O in the water nanopools of w0 = 20 Igepal reverse micelle at four frequencies and bulk water. (Lower) The vibrational population relaxation data for 2,576 cm−1 and a biexponential fit.
Fig. 4.
Fig. 4.
Anisotropy data for bulk water and Igepal w0 = 20 and AOT w0 = 25, both with water nanopool diameters of 9 nm. The decays are quite similar but not identical.
Fig. 5.
Fig. 5.
Data analysis with three component model. (A) Fit to population relaxation data (2,576 cm−1). (B) Fit to anisotropy data (2,576 cm−1).
See this image and copyright information in PMC

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