A large absorption resonance has been observed in the far-infrared spectrum of water confined within inverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in heptane. The amplitude and spectral position of this resonance depend on the size of the water pool. The origin of the THz absorption of these liquid pools is assigned to surface oscillations of the water pool, driven by the interfacial tension of the water−surfactant-oil interface. These data indicate a dramatic restructuring of the density of states of the liquid and are consistent with a theoretical model for the oscillatory modes of liquid droplets. The presence of a large excess in the vibrational density of states of confined water could have far-ranging implications in many biochemical and chemical processes where confined water is present.

The far-infrared absorption spectrum of nanometer-sized water pools at the core of AOT micelles exhibits a pronounced resonance which is absent in bulk water. The amplitude and spectral position of this resonance are sensitive to the size of the confined water core. This resonance results from size-dependent modifications in the vibrational density of states, and thus has far-reaching implications for chemical processes which involve water sequestered within small cavities. These data represent the first study of the terahertz dielectric properties of confined liquids.

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