Proceedings of September Meeting 2016 (NFM16)
Publication date: 14th June 2016
The transport of protonic (H3O+) charges through liquid water occurs very rapidly, much faster than that of any other type of ions. Proton-charge transport in water is believed to involve a "hole-transport" mechanism, in which electron density moving in the opposite direction also contributes to the net charge transport (the so-called Grotthuss mechanism). In a sense, water can be regarded as a protonic semiconductor. Here we show that just as for electronic semiconductors, confinement to nanoscopic dimensions strongly influences protonic charge mobility in water.
To investigate proton-charge transport in nanoconfinement, we prepared protonic nanodots: nanometer-sized spheres of acidic water kept in reverse micelles with continuously adjustable size. Surprisingly, we find that the motion of the confined protonic charges gives rise to a broad resonance in the GHz dielectric spectrum, the frequency of which can be used to determine the charge-diffusion constant. In nanodots with diameters less than about 5 nm, protonic charge transport slows down significantly with decreasing size: in nanodots of about 1 nm diameter, the protonic diffusion constant is two orders of magnitude smaller than in bulk water. This slowing down of the proton-charge mobility can be explained from the more rigid hydrogen-bond network of the nanoconfined water, since proton-charge transfer in water relies on collective hydrogen-bond rearrangements.
Since proton-charge transport often occurs through nanometer-sized volumes of water (with examples ranging from porous minerals, fuel-cell membranes, metal-organic frameworks and zeolites, to the living cell), the observation of a size-dependent mobility is relevant for many branches of physics and chemistry, and might even have biological relevance: in the living cell, water volumes with dimensions both larger and smaller than the "critical size" of 5 nm size occur, and this variation in size may have a biological function as a proton-transport regulation mechanism.
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