High electrical permittivity of ultrapure water at the water-platinum interface

•Applying an apt voltage to a Pt surface plunged in water an exclusion zone appears.•When the exclusion zone appears MBP theory do not works.•Inside the exclusion zone the permittivity of water appears to be very high.

The discharge of a platinum parallel-plate capacitor filled with ultrapure water has been measured. The observed discharge trend can be described by a Modified Poisson-Boltzmann Equation (MPB) only when the voltage is very low. Increasing the applied voltage creates an 'exclusion zone' in which the MPB equation no longer fits, and the system capacitance shows a dependence on the spacing between the two platinum plates. The permittivity of water, calculated considering the system as a plane capacitor, appears to be very high. This implies that the exclusion zone could have extraordinary dielectric properties.

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