Giant Permittivity of Confined Water on Nanodiamonds

Abstract

The Dielectric permittivity (ε) of hydrated detonation nanodiamonds (HDNDs) with variable water content (W) was measured at 1 kHz to 1 MHz frequencies, using a capacitor cell with bare or blocked electrodes. At low frequencies, HDNDs show a giant ε of 104 to 106, i.e., 105 to 107, for the confined water (CW) itself, probably due to microcapacitors formed at particle surfaces. At higher frequency, ε decreases but even at 1 MHz is up to 103 for hydrates and 104 for CW (cf. 78 for bulk water), probably due to disruption of hydrogen bonds. ε as a function of W peaks at W = 6 to 16% and declines on further hydration, while the conductivity continued to increase. CW density, measured by pycnometry, near the surface is ca. 1.5 g/cm3 for nanodiamonds with low hydrogen content but ca. 0.5 g/cm3 for other types, in each case converging to 1 g/cm3 in further layers. Evolved gas analysis of HDND reveals two types of adsorbed water with distinct peaks at 150–180 and 230–300 °C. In view of these and earlier results, the decreased ε of CW determined by atomic force microscopy must be taken with caution.