Distinguishing bulk water from adsorbed water via dielectric measurements [silica gels]

The dielectric properties of silica gels of varying density were measured as a function of frequency and relative humidity. Capacitance and dielectric loss measurements were taken from 10 Hz-10/sup 7/ Hz at humidities ranging from 5 to 90% RH. It is found that the electrical behaviour of these materials is distinctively different when they contain adsorbed water versus molecular or bulk water. Namely, adsorbed water is manifested as an increase in the dielectric constant and dielectric loss primarily at lower frequencies while the higher frequencies remain unchanged. In constrast, the presence of bulk water leads to a uniform increase in the dielectric constant at all frequencies. This behavior is believed to be applicable not only to ceramic insulators but also to a wide range of other materials including polymers and semiconductors.