Dielectric spectroscopy of water at low frequencies: The existence of an isopermitive point

We have studied the relative permittivity of water from 100 Hz to 1 MHz. We have found that there is a frequency where this parameter is independent of temperature, and called this the isopermitive point. Below this point the relative permittivity increases with temperature, above, it decreases. To understand this behavior, we may consider water as a system of two species: ions and dipoles, the first giving rise to the so-called Maxwell–Wagner–Sillars effect, the second obeying the Maxwell–Boltzmann statistics. At the isopermitive point, the effects of both mechanisms in the dielectric response compensate each other.