Dielectric property and micro-structure of water-alcohol solutions

A simple method for measuring dielectric properties of water-alcohol solutions for structural studies was proposed. The apparatus consists of three components : RF-generator, resonator with parallel planar electrodes, and detector. The resonators composed of a coaxial line and a couple of electrodes, generates characteristic resonance frequencies (fR) related to the relative dielectricity of the sample. Shifts of fR with change of temperature, alcohol concentration and the kind of alcohol have been measured for water-alcohol binary solutions. The value of fR was found to be proportional to the ethanol molar fraction (xE) at xE's more than 0.15, but remained constant at lower concentrations. This suggests that the structure of a binary solution changes at this concentration and the relative dielectric permittivity of the system is no longer affected by more alcohol added. This characteristic concentration agrees well with that obtained by the TDR method, as well as that by mass spectrometric measurements. From the measurements for alcohol-alcohol mixtures, a discontinuity of fR was found at a certain temperature for 1:1 methanol-2-propanol mixture.