MICROWAVE DIELECTRIC CHARACTERIZATION OF BINARY-MIXTURES OF WATER, METHANOL, AND ETHANOL

The complex dielectric constants of binary mixtures of water-methanol and water-ethanol in the frequency range from 45 MHz to 26.5 GHz, and binary mixtures of methanol-ethanol in the range from 200 MHz to 26.5 GKz have been measured with various volume fractions around room tempe rature by means of an open-ended coaxial sensor and a network analyzer . Methanol-ethanol mixtures display a near-Debye dispersion while wate r-alcohol mixtures show a Cole-Davidson dispersion. The logarithm of r elaxation time log tau and dielectric decrement Delta epsilon for meth anol-ethanol mixtures show a good linear relation with the volume frac tion of methanol, while log tau and Delta epsilon extracted with the D ebye function for water-alcohol mixtures display a near-linear relatio n with volume fraction of water. Two simple formulas are proposed for identifying the volume fractions of the components in binary mixtures of alcohol-alcohol and water-alcohol from a knowledge of tau and Delta epsilon for the pure liquids and the mixtures. The validity of these formulas has been demonstrated with three blind tests. The relation be tween the mole fraction of water and log tau for water-methanol and wa ter-ethanol mixtures extracted by the use of a Cole-Davidson function clearly shows two linear regions, which implies a change of relaxation mechanism with mole fraction. (C) 1996 American Institute of Physics.