Second-order transition in Rochelle-salt saturated solutions

Abstract

A sui generis second-order transition in water solutions of Rochelle-salt crystal has been seen during measurements of both the solubility and solution densities versus temperature. Both the volumeexpansion coefficients and the temperature differential of solution densities for several compositions along the solubility curve present a jump discontinuity around a saturation temperature of 24'C. Electrical measurements revealed a small peak in the dielectric constant, showing a minute ferroelectric component of solute near 24'C in solution. The loss tangents of solution and the dissolution enthalpy (drawn from the solubility curve) show other characteristics. The excess volume of solute vanishes around 31'C, showing an "ideal" behavior of solution in this region. All data seem to indicate the tartrate molecule NaKC4H406 associates with the four water molecules in solution, just like the water molecules undergoing crystallization. This quasicrystalline structure of the solute is stable in solutions up to about 40'C.A tentative interpretation as a critical phenomenon is made for the volume-expansion coefficient. This response function with a cusplike singularity at 24 C has a critical point exponent of about -,. A oneto-one correspondence within four temperature ranges (between 16 -18'C and 58'C), of the properties of the crystal and solute Rochelle salt, has been experimentally proved in five types of measurements.