LIGHT-SCATTERING INVESTIGATIONS OF THE LIQUID-LIQUID PHASE-TRANSITION OF THE IONIC SYSTEM - TRIMETHYLETHYL-AMMONIUM BROMIDE IN CHLOROFORM

Phase diagrams and light-scattering measurements of solutions of trimethylethyl-ammonium bromide in chloroform (epsilon=4.72 at 25 degrees C) with an ethanol content of 1% are reported. The system has a lower critical point near T=25 degrees C. The critical mole fraction is x(c)=0.0503+/-0.0002, which corresponds to the reduced variables T-c*=0.036 and C-c*=0.029 of the restricted primitive model (RPM) and is slightly below the values predicted by the RPM for the critical parameters. The analysis of the scattering intensity at critical composition gives nu=0.621+/-0.003 for the critical exponent of the correlation length xi with an amplitude of xi(0)=0.87+/-0.01 nm. The system, a solution of a salt of essentially spherical ions of almost equal size in a simple low-dielectric polar liquid, with critical parameters very close to predictions of the RPM, nevertheless has an Ising critical point.