Static and dynamic light scattering studies of critical phenomena. I. Upper and lower consolute points of 3-methylpyridine–D2O

Abstract

To further probe the ’’universality’’ ideas of critical phenomena, the turbidity, light scattering intensity, and Rayleigh linewidth have been measured for the 3-methylpyridine–D2O binary liquid system in the region of both the entropy-driven lower (LCST) and the enthalpy-driven upper (UCST) critical solution temperatures. The LCST was found to be 38.240 °C at XD2O=0.923 and the UCST was 115.440 °C at XD2O=0.9005, both under a nitrogen atmosphere in a sealed cell. In addition to being the first complete light scattering study of two critical points in a single system, the UCST is the highest temperature attempted for such investigations and the LCST study is only the second complete LCST study that has been reported. For the LCST mixture, γ=1.27±0.03 and ν=0.65±0.03; for the UCST system, γ=1.27±0.03 and ν=0.61±0.02. Some deviations from OZ behavior were noted which are related to multiple scattering effects. The critical linewidth data, ranging from the hydrodynamic region (Kξ≪1) to the critical region (Kξ≫1), were compared to the original Kawasaki expression. Deviations similar to those previously reported for other critical systems were observed which resulted from the constant viscosity assumption. Kawasaki viscosities η* at given ΔT values were determined semiquantitatively to infer the different nature of the viscosity anomaly in the UCST and LCST regions. Analysis of the linewidth data also yielded the ’’apparent’’ exponent ν* describing the behavior of the diffusion coefficient. For the LCST system, ν* =0.61±02; for the UCST system, ν*=0.62±02.