Light scattering and viscosity studies of a ternary mixture with a double critical point

We have performed viscosity and dynamic light scattering measurements on the system 3-methylpyridine, water, and heavy water. This system displays a closed-loop coexistence curve which shrinks with increasing H2O/D2O ratio until the upper and lower consolute points merge at a double critical point. Our measurements were performed to study critical phenomena in this mixture in both the phase-separating and non-phase-separating regimes. We have found that the correlation length derived from our measurements diverges in both regimes with a power-law dependence on temperature relative to the double critical point temperature. The power-law exponent was double that usually found in ising-like liquid systems when the system was asymptotically far from the double critical point. Implications of this work for other non-phase-separating yet nonideal systems is discussed.