Light-scattering studies of a ternary mixture: Comparison of field variables for critical phenomena description

We present static turbidity and dynamic light-scattering measurements near the critical double point of a mixture of secondary butyl alcohol, tertiary butyl alcohol, and water. At the critical double point of this mixture an upper two-phase miscibility loop merges with a lower two-phase region. When temperature is used as a field variable to describe the critical behavior of the turbidity and correlation length of the mixture, nonuniversal behavior is seen in both the critical exponents and amplitudes. If, however, the tBA concentration difference from the critical curve is used, universality is found in both exponents and amplitudes. We thus term this concentration difference to be the relevant parameter for proper description of the critical behavior and, following Griffiths and Wheeler [Phys. Rev. A 2, 1047 (1970) ], show this relevant parameter was determined by it never having tangential paths of approach to the critical curve.