Impurity effects in a near-critical binary-fluid mixture

Measurement of the coexistence curve of the binary-fluid mixture methanol-cyclohexane with increasing amounts of water impurity showed essentially a linear dependence of the critical temperature, critical exponent $\beta$, and critical concentration with impurity. Adding 0.10 vol% water impurity caused the critical temperature to increase by 3.8 K, the effective critical exponent $\beta$ to increase by 0.0021, and the critical concentration, as measured by volume fraction of methanol, to increase by 0.004. Surface wetting was also observed in the impure samples and observations are reported on one impure mixture.