Rotational behavior of fluorescent probe molecules near the critical point of cyclohexane + methanol binary mixtures

We report measurements of the emission anisotropy ratio of fluorescent probe molecules dispersed in a critical binary mixture of cyclohexane + methanol near its critical mixing point. Steady-state fluorescent depolarization measurements show that the fluorescent anisotropy ratio, which is related to a rotational diffusivity of the fluorescent molecules, tends to increase to a maximum value as a function of temperature when the critical point is approached. The effective increase in the fluorescent anisotropy ratio has been discussed in terms of singly and doubly scattered light due to order parameter fluctuations activated thermally near the critical point. A small but noticeable possible decay of the rotational diffusivity has been detected through measurements of the fluorescent anisotropy ratios.