Concentration Fluctuations in the Vicinity of the Liquid/Liquid Coexistence Curve of a Binary Mixture with a Lower Critical Point

Experimental evidence is presented demonstrating that the slowing down of the diffusion process in binary liquid mixtures with a miscibility gap approaching the temperature of phase separation at constant noncritical composition and approaching the critical point along the liquid/liquid coexistence curve is caused by an increase of the correlation length of local composition fluctuations. This conclusion is drawn from dynamic light scattering data obtained with the system 2‐butoxyethanol/water in the vicinity of its lower critical point. The (hydrodynamic) correlation length is calculated from the diffusion coefficient data in combination with viscosity data taken from the literature using the analogue of the Stokes‐Einstein relation.