A Rayleigh light scattering study on mixing states of 2-propanol-water binary mixtures widely used as mobile phase in separation.

Rayleigh light scatterings of 2-propanol-water binary mixtures at 546 nm have been measured by using a conventional fluorophotometer at 25 degrees C. Mean square concentration fluctuations and Kirkwood-Buff parameters of the mixtures in a range of 2-propanol mole fraction of 0.0</=x(2pr)</=0.4 (0-75% (v/v)) have been calculated from the scattering intensities. On the basis of these parameters, it has been concluded that at x(2pr)<0.05 (15% (v/v)) 2-propanol is well solvated by water molecules, and hydrates consisting of one 2-propanol molecule and 20-30 water molecules are formed in the mixtures. However, at 0.05</=x(2pr)</=0.3 (15-65% (v/v)) large clusters mainly composed of 2-propanol molecules are formed in the mixtures, and these clusters are further reduced into small 2-propanol aggregates at x(2pr)>/=0.4. The usage of 2-propanol as organic modifier and its effects on CE and RPLC separations are also discussed in terms of microheterogeneity in the mobile phase.