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°C. Mean square concentration fluctuations and Kirkwood–Buff parameters of the mixtures in a range of 2-propanol mole fraction of 0.0≤x2pr≤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 x2pr<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≤x2pr≤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 x2pr≥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.