Water fluctuation in methanol, ethanol, and 1-propanol aqueous-mixture probed by Brownian motion.

The origin of the driving force in Brownian motion is the collision between the colloidal particle and the molecules of the surrounding fluid. Therefore, Brownian motion contains information on the local solvent structures of the surrounding colloid. The mean square displacement in a water-ethanol mixture is greater than that anticipated from the macroscopic shear viscosity, indicating that the microscopic movement of Brownian motion involves the local information on the water-ethanol mixture on a molecular level, i.e., an inhomogeneity in the Brownian particle size (∼1 m). Here, the Brownian motion of mixtures of water and methanol, ethanol, and 1-propanol are systematically investigated. Similar discrepancies between the microscopic and macroscopic viscosities are observed at low alcohol molar concentrations, for all the alcohol mixtures. This means that inhomogeneity with water fluctuation is important in explanation of the unusual Brownian diffusions of alcohol aqueous solutions. The Brownian motion also reveals a thermal energy conversion mechanism between translation and rotation.