Determination of Local Structure and Moving Unit Formed in Binary Solution of t-Butyl Alcohol and Water

Abstract

Abstract The concentration dependences of the mutual diffusion coefficients at 24, 37, and 63 °C were observed for a binary solution of t-butyl alcohol (TBA) and water by the use of the light scattering method. The observed concentration dependences of the mutual diffusion coefficients were explained well by postulating the existence of a “moving unit”–a group of molecules which move together for a time much longer than the velocity correlation time. The structure of the moving units which are formed in the solution are (H2O)11TBA at 24 °C and (H2O)20TBA at 63 °C in the concentration range of 0<xTBA<0.1, where xTBA is the mole fraction of TBA. Taking into account the local structures, which had been determined from the concentration dependences of the mean-square concentration fluctuation values, we could conclude that water molecules form a polyhedron which encages a TBA molecule and that these cages then gather together to form an aggregate. It is essential that these polyhedra do not share their faces with each other in the aggregate. It was also concluded that the number of cages which form an aggregate increases with an increase in the temperature. This suggests the existence of a pseudo-critical temperature for this system. That is to say, the TBA–water solution can separate into two phases under high pressures, a TBA-rich phase and a water-rich phase, in the latter of which almost all the molecules form polyhedra.