Concentration fluctuations and cluster dynamics of 2-butoxyethanol-water mixtures by small-angle neutron scattering and neutron spin echo techniques

The Ornstein-Zernike (OZ) correlation length and cluster dynamics of 2-butoxyethanol (BE: C4H9OC2H4OD)-heavy water mixtures have been measured at mole fractions of BE, xBE=0.040, 0.065, and 0.090 by using small-angle neutron scattering (SANS) and neutron spin echo (NSE) techniques at 8-38 °C, below a lower critical solution temperature (LCST) of 39 °C where the BE-water mixtures undergo phase separation. The SANS data have shown that with increasing temperature, both concentration fluctuations and correlation length of the BE-water mixtures increase rapidly. This indicates that molecular aggregation progresses toward phase separation. The values of the diffusion coefficient obtained from NSE are about 30 times smaller than that of pure water previously obtained with a tracer method, suggesting that NSE investigates cluster dynamics, i.e., the collective motion of molecular aggregation in the mixtures. The diffusion coefficient increases with increasing temperature in spite of the enhancement of the molecular aggregation. It is concluded from both SANS and NSE results that the formation of the molecular aggregation in the mixture is transitory with nano-second lifetime.