Anomalous Hydrodynamic Radius of Polyethylene Glycol Molecules in Mixed Solvents Containing a Hydrotope

Anomalous hydrodynamic radius of polyethylene glycol molecules in mixed solvents containing a hydrotrope

Xiong Zheng^1,2, Mikhail A. Anisimov¹, Jan V. Sengers¹, and Mao-Gang He², (1) University of Maryland, College Park, MD, (2) Xi’an Jiaotong University, Xi’an, Shaanxi Province, PR China

Binary and ternary solutions of polyethylene glycol (PEG) in water, in a hydrotrope - tertiary butanol (TBA), and in TBA/water mixtures in a wide range of molecular weights (200-20,000 g/mol) have been investigated by using dynamic light scattering (DLS). The hydrodynamic radius of the PEG molecules in dilute aqueous solutions is in good agreement with the literature data. In ternary PEG/TBA/water solutions, the DLS correlation function reveals the presence of two relaxation modes: a relatively fast mode associated with the concentration fluctuations and a slow mode associated with the Brownian diffusion of the PEG molecules. We have found that the hydrodynamic radius of the PEG coils for all molecular weights studied exhibits an anomaly around the maximum correlation length of the concentration fluctuations. At this maximum, the radius is twice the value found in aqueous and TBA solutions. We interpret this phenomenon in terms of the Shultz-Flory theory as a result of interplay between preferential adsorption and nonideality of the TBA/water mixed solvent.