Concentration Fluctuations near Lower Critical Solution Temperature in Ternary Aqueous Solutions

Abstract

The effects of osmolytes, such as trimethylamine N-oxide (TMAO), on the phase transition in an aqueous solution of polymer, protein, or DNA are very complex, and several phase transitions can occur. In order to explore such effects, we take a simple ternary system of poly(N,N-diethylacrylamide) and a favorable solvent pair TMAO and water to monitor the reduction in the miscibility of the polymer using small-angle neutron scattering (SANS) and dynamic light scattering (DLS). The result shows that the LCST of PDEA solutions is significantly depressed by TMAO. Although DLS data clearly show that aggregates can form as precursors in the homogeneous one-phase region depending on the TMAO concentration, after removing the low-q aggregate region, the three-component system still obeys the mean-field theory. Based on the ternary random phase approximation (RPA) theory, three Flory–Huggins interaction parameters, i.e., χD2O–TMAO, χPDEA–TMAO, and χPDEA–D2O, are obtained to reveal the microscopic origin of this shif...