Ludwig-Soret effect of poly(N-isopropylacrylamide):: Temperature dependence study in monohydric alcohols

We report the experimental results of the Ludwig-Soret effect for poly(N-isopropylacrylamide) (PNiPAM) in alcohols measured in the temperature range from 20 to 45 degrees C by means of thermal diffusion forced Rayleigh scattering (TDFRS). Alcohols, used in this study as solvent, are monohydric alcohols (methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and tert-butanol). In the studied temperature range, PNiPAM in methanol shows a positive Soret coefficient, whereas PNiPAM has a negative Soret coefficient in larger alcohols, i.e., 1-propanol, 2-propanol, 1-butanol, and tert-butanol. In ethanol the temperature dependence of the Soret coefficient of PNiPAM shows a sign change from positive to negative with increasing temperature at T = 34 degrees C. The temperature dependence of the Soret coefficients for these alcohol solutions is discussed in terms of cohesive energies such as the Hildebrand solubility parameter. It is indicated that the balance between the hydrogen-bonding capability and the hydrophobic association plays a dominant role for the sign change in the Soret coefficient and the thermal diffusion coefficient of PNiPAM.