Flory temperature and upper critical solution temperature of gelatin solutions

The Flory temperatures (θ) measured by turbidity experiments performed on gelatin solutions were found to be 12 ± 0.3, 13 ± 0.3, 14 ± 0.3, 14.5 ± 0.3, and 15 ± 0.3 °C for salt concentrations 0.1, 0.075, 0.05, 0.025, and 0 M (NaCl), respectively. Estimated persistence length (l_p) of this weakly charged polyelectrolyte could be deduced from the Benoit and Doty (J. Phys. Chem. 1953, 57, 958) relationship with the approximation that this biopolymer assumes a compact near-globular shape at Flory temperature, implying l_p = 9(R_h)²/(5L_m), where L_m is the contour length and R_h is the hydrodynamic radius. It was found that l_p ≈ 2.2 ± 0.2 nm at room temperature (20 °C), invariant of salt concentration. The Flory expansion factor (α = R_h(T)/R_h(θ) = 1.5±0.2) was found to be almost constant. θ-Composition for this biopolymer was deduced from turbidimitric titration of aqueous gelatin solutions with the alcohols methanol, ethanol, 2-propanol, and tert-butyl alcohol. It appears that hydrophobic interactions play a crucial role in causing chain collapse at θ-temperature and composition.