Thermodynamics of Conformational Transition and Chain Association of iota-carrageenan in Aqueous Solution: Calorimetric and Chirooptical Data

Isothermal microcalorimetry, differential scanning calorimetry (DSC), and chirooptical data obtained for iota-carrageenan in NaCl, LiCl, and NaI aqueous solutions are presented. The experiments have been performed as a function of concentration both for the polymer and for the simple salt as a cosolute. The experimental findings consistently show the occurrence of a salt-induced disorder-to-order transition. From microcalorimetric experiments the exothermic enthalpy of transition Delta H-tr is obtained as the difference between the theoretical, purely electrostatic Delta H-el enthalpy change and the actual mixing enthalpy Delta H-mix measured when a iota-carrageenan salt-free solution at constant polymer concentration is mired with a I:I electrolyte solution of variable concentration. IPL the case of added NaCl, the absolute values of enthalpy changes \Delta H-tr\ I are in good agreement with those obtained for the opposite process, at comparable polymer and salt concentrations, from DSC melting curves. The microcalorimetric results show that the negative maximum value of Delta H-tr corresponding to the interaction of Li+ counterion with iota-carrageenan polyion results to be significant lower than the corresponding values obtained for Na+ counterion. Ar variance with the microcalorimetric data, chirooptical results show that the salt-induced disorder-to-order transition, occurring in the 0.02-0.2M salt concentration range, appears to be complete at a concentration of about 0.08-0.1M of rite simple ion, ii-respective of the polymer concentration and of the nature of added electrolyte. (C) 1998 John Wiley & Sons, Inc.