The thermodynamic characteristics of the conformational transitions of native xanthan

Xanthan shows a well-known conformational transition in salt free solution at slightly elevated temperature, upon charging. Recently, a second transition was reported at higher temperature, when potentiometric results were analyzed using a Henderson-Hasselbalch representation of the titration curves. The molar fractions of the xanthan monomers in the different conformational states A, B, and C could then be determined as a function of the degree of proton dissociation, θ. Here it is shown that a simple linear dependence of the standard free enthalpy changes (ΔG°) of the transitions on θ adequately describes the changing molar fractions throughout the titration curves. From a temperature series of the first transition (A → B), the dependence of the changes of standard enthalpy and entropy on θ ΔH°(θ), and ΔS°(θ) are inferred. Surprisingly, irrespective of the degree of dissociation, the A → B transition is dominated by the change of standard entropy of the transition. Beyond θ ≈ 0.3 the B state is entropy stabilized relative to the A state. The dependence of the thermally induced transitions on the polymer charge is illustrated with viscosity data.