The thermodynamic characteristics of the conformational transitions of native xanthan

✍ Scribed by Leon Bezemer; Maxim E. Kuil; Jaap C. Leyte

• Publisher: Elsevier Science
• Year: 1994
• Tongue: English
• Weight: 805 KB
• Volume: 263
• Category: Article
• ISSN: 0008-6215
• DOI: 10.1016/0008-6215(94)00171-5

✦ Synopsis

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, 0. Here it is shown that a simple linear dependence of the standard free enthalpy changes (AG") of the transitions on f3 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 0, dH*( 0), and AS*( 13) 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 0 = 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,