Pectic substances—water. II. Thermodynamics of water vapor sorption

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Abstract
The differential thermodynamic functions of water vapor sorption for the different forms of polygalacturonide chains indicate that in both the acid and sodium salt forms the swelling of polymer during vapor sorption opens new polar groups for further adsorption. In the highly esterified pectin no new polar group will be available through swelling, indicating that the methayl group shields the carboxylic oxygens from hydrogen bonding with water. The differential heat and entropy functions closely follow each other in the investigated systems. The high negative differential entropy values are explained by the contribution of the solid polymer which shows increasing order as indicated by an increase in crystallinity during the water vapor sorption. The mechanisms of water vapor sorption on the different forms of polygalacturonide chain were deduced from the differential thermodynamic functions. The constants of the B.E.T. and of the Hailwood and Horrobin theories of adsorption isotherms are in agreement with the proposed mechanisms.