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The enthalpy of dissociation of poly(acrylic acid) and of poly(methacrylic acid) in water and in 0.5N NaCl at 25°C has been measured over a wide range of degrees of neutralization of the polyelectrolytes. In the case of poly(methacrylic acid) the calorimetric data permit the direct evaluation of the enthalpy of conformational transition of the polymer. For this transition, with the aid of standard free energy data derived from potentiometric titrations, the change in entropy was also estimated. The relative accuracy of the thermodynamic data, and the possibility of deriving therefrom information on the mechanism of transitions of the type, globular coils → expanded coils for partially hydrophobic synthetic polyelectrolytes in aqueous solution are discussed.

The interaction of Cs+ ions with c-carrageenan in aqueous solution was studied by using spectroscopic (133Cs NMR, optical rotation, and turbidimetry) and thermodynamic (isothermal microcalorimetry and dilatometry) techniques. All data collected seem to confirm the previous evidence of the occurrence of a conformational transition, induced by temperature and/or ionic strength. From NMR and calorimetry data the relative amount of species in equilibrium (disaccharide repeat units present as disordered coil, single helices, and dimers of single helices) is computed. Comparison of calorimetric and dilatometric data with theoretical predictions allows one to discriminate between overall and purely electrostatic contributions to the binding of Cs+ ions by x-carrageenan.

The effect on the conformational-aggregational properties of pectate in aqueous solution brought by the addition of specific ions (H+, Ca 2 +, Cu 2 +) was studied by osmometric, microcalorimetric, dilatometric, and rheological methods. Evidence is provided for the intramolecular nature of the pH induced conformational transition. The addition of divalent ions brings about at the same time a conformational change of the chain of pectate and chain--chain association.The study of the interaction of pectate (and its partially esterified "derivatives", pectins) with specific ions in aqueous solutions is of fundamental importance to understand the properties of their solutions and gels at the molecular level. Indeed, pectate aqueous solutions in the presence of ions have been subjected to many investigations (1-5). However, only in a few cases has the "course" of the interaction been followed over a wide range of the ion to polymer molar ratio, R. The scrutiny of such a dependence for thermodynamic functions like ΔΗ has allowed us to disclose an intramolecular coopera tive conformational transition of pectate upon changing pH (5). Other properties investigated were consistent with the above interpretation. It was also suggested that the intramolecular conformational transition was a prerequisite for the further aggregation of chain mo lecules occurring in the gelling conditions at low pH. Direct evidence from molecular weight measurements will be provided here in favor of such a transition.Among the other parameters able to induce similar macroscopic effects, the addition of divalent cations is particularly effective. Literature data are in almost complete agreement with the view that an aggregation of chains induced by ions is responsible for the gel forma-

SynopsisOn the basis of measurements of enthalpy of dissociation and of dilution, an intramolecular conformational transition induced by pH change is shown for pectic acid in aqueous solution. Additional evidence is given by potentiometic, viscometric, and chiroptical results. The transition from a more rigid, probably H-bonded, structure prevailing at low pH to a more extended one at around neutrality is accompanied by a AH value of about 500 cal/equiv and a AS value of 1.6 cal/equiv K in water at 25OC. The addition of salts increases the stability of the rigid conformation without changing the general features of the phenomenon. Dilatometric measurements suggest that the transition is accompanied by practically no change in the overall solvation of the polymer chain.