Chaleurs de dilution de solutions aqueuses d'acide polyméthacrylique et de ses sels de sodium et de rubidium

Heats of dilution of aqueous solutions of two polymethacrylic acid samples having a viscosity average molecular weight of 5000 (PMA-I) and 140 000 (PMA-II) respectively and of isobutyric acid (BA) have been measured at 25 °C using a Tian–Calvet microcalorimeter. The same measurements have been repeated in presence of HCl (0.2 N). Aqueous solutions of sodium and rubidium salts of PMA-I and II have also been investigated. Dilution of BA solutions is endothermic at high solute concentration and then becomes exothermic as concentration decreases and the presence of HCl does not affect the results. Dilution of PMA solutions is exothermic over the investigated concentration range in agreement with the results of Eliassaf and co-workers. PMA-I gives a more exothermic heat of dilution than PMA-II in the lower concentration range. The presence of HCl does not affect appreciably the results for PMA-II but it makes the dilution of PMA-I solutions less exothermic. The results are discussed in the light of Liquori et al.'s model for PMA chain which may adopt a globular or an extended solvated coil depending on solvent properties. Heats of dilution of aqueous solutions of sodium salts of both samples I and II are endothermic over the entire concentration range investigated whereas the heats of dilution of rubidium salts are exothermic.