Precipitation of perfluorocarboxylic acids (n-C3HTOOH, n-C7H~sCOOH, and n-C9F19COOH ) in aqueous solutions of different metal nitrates (K § Ag § Ca 2 § Ba 2 § , Zn 2 § AP § La ~*) was studied by solubility and calorimetric measurements. The free energy of precipitation per single surfactant chain was found to depend on the chain length but not on the charge of the reacting cation. It was shown that the precipitation of perfluorocarboxylates of multivalent cations was governed by positive entropy change and a low enthalpic contribution, whereas the soaps of monovalent cations exhibited the opposite behavior. The entropy of precipitation was not affected by the chain length of the surfactants in the presence of monovalent cations, but it increased for cations of higher charge.
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