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However, in the case of sodium dodecanoate, interaction with Ca
2+
occurs from
the ¿ rst addition of carboxylate to the divalent ion solution. From these results we can
conclude that, in the case of Ca
2+
, the interaction with carboxylates is not only affected
by ionic interactions, but also by the interactions involving the hydrophobic chain of
the amphiphilic ion. The differences in behavior can be justi¿ ed by the higher charge
density of the hydrated ions of Pb
2+
when compared with Ca
2+
, because their radii
of hydration are 4.01 Å and 4.12 Å [44], respectively. In addition, crystallographic
evidence has been presented that the lead(II) carboxylate bond has appreciable cova-
lent character [45] here, as would be expected from it being a softer acid than Ca(II).
Another important point that cannot be neglected in the analysis of the experimental
results presented in Figure 1 is related with the possible inÀ uence of hydrolysis pro-
cesses in the interaction between the carboxylate and divalent metal ions [10].
Figure 2 shows the variation of pH in a titration of a 1 mM solution of divalent ion
with sodium carboxylates having different alkyl chain lengths. It is interesting to note
that in the case of solutions containing Ca
2+
, there is a pH increase with the addition of
octanoate and decanoate, following a behavior similar to what occurs in the absence of
divalent ions (Figure 3), and that the hydrolysis of alkanoates increases with increas-
ing the alkyl chain length. This behavior, which is consistent with that observed by
analysis of electrical conductivity measurements, suggests that in this concentration
range Ca
2+
/carboxylate interactions do not exist or are very weak.
The addition of C
11
COO
í
to solutions of Ca
2+
or Pb
2+
, initially leads to a pH de-
crease, which can be explained by the breakup of dimers/oligomers of carboxylates
following the interaction with divalent metals, with the consequent release of hydro-
nium ions [46]. As was observed in the conductivity measurements, the presence of
Pb
2+
leads to more signi¿ cant changes in the macroscopic physical-chemical proper-
ties of those mixed solutions.
FIGURE 2
(Continued)
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