Potentiometric Study on the Interactions between Divalent Cations and Sodium Carboxylates in Aqueous Solution - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 24

20], and electron spin resonance spectroscopy [21], self-diffusion measurements [22],
ultra¿ ltration [23], ellipsometry [13], and time-resolved À uorescence quenching [24].
Although these systems have been widely studied and a number of reviews have
been reported [25-29], there is not much information about the formation and structure
of the resulting aggregates, which complicates the development of models of such
systems in solution.
The long chain carboxylates of divalent metal ions (metal soaps) are an important
group of compounds, which ¿ nd applications as emulsi¿ers, paint driers, grease thick-
eners, dispersant agents, and so on [30-33]. They are also used in solvent extraction
procedures [34], and may ¿ nd interesting material applications in metal organic meso-
gen systems [35, 36]. They are in general insoluble in water, but dissolve in a variety
of organic solvents.
Of particular importance is the interaction of divalent metal ions, such as
calcium(II) and lead(II) with carboxylates. Lead and calcium are ions of high interest
due to their industrial, environmental, and biological importance, and the carboxylates
of these ions play an important role on, for example, lipid organization in human hair
[37] or in aged traditional oil paint [38]. In addition, the two ions have contrasting
complexation behavior, with the chemically “harder” [39] calcium(II) forming a more
ionic bond with the carboxylate group.
In this chapter, the effect of divalent cations, Pb 2+ and Ca 2+ , and carboxylate anions
with different alkyl chain lengths (octanoate, decanoate, and dodecanoate) on solution
properties, at 25ºC, is studied in water using conductimetric and potentiometric tech-
niques. The effect of metal ion hydration on the interaction process is also analyzed.
With the present study, we feel that we can contribute to a deeper understanding of
the solution structure of these salts which will be valuable in the development of new
potential applications for these systems.
The effect of the divalent cations Pb 2+ and Ca 2+ on the properties of carboxylates
with different alkyl chain lengths (octanoate, decanoate, and dodecanoate) in aqueous
solution at 25ºC, is evaluated using conductimetric and potentiometric data. Electrical
conductance data show strong interaction for concentration ratio (carboxylate divalent
ion) up to around two, suggesting an interaction stoichiometry of 2:1. With lead(II),
the concentration of free metal ion in solution has also been determined by using a
selective electrode, and stability constants are proposed. In the presence of lead ions,
the length of the surfactant alkyl chain has a direct inÀ uence in the hydrolysis process,
followed by pH measurements, and this effect becomes more pronounced upon in-
creasing the number of methylene groups of the carboxylate .
3.2 EXPERIMENTAL
Lead (98%) and calcium nitrate tetrahydrates were purchased from Panreac Quimica
SA and Riedel-de Haën, respectively. Sodium octanoate (99%), sodium decanoate
(98%), and sodium dodecanoate (99-100%) were purchased from Sigma. These re-
agents were used as received, and all solutions were prepared using Millipore-Q water.
No control was made on the pH, which was the natural value for each solution.
Electrical conductance measurements were carried out with a Wayne-Kerr mod-
el 4265 automatic LCR meter at 1 kHz, through the recording of solution electrical