Interaction of PbF2 and Triton X-100 Micellar Aggregates Studied by Potentiometric pF-Stat and Dynamic Light Scattering Method

Abstract

The kinetics and mechanism of crystal growth of α-lead fluoride at 25 °C in micellar solution of nonionic surfactant Triton X-100 was studied at dynamic steady-state condition using the pF-stat method. Dynamic light scattering (DLS) measurements were used to characterize the size of submicrometer particles of micelles and ionic microcrystals in these mixed colloidal systems. The supersaturated solution, with relatively low ionic strength, I = 0.018 M (mol dm-3), activity ratio of lattice ions a(Pb2+)/a(F-) = 1/2, and attained isoelectric pH 5.3 and two different aggregation states of micelles, was used for growth of well-defined α-PbF2 seeds. The change of the PbF2 growth rate with Triton X-100 concentration above its critical micellization concentration, cmc, up to about 30 times cmc, was a consequence of the changing the supersaturation due to interactions of the lattice ions with two differently packed Triton X-100 micellar aggregates being formed in solution below and above c1 (c1 ∼ 10cmc). With an increase of the concentration of Triton X-100 above cmc, first a promotion of crystal growth up to about five times was evidenced, followed by a leveling off or an inhibition of growth rate for two to three times at c1, and afterward a second increase of growth rate was determined from the potentiometric pF-stat data. The kinetic, potentiometric data obey the linear growth rate equation in both these two separate regions, which means that adsorption of the lattice ions modified by the interactions with surfactant self-organized assemblies is the rate-determining mechanism. The thermodynamic activity solubility product of lead fluoride in micellar solution of Triton X-100 above c1, at 298.15 K, is about three to five times lower than that in water; i.e., it is about (3 ± 2) × 10-9, as obtained by potentiometric titration. The two association constants of surfactant aggregates and lattice ions were evaluated, and the first one is about 7 ± 1 times greater than the second one, below and above c1. The kinetics of growth was also followed by measuring the change in crystallite size, expressed as a hydrodynamic diameter, determined by the DLS method, in time and with mass of PbF2 grown, respectively. The systems consisted of submicrometer and micrometer classes of crystals; i.e., bimodal distribution was preserved during the whole process. The apparent median hydrodynamic diameter of the Triton X-100 micelles in PbF2 supersaturated solution at c1 was ∼5.3 nm (at q = 0, and it was angular independent, with the polydispersity index dw/dn = 1.01, nearly monodisperse micelles). They increased with concentration up to a limiting value of ∼20 nm (dw/dn = 1.10), which was reached at about 10 times lower Triton X-100 concentration as compared to the same in pure water (salt effect).