Kinetics of crystal growth of α-PbF₂ and micellization of non-ionic surfactant Triton X-100 at steady-state condition

Abstract

A competition between the crystal growth rate of orthorhombic lead fluoride at 25°C and the micellization of non-ionic surfactant Triton X-100, due to iits adsorption/desorption and interaction with the lattiice ions, was investigated at steady-state conditions, using pF-stat method. The relatively low supersaturated, equimolar solutions of constant composition at isoelectric pH 5.2–5.3 were used for growth of well-defined seeds, in order to modify the kinetics of growth without affecting the anisotropic optical properties of these crystals.

The promotion of rate up to five times, in two steps, took place if Trition X-100 micellar solution is in the range between CMC and c ₁ (10 × CMC), which is characterized with the ratio between the activity of free fluoride ions and Triton X-100 concentration greater than 1. Above the c ₁ of Triton X-100 the inhibition of crystallization rate, two to three times, is evidenced, caused by the depletion effect, and the mentioned ratio is smaller tha 1. The main role in the proposed mechanism plays α-PbF₂ crystal surface with F⁻ active sites being attached by spherical (below c ₁) and above it by elongated bilayer chains of primary micelle aggregates. Thiis is confirmed by IR spectra of solids and was directly visualized by polarizing microscope examinations. Both of these effects are quantitatively expressed as a degree of promotion/inhibition, using the rates in the absence and presence of non-ionic surfactant. The pF-stat kinetic data were also analyzed in the sense of adsorption/desorption process through the adapted Langmuir adsorption isotherm. The mutual changes in Gibbs free energies of micellization and micellar bilayer formation, the adsorption/desorption (attachment) of Triton X-100 and crystal growth of α-PbF₂ are discussed.