Non-ionic surfactant adsorption on lead fluoride under dynamic steady-state conditions

Abstract

The adsorption of non-ionic surfactant Triton X-100 on the surface of lead fluoride microcrystals has been examined as a function of the mutual mass ratio of surfactant and α-PbF 2 crystals during their growth under dynamic steady-state conditions, i.e. using the potentiometric pF-stat method, known as the constant composition method, which means: constant supersaturation (constant activity of lattice ions during the crystal growth process); constant pH eq = 5.30; constant relatively low ionic strength I = 0.018 mol dm −3; temperature 25°C; the same method of preparation of the stable supersaturated solution; the same seed crystals and speed of mixing; constant Triton X-100 concentration above the cmc. The concentration of unadsorbed Triton X-100 in supersaturated solution, i.e. the adsorption isotherms, were determined by UV-visible spectrophotometry at different growth times. A very broad concentration region was examined from the cmc up to 33 × cmc. Two-step adsorption isotherms, i.e. LS-type, were obtained for all systems under dynamic steady-state conditions, which then led to the L-type isotherm in the same concentration region of non-ionic surfactant, when systems were allowed to stay for 1 day after running the growth experiment (batch thermodynamic equilibrium adsorption isotherm). Strong interactions between the terminal OH group of the Triton X-100 end molecule and F − ions at the lead fluoride crystal surface, in the region between cmc and about 10 × cmc ( c 1), led to the first step of the isotherms, and also to the one-step, L-type equilibrium isotherm afterwards. The second step of the LS-type isotherms, which appears in the region above c 1, is a consequence of the cooperative process between the surfactant's hydrophilic chains and/or of the hydrophobic parts of their molecules. The experimental data for the LS-type isotherms were fitted by a recently proposed equation [B.-Y. Zhu, T. Gu, Adv. Colloid Interface Sci. 37 (1990) 1] and the following parameters were estimted: maximum adsorbed amounts; hemimicelle concentrations; aggregation number of hemimicelles; adsorption constants k 1 and k 2 for both processes, taking place at first and second steps of the LS isotherms; the Gibbs free energy of hemimicellization at 25°C. The aggregation number of the hemimicelles was found to be around six and the free energy of hemimicellization was about −10 kJ mol −1.