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

Autori
Stubičar, Nada ; Stubičar, Mirko ; Zadro, Krešo

Izvornik
Colloids and surfaces A : physicochemical and engineering aspects (0927-7757) 135 (1998), 1-3; 245-252

Vrsta, podvrsta i kategorija rada
Radovi u časopisima, članak, znanstveni

Ključne riječi
Adsorption under dynamic condition; Lead fluoride; Non-ionic surfactant (Triton X-100); Potentiometric pF-stat method.

Sažetak
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 alpha-PbF2 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/dm3; temperature 25 oC; 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 33xcmc. 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 10xcmc (c1), 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 c1, 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 estimated: maximum adsorbed amounts; hemimicelle concentrations; aggregation number of hemimicelles; adsorption constants k1 and k2 for both processes, taking place at first and second steps of the LS isotherms; the Gibbs free energy of hemimicellization at 25 0C. The aggregation number of the hemimicelles was found to be around six and the free energy of hemimicellization was about -10 kJ/mol.

Izvorni jezik
Engleski

Znanstvena područja
Fizika, Kemija

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