engleski

Factors Influencing Nucleation from Solutions Supersaturated to Different Crystal Hydrates

The influence of the reactant concentrations, the stirring mode and some additives (serine, Ser, glutamic acid, Glu, ornithine, Orn, tryptophan, Trp, histidin, His, sodium dodecyl sulphate, SDS) on the mode of nucleation of calcium oxalate hydrates (the thermodynamically stable monoclinic monohydrate, COM, the metastable tetragonal dihydrate, COD, and triclinic trihydrate, COT) and of dodecylammonium nitrate, DDA on the formation of the hexagonal and cubic modifications of AgI has been investigated. To obtain the above information, changes in the induction times, the number of particles, the volume and composition of the precipitates have been considered. The change in the mechanism of nucleation from heterogeneous to homogeneous (achieved as a consequence of an increase in supersaturation) has a striking effect not only on the particle number and size distribution, but also on the hydration of the crystalline phase. The interfacial energies and radii of the homogeneous nuclei (as determined from the critical concentration for homogeneous nucleation) are σ(COD) = 123 ± 6 mJ m−2, r* (COD) = 0.56 nm, σ(COT) = 135 ± 6 mJ m−2 and r* (COT) = 0.54 nm respectively. For the heterogeneous nucleus σHET = 53 ± 5 mJ m−2 has been determined from induction periods. In the range of reactant concentrations where heterogeneous nucleation is dominant, the mode of agitation influenced the induction time and the type of the dominant nucleating phase. Introducing stirring promoted the formation of COT whereas in unstirred systems COM was preferentially formed. The addition of amino acids and/or surfactants also influenced the composition of the nascent precipitates. Ser, Glu, Orn promoted the formation of COM while in the presence of Trp, His and SDS the formation of COD was enhanced. DDA promoted crystallization of the cubic modification of AgI whereas without the surfactant the hexagonal form prevailed. The effect of the surfactants (SDS and DDA) was related to the respective c.m.c.

hydrates ; distribution

Rad je prezentiran na skupu Congress on Applications of New Trends in Colloid and Surfactant Science, održanom od 5.-8.6.1989., Torino, Italija.