Coulombic and stereochemical factors of colloid stability of precipitating systems

Abstract

Applying the method of continuous variation of concentration of precipitating components the typical precipitation bodies (PB)(as a plot of logarithm of concentration of the cationic component against anionic one) for various precipitates are given. The boundaries of PB (precipitating/non-precipitating limits) reflect the existence of species controlling the transition from liquidus structures to solidus phase (genotypical processes), and also phenotypical factors influencing the formation of sub-systems. The main types of PB are named according to the dominant processes of transition of solute species in the solid phase : (A) neutralization; (B) ionic solubility; (C) formation of ionpairs and associates; and (D) unsymmetrical ionic reactivity. Examples of pure and mixed types of PB are given. For an explanation of internal features PB, the precipitating systems are treated as composed of five sub-systems : (i) formation of complex species (simple and monomers, as well as polynuclears and polymers); (ii) embryonation (aggregates without core formation); (iii) nucleation and direct growth (the growing core); (iv) micellation (formation of primary particles with colloid individuality); and (v) secondary aggregation (secondary structures of micro- and macro-particles). It is assumed that the transition from ideal liquidus structures (ILS) to ideal solidus structures (ISS) usually proceeds through stages characterized by the development of methorical structures and textures (MS & T). A change of the shape of PB with the change of medium is demonstrated on precipitating system of barium sulphate in various water + ethanol solutions. With respect to the flocculation-stabilization effects of large ions, non-ionic and ionic macromolecules on sols in statu nascendi, and preformed sols, the adsorption capacity, the electrokinetic potential, and the exchange fraction of the sols are correlated with the micelle formation (surface tension) of the surface active substances (SAS). In general, the ways and means of characterization of precipitating system and sub-systems, with special emphasis on factors of colloid stability, are suggested and discussed.