Schulze-Hardy rule, coagulation

When foreign electrolytes which do not react with the soap are present, the ions responsible for bringing about coagulation are the ones with charges opposite in sign to those on the dispersed particle. The effect of the valence of the ion follows the Schulze-Hardy rule for the precipitation of sols by electrolytes wherein the coagulating power of an ion increases considerably with the increasing valence of the ion. 

Inert electrolytes, i.e., ions which are not specifically adsorbed, compress the double layer and thus reduce the stability of the colloids (Fig. 7.4). A critical coagulation concentration, Cs or ccc, can be defined (see Eqs. (4) and (5) in Table 7.3) which is independent of the concentration of the colloids (Schulze-Hardy Rule). 

Under what conditions are colloids stable Explain qualitatively (with schematic diagrams) the forces between colloidal particles. How does the force of repulsion between them vary with concentration As the concentration of the colloid increases, there is the tendency to coagulate and in fact the critical concentration for coagulation gets less as the valence of the ions present increases (Schulze-Hardy rule). Give a detailed, although qualitative, rationalization of this law. (Bockris)... 

Critical coagulation concentrations - Schulze-Hardy rule... 

The critical coagulation concentration (c.c.c.) of an indifferent (inert) electrolyte (i.e. the concentration of the electrolyte which is just sufficient to coagulate a lyophobic sol to an arbitrarily defined extent in an arbitrarily chosen time) shows considerable dependence upon the charge number of its counter-ions. In contrast, it is practically independent of the specific character of the various ions, the charge number of the co-ions and the concentration of the sol, and only moderately dependent on the nature of the sol. These generalisations are illustrated in Table 8.1, and are known as the Schulze-Hardy rule. 

In the early work of Schulze ( 0, Linder and Picton (2) and Hardy (3) the sensitivity of colloidal dispersions to the addition of electrolytes was clearly demonstrated. Then in 1900 Hardy (4) showed that the stability of sols was connected with the electrophoretic mobility of the particles and he demonstrated, i) that the valency of the ion opposite in charge to that of the sol particles determined the ability of an electrolyte to coagulate a sol and that, ii) the effectiveness of the electrolyte increased rapidly with increase in valency of the counter-ion. These observations formed the basis of the so-called Schulze-Hardy rule. 

The transition from stable dispersion to aggregation usually occurs over a fairly small range of electrolyte concentration. This makes it possible to determine aggregation concentrations, often referred to as critical coagulation concentrations (CCC). The Schulze-Hardy rule summarizes the general tendency of the CCC to vary inversely with the sixth power of the counter-ion charge number (for indifferent electrolyte). 

The transitions from stable dispersion to aggregation just described in terms of the critical coagulation concentrations and the Schulze-Hardy rule, apply best to suspensions in which the particles have only one kind of charge. However, clay particles can carry positive and negative charges at the same time, on different parts of the particle. See Section 5.6.2. 

If an emulsion is stabilized by electrical repulsive forces, then demulsification could be brought about by overcoming or reducing these forces. In this context the addition of electrolyte to an O/W emulsion could be used to achieve the critical coagulation concentration, in accord with the Schulze-Hardy rule. 

I. M. Metcalfe and T. W. Healy, Charge-regulation modeling of the Schulze-Hardy rule and related coagulation effects, Faraday Discuss. Chem. Soc. 90 335 (1990). 

Schulze-Hardy rule (for coagulation of colloids) 1.5.67, 1.6.83, 3.129ff... 

Schulze first showed that inorganic colloids are especially sensitive to electrolytes of high charge, and Hardy pointed out that their stability is closely related to mobility in an electric field. The Schulze-Hardy rule states that the sensitivity of lyophobic colloids to coagulating electrolytes is governed by the charge of the ion opposite that of the colloid and that the sensitivity increases more rapidly than the charge of the ion. 

Critical Coagulation Concentration (CCC) The electrolyte concentration that marks the onset of coagulation. The CCC is very system-specific, although the variation in CCC with electrolyte composition has been empirically generalized. See also Schulze-Hardy Rule. 

Schulze-Hardy Rule An empirical rule summarizing the general tendency of the critical coagulation concentration (CCC) of an emulsion or other dispersion to vary inversely with the sixth power of the counterion charge number of added electrolyte. See also Critical Coagulation Concentration. 

The minimum concentration of ions needed to produce fast coagulation is called the critical coagulation concentration (c.c.c.). The c.c.c. values usually depend strongly on the counterion charge (Schulze-Hardy rule)238. ... 

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