Colloids critical coagulation concentration

In a number of recent publications (1, 2) microcrystailine cellulose dispersions (MCC) have been used as models to study different aspects of the papermaking process, especially with regard to its stability. One of the central points in the well established DLVO theory of colloidal stability is the critical coagulation concentration (CCC). In practice, it represents the minimum salt concentration that causes rapid coagulation of a dispersion and is an intimate part of the theoretical framework of the DLVO theory (3). Kratohvil et al (A) have studied this aspect of the DLVO theory with MCC and given values for the CCC for many salts, cationic... 

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). 

TABLE 13.1 Critical Coagulation Concentration Values (in Moles Liter-1) for Mono-, Di-, Tri-, and Tetravalent Ions Acting on Both Positive and Negative Colloids (Numbers in Parentheses) and CCC Values Relative to the Value for Monovalent Electrolytes in the Same System (Numbers Outside Parentheses)3... 

Some other practical situations where particle aggregation is important include the precipitation of colloidal mud at the mouth of a river due to the salinity of the sea-water exceeding the critical coagulation concentration, land (e.g. mountainside) stability, building and road foundations, the retention of a porous structure in filtration, mineral processing117 and paper making. Control of particle aggregation is also of primary importance in adhesives, inks, pharmaceuticals, cosmetics, foodstuffs and lubricants. 

Fig. 6.8. Log-log plot of Wrap versus electrolyte concentration for hematite ( Fe203) colloids suspended in either CaCl2 or NaCI solution at pH 10.5. Arrows indicate critical coagulation concentrations [Eq. 6.76 data from L Liang, Effects of surface chemistry on kinetics of coagulation of submicron iron oxide particles (a-Fe2Oi) in water, Ph.D. dissertation, California Institute of Technology, Pasadena, CA, 1988. Environmental Quality Laboratory Report No. AC-5-88].

The critical coagulation concentration for a colloid suspended in an aqueous electrolyte solution is determined by the ions with a charge opposite in sign to that on the colloid and is proportional to an inverse power of the valence of the ions. 

LOG MOLAR CONCENTRATION FIGURE 10J29 Colloid stability ratio for different salt concentrations showing the critical coagulation concentration, CCC. Data from Barringer [25]. 

Hsu JP, Liu BT. Critical coagulation concentration of a colloidal suspension at high particle concentration. J Phys Chem B 1998 102 334-337. 

The ratio of the critical coagulation concentration value for mono, di- and tri-valent ions has been reported to be 500 10 1 as well as 100 20 1 [4]. Coagulation concentrations of various electrolytes for colloidal suspension of arsenic sulfide are listed in Table 5.26. 

Figure 4. Schematic variation of the DLVO theoretical stability domain for a pHiep colloid of pH 2 (critical coagulation concentration, c.c.c. isoelectric point, i.e.p.). The insert shows this theoretical prediction compared to that observed...

In the other limiting case of weakly charged colloidal particles y zeq>0 / 4kT (see Chapter III,3). In this case the critical coagulation concentration of electrolyte is a weaker function of counter-ion charge, given by the equation originally derived by Derjaguin ... 

---