INTERPARTICLE ASSOCIATIONS

Across any interface separating two phases there is, in general, a difference in potential. This potential difference may be pictured as an electrical double layer. One phase acquires a net negative charge and one acquires a net positive charge. Such double layers exist not only at plane surfaces but also around solid particles, such as clays, suspended in a liquid medium. Stern (in van Olphen38) proposed that the charge on the solid is rigidly fixed and on top of this there may be a practically immobile layer of oppositely charged particles of liquid. Further out from the surface there is a diffuse layer of charge, which may have a sign either the same as or opposite to that of the adsorbed layer. This diffuse layer is free to move, for example under the influence of an applied potential difference. The potential drop in the diffuse layer is known as the zeta potential68 (Figure 6).

The types of aggregates that form in clay suspensions result from the state of balance between:

1. van der Waals attractive forces which are responsible for the attraction between clay platelets. If these forces dominate then flocculation may occur, however, interparticle forces are inversely proportional to the seventh power of the distance. These forces are essentially unaffected by electrolyte concentration.

2. Electrostatic repulsive forces that exist between the clay double layers which have negative charges as a result of isomorphous substitution. These forces decrease with increasing electrolyte concentration.

Therefore, changes of the physico-chemical conditions of the system result in a change in the balance of these forces, which result in changes in the type of aggregate69. Among different chemical factors, the kind and concentration of electrolyte and the pH exert the

Clay Particle

Electrical Potential

Stern Layer [0.5 - 0.6 nm]

Outer Surface of Stern Layer a

Gouy Layer

Shearing Surface (Zeta Potential] Distance to Clay Particle Surface

d _x

b

Double Layer Thickness Stern Potential

Surface Potential

Figure 6. Showing the Stern-Gouy electrical double layer, (a) Charge

distribution as related to distance, (b) potential distribution as related to distance. The x scale from the origin to d is exaggerated

approximately 3 times for clarity.

2.6.2 Interparticle Associations in Clay Suspensions.

The significance of the exchangeable cation has already been discussed (Section 2.3.5). However, these cations are also primarily responsible for the stability of clay suspensions, van Olphen71 carried out rheological studies of homo-ionic suspensions of sodium montmorillonite and showed significant changes in suspension viscosity as a result of increasing NaCl concentrations. The changes result from deflocculation72 and aggregation73. For smectite platelets in aqueous suspension there are three possible modes of particle association (Figure 7)74.

1. Association between an edge surface and a siloxane planar surface, edge-face (E-F). 2. Association between edge surfaces of neighbouring particles, edge-edge (E-E). 3. Association between siloxane planes of two parallel platelets, face-face (F-F).

Edge-Face Edge-Edge Face-Face

F-F

E-F E-E

Figure 7. Showing the three basic modes of particle association.

In highly dilute suspension only a few floes form which do not have sufficient mass to sediment out of solution. But, as the electrolyte concentration increases, the double layers at both the negative basal surface and positive edge sites are compressed and at the critical flocculation concentration (CFC) both E-F and E-E associations can occur. - As the electrolyte concentration increases further the F-F associations can occur and ‘oriented aggregates’ are formed75. These aggregates have sufficient mass to readily sediment out of solution and flocculation is observed.

In summary, the interparticle associations give rise to three different suspensions:

1. Deflocculated Suspensions.

An overall repulsive force between clay platelets results in particles being well dispersed or peptised (Figure 8a). If each particle is given a similar charge the suspension can be made to peptise.

\

\

\ / /

/

4

\

b. Aggregated and deflocculated

d. E-F Flocculated and dispersed

f. E-F Flocculated and aggregated

'VV'V

g. E-E and F-F Flocculated and aggregated a. Dispersed and deflocculated

c. E-E Flocculated and dispersed

2. Flocculated Suspensions.

E-E (Figure 8c) and E-F (Figure 8d) bonds form as a result of a net attractive force between the clay platelets. ‘Floes’ form in dilute suspensions. A rigid ‘card-house’ structure forms in concentrated suspensions (Figure 8d).

3. Aggregated Suspensions.

A dominance of F-F associations, as a result of clay plates stacking one upon the other, leads to an aggregated system. The stacked sheets may be disaggregated by mechanical sheer forces or further hydration. The aggregates themselves may form E-E (Figure 8e) and F-F type (Figure 8f) associations and be deflocculated (Figure

8b) or flocculated (Figure 8e, f and g).

2.6.2.1 Controlling Flocculation o f Clay Suspensions.

Flocculation can also be controlled by the addition of polymers and certain organic cations and by controlling the nature of the exchange cation associated with the clay. As discussed above (Section 2.3.5), predominantly polyvalent cation exchanged clays are more flocculated because of the association between the cation and more than one exchange site on more than one clay platelet. Hence, to deflocculate a polyvalent cation exchanged clay, the resident cations must be replaced by monovalent ones. It has been shown that tactoid sizes (with the number of plates per tactoid relative to Li+ exchanged montmorillonite), increased in the order Li+ < Na+ < K+ < Mg2+ < Ca2+, varying from 1.5 for Na+ to 6.1 for Ca2+ exchanged montmorillonite, i.e. a suspension of Ca2+ exchanged montmorillonite will be more flocculated than a Li+ exchanged montmorillonite suspension76.

2.6.2.1.1 Face Charge Reversal

Upon the addition of certain organic cations, e.g. long chain quaternary ammonium salts, the double layer of the clay platelet is suppressed leading to flocculation in the system.

Upon further addition, the hydrocarbon chains of the adsorbed cation, which are pointing towards the solution, are attracted by van der Waals forces to the hydrocarbon chains of the newly added salt. Hence the cationic groups of the second layer now point towards the solution and a positive particle is formed77. This is not an economically practical method of reversing the net negative charge on the face of clay platelets since large quantities of cations are required.

2.6.2.1.2 Edge Charge Reversal

A more economically viable option of controlling flocculation is the addition of small amount of salts containing large polyanions, such as polyacrylate or silicate. These anions are chemisorbed at the positive sites on the edges of broken octahedral sheets thus making the edge sites negatively charged. These sites represent only a small percentage of the total surface area and so only a small quantity of salt is required.

An alternative method of edge charge reversal is to raise the pH of the clay suspension above pH 8.0. This will increase the number of negative silic acid groups on the platelet edges.