I

aHjO/fl CLAY

Figure 4-24. Lattice expansion of Na Wyoming bentonite with varying water contents. The dashed line represents the relationship which holds if the lattice expansion is continuous throughout the range of water contents. (From Might et a/.13 Courtesy of Journal of Chemical Physics.)

Figure 4-25. Schematic representation of edge-to-face bonds. (From Norrish and Rausell-Colombo44 Courtesy of Pergammon Press Ltd.)

forces, but not uniform alignment throughout the suspension. Neighboring platelets may flex according to their relative positions and the relative magnitude of their surface and edge potentials. Thus, when the edges are positive, the platelets will flex toward a negative basal surface, as shown in Figure 4-25.44 When the edges are negative, the stronger basal repulsive potential will cause the platelets to align parallel, when not prevented from doing so by mechanical interference. Addition of a thinner reverses positive edge potentials, and increases the repulsive forces between the edges.

it is also to be expected that the layers of crystalline water on the basal surfaces will tend to maintain parallel orientation of the platelets, the only question being to what distance from the surface the water structure is effective.

Clay Mineralogy and Colloid Chemistry 175 ---N----

Clay Mineralogy and Colloid Chemistry 175 ---N----

Figure 4-26. Schematic representation of the structure of clay gel: a—before freezing; b—after freezing; N—ice crystallization nuclei; B—edge-to-face bonds. (From Norrish and Rausell-Colombo. Courtesy of Clay Minerals Group, Mineralogical Society.)

Attempts have been made from time to time to deduce gel structure by flash-freezing a gel, and then drying it under a vacuum. The process leaves a skeleton framework, presumed to be that of the original gel.45 The validity of this assumption is dubious, because X-ray diffraction measurements, made while the gel was being flash frozen in the diffractometer, have shown that the inter-platelet spacing contracts at the moment of freezing,46 forming large pores, as shown in Figure 4-26.

Polymers

The polymers discussed in this chapter are organic colloids. They are com posed of unit cells (monomers) such as the cellulose cell shown in Figure 4-27, linked together either in straight or branched chains to form macromolecules A single macromolecule may contain hundreds, even thousands, of unit cells, and thus is well within the colloidal size range.

Polymers are used in drilling muds when the desired properties cannot be obtained with colloidal clays. For instance, starch, which was the first colloid used in drilling muds, was introduced to provide filtration control in salt-water muds'4 because starch is stable in salt water, whereas clays are not.

Starch is a natural polymer, but most polymers used in muds are synthetic. Numerous synthetic polymers have been developed for various purposes, and new ones are constantly being introduced. Only the main types and the principles governing their behavior will be discussed in this chapter. Further details may be found in Chapter 11 and in two reviews in the technical literature.4ii w

Synthetic polymers are often made by modifying natural polymers. For instance, carboxymethylcellulose (CMC) is made by reacting cellulose uith chloracetic acid and NaOH, substituting CH3COO Na+ for H, as shown in Fit; ure 4-28. Note that there are three OH groups on each cellulose unit, each capable of substitution. The term degree of substitution (DS) refers to the average number of carboxy groups on the chain per unit cell.

The carboxy group has two important functions: first, it imparts water solubil ity (strictly speaking, water dispersibility) to the otherwise insoluble cellulose polymer. Secondly, dissociation of Naf creates negative sites along the chain. Mutual repulsion between the charges causes the randomly coiled chains to

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