Negative tactoids

In the de-stabilization of lyophilic colloids when coa-cervation occurs, the dispersed phase can initially separate into small, anisotropic droplets having shapes such as cylinders, called tactoids . With concentrated colloids, droplets of dilute colloid can separate out within the concentrated colloid these droplets are sometimes referred to as negative tactoids . 

Fig. 21. Negative tactoids of isotropic solutions in the anisotropic phase of tobacco mosaic virus.

Fig. 23. Positive and negative tactoids in colloids with rodhke particles.

Dispersion-flocculation processes are generally controlled by double layer swelling, adsorbed hydrolyzed Fe or Al, and chemical bridging (tactoid formation) (Stumm and O Melia, 1968). Once dispersed, clay colloids are kept dispersed by repulsive double layers (Van Olphen, 1971). The force of repulsion is related to the thickness of the double layer (see Chapter 4). This dimension is represented by the ions concentrated near the oppositely charged colloid surface. Any colloid that has a net negative or a net positive charge repulses a like-charged colloid. 

D. G. Edwards, A. M. Posner, and J. P. Quirk, Repulsion of chloride ions by negatively charged clay surfaces. Part 2. Monovalent cation montmorillonites, Trans. Faraday Soc. 61 2816 (1966). Part 3. Di- and trivalent cation clays, Trans. Faraday Soc. 61 2820 (1966). L. L, Schramm and J.C.T. Kwak, Interactions in clay suspensions The distribution of ions in suspension and the influence of tactoid formation, Colloids Surfaces 3 43 (1982). 

The total clay structure is formed by hundreds of layered platelets stacked into particles or tactoids 8-10 /tm in diameter. The imbalance of the surface negative charges is compensated by exchangeable cations typically Na" and Ca and forms two different MMTs Sodium MMT(Na" MMT) and calcium MMT(Ca MMT), respectively (Azeredo, 2009). The MMT has a property that it undeigoes intercalation and swelling in the presence of water and organic cations. 

A second group of nanocomposites mentioned earlier is characterized by exhibiting negative deviations from the tortuous path model. These account for another significant number of composites. In these composites this negative deviation can be explained by one of two possible mechanisms. The first is the one mentioned in the previous example where tactoids predominate in the composite. In these cases the aspect ratio of the tactoids has not been determined but the data is compared to the tortuous path equation predictions utilizing the aspect ratio of the completely exfoliated clay. The solution in these cases is to utilize X-ray diffraction and electron microscopy to determine the true aspect ratio of the tactoids. This was done in the previous example. 

---