Tactoids

Ogata et al. (1997) first prepared PLA/ organoclay (OMMT) blends by dissolving the polymer in hot chloroform in the presence of dimethyl distearyl ammonium modified MMT (2Ci8MMT). XRD results show that the silicate layers forming the day could not be intercalated in the PLA/MMT blends, prepared by the solvent-cast method. Thus, the clay existed in the form of tactoids, consisting several stacked silicate monolayers. 

These tactoids are responsible for the particular geometrical structures formation in the blends, which leads to the formation of superstructures in the thickness of the blended film. The Young s modulus of the hybrid is increased by this kind of structural feature. After that, the preparation of intercalated PLA/ OMMT nano-composites with much improved mechanical and thermal properties was reported by Bandyopadhyay et al. (1999). 

The assumption that the water is adsorbed in uniform layers on all the clay surfaces for a wide range of mixtures has been criticized (2, 20). The argument is that the individual clay particles in the clay-water mixture do not expand beyond a certain distance regardless of the quantity of water which is added. The clay layers group themselves into tactoids resulting in two populations of water those molecules which are found between the tactoids and those directly perturbed by the clay layers. If true, this would invalidate the procedure used to calculate the thermodynamic properties of the adsorbed water. However, other workers have reported complete delamination of certain smectites (21., 22). It is not clear under what conditions tactoids will form, or not, and this uncertainty is underlined in (21) (see remarks by Nadeau and Fripiat, pages 146-147). 

T. Lan, T.J. Pinnavaia, Mechanism of clay tactoid exfoliation in epoxy-clay nanocomposites, Chem. Mater., vol. 7, pp. 2144-2150,1995. 

Rod-like crystals (Eig. 4.15 b) are formed from partly neutralized Fe solutions (0 < OH/Ee < 3) (Mackay, 1962 Atkinson et al., 1977 Paterson Tait, 1977). They are usually monodisperse, around 50 nm long, 6 nm wide and also elongated in the [010] direction. In concentrated suspensions, these rods associate to form tactoids,... 

Langmuir, . (1938) The role of the repulsive and attractive forces in the formation of tactoids, thixotropic gels, protein crystals and coacer-vates. J. Chem. Phys. 6 873-896... 

From macroscopic observations, it appears that in the DMDBTDMA-dodecane system the nature of the third phase (liquid, gel, or solid) (140) depends to a large extent on the extracted species. In some cases, microphase separations can be obtained, that is, the coexistence of a more crystalline phase with domains of diluted phase that do not separate upon centrifugation. In classical colloidal literature (141), this situation is described as a dispersion of tactoids in the form of small amounts of liquid crystals, giving macroscopically a gel. 

In terms of structure, the FF association leads to the build-up of successively larger stacks of partides called oriented aggregates, or tactoids [321-323], The EE and EF assodations produce floes and can lead to voluminous, three-dimensional assemblages, often described as card-house structures. Figure 1.7 provides an illustration of these modes of interaction. Descriptions of the details surrounding the transitions from one structure to another are given by van Olphen [1],... 

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 . 

In clay suspensions the thin sheet-like or plate-like particles can aggregate to form stacks of particles in face-to-face orientation, which are termed tactoids . 

Clays, and especially swelling clays, are made from anisotropic microcrystals displaying large specific surface areas. However, because these microcrystals have a strong tendency to aggregate into tactoids, only a fraction of the surface area is available for surface reactions. 

In addition, a tactoid containing n sheets in such a disordered state is separated from an adjacent tactoid containing n sheets by inter-tactoid pores. If two adjacent tactoids are tilted with respect to one another, very complicated architecture may exist within the suspension. The description of such architectures should be performed in terms of correlation functions relating the spaces occupied by solids or by voids. Such an analysis is difficult for anisotropic objects. 

Any information on these architectures would be, however, worthwhile because the availability of the surface is ruled to a large extent by the size of the tactoids, and by their mutual arrangements. 

Water Associated with the External Contour of Tactoids. The existence of tactoids can be evidenced by studying th longitudinal relaxation time of the nuclear magnetic resonance of H or H in aqueous slurries (7-8). In simple terms, the relaxation rate, e.g. the inverse of the relaxation time, is the sum of three terms... 

T 1, T 1 and T 1 are the proton (or deuteron) relaxation rates or the inter tacto id water (a), of the water at the external surface of the tactoid (5), and of the few layers ( 3) of water within the interlamellar space (i), respectively. The molar fractions... 

Figure 1. Computer simulation of the formation of clay tactoid by a process of aggregation of lCr particles. Ration length to thickness 9/1. In the three topmost diagrams each rectangle endorses a portion of the tactoid shown enlarged below, df is represented by the thickness of the line enclosing the sheets in contact. Adapted from ref. 6. N.B. With respect to the next section, the molar fraction xi consists of the water in the interlamellar space and in the closed spaces within the tactoid. The molar fraction xb is in a 10 A thick layer on the external contour of the tactoid.

T can be calculated (7). Therefore, it is possible to carry out relaxation time measurements in such a way as to obtain xfa, the volume fraction of the solvent bouijid to the contour surface of the tactoid. Indeed, in that case T. appears as a linear function of the content in solid (Figure 2) because x x l. If the interlayer water were taking part in the exchange (that s, if T T ) the slope (xfa + x.) of the linear relationship would be larger than that observed. On th other hand, if x was changing with the solid content, then T would not be linear with respect to the concentration in solid. 

Thus, a linear relationship means that in suspensions of smectites with clay/water (w/w) concentrations ranging from 0.1 to 10%, the external surface of a tactoid and the number of water layers on this surface are constant in first approximation. Increasing the concentration in solids in this range increases the number of tactoids, whereas their average size remains constant. 

The size of the tactoids is a function of the nature of the charge balancing cation, of the ionic strength within the suspension, and of the microcrystalline morphology. 

Beyond some critical concentration in solid, aggregation of tactoids occurs undoubtedly. The critical concentration depends, obviously, on many factors, but at the present time we are not aware of any experimental studies on this matter, nor on the factors controlling the critical concentration. This further aggregation process yields larger objects with complicated texture and porous structure. Upon drying thick pastes, some kind of cards house structure has been reported (3). However, irregular stacks composed of tactoids with face to face aggregation have been commonly observed (9). 

Figure 2. Variation of the proton relaxation rate Tj 1 vs. the solid content (Cf) in water for various smectites. The contour (external) surface area of hectorite is ca 80 m2/g, the average number of sheets within a tactoid being about 10. (Reprinted with permission from ref 7. Copyright 1982 Academic.)...

If hectorite was pretreated at a temperature high enough to collapse the tactoid, and subsequently exchanged with a mixture of Ru(bpy), Co(NH4)5 Cl and Ru02, then photooxidation is observed in the colloidal suspension because the catalyst is in contact with the reagents, all components being on the external surface (Figure 7b). 

This has2been shown to be correct by using cis-Ru(bpy)2(H20)2 as catalyst then all the components for the reaction are within the interlamellar space and photooxidation proceeds at a rate comparable with that observed in solution (Figure 7c). These observations outline the importance of the textural arrangement of the clay sheets with respect to the availability of the surface sites, in line with what has been previously discussed in the paragraph dealing with the concept of tactoids. 

Tactoids formed by phyllosilicate gels. Orientation under application of a strong magnetic field. 

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