Colloidal crystalline phase

Similarly, charged solid particles (such as latex spheres) —kinetically stable lyophobic colloids —may exist in colloidal crystalline phases (with body-centered or face-centered cubic structures) as a consequence of thermodynamically favored reduction in free energies (see Chapter 13). Even neutrally charged spherical particles ( hard spheres ) undergo a phase transition from a liquidlike isotropic structure to face-centered cubic crystalline structures due to entropic reasons. In this sense, the stability or instability is of thermodynamic origin. 

Figure 2.1 The hard-sphere phase diagram. Below volume fraction < (f>] = 0.494, the suspension is a disordered fluid. Between <) >i = 0.494 and 02 = 0.545, there is coexistence of this disordered phase with a colloidal crystalline phase with FCC (or HCP) order the colloidal crystalline phase is the equilibrium one up to the maximum close-packing limit of 0cp = 0.74. Nonequilibrium colloidal glassy behavior can also occur between

In contrast to the above-described kinetic stability, colloids may also be thermodynamically stable. A stable macromolecular solution is an example we have already discussed. Formation of micelles beyond the critical micelle concentration is another example of the formation of a thermodynamically stable colloidal phase. However, when the concentration of the (say, initially spherical) micelles increases with addition of surfactants to the system, the spherical micelles may become thermodynamically unstable and may form other forms of (thermodynamically stable) surfactant assemblies of more complex shapes (such as cylindrical micelles, liquid-crystalline phases, bilayers, etc.). 

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. 

Kinetic studies have been important in many areas of chemistry, but, to date, classic kinetic approaches have not played a major role in the area of colloidal science related to self-organisation of surfactants. Although micellar kinetics were investigated in some detail in the 1970s, related work involving other self-assembly systems, e.g., vesicles, lyotropic liquid-crystalline phases, have been limited. 

The deposition of unstable amorphous precursor phases requires a hydrated medium that has a very high ion concentration, and an inordinately high supersaturation relative to the corresponding crystalline phases. In order to stabilize, even transiently, such high supersaturations, specialized inhibitors of crystallization probably need to be present. Such high concentrations of ions cannot be regarded as a solution, but rather a structured colloidal phase. Concepts such as mechanisms of diffusion, levels of supersaturation and consequently kinetics of crystallization, must be reconsidered if crystallization does not occur from free solution. 

But it was not to be. Try as we might, the difference in scattering lengths between the 6Li and 7Li isotopes was too small to permit us to measure the lithium ion distribution in the swollen state. We had to content ourselves with the results for the crystalline phase, where the behavior of the lithium ions is different from that of the larger alkali metal cations [27], Potassium and cesium ions bind directly to vermiculite clay surfaces rather than hydrating fully. Because only lithium-substituted vermiculites of the alkali metal series will swell macroscopically when soaked in water, it seems that interlayer cations must form fully hydrated ion-water complexes if the particles are to expand colloidally. This conclusion has since been supported... 

I should note that the limit of sensitivity of these experiments restricts us to saying that the phase transitions are simultaneous only in the sense that they both occur between -1 and 0°C (in either direction). More subtle variations, of the order of 0.1°C, would not have been detectable. According to Debye-Huckel theory [3], the depression of the freezing point of pure water is 0.18°C in a 0.1 M uni-univalent electrolyte solution. We would expect the clay to cause a further small depression in the freezing point, as discussed below. Within these limits, the temperature where both the freezing transition and the gel-crystalline phase transition occur is the same in our model clay colloid system, and it can be concluded to be the ordinary freezing point of the soaking solution. 

Caboi, F., Borne, J., Nylander, T., Khan, A., Svendsen, A., and Patkar, S. (2002). Lipase action on a monoolein sodium oleate aqueous eubic liquid crystalline phase aNMR and x-ray diffraction study. Colloids Surf. B. 26, 159-171. 

Miller, C.A., Ghosh, O., and Benton, W.J., Behavior of dilute lamellar liquid-crystalline phases. Colloids Surf., 19, 197, 1986. 

Since the chain length of tallow components is essentially between C14 and Cis, the total number of different structures in the softener active exceeds 15 [26], The raw material also contains mono- and usually tri-tallow derivatives. Most of those molecules are not water soluble and do not associate into micelles, but form stable colloidal dispersions in water. Maltese crosses can be observed using an optical microscope under polarized light (Figure 12.7), revealing the presence of strongly birefringent particles, typical of liquid crystalline phases [88,89],... 

The liquid crystalline phase makes DHTMAC extremely viscous when the concentration exceeds 5%. Because of this inherently high viscosity and the high volume fraction of colloidal particles, the formulation of concentrated products is difficult. In practice, DHTDMAC is never used at concentrations above 15%. In contrast, esterquats or other quaternaries bearing unsaturated fatty chains are more suitable for producing concentrated dispersions. As a result of their more disordered fatty chain packing, the particles they form are cubic or isotropic but not lamellar, and the resulting phase viscosity is lower and more stable [3],... 

Jung, M., German, A.L. and Fischer, H.R. (2001) Polymerisation in lyotropic liquid-crystalline phases of dioctadecyldimethylammonium bromide. Colloid Polym. Sci, 279, 105-113. 

Association colloids are formed by fairly small molecules that associate spontaneously into larger structures. A clear example is the formation of micelles, i.e., roughly spherical particles of about 5nm diameter, by amphiphilic molecules like soaps see Figure 2.8. At high concentrations, such molecules can in principle form a range of structures, called mesomorphic or liquid crystalline phases, which are briefly discussed in Section 10.3.1. To be sure, the whole system is called a mesomorphic phase, not its structural elements. 

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