Colloidal structural transitions

Wasan DT, Nikolov A (1999) Structural transitions in colloidal suspensions in confined films. In Manne S, Warr GG (eds) Supramolecular Structure in Confined Geometries. ACS Symp Ser 736. American Chemical Society, Washington, pp 40-53... 

E. R. Weeks, J. C. Crocker, A. C. Levitt, A. Schofield, and D. A. Weitz, Three-dimensional direct imaging of structural relaxation near the colloidal glass transition. Science 287, 627-631 (2000). 

The SAXS diagrams show a very intricate transition hydrated solid gel in the Li-exchanged solids. Three phases are coexisting a hydrated solid with two water layer, a gel and a very disordered solid. This study should allow to elucidate the formation process of a colloidal structure in such systems. 

It has been already pointed out that the energy of interaction between dispersed particles depends on the particle size. As a result, for large particles, and especially for anisometric ones, oriented in a certain way with respect to each other, the presence of a secondary minimum may be of importance. For such particles this secondary minimum may be sufficiently deep in comparison with kT. In some cases these systems may experience a peculiar colloid phase transition from a free disperse system (at low concentrations of dispersed phase) to crystal-like periodic structures consisting of colloidal particles in equilibrium with the dilute sol consisting of single particles. Such periodic structures are observed in some biological systems, e.g. in tobacco mosaic virus, in V205 sols and in latexes. 

The incorporation of lecithin in different amounts (10-50% related to the triglyceride) did not influence the melting or crystallization temperature of the hard fat in the bulk phase neither in the nanoparticles but led to an accelerated polymorphic transition to the stable 6-modification particularly in the nanoparticles. No additional colloidal structures derived from the emulsifiers (lecithin and Solutol) could be detected in the aqueous phase by electron microscopy and the results of H- and P-NMR studies indicate that lecithin and Solutol are nearly completely attached to the particle surface. However, from the results it was also concluded that due to the redistribution of the lecithin from the lipid matrix and its enrichment in the particle interface, neither an increase in drug load (into the solid... 

Order (crystal structure)-disorder (liquid structure) transition in ionic colloidal dispersions has been intensively studied [1]. Since the driving force of the ordering is electrostatic, the order-disorder transition point for the colloidal system is largely determined by the surface charge density of the particle and the salt concentration of the dispersion, Cg, in addition to the volume fraction of the particles, . A number of experimental studies have so far been made to determine the transition point as a function of Cs and 0. However, little attention has been paid on the influence of the surface charge density. This seems to be... 

Kogan A, Aseiin A and Garti N. 2007. Improved soluhUization of carbamazepine and structural transitions in nonionic microemulsions upon aqueous phase dilution. Journal of Colloid and Interface Science 637-647. 

For the mainly oil-soluble Span 20 siufactant, however, the lifetimes are much less and films rupture prematurely, in line with predictions based on Bancroft s rule. At concentrations well above the CMC where the effective volume fraction of micelles is significant (>5 vol%), thin liquid films may drain in a stepwise fashion by stratification. This phenomenon, seen initially with foam films, was explained by the formation of periodic colloidal structures inside the film that results in layering of the micelles. At a step-transition, a layer of micelles leaves the film and the film thickness decreases by approximately the effective micellar diameter. It can also occur in emulsion films shown recently for hexadecane-aqueous sodium case-inate-hexadecane systems. The step-height seen of around 20 nm is very close to the measured diameter of the casein micelles of between 20 and 25 nm. The layering ultimately increases the lifetime of a film, but a critical film area exists below which step transitions are inhibited such thick films containing layers of micelles are even more stable. 

Meulenberg, R.W., Jennings, T., and Strouse, G.F. (2004). Compressive and tensile stress in colloidal CdSe semiconductor quantum dots, Phys. Rev. B 70, 235311. Jarrold, M.F. and Constant, V.A. (1991). Silicon cluster ions Evidence for a structural transition, Phys. Rev. Lett. 67, 2994-2997. 

It is of special interest for many applications to consider adsorption of fiuids in matrices in the framework of models which include electrostatic forces. These systems are relevant, for example, to colloidal chemistry. On the other hand, electrodes made of specially treated carbon particles and impregnated by electrolyte solutions are very promising devices for practical applications. Only a few attempts have been undertaken to solve models with electrostatic forces, those have been restricted, moreover, to ionic fiuids with Coulomb interactions. We would hke to mention in advance that it is clear, at present, how to obtain the structural properties of ionic fiuids adsorbed in disordered charged matrices. Other systems with higher-order multipole interactions have not been studied so far. Thermodynamics of these systems, and, in particular, peculiarities of phase transitions, is the issue which is practically unsolved, in spite of its great importance. This part of our chapter is based on recent works from our laboratory [37,38]. 

---