Colloid structure

FI . 16 An aggregated colloidal structure. Neighboring particles touch each other and a complicated network builds up. 

A solid emulsion is a suspension of a liquid or solid phase in a solid. For example, opals are solid emulsions formed when partly hydrated silica fills the interstices between close-packed microspheres of silica aggregates. Gelatin desserts are a type of solid emulsion called a gel, which is soft but holds its shape. Photographic emulsions are gels that also contain solid colloidal particles of light-sensitive materials such as silver bromide. Many liquid crystalline arrays can be considered colloids. Cell membranes form a two-dimensional colloidal structure (Fig. 8.44). 

T-lymphocytes seem to be excellent carriers, inclusion of therapeutical nanoparticles into immune cells as a new strategy for localized therapy (Steinfeld et al, 2006). Nanoparticles are colloidal structures with diameter smaller than lOOOnm and can therefore penetrate through fine capillaries into the cells. Several of these are under investigation (Yih et al, 2006). 

The highly dynamic colloidal structures described in this chapter result in considerable complexity in behaviors. This complexity has resulted in relatively slow improvement in our understanding of colloidal systems despite the fact that the structure of micelles was in essence described almost a century ago already. Results from a series of relatively recent approaches to describe colloidal aggregates are now beginning to coalesce into a model of colloidal structures incorporating the dynamic and nonhomogeneous structures of these aggregates. 

In one example, the colloidal structure, is made by sedimentation of polystyrene beads, giving voids in the range 120-1000 mn, and the voids are filled with TiO generated from titanium tetrapropoxide. The polystyrene bead lattice is then removed by calcining to give an iridescent material, but not with a full photonic band gap. In this case one of the controlling factors is the refractive index of the matrix, which needs to be greater than 2.8. 

Any conclusions about the organization of different components within the dispersions should take the ultrastructure of the systems into consideration. The surface-active agents that act as stabilizers for the nanoparticles are often able to form additional colloidal structures, such as vesicles or micelles, by self assembly. In addition to a potential importance in the formation and stability of the dispersions, such structures contain lipophilic domains that may represent alternative compartments for the localization of incorporated drugs. As a consequence, their presence may affect drug incorporation and release. 

Transmission electron microscopy (TEM) can provide valuable information on particle size, shape, and structure, as well as on the presence of different types of colloidal structures within the dispersion. As a complication, however, all electron microscopic techniques applicable for solid lipid nanoparticles require more or less sophisticated specimen preparation procedures that may lead to artifacts. Considerable experience is often necessary to distinguish these artifacts from real structures and to decide whether the structures observed are representative of the sample. Moreover, most TEM techniques can give only a two-dimensional projection of the three-dimensional objects under investigation. Because it may be difficult to conclude the shape of the original object from electron micrographs, additional information derived from complementary characterization methods is often very helpful for the interpretation of electron microscopic data. 

Interaction with such properties does, however, not allow the investigator to answer the question of where the drug is localized in the particles (on the surface or in the crystal lattice). Thermal interactions were also observed when an incorporated drug or a second type of triglyceride formed a separate phase within the nanoparticles [3,37,64,68]. Drug release studies can provide supportive information on the accessibility of the drug to the aqueous phase [72,108], but separation of the effects from the nanoparticles from those of additional colloidal structures — if present — may be difficult. 

The invention of the electron microscope in the 1930s by Knoll and Ruska cleared the way for scientists to take an even closer look at vesicles and other colloidal structures [5]. Improving the resolution of the optical microscope roughly by the factor that the optical microscope improved that of the unaided eye, the finer structures of colloidal systems became visible. With the electron microscope, single bilayers can be made visible and the distance between lamellae can be determined. Thus, the structure of a given system can be determined to up to 1/10000000 of a millimeter, which is about the distance of six atoms in a molecule. The most impressive results are obtained with the freeze fracture and cryo-TEM methods [6]. 

Much stronger kinetic stabilization can be expected for processes leading to the inclusion of radionuclide ions into the colloid structure (Fig. 7, lower part). Spectroscopic indications for such processes have indeed been found again by TRLFS for the Cm(III) interaction with colloidal and particulate amorphous silica, calcite and CSH phases (Chung et al. 1998 Stumpf Fanghanel 2002 Tits et al. 2003). The incorporation of actinide ions into colloidal precursor clay phases has been recently investigated as a possible mechanism in natural... 

The colloidal system consists of 5 X 10-2 M Cu(AOT)2-isooctane-water. The colloidal structure is changed by increasing the amount of water in Cu(AO-T)2-isooctane solution (56,57). Syntheses are performed in various regions of the phase diagram (57) (Fig. 9.3.2). 

From these data it is concluded that the size, shape, and polydispersity of nanoparticles depend critically on the colloidal structure in which the synthesis is performed. This is well demonstrated when, by changing the water content, similar colloidal structures (reverse micelles or interconnected cylinders) are obtained ... 

In this chapter we have outlined how the use of a universal thermodynamic approach can provide valuable insight into the consequences of specific kinds of biopolymer-biopolymer interactions. The advantage of the approach is that it leads to clear quantitative analysis and predictions. It allows connections to be made between the molecular scale and the macroscopic scale, explaining the contributions of the biopolymer interactions to the mechanisms of microstructure formation, as well as to the appearance of novel functionality arising from the manipulation of food colloid formulations. Of course, we must remind ourselves that, taken by itself, the thermodynamic approach cannot specify the molecular or colloidal structures in any detail, nor can it give us information about the rates of the underlying kinetic processes. 

Dickinson, E. (1997). Aggregation processes, particle interactions and colloidal structure. In Dickinson, E., Bergenstahl, B. (Eds). Food Colloids Proteins, Lipids and Polysaccharides, Cambridge, UK Royal Society of Chemistry, pp. 107-126. 

For a colloidal system containing a mixture of different biopolymers, in particular a protein-stabilized emulsion containing a hydrocolloid thickening agent, it is evident that the presence of thermodynamically unfavourable interactions (A u > 0) between the biopolymers, which increases their chemical potentials (thermodynamic activity) in the bulk aqueous phase, has important consequences also for colloidal structure and stability (Antipova and Semenova, 1997 Antipova et al., 1997 Dickinson and Semenova, 1992 Dickinson et al., 1998 Pavlovskaya et al., 1993 Tsap-kina et al., 1992 Semenova et al., 1999a Makri et al., 2005 Vega et al., 2005 Semenova, 2007). 

Other examples of thermodynamically stable colloidal structures are highlighted in Vignette 1.6. 

The colloidal structures described above are dictated by thermodynamics, and the resulting structures are thermodynamically stable. Similar thermodynamically stable structures can develop even in a copolymer melt (i.e., there is no other polymer or solvent). Such colloidal systems differ from kinetically stable lyophobic dispersions of the type discussed in Vignettes 1.4 and 1.5. 

Measurement of the static structure factor using scattering techniques also provides us with a nonintrusive method to probe the structure of dispersions and the nature of interaction forces in colloids. Structural changes in colloids are particularly of interest in colloid-based techniques for fabrication of structural and special-purpose ceramics. 

The colloidal structures we examine in this chapter are formed as a result of physical interactions among amphipathic molecules, rather than by covalent bonding. This sort of physical association has been recognized for a long time, although contemporary students may be relatively (or totally ) unaware of it. It is interesting to note that in the early days of polymer chemistry, macromolecules were believed to be physically associated rather than covalently bonded structures. The birth of modern polymer chemistry can be traced to the acceptance of the covalent character of these substances. Associated structures do exist, however, and we see by the end of this chapter that their investigation is a very lively area of chemical research. 

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