Colloids network

Physical Chemistry of Polymer solutions, Gels, networks, Colloids,... 

Network colloids have two phases forming an interpenetrating network, for example, a polymer matrix. 

In the above examples, which may be called simple colloids, a clear distinction can be made between the disperse phase and the dispersion medium. However, in network colloids this is hardly possible since both phases consist of interpenetrating networks, the elements of each being of colloidal dimensions. Porous solids, in which gas and solid networks interpenetrate, two-phase glasses (opal glasses), and many gels are examples of this category. 

A fourth class of colloids often encountered is that of the network colloids. Such systems are difficult to define exactly because they consist of two interpenetrating networks, which make it hard to specify exactly which is the dispersed phase and which is the continuous phase. Classic examples of network colloids would be porous glass (air-glass), opal glass (solid-solid dispersion), and many gels. Practical examples of many of the colloids mentioned above are given in Table 10.2. 

Melgar-Lesmes P, Morral-Ruiz G, Solans C, Jose Garcia-Celma M. Quantifying the bioadhesive properties of surface-modified polyurefhane-urea nanoparticles in tire vascular network. Colloids Surf B Biointerfaces June 1, 2014 118 280-8. 

In network colloids the definition of colloids in terms of dispersed phase and dispersion medium breaks down since the networks consist of interpenetrating continuous channels. Examples include porous solids, where a solid labyrinth contains a continuous gas phase. There are also examples of colloids where three or more phases coexist, two or more of which can be finely divided. These are called multiple colloids. An example is an oilbearing porous rock, since both oil and water will be present within the solid pores. 

Llorente MA, Andrady AL, Mark JE. Model networks of end-linked polydimethylsiloxane chains. XIII. The effects of junction fimctional-ity on the elastic properties of the bimodal networks. Colloid Polym Sci 1981 259 1056-61. 

Sahiner, N., 2006. In situ metal particle preparation in cross-linked poly(2-acrylamido-2-methyl-1-propansulfonic acid) hydrogel networks. Colloid Polymer Science 285, 283—292. 

The common colloidal dispersions (e.g. food or paint) are thermodynamically unstable, while association colloids (surfactants) and polymer/protein solutions are thermodynamically stable. In addition, there can be multiple or complex colloids which are combinations of the above, e.g. dispersion, emulsion, surfactants and/or polymers in a continuous phase. Finally, network colloids, also called gels, are sometimes considered to be a separate category. 

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

Colloidal dispersions often display non-Newtonian behaviour, where the proportionality in equation (02.6.2) does not hold. This is particularly important for concentrated dispersions, which tend to be used in practice. Equation (02.6.2) can be used to define an apparent viscosity, happ, at a given shear rate. If q pp decreases witli increasing shear rate, tire dispersion is called shear tliinning (pseudoplastic) if it increases, tliis is known as shear tliickening (dilatant). The latter behaviour is typical of concentrated suspensions. If a finite shear stress has to be applied before tire suspension begins to flow, tliis is known as tire yield stress. The apparent viscosity may also change as a function of time, upon application of a fixed shear rate, related to tire fonnation or breakup of particle networks. Thixotropic dispersions show a decrease in q, pp with time, whereas an increase witli time is called rheopexy. 

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

Pietralla M, Kilian H-G (eds) (1987) Permanent and transient networks. Prog Colloid Polym Sci vol 75... 

Djabourov M., Grillon Y., Leblond J. The sol-gel Transition in gelatin viewed by Diffusing colloidal probes. Polymer Gels and Networks 3 (1995) 407-428. 

Jansons, KM Phillips, CG, On the Application of Geometric Probability Theory to Polymer Networks and Suspensions, I, Journal of Colloid and Interface Science 137, 75, 1990. 

Agatonovic-Kustrin S, Alany RG. Role of genetic algorithms and artificial neural networks in predicting phase behaviour of colloidal delivery systems. Pharm Res 2001 18 1049-55. 

Figure 6.2. (a). Colloidal silica network on the surface of spores from Isoetes pantii (quill wort). Scale = 20 pm. (b). Polystyrene networks and foams produced as a biproduct of colloidal latex formation. Both types of colloidal system are typical of the diversity of patterns that can be derived from the interactions of minute particles. Scale (in (a)) = 50pm. 

Bifunctional spacer molecules of different sizes have been used to construct nanoparticle networks formed via self-assembly of arrays of metal colloid particles prepared via reductive stabilization [88,309,310]. A combination of physical methods such as TEM, XAS, ASAXS, metastable impact electron spectroscopy (MIES), and ultraviolet photoelectron spectroscopy (UPS) has revealed that the particles are interlinked through rigid spacer molecules with proton-active functional groups to bind at the active aluminium-carbon sites in the metal-organic protecting shells [88]. 

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