Thin colloidal films

Any real sample of a colloidal suspension has boundaries. These may stem from the walls of the container holding the suspension or from a free interface towards the surroundings. One is faced with surface effects that are small compared to volume effects. But there are also situations where surface effects are comparable to bulk effects because of strong confinement of the suspension. Examples are cylindrical pores (Fig. 8), porous media filled with suspension (Fig. 9), and thin colloidal films squeezed between parallel plates (Fig. 10). Confined systems show physical effects absent in the bulk behavior of the system and absent in the limit of extreme confinement, e.g., a onedimensional system is built up by shrinking the size of a cylindrical pore to the particle diameter. 

Keywords Macroions, layering, thin colloidal films, solvent excluded volume effects, struc-... 

To prepare surface-modified colloidal membranes, a thin colloidal film was assembled from440 11 nm and 184 9 nm silica spheres [26,28] on the surface of 25 pm radius, disk-shaped Pt microelectrodes shrouded in glass [26], These electrodes were... 

FIGURE 8.3 Voltammetric responses of a Pt electrode (a) bare (bottom), after opal assembly (middle), and after chemical modification of the thin colloidal film with 3-aminopropyltriethoxysilane (top) (b), (c) bare (top), after opal assembly and after chemical modification of the thin colloidal film with 3-aminopropyltriethoxysilane (middle) (d) voltammetric responses of a Pt electrode as a function of pH for unmodified (triangles) and modified (circles) opal electrode. Reproduced with permission from Reference 26. Copyright 2005 American Chemical Society. 

An experiment with resultant pulling velocity 0.049 0.003 p,m/s, volume fraction (p = 1.00% and 1 mM NaCl was carried out in order to obtain a thin colloidal film. The... 

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian Hquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin Hquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a large number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the lUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

Zhu T., Zhang X., Wang J., Fu X.Y., Liu Z.F., Assembling colloidal Au nanoparticles with functionalized self- assembled monolayers. Thin Solid Films 1998 329 595-598. 

Colloidal liquid aphrons (CLAs), obtained by diluting a polyaphron phase, are postulated to consist of a solvent droplet encapsulated in a thin aqueous film ( soapy-shell ), a structure that is stabilized by the presence of a mixture of nonionic and ionic surfactants [57]. Since Sebba s original reports on biliquid foams [58] and subsequently minute oil droplets encapsulated in a water film [59], these structures have been investigated for use in predispersed solvent extraction (PDSE) processes. Because of a favorable partition coefficient for nonpolar solutes between the oil core of the CLA and a dilute aqueous solution, aphrons have been successfully applied to the extraction of antibiotics [60] and organic pollutants such as dichlorobenzene [61] and 3,4-dichloroaniline [62]. 

Marinova, K., Gurkov, T., Velev, O., Ivanov, I., Campbell, B., Borwankar, R. (1997). The role of additives for the behaviour of thin emulsion films stabilized by proteins. Colloids and Surfaces A Physicochemical and Engineering Aspects, 123-124, 155-167. 

A foam is a colloidal dispersion in which a gas is dispersed in a continuous liquid phase. The dispersed phase is sometimes referred to as the internal (disperse) phase, and the continuous phase as the external phase. Despite the fact that the bubbles in persistent foams are polyhedral and not spherical, it is nevertheless conventional to refer to the diameters of gas bubbles in foams as if they were spherical. In practical occurrences of foams, the bubble sizes usually exceed the classical size limit given above, as may the thin liquid film thicknesses. In fact, foam bubbles usually have diameters greater than 10 pm and may be larger than 1000 pm. Foam stability is not necessarily a function of drop size, although there may be an optimum size for an individual foam type. It is common but almost always inappropriate to characterize a foam in terms of a given bubble size since there is inevitably a size distribution. This is usually represented by a histogram of sizes, or, if there are sufficient data, a distribution function. 

The negative derivative with respect to distance of the Gibbs energy of interaction per unit area yields a force per unit area between colloidal species, termed the disjoining pressure . Example In a thin liquid film, the disjoining pressure equals the pressure, beyond the external pressure, that has to be applied to the liquid in the film to maintain a given film thickness. 

A dispersion of gas bubbles in a liquid, in which at least one dimension falls within the colloidal size range. Thus a foam typically contains either very small bubble sizes or, more commonly, quite large gas bubbles separated by thin liquid films. The thin liquid films are called lamellae (or laminae ). Sometimes distinctions are drawn as follows. Concentrated foams, in which liquid films are thinner than the bubble sizes and the gas bubbles are polyhedral, are termed polyederschaum . Low-concentration foams, in which the liquid films have thicknesses on the same scale or larger than the bubble sizes and the bubbles are approximately spherical, are termed gas emulsions , gas dispersions , or kugelschaum . See also Evanescent Foam, Froth, Aerated Emulsion. 

Thin Liquid Films in Surface and Colloid Science, Vol. 3, Matijevic, E. (Ed.), Wiley New York, 1971, pp. 167-239. 

A. M. Cazabat, N. Fraysse, and F. Heslot, Thin wetting films, Colloids Surf. 52, 1-8 (1991). 

D. E. Tambe and M. M. Sharma, Hydrodynamics of thin liquid-films bounded by viscoelastic interfaces, J. Colloid Interface Sci. 147, 137-151 (1991) Factors controlling the stability of colloid-stabilized emulsions. 1. An experimental investigation, J. Colloid Interface Sci. 157, 244-253 (1993) Factors controlling the stability of colloid-stabilized emulsions. 2. A model for the rheological properties of colloid-laden interfaces, J. Colloid Interface Sci. 162, 1-10 (1994) Factors controlling the stability of colloid-stabilized emulsions. 3. Measurement of the rheological properties of colloid-laden interfaces, J. Colloid Interface Sci. 171, 456-462 (1995). 

The special properties of thin liquid films, in particular of foam films, involve studying various colloid-chemical aspects, such as kinetics of thinning and rupture of films, transition from CBF to NBF, isotherms of disjoining pressure, thermodynamic (equilibrium) properties, determination of the electrical parameters of surfactant adsorption layer at the liquid/gas... 

The treatment of foam films is important in its own right, thin liquid films being an effective tool and a luring scientific topic in surface and colloid science. In all justice we must say that nowadays this scientific domain exists wholly in its own account and its numerous practical... 

Interaction between double layers, one of the building bricks of colloid stability, is an important theme planned for Volume IV. It has a large number of spin-offs, in, for instance ion exchange, thin wetting films, free films, membranes, association colloids, vesicles, polyelectroljdes, emulsions and rheology. The dramatic influence of electroljrtes on these phenomena finds its origin in the changes in the double layer, discussed in this chapter. 

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