Complete colloid films

If however the salt employed does not cause flocculation or coacervation, the electrophoretic velocity is measured of small particles (e.g., SiOg) suspended in the sol and covered by a complete colloid film (see p. 277 2b). 

As no flocculation, occurs the reversal of charge concentrations needed for the calculation of R.H.N. were determined on suspended particles (for instance Si02) which become covered with a complete colloidal film. For particulars see p. 277 2 b. 

A complete colloid film will be said to be present on the surface of a particle, if its electrophoretic properties are solely determined by the sol in which the particle is immersed. Such complete colloid films wholly mask the electrophoretic properties of the original phase boundary of the particle. 

Fig. 6 gives an example, from which may be concluded that with the sol concentration chosen, the five different kinds of "particles used as "electrophoretic indicators are covered with a complete colloid film. To avoid misinterpretation we must add that the adjective "complete does not give any information on the structure of the colloid film formed. Thus complete does not mean, for instance that a monomolecular colloid film on the particle surface would have been completed. It only means that the mono- di- or perhaps poly-molecular colloid film formed possesses such a boundary with the surrounding sol, that its electrophoretic properties have become independent of those of the original particle surface. 

In using this method, care must be taken to choose such a sol concentration as to ensure that the particles are really covered with a complete colloid film. For... 

Since the purified egg lecithin does not flocculate with the salts used, the electrophoresis measurements were carried out on suspended SiOa particles, which at the chosen sol concentration (0.049%) were covered with a complete colloidal film (this is even the case from a 10 X smaller lecithin concentration onwards (see Fig. 8). 

It seems plausible to assume that the formation of a complete colloid film is optimal just at the reversal of charge point of a colloid Evidently this plays an important role in the competition of the phosphatide and nucleate. Thus at a the colloid film on the adsorbed particles consists practically only of phosphatide, at c only of nucleate. The remarkable retrograde reversal of charge (point 6) on increase of the CaCl2 concentration can thus be seen as result of the gradual replacement of positive phosphatide by negative nucleate. 

In 2b (p. 277) we studied the r61e of colloid concentration on the formation of complete colloid films at the reversal of charge point of the colloid. As therefore we already used conditions most favourable to film formation, this second factor (the state of charge of the colloid) could not manifest itself. Certain irregular forms of U-log C curves (see p. 427, 428, Fig. 55 and 56) can be explained by this second factor. 

Polymer dispersed nematic films are made by one of two distinct processes. In one, the nematic is emulsified in either an aqueous solution of a film-forming polymer (for example, poly vinyl alcohol) or an aqueous colloidal dispersion (for example, a latex). This emulsion is coated onto a conductive substrate and allowed to dry, dming which time the polymer coalesces around the nematic droplets. Laminating a second conductive substrate to the dried film completes the device. Alternatively, the nematic is mixed with a precursor to the polymer to form an isotropic solution. When polymerization is initiated, typically with heat or light, nematic droplets nucleate in situ as the polymer chains grow. 

Another generator that was successfully used in the formation of polymer colloids, particles of mixed composition, and coated particles is shown in Figure 1.5.3f, in which the falling film tube is replaced by a boiler (38). The complete apparatus described in Figure 1.5.3 makes it possible to produce cores and shells in a continuous process (39). 

A variation of the CD process for PbSe involved deposition of a basic lead carbonate followed by selenization of this film with selenosulphate [64]. White films of what was identified by XRD as 6PbC03-3Pb(0H)2-Pb0 (denoted here as Pb—OH—C) were slowly formed over a few days from selenosulphate-free solutions that contained a colloidal phase and that were open to air (they did not form in closed, degassed solutions). CO2 was necessary for film formation—other than sparse deposits, no film formation occurred of hydrated lead oxide under any conditions attempted in this study. Treatment of these films with selenosulphate solution resulted in complete conversion to PbSe at room temperature after 6 min. The selenization process of this film was followed by XRD, and it was seen to proceed by a breakdown of the large Pb—OH—C crystals to an essentially amorphous phase of PbSe with crystallization of this phase to give finally large (ca. 200 nm) PbSe crystals covered with smaller (15-20 nm) ones as well as some amorphous material. 

Uses of gelatin are based on its combination of properties reversible gel-to-sol transition of aqueous solution viscosity of warm aqueous solutions ability to act as a protective colloid water permeability and insolubility in cold water, but complete solubility in hot water. It is also nutritious. These properties are utilized in the food, pharmaceutical, and photographic industries. In addition, gelatin forms strong, uniform, clear, moderately flexible coatings which readily swell and absorb water and are ideal for the manufacture of photographic films and pharmaceutical capsules. 

Grimley (G10, Gil) used an ultramicroscope technique to determine the velocities of colloidal particles suspended in a falling film of tap water. It was assumed that the particles moved with the local liquid velocity, so that, by observing the velocities of particles at different distances from the wall, a complete velocity profile could be obtained. These results indicated that the velocity did not follow the semiparabolic pattern predicted by Eq. (11) instead, the maximum velocity occurred a short distance below the free surface, while nearer the wall the experimental results were lower than those given by Eq. (11). It was found, however, that the velocity profile approached the theoretical shape when surface-active material was added and the waves were damped out, and, in the light of later results, it seems probable that the discrepancies in the presence of wavy flow are due to the inclusion of the fluctuating wavy velocities near the free surface. 

Example 11.4. McGuiggan et al. [492] measured the friction on mica surfaces coated with thin films of either perfluoropolyether (PFPE) or polydimethylsiloxane (PDMS) using three different methods The surface forces apparatus (radius of curvature of the contacting bodies R 1 cm) friction force microscopy with a sharp AFM tip (R 20 nm) and friction force microscopy with a colloidal probe (R 15 nm). In the surface force apparatus, friction coefficients of the two materials differed by a factor of 100 whereas for the AFM silicon nitride tip, the friction coefficient for both materials was the same. When the colloidal probe technique was used, the friction coefficients differed by a factor of 4. This can be explained by the fact that, in friction force experiments, the contact pressures are much higher. This leads to a complete penetration of the AFM tip through the lubrication layer, rendering the lubricants ineffective. In the case of the colloidal probe the contact pressure is reduced and the lubrication layer cannot be displaced completely. 

The function of a protective colloid is to lower cr01 to a minimum. In practical language, wetting is an attempt by a surfactant to accomplish this by lowering the contact angle, which enables liquids to spread over each other, on its mission to make the phases mutually miscible. Relative to solvent and component, the concentration of a protective colloid is quite low, but it accumulates at the interface, theoretically as a thin film. Micromolecules that wet surfaces dissolve completely in the solvent. One unique property of micellar surfactant electrolytes is their ability to solubilize some otherwise insoluble organic molecules (Adamson, 1990). 

Figure 5.7. Attachment of spherical particles or droplets to (solid) surfaces, (a) h, no interaction (b) h of colloidal range interaction determined by the disjoining pressure across phase 2 (c) attachment (d) attachment of a rectangular particle (e) spreading of an attached drop until the contact angle is a (f) droplet deforms but does not wet (a = 180°) (g) complete wetting (h) partial wetting on a completely wetting film.

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