Surface viscosity colloidal systems

The effective surface viscosity is best found by experiment with the system in question, followed by back calculation through Eq. (22-55). From the precursors to Eq. (22-55), such experiments have yielded values of [L, on the order of (dyn-s)/cm for common surfactants in water at room temperature, which agrees with independent measurements [Lemhch, Chem. Eng. ScL, 23, 932 (1968) and Shih and Lem-lich. Am. Inst. Chem. Eng. J., 13, 751 (1967)]. However, the expected high [L, for aqueous solutions of such sldn-forming substances as saponin and albumin was not attained, perhaps because of their non-newtonian surface behavior [Shih and Lemhch, Ind. Eng. Chem. Fun-dam., 10, 254 (1971) andjashnani and Lemlich, y. Colloid Inteiface ScL, 46, 13(1974)]. 

The principles of colloid stability, including DLVO theory, disjoining pressure, the Marangoni effect, surface viscosity, and steric stabilization, can be usefully applied to many food systems [291,293], Walstra [291] provides some examples of DLVO calculations, steric stabilization and bridging flocculation for food colloid systems. 

Ophthalmic delivery For administration of water-insoluble drugs into the eye, colloidal systems are used. Surface active polymers, such as polyvinyl alcohol, poloxamer, and methylcellulose are often added as particle stabilizers and viscosity enhancers. Particle size in such colloidal systems should not exceed 5-10 pm in diameter. Emulsions, liposomes, and micro- and nanoparticles, all stabilized by surfactants, are used in topical ophthalmic drug delivery [56]. For instance, an ophthalmic preparation of cyclosporin A, Restasis , is an emulsion stabilized by polysorbate 80 and polymer carbomer 1342. 

Solubility Viscosity Surface activity pH Colloidal systems... 

The presence of a chromophore group in the hydrophilic or hydrophobic moieties in the surfactant molecular structure makes it sensitive to different physical responses, in particular, for the control of physicochemical parameters of colloidal systems such as surface activity, aggregation structure, viscosity, microemulsion separation, and solubilization. 

Polyelectrolytes provide excellent stabilisation of colloidal dispersions when attached to particle surfaces as there is both a steric and electrostatic contribution, i.e. the particles are electrosterically stabilised. In addition the origin of the electrostatic interactions is displaced away from the particle surface and the origin of the van der Waals attraction, reinforcing the stability. Kaolinite stabilised by poly(acrylic acid) is a combination that would be typical of a paper-coating clay system. Acrylic acid or methacrylic acid is often copolymerised into the latex particles used in cement sytems giving particles which swell considerably in water. Figure 3.23 illustrates a viscosity curve for a copoly(styrene-... 

The huge variety of emulsions used as food, medicinal, cosmetic, and other industrial products make these colloids important practical systems in which the surface monolayers exert considerable influence. We have already discussed the use of lecithin to control the viscosity and the texture of chocolate in Vignette IV in Chapter 4. 

Colloids are either hydrophilic (water-loving) or hydrophobic (water-hating). Hydrophilic colloids (e.g., proteins, humic substances, bacteria, viruses, as well as iron and aluminum hydrated colloids) tend to hydrate and thereby swell. This increases the viscosity of the system and favors the stability of the colloid by reducing the interparticle interactions and its tendency to settle. These colloids are stabilized more by their affinity for the solvent than by the equalizing of surface charges. Hydrophilic colloids tend to surround the hydrophobic colloids in what is known as the protective-colloid effect, which makes them both more stable. 

For monodisperse or unimodal dispersion systems (emulsions or suspensions), some literature (28-30) indicates that the relative viscosity is independent of the particle size. These results are applicable as long as the hydrodynamic forces are dominant. In other words, forces due to the presence of an electrical double layer or a steric barrier (due to the adsorption of macromolecules onto the surface of the particles) are negligible. In general the hydrodynamic forces are dominant (hard-sphere interaction) when the solid particles are relatively large (diameter >10 (xm). For particles with diameters less than 1 (xm, the colloidal surface forces and Brownian motion can be dominant, and the viscosity of a unimodal dispersion is no longer a unique function of the solids volume fraction (30). 

Lucassen J and Hansen RS (1966) Damping of waves on monolayer-covered surfaces. I. Systems with negligible surface dilational viscosity. J Colloid Interface Sci 22 32-44... 

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