Dispersed particles, emulsions forces

A dispersion Is a system made of discrete objects separated by a homogeneous medium In colloidal dispersions the objects are very small In at least one dimension. Colloidal sizes range from 1 to 100 nm however these limits are somewhat arbitrary, and It Is more useful to define colloids as dispersions where surface forces are large compared to bulk forces. Here we are concerned with systems where the dispersion medium Is a liquid examples are droplets In emulsions or mlcroemulslons (oll/water or water/oll), aggregates of amphiphilic molecules (surfactant micelles), foams, and all the dispersions of solid particles which are used as Intermediates In the manufacture of ceramics. At this stage we are not too concerned with the nature of the constituents, but rather with the structures which they form this Is a geometrical problem, where the system Is characterized by Its surface area A, by the shapes of Its Interfaces (curvatures - b ), and by the distances between opposing surfaces (d — concentration parameter). 

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). 

Energy can also be stored in other ways on a microscopic scale, e.g., by electrical charges being forced near each other in colloidal systems and by emulsion drops being distorted from the spherical shape. In this case, the surface tension gives them stabilizing surfactant layers on dispersed particles being pressed into each other. 

Many liquid and semi-liquid pharmaceutical preparations are disperse systems. Disperse systems are defined as systems in which a substance is distributed as particles (discontinuous) into a dispersion medium (continuous). Three types of disperse systems will be discussed which are pharmaceutically relevant colloidal systems, suspensions and emulsions. In both colloidal systems and suspensions, solid particles are dispersed in a liquid. The difference is that in colloidal systems the particles do not settle, while they do in suspensions. This difference is caused by the size of the particles. In colloidal systems, the particles are so small (1 mn - 1 pm) that the Brownian motion (diffusion caused by thermal energy) is stronger than the force of gravity so that they remain suspended in the liquid and do not settle. In suspensions, the particles are larger (>1 pm) and as a consequence the force of gravity is stronger than the Brownian motion which makes them settle (if the density of the particles is larger than that of the dispersion medium). Emulsions consist of non-miscible liquids. Two types of emulsions will be discussed oil drops dispersed in water (oil-in-water emulsion or o/w emulsion) and water drops dispersed in oil (water-in-oil emulsion or w/o emulsion). There are also more complex structures such as w/o/w emulsions and bi-continuous systems. However, these systems will not be discussed. 

Conclusions. The zeta-potential helps to evaluate the repulsive forces between the dispersed particles. It was often observed that zeta-potential values of 50 mV minimum are needed to get satisfactory stability of the dispersions. Nevertheless, a lot of emulsions are satisfactorily stabilized with nonionics which induce lower potential values. 

Interaction forces between dispersed particles and emulsion stability... 

Preparation of Emulsions. An emulsion is a system ia which one Hquid is coUoidaHy dispersed ia another (see Emulsions). The general method for preparing an oil-ia-water emulsion is to combine the oil with a compatible fatty acid, such as an oleic, stearic, or rosia acid, and separately mix a proportionate quantity of an alkah, such as potassium hydroxide, with the water. The alkah solution should then be rapidly stirred to develop as much shear as possible while the oil phase is added. Use of a homogenizer to force the resulting emulsion through a fine orifice under pressure further reduces its oil particle size. Liquid oleic acid is a convenient fatty acid to use ia emulsions, as it is readily miscible with most oils. 

Emulsifier is not a necessary component for emulsion polymerization if ihe following conditions are satisfied The particles are formed by homogeneous nucleation mechanism, and the particles are stabilized by factor(s) olher than emulsifier. As to the latter, the sulfate end group that is the residue of persulfate initiator serves for stabilization of dispersion via interparticle electrorepulsive force (20). When the stabilization mechanism works well, a small number of particles grow during polymerization without aggregation, keeping the size distribution narrow. Finally stable, monodisperse, anionic particles are obtained. 

When conventional surfactants are used in emulsion polymerization, difficulties are encountered which are inherent in their use. Conventional surfactants are held on the particle surface by physical forces thus adsorption/des-orption equilibria always exist, which may not be desirable. They can interfere with adhesion to a substrate and may be leached out upon contact with water. Surfactant migration affects film formation and their lateral motion during particle-particle interactions can cause destabilization of the colloidal dispersion. 

In this equation, viscosity is independent of the degree of dispersion. As soon as the ratio of disperse and continuous phases increases to the point where particles start to interact, the flow behavior becomes more complex. The effect of increasing the concentration of the disperse phase on the flow behavior of a disperse system is shown in Figure 8-41. The disperse phase, as well as the low solids dispersion (curves 1 and 2), shows Newtonian flow behavior. As the solids content increases, the flow behavior becomes non-Newtonian (curves 3 and 4). Especially with anisotropic particles, interaction between them will result in the formation of three-dimensional network structures. These network structures usually show non-Newtonian flow behavior and viscoelastic properties and often have a yield value. Network structure formation may occur in emulsions (Figure 8-42) as well as in particulate systems. The forces between particles that result in the formation of networks may be... 

---