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The colloidal state

Colloid science deals with systems comprising molecules/particles with different dimensions, ranging from smaller (nanometer (10^-9m) to larger (micrometer (10^-6m) molecules/particles. As these systems contain a mixture of particles with different sizes, colloid systems are heterogeneous in nature. Colloid systems are very important as they have a wide range of applications, including adhesion, chromatography, detergency, precipitation, road surfacing, food processing, grinding, heterogeneous catalysis, ion exchange, lubrication, sugar refining, sewage disposal, and wetting. It is important to note that systems in the colloidal state may be desirable or undesirable depending on its applicability. Therefore, in depth knowledge of the formation and destruction of colloidal systems is necessary.

Colloid science includes the fields of physics and physical chemistry. The natural laws of physics and chemistry that are associated with the behaviour of matter in bulk and molecular states also apply to the colloidal state (Shaw, 1991). The nature of colloidal systems is reliant on certain factors that are also dependent on the physical and chemical properties of the systems. These factors are particle size, particle shape and flexibility, surface (as well as electrical) properties, particle – particle interactions, and particle – solvent interactions (Shaw, 1991).

Colloidal dispersions

Due to their high surface-free energy, colloidal dispersions are thermodynamically unstable. Systems of this kind are irreversible when considering such processes as phase separation and so may not be easily reformed back to their original state. Particles in colloidal dispersion are large enough for easy and precise separation between the particles and the medium in which they are dispersed and so simple colloidal dispersions can be taken as two-phase systems. For these simple systems, the dispersed phase can be taken as the phase forming the particles while the dispersion medium can be taken as the medium in which the particles are dispersed. For a given dispersion system, its physical nature depends mostly on the physical properties of the individual constituent phases rather than the overall composition. For such systems as an oil-in-water emulsion and a water-in-oil emulsion, a notable fact is that irrespective of similarities in the overall composition of these systems, their physical properties may be different. This suggests that the individual constituents are very influential when determining the nature of dispersion. Sols, a term which differentiates colloidal from macroscopic suspensions and emulsions – a mixture of two or more immiscible materials (e.g. particles and liquid) – are the most important types of colloidal dispersion.

In colloidal dispersions, interfaces exist between the dispersed phase and the dispersion medium due to the large area-to-volume ratio exhibited for the particles. At these interfaces, surface properties such as electric double layer effects and adsorption are eminent and influential in determining the physical properties of the overall system. As such, the particular material around the molecular layer of the interface, is the most influential on interactions between particle-particle and particle-dispersion medium (Shaw, 1991). In colloidal dispersions, interfaces and therefore the overall properties of the system can be modified considerably when small amount of suitable materials are added. This is possible even though a large area-to-volume ratio exists in these dispersions. For instance, small amounts of calcium ions for thickening or phosphate ions for thinning can be added to some clay suspensions for significant modifications in its consistency (Shaw, 1991).

Furthermore, terms such as lyophilic and lyophobic can, in a way, be used to describe colloidal systems especially when considering the phenomenon of surface activity (Verwey & Overbeek, 1999). Lyophilic systems can be described as liquid-loving in nature while lyophobic systems can be regarded as liquid-hating. These terms are often used to describe the tendency of a surface to become wet or solvated. For an aqueous liquid medium, the terms hydrophilic and hydrophobic are often used.  With regards to surface activities, lyophilic surfaces can be made lyophobic, and vice versa. The molecules of surface active materials contain both hydrophilic and lipophilic (described as oil-loving) regions which gives them a strong affinity for interfaces (Shaw, 1991). In relation to colloidal systems, the term lyophobic refers to the liquid dispersion of solid or liquid particles created by chemical or mechanical action. However, there are situations where lyophobic systems behave in a lyophilic manner. In such cases, for systems such as lyophobic sols (e.g. powdered alumina or silica in water dispersions), there is a strong affinity between the particles and the dispersion medium. This obviously suggests the particles are lyophilic. This is different from the behaviour of a real lyophobic system where an affinity between particles and the dispersion medium does not exist. The implication is that the particles would not be wet and therefore no dispersion would occur. This disparity observed in lyophobic systems (where they behave in a lyophilic manner) can exist in certain lyophilic systems. In such cases, although lyophilic describes soluble macromolecular material, lyophobic regions exist. An example to buttress this is in the case of proteins that are somewhat hydrophobic (hydrocarbon region) and somewhat hydrophilic (peptide affiliations including amino and carboxyl groups) (Shaw, 1991).

Structural Characteristics

Particle Shape

A very important factor in determining the general properties of colloidal systems is particle asymmetry (Shaw, 1991). Taking shape into consideration, colloidal particles can be classified as corpuscular, laminar or linear. Shapes can range from complex to simple but for clarity, relatively simple shapes are usually considered (Figure 1). Of the simple shapes, spheres are considered to be the simplest; however, there are a wide variety of colloidal systems containing particles with spherical or almost spherical shape. Systems such as latex, liquid aerosols, and emulsions contain spherical particles. Certain protein molecules are almost spherical. Further, certain particles that are not full spheres but are sufficiently symmetrical can be regarded as spheres. An example is the crystallite particles in dispersions such as gold and silver iodide sols.

Unlike spherically shaped particles, corpuscular particles are usually considered as ellipsoids of revolution, a shape possessed by many proteins. The axial ratio (which is the ratio of the single half-axis a to the radius of revolution b) is a characteristic function of an ellipsoid of revolution. It is greater than unity for a prolate ellipsoid and less than unity for an oblate ellipsoid (Shaw, 1991). A prolate takes the shape of a rugby football while an oblate is discus shaped. An example of systems containing plate-like particles is the iron (III) oxide and clay suspensions. Examples of long thread-like straight or branched-chain molecules are high-polymeric materials. These materials usually possess high mechanical strength and durability due to the presence of attractions between inter-chains or cross-linking and entanglement of the polymer chains. This inter-chain attraction or cross-linking occurs as a result of covalent bonding including hydrogen bonding or van der Waals forces. In the case of corpuscular or laminar shaped particles, this is not feasible. Thread-like polymeric materials can exist naturally in both plants and animals as they contribute to their structural outcome. Cellulose fibres form a basis for the assembly of plant life. Furthermore, thread-like polymeric materials found in animals such as linear protein material as seen in collagen in skin, sinew and bone, myosin in muscle and keratin in nails and hair, form a building block for animal life. Corpuscular-shaped particles can be produced in body fluids from the folding of coiled polypeptide chains of globular proteins which circulate in the fluids. A notable fact is that in the event of particle aggregation, a wide variety of shapes can be formed which may not be similar to the original shape of the primary particles.

Flexibility

Compared to most of the particle shapes, thread-like high polymer molecules exhibit high flexibility due to rotation about carbon-carbon and other bonds. Moreover, this rotation does not give full assurance of flexibility as the shapes of these molecules changes more often under the influence of thermal motion. For further insight, the polymer molecules can be considered as random coils. Also, the effects of steric and excluded volume negate the creation of a completely random configuration which makes the tendency of dissolved linear polymer molecules more extended than random coils, taking into consideration the magnitude of polymer-polymer and polymer-solvent forces. In the event of polymer chain segments sticking together, the random coils will be closely packed including an occurrence of precipitation. On the other hand, loosely packed coils occur when due to strong solvation and/or electrical repulsion the polymer segments avoid one another.

Solvation

In terms of solvation, colloidal particles can be solvated to the level of about one molecular layer and the resulting tightly bound solvent can be taken as a part of the particle. Most often, when a huge number of flocculent hydroxide precipitates are formed, solvents can be immobilised by mechanical entrapment within particle aggregates. A continuous three-dimensional network may be formed for cases where the polymer chains in long thread-like molecules cross-link chemically or physically, and/or become mechanically entangled (Shaw, 1991). A gel can be formed if all the solvent within this network becomes mechanically trapped and immobilised, making the entire system appear to be solid.

Polydispersity

For a monodispersed system, the molecules or particles are all similar and so the term’s relative molecular mass and particle size applies. From the initial definition given, colloidal systems are polydispersed in nature since the molecules or particles in a given system vary in size. Generally, colloidal particles and polymer molecular sizes tend to have uneven distributions based on their stepwise build up. This observation is as seen in the Poisson distribution which gives a good approximate, as shown in (Figure 2). The individual contribution of the different molecules or particles to the measured property of the system determines the importance of the word ‘average’.

Figure 2          Particle diameter distributions for a polydispersed colloidal dispersion shown (a) in histogram form, and (b) as a cumulative distribution (Shaw, 1991).

A number-average relative molecular mass is generated by osmotic pressure which depends on the number of solute molecules present.

(1)             

Where  is the number of molecules of relative molecular mass .

Most often, the property being measured is largely influenced by the contribution of the larger particles. A mass-average relative molecular mass or particle mass is derived if the contribution of each particle is proportional to its mass. It is given as:

(2)           

Generally, for polydispersed systems,  (mass average)   (number average), except in the case of monodispersed systems in which case these averages will be equal. The measure of polydispersity is the ratio of  (mass average)/ (number average) (Shaw, 1991).

Preparation and purification of colloidal systems

Colloidal dispersions

Generally, colloids can be formed either by degradation of bulk matter or aggregation of small molecules, particles or ions (Shaw, 1991). Although bulk material can be degraded, as is carried out in industrial grinding such as in colloid mills, a complete segmentation of sub-degraded materials may be difficult to accomplish as smaller particles may reunite. This may be due to the influence of mechanical forces as well as the attracting forces between the particles. During grinding, a stage is reached where particle distribution reaches an equilibrium, in which case a more desirable dispersion can be achieved by adding an inert diluent to prevent particles from meeting each other. Wet-milling with the incorporation of surface-active material may be another option. Another method of dispersion which may seem to achieve better results is when a sol is prepared by an aggregation method: a molecularly dispersed supersaturated solution is formed paving the way for the separated precipitation of the material (sol).

Nucleation and growth

During precipitation, the creation of a new phase involves two stages: nucleation and crystal growth. As nucleation is the formation of centres of crystallisation, the relative rates of these processes is influential in determining the particle size of the precipitate formed. For a desirable level of dispersion to be achieved, the rate of nucleation has to be higher than that of crystal growth. The degree of supersaturation can actually be reached before phase separation occurs, which influences the initial rate of nucleation and creates room for easy preparation of the colloidal sols when the material involved has very low solubility. For very soluble materials like calcium carbonate, there is the possibility that smaller particles can redissolve and recrystallise on the larger particles as the precipitate time line increases.

The rate of particle growth depends mainly on such factors as (Shaw, 1991):

1. The amount of available material
2. The viscosity of the medium which controls the rate of diffusion of material to the surface of the particle
3. The simplicity of appropriate orientation of material and inclusion into the crystal lattice of the particle
4. Particle-particle aggregation
5. Adsorption of impurities which hinder growth on the surface of the particle.

Monodispersed sols

As the creation of new nuclei and the growth of already existing nuclei occur alongside each other, methods of aggregation often lead to the creation of polydispersed sols. This means particles are created from nuclei which were formed at different times. Experimentally, monodispersed systems can be prepared with conditions involving the restriction of nucleation to rather short periods at the initial stage of sol creation. This can be derived by using very small particles to seed supersaturated solution or alternatively using conditions which lead to a short fracture of homogeneous nucleation.

References

1. Shaw, D. J., (1991). Introduction to colloid and surface chemistry. (4^th) Butterworth-Heinemann.
2. Verwey, E. J. W., Overbeek, J. TH. G. (1999). Theory of the stability of lyophobic colloids. Elsevier.