Dispersion systems are heterogenous systems, which contain small particles of dispersed phase distributed throughout dispersion medium. Some examples of dispersion systems include aerosols (droplets of liquid or particles of solid matter are sprayed in gaseous medium), emulsions (formed by immiscible fluids), suspensions (solid particles of dispersed phase are distributed in liquid), and so on. Most natural dispersion systems are complex by their composition. All the biological fluids and tissues of our body, as well as sea-water, soil, cosmetics and food belong to dispersion systems.
Among the dispersion systems, we usually emphasize colloidal solutions, defined as systems with nanosized solid particles of the dispersed phase (1–100 nm, it is 10–9–10–7 m), distributed in liquid dispersion medium. Because of the specific size and charge of particles, these colloids have a certain aggregative and sedimentation stability (the particles do not form any aggregates and do not settle down). Colloidal solutions take an intermediate position between true solutions (the size of ions, molecules is less than 1 nm) and coarsely dispersed systems (the size of particles is more than 100 nm). Examples: aqueous solutions of sodium chloride or glucose are true solutions; slurry of enterosorbent or starch in water — suspensions (coarsely dispersed systems). But if we prepare a glue, dissolving sodium silicate in water or adding starch suspension to boiling water (paste forms due to starch hydrolysis, because the fragmentation of large molecules happens), such solution is a typical colloid and characterized with some specific properties (see below), which appear due to nanometer size of particles. Nowadays, a lot of colloidal plasma substitutes and other drugs are used in practical medicine.
The size of colloidal particles may be found with the help of ultramicroscope or more modern methods: transmission electron microscopy (TEM), scanning electron microscopy (SEM), or may be calculated by formulae.
Properties of colloidal solutions
1. Optical properties.
Colloidal solutions are able to scatter light. If a beam of light passes through a colloid, it becomes visible and forms the opalescent cone (Tyndall effect).
In accordance with Rayleigh’s light scattering law, the intensity of light scattering is inversely proportional to the wavelength, raised to the fourth power (I~1/λ4). As a result, short waves (blue part of the spectrum) are scattered more intensively during the light passage, and colorless colloids become bluish. This phenomenon gives an explanation for the blue color of the atmosphere, because our air is an aerosol, which contains the particles, comparable with a half wave-length (this is a requirement of Rayleigh’s law).
Opalescence is usually observed in colloidal solutions with colorless particles. This is a difference in colors of transmitted and scattered lights, distinctive «shine» of colloids. In colored colloids, the colour of colloidal particles depends not only on their nature, but also on their size, shape and charge.
2. Electrokinetic properties.
As the colloidal particles carry the charge, there are the following phenomena: electrophoresis (movement of dispersed phase in an electric field), electroosmosis (movement of dispersion medium in an electric field), formation of sedimentation potential and potential of streaming. All these phenomena have many applications. For example, electrophoresis is not only a physiotherapeutic procedure in medicine and cosmetology, but also popular in clinical diagnostics and scientific researches in order to separate protein molecules and nucleic acids. Moreover, bacteria and viruses carry the charge, which promotes their movement in an electric field, thus electrophoresis is used in medicobiological investigations. Electroosmosis is applied for drug purification. Streaming potential appears during the blood flow in vessels and allows to explore the heart work and the blood vessels state.
3. Molecular-kinetic properties.
The rate of Brownian motion, diffusion and osmosis are much lower in colloids comparing with the same in the true solutions.
Hydrophilic and hydrophobic colloidal solutions
There are lyophilic and lyophobic colloids differ by the intensity of interactions between the dispersed phase and dispersion medium. As we discuss only aqueous solutions in this chapter, the terms “hydrophilic” and “hydrophobic” colloids, or sols are used here and further.
Let us examine the structure of colloidal particle, which is called micella.
Hydrophilic micellas are spheres in biopolymer solutions (proteins, polysaccharides, nucleic acids and so on), SAA (with the long chains mainly), and some organic pigments. The polar parts of molecules stay outside the sphere, and hydrophobic — hide inside. The same principle works by the formation of the lipid bilayer in biological membrane, highest structures of proteins and nucleic acids. Micellas may get more complicated shape, which depends on concentration, temperature, and presence of other substances in a solution. Hydrophilic micella is surrounded by hydrated covers, that is why such solutions are rather stable.
Hydrophobic micellas are mostly formed by insoluble or slightly soluble inorganic compounds or organic molecules, which does not contain polar groups. The sediments need a stabilizer for their existence in colloidal form. A stabilizer creates the protection layers, which prevent the aggregation of colloidal particles. Sometimes such stabilizer may present in a solution (biological fluids) or added on purpose, but most of the time excess of one reaction reagent serves as a stabilizer (in vitro preparation). For example, calcium phosphate Са3(PO4)2 formed in presence of an excess of soluble calcium salts has the positive nucleus of micella, as Са2+ adsorbs on the surface of crystals trying to build crystal lattice of this compound. And vice versa, if a surplus of phosphate ions PO43– is present, it charges the nucleus of micella negatively. The processes of a similar nature occur during the concrement formation (calculus, stones, plaques) in the body. These insoluble compounds usually contain calcium and magnesium oxalates, phosphates, carbonates, sodium urate (insoluble salt of uric acid), hydroxyapatite (basic calcium phospate, the main component of bone and dentine tissue).