These are eterogeneous systems, resulting from the mixing of two or more substances, in which are distinguishable different phases. Some common eterogeneous systems are: Aerosol ( solids or liquids dispersed in a gas); Emulsion (liquids dispersed in a liquid); Suspension (dispersion of a solid in a liquid). In these systems we can distinguish a dispersing phase and a dispersed phase, the dispersing phase is the continuous phase in which the dispersed phase is distributed. Normally in these systems the phases tends to separate under gravitational force.
Colloidal dispersions. The borderline between dispersions and true solutions is not well defined. In fact, there exist a number of dispersions apparently homogenous but having properties differents from those of true solutions, these are called colloidal dispersion or colloidal solutions. In colloidal solutions, the dispersed particles are larger than those in a true solutions but not enough large to be visible by an optical microscope (as in emulsions and suspensions). According to the dimensions of particles the dispersed systems can be classified as:
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Suspensions or Emulsions In which have particles larger than 10-5 cm visible by an optical microscope |
Colloidal systems In which the dimensions of partcles range between 10 -5 e 10-7 cm |
True solutions In which the dimensions of particles are lesser than 10 -7 cm (10 Angstrom. |
The particles forming a colloidal solutions can be macromolecules like proteins or molecular aggregates (micelles). The formation of a micelle dipends on the dipersing phase, for example sodium chloride in water form a true solution but it forms a colloidal system in ethyl alcohol. Commonly, some compounds are said colloidal but this not very correct since the colloidal state depends on the dispersing phase. An important differences between a true solution and a colloidal solution is that in a colloidal systems there exist a well defined interface separating dispersed phase and dispersing medium. At this interface take place important penomenons as adsorption, coagulation, formation of a layer of electrical charges. In colloidal systems this interface is more important than chemical composition in determining the properties of the solution.
Colloidal solutions (generally indicated as Sol) can be classified as
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Hydrophilic
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Colloidal solutions formed by particles having affinity with the solvent (water in this case). The particles are generally high molecular weight polymers such as proteins and starch. These systems have properties similar to true solutions and are very stable. Furthermore, the disperse phase has the tendency to combine with the dispersion medium to produce a semisolid material (gel), this phenomenon can be reverted by addition of dispersion medium. For this reason hydrophilic colloidal solutions are said reversible. |
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Hydrophobic
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On the contrary hydrophobic colloidal solutions are formed by particles with poor affinity to the solvent. Particles are generally aggregates (micelles) of inorganic compounds. They are relatively unstable and can coagulate by addition of small amount of electrolytes or by increasing the temperature. Some of them can form gel (ferric hydroxide, alluminium hydroxide, silicic acid) however they can not reverted to sol by the addition of solvent and are said irreversible. |
Methods of separations. The disperse particles can be separate from the dispersion medium and from true solutes by means of membranes such as cellophane (transparent cellulose material) or collodion (nitrocellulose). These membranes acts as filters having pores allowing only the passage of small molecule and ions thus retaining colloidal particles. This methods of separation is called dialysis. Dialysis can be also accomplished by applying an electrical potential (electrodialysis) on the two sides of the membrane thus accelerating the process of separation. Another methods of separation is ultrafiltration (SEE unit operations).
Osmotic properties. We have already seen that the osmotic pressure depends on the number of particles in a solution. For the high molecular weight of particles, the number of molecules in a colloidal solution is small compared to those in a true solution and thus the osmotic pressure of a colloidal solutions may be very low and cannot be accurately measured. However, if the molecular weight of particles is not very high the measure of osmotic pressure can be accurately accomplished to achieve the determination of the average molecular weight of particles.