If a substance in dispersed state is evenly distributed within a mass of another substance, in colloid chemistry such system is called dispersed. The dispersed substance is usually referred to as dispersed or discontinued phase, and the substance where the phase is dispersed is known as dispersing or continuous medium.
All dispersed systems are classified in regard to degree of particle dispersion (size) or the aggregate state of the components of the system (phase and medium).
Depending on the size of the particle of the discontinued phase, the systems are divided into three groups:
- Coarse-grained (emulsions or suspensions) with particles larger than 10-4 cm (1 micron);
- Fine-dispersed (colloids) with particles in the range of 10-4-10-5 cm (1-0.1 micron);
- Molecular solutions (true solutions), with particles in the range of 10-5-10-7cm (0.1-0.001 micron).
There is no definitive line between the systems, since there are areas with fuzzy borders, the location of which depends on the properties of the matter in the dispersed phase (DP) and dispersing medium (DM), i.e. on their chemical nature.
The measure of subdivision of each dispersed system is usually considered the lateral size of the dispersed phase (for spherical systems it is equal to the diameter of the particle d), or the reciprocal to it D = 1/d, which is usually referred to as degree of dispersion. Several authors consider the mean surface area
Smean., i.e. the interphase surface. These two values are interrelated: the smaller the particle size, the higher the degree of dispersion or the mean surface area.
Molecular systems (high dispersal or true solutions), which have no mean surface, i.e. practically single phase, are not considered in this work.
Coarse and fine dispersion systems are usually classified according to aggregate state of the phases which phase separation surface (border). One such classification is as follows. First of all, all dispersed systems are divided into three aggregate state groups: liquid (L), solid (S) and gaseous (G). The dispersed phase (discontinued phase) is assigned index 1, and the dispersal medium (continuous phase) is assigned index 2.
With such system, any combination of dispersed systems can be recorded as phase ratio. For instance, T1/L2 is a solid dispersed phase in liquid dispersal medium, i.e. suspension.
The dispersed systems with liquid dispersion medium of most interest to road construction are as follows:
- Coarse dispersion systems:
- Free dispersion:
- T1/L2 – suspensions (such as clay in water);
- L1/L2 – emulsion (oil in water);
- Bound dispersion:
- G1/L2 – foams (such as foamed bitumen).
It is worth noting that in concentrated dispersed systems (foams), such as foamed bitumen, not only the dispersion phase degree of dispersion, but also that of dispersing medium, is high. In this case, the dispersing medium becomes the second dispersed phase.
- Fine dispersion (colloid) systems:
- S1/L2 – colloid solution (e.g., asphaltene in oil).
A common characteristic of dispersion system is the interaction of dispersed phase with the dispersing medium. From this view point, systems are divided into free or bound dispersion. The former include systems, where dispersed particles are not in contact and can move freely under the influence of Brownian agitation or gravity. Such systems are usually called sols.
In bound systems the particles are bound by intermolecular forces, forming structures unable to move and only capable of vibration. Such systems are called gels. Examples of such systems are foams, concentrated emulsions and pastes. Gels can be formed in one of two ways: coagulation of the system or molecular adhesion of the particles. In the former case, the even structure of the system is lost, where as in the latter case it remains.
In the systems with liquid dispersing medium the interaction of substances of dispersed phase and the medium always happens at the border of phase separation due to intermolecular forces. However, the degree of manifestation of such interaction may vary. In this respect, systems may be lyophilic, with strong intermolecular interaction, and lyophobic, with weak interaction.
True lyophilic systems consist of dispersed phase permeated completely by the dispersion medium, i.e. the systems is seemingly single phase (e.g. solution of rubber in benzol or gelatin in water). An example of lyophobic system is the emulsion of oil in water.
Lyophilic systems are stable in time, whereas lyophobic systems are unstable and tend to degrade over time, as the dispersed phase particles grow due to molecular forces. This phenomenon is called coagulation.
Coagulation of sol forms a gel. The reverse process (transformation of gel into sol) is called dispergation (peptizing).