A dispersed system is a mixture between two or more substances, either simple or compound, in which there is a discontinuous phase. They are systems in which a substance is dispersed within another substance. The dispersions can be homogeneous or heterogeneous; the dispersed phase, typically some particle, may or may not be distinguished from the medium in which it is dispersed.
Dispersions can be found in many substances in pharmaceuticals. From solutions of quite large molecules, such as albumin and polysaccharides, to nano and micro liquid suspensions, and coarse emulsions and suspensions.
Having physically distinctive phases allows dispersions to have different properties than true solutions, such as particle aggregation and fit.
In any dispersed system there are two different phrases: the dispersed and the dispersant. The dispersed phase refers to the one that is distributed in the other phase, which is called dispersant.
Dispersed systems can be classified in several different ways, including how large the particles are relative to the continuous phase particles, regardless of whether precipitation occurs.
Main types of dispersed systems
Suspensions
A suspension is a heterogeneous mixture that contains solid particles that are large enough to settle.
In suspensions, the heterogeneous mixture shows the solute particles suspended in the medium and not completely dissolved. They can be gross or coarse dispersions, or fine dispersions.
The particles in the suspension are visible to the naked human eye. In suspensions, the particles are floating freely in a solvent.
The internal phase (solid) is dispersed through the external phase (fluid) through mechanical agitation, with the use of certain excipients or suspending agents.
A clear example of a suspension is sand or soil in water. Suspended soil particles will be visible under a microscope and will eventually settle out over time if left undisturbed.
This property differentiates colloids from suspensions, since in colloids the particles are smaller and do not settle.
In turn, colloids and suspensions are different from solutions since the dissolved substance does not exist as a solid, and the solvent and the solute are homogeneously mixed.
A suspension of liquid droplets or fine solid particles in a gas is called an aerosol. For example, in the atmosphere they can be found in the form of soil particles, sea salt, nitrates, and cloud droplets.
Suspensions are classified on the basis of their dispersed phase and the dispersion medium. The dispersion medium is essentially a solid, while the dispersed phase can be a liquid, a gas or a solid.
From a thermodynamic point of view, suspensions are unstable. However, it can stabilize over a period of time, which determines its useful life. This is useful in industries in establishing a quality product for consumers.
Colloids or colloidal systems
A colloid is a mixture in which one substance of microscopically dispersed insoluble particles is suspended through another substance.
Colloids can sometimes have the appearance of a solution, so they are identified and characterized by their physical-chemical and transport properties.
Unlike a solution, where the solvent and solute constitute only one phase, a colloid has a dispersed phase (the suspended particles) and a continuous phase (the suspension medium).
To qualify as a colloid, a mixture must not settle or it must take a long time to noticeably settle.
The particles of the dispersed phase have a diameter of approximately 1 and 1000 nanometers. These particles are normally visible under a microscope.
Homogeneous mixtures with dispersed phase in this size can be called colloid aerosols, colloid emulsions, colloid foams, colloid dispersions or hydrosols.
The dispersed phase particles are severely affected by the chemical surface present in the colloid.
Some colloids are translucent by the Tyndall Effect, which is the scattering of light particles in the colloid. Other colloids may be opaque or may have a slight color. In some cases, colloids can be considered as homogeneous mixtures.
Colloids can be classified into:
- Hydrophilic colloids: Colloid particles are attracted directly to water. They are also called reversible sols.
- Hydrophobic colloids: They are the opposite of the above; hydrophobic colloids are repelled by water. They are also called irreversible sols.
True solutions
A solution is a homogeneous mixture composed of two or more substances. In such mixtures, the solute is the substance that dissolves in another substance - known as a solvent.
The process of combining a solution occurs on a scale where the effects of chemical polarity are involved, resulting in interactions specific to solvation.
Usually, the solution assumes the solvent phase when the solvent is the largest fraction of the mixture. The concentration of a solute in a solution is the mass of solute expressed as a percentage of the mass in the complete solution.
The solute particles in a solution cannot be seen with the naked eye; a solution does not allow light rays to scatter. The solutions are stable, they are composed of a single phase and their solute cannot be separated when filtered.
The solutions can be homogeneous, in which the components of the mixture form a single phase, or heterogeneous, in which the components of the mixture are of different phases.
The properties of the mixture, such as concentration, temperature and density, can be uniformly distributed throughout the volume, but only in the absence of diffusion phenomena or after its completion.
There are several types of solutions, including:
- Gaseous solutions, such as air (oxygen and other gases dissolved in nitrogen)
- Liquid solutions, such as gas in liquid (carbon dioxide in water), liquid in liquid (ethanol in water), and solid in liquid (sugar in water)
- Solid solutions, such as gas in solids (hydrogen in metals), liquids in solids (hexane in paraffin) and solid in solid (alloys and polymers)
References
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- Coarse dispersion (suspension). Recovered from wikipedia.org.
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