8 junio, 2024

Chemical suspensions: what they are, characteristics, types, examples

We explain what a chemical suspension is, its characteristics, its components, types that exist, and we give several examples.

What are chemical suspensions?

The chemical suspensions They are a heterogeneous mixture formed by a solid dispersed in a liquid that does not dissolve. Suspensions are unstable solutions, since the solute has the peculiarity of sedimenting over time.

A suspension is a biphasic heterogeneous system, where the solute makes up the solid phase dispersed in a liquid medium, or dispersing phase. This dispersant phase can also be a gas or a mixture of gases in which the solid particles remain suspended.

The solute in the suspensions contains solid particles with a larger size than those that lie present in a true solution and the colloids. Therefore, it is at the larger particle size extreme for these substances (true solution < colloid < suspension).

The approximate size of the dispersed particles of the suspensions is greater than ten thousand angstroms. An angstrom, Å, is a unit of length equal to one ten billionth of a meter. It can also be said that one Å angstrom is equal to one ten-thousandth of a micron (1Å=0.0001 µm).

The formation of a suspension then depends on the size of the solute particles, its solubility properties, and its miscibility characteristics.

Characteristics of a chemical suspension

– It is a heterogeneous system, formed by two phases: a solid internal one, and an external one formed by the fluid or dispersing phase.

– The solid phase contains a solute that does not dissolve in the dispersing liquid, and therefore remains free-floating or suspended. This implies that the solute is kept, from the physical and chemical point of view, separated from the liquid phase.

– The particles that make up the solute are generally solid, large in size, and are visible to the naked eye.

– The size of the solute particles in the suspensions is close to or greater than 1 micron (1 µm).

– Due to its size, weight and over time, the solute has a tendency to sediment.

– The suspensions are characterized because they are easily resuspended, and quickly become homogeneous after mechanical agitation.

To keep suspensions stable, the pharmaceutical industry generally adds surfactants, stabilizers, or thickeners.

– The suspensions have a cloudy appearance, they are not clear or transparent, like homogeneous solutions.

– The components of heterogeneous mixtures, such as suspensions, can be separated by applying physical methods such as filtration.

Composition of chemical suspensions

As a biphasic system, suspensions consist of two components: the solute or dispersed phase, and the dispersing phase.

dispersed phase

The solute or dispersed phase is made up of solid particles in the suspension mixture. It does not dissolve, because it is lyophobic, that is, it hates the solvent due to its differences in polarity. The more lyophobic the solute, the shorter its settling time and suspension life.

Likewise, when the solute particles abhor the solvent, the greater their tendency to clump together to form larger aggregates, enough so that their sizes are no longer on the order of microns, as mentioned above. And then gravity does the rest: it pulls them to the bottom.

dispersing phase

The dispersant of the suspensions or external phase, in general, is liquid in nature, however, it can be gaseous. Suspension components can be separated by physical processes such as filtration, evaporation, decantation or centrifugation.

The dispersing phase is characterized by being molecularly smaller and more dynamic. However, by increasing its viscosity, it prevents the suspended solute from tending to aggregate and settle.

surfactant agents

Suspensions may contain surfactants or other dispersants to prevent solid phase particles from settling. Likewise, stabilizing substances can be added to the suspension, which increase the solubility and prevent the deterioration of the particles.

If one could hypothetically add a specific gas that fulfilled this function to a dusty room, all the dust would be removed from the objects when they were resuspended, and thus, it would be enough to blow fresh air to remove all the dust.

Types of chemical suspensions

There are different types of suspensions that can be classified according to the medium or dispersion phase, the sedimentation capacity, and in pharmacological matters, depending on the route of administration.

Depending on the dispersion medium

Dispersion media for suspensions are generally liquid. However, there are also gaseous media.

mechanical suspensions:sWith the most common suspensions, formed by the solid-liquid phases, already described, such as sand in a container with water.

Aerosol sprays:eThis is a type of suspension made up of fine solid particles plus liquid droplets suspended in a gas. Example of this suspension is found in the atmosphere and its layers of dust and ice.

According to the sedimentation capacity

There are suspensions that according to the sedimentation capacity can be classified as deflocculated suspensions and flocculated suspensions.

deflocculated:eIn this type of suspension, the repulsive force between the particles is important and they remain separated, without flocculating. In the initial phase of suspension formation no aggregates are formed.

flocculated:sThey are suspensions in which there is little repulsion between the solute particles and have a tendency to form flocs. The sedimentation rate of the solid phase is fast and the formed sediment is easily redispersible.

Examples of chemical suspensions

Atmosphere: is an example of an aerosol-type suspension, as it contains many suspended solid particles. The atmosphere contains soot, fine dust particles, sulfates, nitrates, among other compounds, interspersed with water droplets from the clouds.
mud or mire: It is a mixture of water with sand. Turbid rivers, when the water carries a large amount of sediment, form a suspension.
Fruit juices: they are suspensions, since the pulp of the fruits floats in the liquid medium. If we want to dispose of the liquid medium, we must decant or filter the mixture.
cloudy river water: the sediments carried by the river form the suspension.
watercolors: they are a suspension that is deposited on the paper, where it filters the water and collects the color pigment.
powder medicines: to keep them in suspension and not settle to the bottom you have to stir them.
exfoliating creams: where there are small particles forming solid grains in the cream to fulfill the exfoliation function.
Milk: animal fat particles are in solution with water. Because they are less dense than the dispersant, they tend to settle on the surface over time.
Paint: is a suspension of color pigments in an aqueous or oily medium. If it is not stirred, it can come to separate.
Seawater: in the shore area a suspension with sand particles can be considered, although it is a suspension of limited duration.
salad dressings: they contain vegetable particles suspended in oil or vinegar, they have a viscous dispersant that keeps them in a state of rest.

Other examples of common suspensions

Horchata water.
Cocoa in milk or water.
Moisturizing creams or facial creams.
Penicillin.
Insulin.
Amoxicillin (antibiotic).
Powder makeup.
Ash in a volcanic eruption.
Milk powder.

References

Soult, A. Colloids and suspensions. Retrieved from chem.libretexts.org
Reid, D. What is Suspension in Science? – Definition, Types & Examples. Retrieved from study.com

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