CHEMICAL SUSPENSIONS: CHARACTERISTICS, COMPOSITION, EXAMPLES - CHEMISTRY - 2024

The chemical suspensions are a heterogeneous mixture of a solute that is not dissolved in the solution. Suspensions are unstable solutions, since the solute has the peculiarity of sedimenting over time.

But exactly what is a suspension? It is a heterogeneous biphasic system, where the solute composes the solid phase dispersed in a liquid medium or dispersant phase. This dispersing phase can even be a gas or a mixture of gases in which the solid particles remain suspended.

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The solute in suspensions contains solid particles larger than those found in a true solution and colloids; therefore, it is on the extreme end of the largest particle size for these substances (true solution

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

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

The solute in the emulsions has zero miscibility, that is, the solute does not have the ability to dissolve. But with the addition of an emulsifying agent (or emulsifier), it is possible to stabilize the emulsion; This is, for example, the case of mayonnaise, where the egg white acts as an emulsifier.

In the pharmacological industry there are a variety of suspensions whose solid and insoluble solute is the active principle of a drug. These particles are dispersed in the medium, with the help of excipients the solute can be kept suspended in the mixture.

Examples of the simplest suspensions include the one formed by a mixture of sand with water; the dust that is suspended in the air and that by gravity is deposited on the surfaces; sunscreen, among many others.

Suspension characteristics

There are many characteristics that allow a suspension to be defined and clearly differentiated from true solutions and colloids:

Physical

-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, separate from the liquid phase.

-The particles that make up the solute are generally solid, large in size, and 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 in that they are easily resuspended, and quickly homogenize after mechanical stirring.

- To keep suspensions stable, generally the pharmaceutical industry adds surfactants, stabilizers or thickening agents.

-The suspensions have a cloudy appearance, are not clear or transparent; as are homogeneous solutions.

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

Sedimentation time

Perhaps one of the first questions to ask yourself as to whether a substance is a suspension or a colloid is the sedimentation time of the solute. In true solutions, the solute will never clump together to form a precipitate (assuming the solvent does not evaporate).

For example, if sugar is dissolved in water, and the unsaturated solution is kept covered to prevent solvent leakage, no sugar crystals will form at the bottom of the container. The same is true for colored solutions of various indicators or salts (such as CuSO ₄ ∙ 5H ₂ O).

However, in suspensions the solute does end up clumping at a certain time, and as a result of the increase in their interactions, they settle to the bottom. Therefore, they exist for a very short period of time.

Another example is found in redox reactions where KMnO ₄, with an intense purple color, participates. By reducing or gaining electrons, oxidizing the chemical species of interest, a brown precipitate of MnO _{2 is formed}, which remains suspended in the reaction medium; very small brown grains.

After a certain time (minutes, hours, days), the suspension of MnO ₂ in the liquid ends up settling to the bottom like a "brown carpet".

Stability

The stability of the suspensions is related to the resistance to the change of their properties over time. This stability is achieved with control of several factors including the following:

-Suspensions must be easily resuspendable by mechanical stirring.

-The control of the viscosity of the dispersion, which reduces the sedimentation of the solute; therefore, the viscosity must be high.

-The smaller the size of the solid phase particles, the greater the stability of the suspensions.

-It is useful to incorporate substances such as surfactants, emulsifiers or antifreezes into suspensions. This is done in order to decrease the aggregation or flocculation of the internal phase particles or solid particles.

-A constant control over the temperature must be maintained during the preparation, distribution, storage and use of the suspensions. To ensure their stability, it is important not to subject them to sudden changes in temperature.

Composition

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

Scattered 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 abhors the solvent for its differences in polarity. The more lyophobic the solute, the shorter its sedimentation time and the life of the suspension.

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

This is where the stability of the suspensions lies. If the aggregates are in a viscous medium, then more difficulties will be found for them to interact with each other.

Dispersing phase

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

The dispersant phase is characterized by being molecularly smaller and 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 a specific gas that would fulfill this function could be hypothetically added to a dusty room, all the dust would be removed from the objects when resuspended; and thus, it would be enough to blow fresh air to remove all the dust.

Differences between suspension, colloids and true solutions

It is important to highlight some differences between suspensions, colloids and true solutions to better understand their composition.

-Colloids and true solutions are homogeneous mixtures, and therefore, have a single phase (visible); while suspensions are heterogeneous mixtures.

-Another difference between them lies in the size of the particles. In a true solution, the size of the particles ranges from 1 to 10 Å, and they dissolve in the solvent.

-In true solutions the solute does not remain solid, it dissolves forming a single phase. Colloids are an intermediate type of mixture between true solutions and suspensions.

-A colloid is a homogeneous mixture, formed by solutes whose particles have a size ranging from 10 to 10,000 Å. In both colloids and suspensions, the solute remains solid and does not dissolve.

-The solute of the colloid remains suspended in the dispersing phase, does not tend to sediment and is not visible to the naked eye. Milk is one of many examples of a colloidal solution. In suspension, the solute tends to settle and is visible with the naked eye or with a light microscope.

Types

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

-According to the dispersion medium

The dispersion media of the suspensions are generally liquid, however there are also gaseous media.

Mechanical suspensions

They are the most common suspensions, formed by the solid-liquid phases, already described; like sand in a bowl of water. However, there are suspensions such as the aerosols described below.

Aerosol sprays

This is a type of suspension made up of fine solid particles plus liquid droplets suspended in a gas. An example of this suspension is found in the atmosphere and its layers of dust and ice.

-Depending on the sedimentation capacity

There are suspensions that according to their sedimentation capacity can be classified into deflocculated suspensions and flocculated suspensions.

Defloculated

In this type of suspension, the repulsive force between the particles is important and they are kept separate, without flocculating. In the initial phase of suspension formation, no aggregates are formed.

The sedimentation rate of the solute is slow and it is difficult to resuspend the sediment once it has formed. In other words, even if they were agitated, the particles will not resuspend; This is especially the case with gelatinous solids, such as Fe (OH) ₃.

Flocculated

They 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 sediment formed is easily redispersible.

-Depending on the route of administration of the suspension

There are oral suspensions, which are easy to administer and generally have a milky appearance. There are also suspensions for topical use, presented as creams, ointments, emollients, protectants, which are applied to the skin or mucous membranes.

There are suspensions that can be applied by injections, and in aerosol, such as salbutamol, which is a bronchodilator.

Examples

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There are numerous examples of suspensions in nature, in products and foods, and in the pharmaceutical drug industry.

In nature

The 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 clouds.

Another example of suspension found in nature is mud or mud, which is a mixture of water and sand. Turbid rivers, when the water carries a quantity of sediment, form a suspension.

In the kitchen

The mixtures made in the kitchen by joining flour with water constitute an emulsion: with rest the flour tends to settle. Fruit yogurts are examples of foods that are suspensions. Fruit juices that have not been strained are examples of suspensions.

Likewise, the chocolate chips in a glass of chicha constitute a rather heterogeneous and unstable suspension. Leaving the chicha at rest, sooner or later a layer of chocolate will form at the bottom of the glass.

In the pharmaceutical industry

Suspensions used to combat parasitic infections, such as mebendazole, are known. There are also the intestinal astringents that contain magnesium and aluminum salts, mixed with pectin and kaolin.

These pharmacological suspensions can have different routes of administration: topical, oral or injectable. They will have different uses, that is, they serve for the treatment of various diseases.

There are ophthalmic and otic suspensions, among others. It is recommended that the suspension be resuspended, well before consuming to guarantee the dose prescribed by the doctor.

Glass of sand vs glass of stars

Some poetic phrases say: the white stars suspended in the sky.

Although the comparison between a glass of water with suspended sand and a “cosmic glass” of stars is totally disproportionate (and far-fetched), it is interesting to consider for a moment the universe as a huge suspension of stars (and countless other bodies). celestial).

If so, they would not move away from each other; rather, on the contrary, they would end up grouping together to form a layer of stars at the bottom of said cosmic vessel.

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