Suspension, types and, properties, its preparation and destruction.
A suspension is a dispersed system where the dispersed medium is liquid and dispersed phase is solid, which puts it in the category of coarse.
Suspension – a dispersion system in which dispersed medium is liquidand dispersed phase is solid, which puts it in the category of coarse. In this phase there may be several and they are represented most often in powder form.
A striking example of a suspension is considered to be the cement mortar, this may also include paint-based enamel, drilling mud. However, the most popular suspensions in the pharmaceutical industry, because it is one of the most popular dosage forms.
The name of the system received from the Latin word suspensio meaning “hanging”. In essence , the suspension is a suspension in which after the connection of the liquid medium and the solid phase, presented in the form of tiny particles, the sedimentation of the latter (sedimentation) occur very slowly. The reason is that, unlike other systems, the particles size is still large (over 10 micrometers) and small difference in density of its components (phase and environment). Provided that the concentration of the powdery component is large, dispersion medium is formed very quickly.
So, dissolved in water flour is a typical representative of the suspension. If the concentration of the particles of flour are in a liquid in suspension, virtually no settling on the bottom. Over time the sedimentation does occur, appears a dense precipitate, the molecules of which is subject to gravity. During subsequent stirring, the suspension re-acquire their original condition, and quite quickly.
The separation of suspensions as a dispersed system into classes, types and takes into account several key characteristics:
1. The nature of the dispersion medium. This can be water or organic liquid.
2. The particle size of the dispersed phase. Are divided into 3 types depending on the diameter:
– rude – at least 1 micrometer;
– thin – from 0.1 to 1 micrometer;
– turbidity of less than 0.1 micrometer.
3. Concentration. The volume of the dispersed phase can make the system dilute (suspension) or concentrated like a paste.
The diluted slurry is less stable: the particles of the dispersed phase can be easily moved in the environment, the structure of the system as kinetic energy, binding the constituent phases.
In the same paste-like suspensions of the relevant physical forces present, which makes them svjaznodispersnye structured (having a spatial grid) systems.
Concentration or concentration interval directly influences the formation of structure in suspension, while all the components of the suspension, these indicators individually. They include:
– the nature of the dispersed phase;
form of particles of which the phase is;
– temperature of the mating components;
– the presence and forms mechanical effects on suspension.
The presence or absence of this structure directly affect the properties of the resulting dispersed systems.
There are the following main properties:
– molecular kinetic.
Mechanical properties of pastes and suspensions differ significantly. Thus, in dilute suspensions they directly depend on the selected dispersion medium, and the concentrated from of the dispersed phase and the number of contacts between particles.
Mechanical properties are manifested in the fact that in dilute suspensions the particles are moving freely in the liquid, the adhesion between the particles is absent and each particle is kinetically independent. The diluted suspension is svobodnodispersnye structureless system.
In the concentrated suspensions (pastes), on the contrary, between the particles there are forces leading to the formation of a specific structure (spatial grid). Thus, the concentrated suspension is svjaznodispersnye structured system.
Passing through the composition, the light rays are absorbed, scattered or reflected. The most studied and important optical parameter is the ability of the suspension to scatter light. The dispersion light is converted to particles of the system, with the result that it changes the direction and intensity of incident light changes. Such a property is typical for highly dispersed suspensions. In stained suspensions light waves tend to be absorbed, the rest reflected, which give them a cloudy look.
Based on these properties, on the interaction between energy, the resulting electromagnetic radiation of a disperse system, and the particles that comprise the medium composition. The main parameters of the latter will carry them:
– the ratio between the wavelength of electromagnetic radiation and the size of the molecule.
Electrokinetic properties of suspensions occur due to contact between the solid particles of the suspension constituting the dispersed phase, and the electrolyte solution with which the composition comes into contact with. As a result of interaction forms a difference in potentials on the surface of the suspension they are the same polarity, and the layer located at the very surface opposite that leads to the appearance of the double electric layer. If the concentration of the electrolyte is small, the ions with opposite charge are mixed, diffuse nature, and its severity depends on the concentration: the smaller, the brighter is the diffusion.
Charge separation in space and determines the basic electrokinetic properties of the suspension. This:
electrophoresis (movement of particles in the medium) is a process in which phases are mutually shifted, feeling the influence of an electric field;
– electro-osmosis – the movement of the environment, which is the basis of the composition through a porous membrane;
– the emergence of the current;
– the formation of a potential difference in the case when there is displacement of both phases.
All these phenomena often occur in coarsely dispersed systems, but the most commonly used electrophoresis.
These properties arise as a result of chaotic thermal motion of molecules of the dispersion medium, which can be:
If the environment is represented by a liquid or a gas, its particles do not stop moving even for a second, causing happen their collision. Each molecule before collision with another is a certain distance, and its average is called average free path length. Since the kinetic energy of each molecule individually, the figures also differ, and the deviation from the average of the parameter leads to the appearance of molecular-kinetic properties. However, these properties are rather weak due to the fact that the size of the particles in the dispersion medium large and cannot lead to the diffusion (mixing), Brownian motion or osmosis. Weak kinetic resistance causes phase separation in the suspension happens quickly and leads to the release of solid sediment.
Suspension is a classic dispersion medium, so you can get it in two main ways:
– disperazione (dispersed) – fragmentation of larger particles into small;
– condensation (crystallization) – increase the initial size of the particles to the desired parameters.
To dispergation method resorted to when you want to produce a slurry, disperse phase which is a substance insoluble or slightly soluble in the selected environment. If the molecules of the phase do not have the ability to absorb water or dissolve in it, to swell upon contact with fluid, is applied this technique is the manufacture of a thin suspension as the turbidity.
The process is fairly simple: a solid phase slightly moisten with liquid, which presents a disperse medium, and triturated. After adding slightly more of the environment and give the resulting suspension to settle. Under the action of gravity are not enough particles will settle to the bottom, forming sludge, while smaller and light will remain in a suspended state. Top, fine layers, carefully separated and again subjected to coarse grinding. The procedure can be performed several times – as long as necessary to obtain a stable suspension.
Faster this method is, if the particle size reduction uses a rule Deryagina: striking the right balance between the solid phase and the liquid medium at the time of grinding. Optimal parameters considered to be of 0.4-0.6 ml of water or organic liquid to 1 gram of a powdered substance. In this concentration, the friction of the particles against each other is the best, and large granules are quickly and easily broken to the desired size. One more important thing – the appearance of the splitting ability of a dispersed medium, which is possible only with properly selected ratio.
To obtain low concentrated suspensions used methods such as shaking, mixing by hand or using simple mechanisms (mixer). For concentrated compositions (pastes) the optimal will be the classic rubbing.
The condensation method of suspension preparation involves the combination of two substances, each of which is soluble in isolation, but by mutual mixing forming insoluble clumps. Often it is necessary to prepare two separate composition, where the phase and the environment react well with each other, and after connecting them.
An example of this method is to obtain the pharmaceutical composition of the diluted in water to the concentrated alcohol extract or tincture. As a result, the decreased concentrations of alcohols, which leads to the precipitation of constituents of an extract or tincture, the appearance of a coarse precipitate, easily soluble in strong alcohol, but are not able to maintain its structure in the liquid, where the concentration of alcohol is low or absent.
This method is essential oil, resin, lipids, wax, stearin and other products. Contacting with alcohols, they represent the true solutions, but precipitating transformed into a heterogeneous system, usually easily extracted from the liquid. The last parameter depends on the selected as replacement solvent and water solubility of the components themselves.
After obtaining the insoluble phase it is possible to prepare the desired slurry by mixing the selected dispersion medium. Note, however, that each such phase has its own chemical and physical properties and when the inept or unfair treatment is able to form a solid precipitate, to dissolve which requires a lot of effort. Therefore, substances for re-dissolution should be chosen very carefully and take into account all the key parameters.
For more stability of the dispersed system in its composition is administered stabilizers – substances that prevent the adhesion of smaller particles to larger with subsequent deposition of them on the bottom under the force of gravity. Most often used:
– low molecular weight electrolytes;
high molecular compound (IUDs).
In some cases the suspension is required to not only create, but also to carry out the reverse process to break down. For this different methods are used.
The basis of mechanical methods of destruction of suspension is the separation of dispersed phase from the dispersion medium by using mechanical fixtures or devices. Main of them – the use of sedimentation basins, where the composition is stored required for the sedimentation or centrifuge that separates the solid and liquid components in a matter of minutes. These methods are suitable for suspensions with low aggregate stability (the ability of particles to the Association) either as the final stage of destruction.
Thermal methods of destruction of suspensions are two classic ways of exposure:
– decrease the temperature to the critical – freezing and thawing in natural conditions;
– drying – the increase in the concentration of the dispersed phase due to the extraction of from the composition of the liquid medium.
These methods require special equipment, often with high energy consumption, and are not designed for large volumes, therefore are applied exclusively in a domestic environment.
Chemical methods for the destruction of suspensions require the use of certain chemicals – reagents, the compound with which the components of the slurry and leads to the alteration and destruction of its aggregative properties. Destroying the ability of particles to bond together (aggregation), such substances reduce the parameters clumping of small particles into large (coagulation), and their choice depends on the original components of the suspension and of the stabilizer (its availability and the type or lack of).
Electric methods of destruction suitable for suspensions of dispersed systems, the particles which have a charge, i.e., stabilization due to their ionogenic components. Under the influence of an electric field in the destroyed part of a potential difference, which in turn provokes the directed movement of charged particles with subsequent deposition of those on the required electrode.
As thermal techniques implies the availability of expensive equipment and significant energy expenses, not reflected in those branches of industry that require the destruction of large volumes of suspensions.
The formation of suspensions is possible in two ways – artificial, by human hands, and the natural, i.e. the forces of nature. The latter will include the formation of sedimentary rocks and many minerals, the appearance of rivers through reclamation of soil strong and tireless streams and springs. However, the broader the use of a dispersed system created by man. Today’s suspension – is an integral part of these industries:
– mining and metallurgical;
– tanning and other.
It is impossible to imagine modern medicine without these compounds. They are made from the following powders: Amoxiclav, Ursofalk and Augmentin, suprax, Sumamed, enterofuril, Pyrantel, klacid, motilium, paracetamol, amoxicillin, Zinnat, nematol, gaviscon, and Panzer, azithromycin, prazitsid, NIMULID, Maalox , etc.
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