The coarse dispersion system formed by dispersing insoluble solid particles in liquid is called suspension (or suspension)[1]。The linearity of its dispersed phase particles is greater than 1000 nm, which is more thanlyophobic sol The dispersed phase particles are much larger.Most of the dispersion medium used is water, and vegetable oil can also be used.
nature
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Suspension is a kind of dispersion system. The diameter of its dispersion particles is more than 1000 nm, and most of them are aggregates of many molecules, such as mud.The suspension is opaque, uneven, unstable, and cannot pass through the filter paper. After standing, it will be stratified, that is, the dispersed particles will gradually settle under the action of gravity.
The storage of suspension has physical stability problems.The dispersion of solid particles in the suspension is large, and there is a physical interface between the particles and the dispersion medium, which makes the suspended particles have high surface free energy and are in an unstable state.The suspension of hydrophobic particles has greater stability problems than that of hydrophilic particles.To prepare the suspension with slow sedimentation, it can be achieved by reducing the particle size, increasing the viscosity of dispersant and reducing the density difference between solid and liquid. In addition, some macromolecules or surfactants can be added to stabilize the system by reducing the free energy of the interface.
Particle size distribution and determination
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Most suspensions, whether natural or artificial, are multistage dispersion systems, that is, composed of particles of different sizes.In production and scientific research, it is often necessary to know the percentage content of particles of different sizes in the sample, that isParticle size distribution。The most commonly used method to determine the particle size distribution is sedimentation analysis.This method is to measure the particle size distribution in a static medium according to the principle that particles with different sizes fall at different speeds under the influence of gravity.
Since the diameter of the dispersed phase particles in the suspension is larger than that of the colloidal particles, the Brownian motion of the particles can be ignored, and the particles are mainly affected by gravity and decline.In a static dispersion medium, the particles affected by gravity will subside downward with acceleration at the beginning.However, with the rapid increase of the falling speed of particles, the resistance of the dispersion medium to particles also increases greatly, because the resistance is proportional to the falling speed of particles.When a certain falling speed is reached, the resistance and gravity reach balance, and then the particles will not accelerate to fall, but fall at a constant speed.This constant descending speed is calledSettling velocity。The following table lists the experimental values of the settling speed of different clay particles in water, indicating that the smaller the particles are, the less likely they will settle in water, so particle size grading will occur when the particles settle in the medium.
Particle diameter d/(0.001 m)
Settlement speed/(0.01m/s)
Time required for settlement of 0.1 m
zero point five
four point one zero four
2.44s
zero point zero five
zero point one seven seven
56.50 seconds
zero point zero zero five
1.79×10
1 hour and 33 minutes
zero point zero zero zero five
1.79×10
6 days and 11 hours
purpose
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Suspended body can be encountered in nature, industry and agriculture, so it is of great significance to study it.
For example, the water of the Yangtze River and other rivers in China contains a large number of sediment suspensions, which are charged and rarely precipitate in flowing rivers.However, when reaching the estuary, the salt ions in the seawater will neutralize the charge in the sediment particles, and the flow rate will be greatly reduced, so the sediment particles are easy to accumulate under the action of gravity.This is the origin of the Yangtze River Delta and the delta at the mouth of the world's major rivers (such as the Nile).
In medical treatment, some insoluble drugs are often prepared into suspensions for use.Penicillin potassium (sodium), etc. used to treat tonsillitis, should be added with appropriate amount of water for injection before use. After shaking up, it will become a suspension for intramuscular injection.When X-ray is used to check gastrointestinal diseases, let the patient take barium sulfate suspension (commonly known as barium meal), etc.
In agricultural production, in order to use pesticides reasonably, insoluble solid or liquid pesticides are often mixed into suspensions or emulsions to spray crops affected by diseases and pests.In this way, the pesticide solution is less lost, more attached to the leaf surface, and the pesticide solution is sprayed evenly, which is not only convenient to use, but also saves pesticides and improves the efficacy[2]。
For another example, when painting walls, hydrated lime powder (or wall coating) is often prepared into a suspension (containing a small amount of glue), which is evenly sprayed on the walls.In addition, graphite and carbon suspensions (crystallization centers) are often added to industrial boilers to prevent scaling.
