colloid

[jiāo tǐ]

A more homogeneous mixture

Collection

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Colloid, also called Colloid colloidal dispersion (colloidal dispersion) Homogeneous mixture The colloid contains two substances in different states, one dispersed phase , another continuous phase 。 Part of the dispersion is composed of tiny particle or liquid drop Composed of, Dispersant The particle diameter is 1~100nm Dispersion system Is colloid; Colloid is a kind of dispersion whose particle diameter is between Dispersion system A kind of dispersion system between and solution, which is a highly dispersed heterogeneous system.

Colloids are not necessarily colloidal substances, nor are they necessarily liquids. For example: Ferric hydroxide colloid , clouds, fog, etc.

Chinese name: colloid
Foreign name: Colloid
Dispersion diameter: 1~100nm

Appearance: transparent
stability: Relatively stable (stable polymer solution)
Nature: dialysis 、 adsorbent 、 Coalescence precipitation
Application: Water purification, fertilizer conservation, medical dialysis, etc

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nature

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Colloid is a dispersion system, which is a collection of some particles with the same or similar structure. There are several particles to form a colloidal particle, so generally when 1mol of material forms a colloid, the number of colloidal particles (colloidal particles) is less than 1mol.

Tindall effect

The phenomenon of colloidal particles scattering light to form a bright "path" is called the Tindall effect (Tindall phenomenon), because light is scattered by colloidal particles, rather than the colloidal particles themselves emitting light. When the sun shines into the dark room from the window gap, or the light shines into the dense forest through the gap between the leaves, it can be observed Tindall effect ； The formation of the light column from the projection room to the screen during film playing also belongs to the Tindall effect.

Tindall phenomenon

Metastability

The stability of colloid is between solution and Turbid liquid It can exist stably under certain conditions, belonging to Metastable system 。 Colloid has Metastability Two reasons: reason one: colloidal particles can be charged by adsorption Colloidal particles With the same kind of charge, the same kind of charge will repel each other (to make the colloid agglomerate, it is necessary to overcome the repulsive force and eliminate the charge carried by the colloidal particles). Reason 2: Colloidal particles are constantly doing Brownian motion , formed when it is the same as gravity Settlement equilibrium Status of.

structure

according to Fayans Regular (ions that can form insoluble matter with the constituent ions of the crystal will be preferentially adsorbed, preferentially adsorbing ions with the same composition), colloidal particles are colloidal particles, colloidal particles and diffusion layer Together to form micelles, and the colloidal particles include the core and Adsorption layer 。

electrophoresis

The colloidal particles have large Specific surface area (specific surface area=surface area/particle volume), so it has strong adsorption capacity Surface adsorption The ions in the solution so that the colloidal particles are charged. Different colloidal particles adsorb ions with different charges. Generally speaking, metal hydroxide The colloidal particles of metal oxides absorb cations, and the colloidal particles are positively charged, such as Fe (OH) three ，Al(OH) three ，Cr(OH) three ，H two TiO three ，Fe two O three ，ZrO two ，Th two O 3。 Nonmetallic oxide 、 metal sulfide The colloidal particles attract anions. The colloidal particles are negatively charged, such as As two S three ，Sb two S three ，As two O three ，H two SiO three Au, Ag, Pt. In addition, soil particles are also negatively charged. There are also some uncharged colloids, such as starch colloids and polyethylene glycol colloids. The colloidal particles can be charged, but the whole colloid is electrically neutral.

Since colloidal particles have the same charge and repel each other, it is not easy for colloidal particles to gather, which is an important reason for colloid to keep stable. As the colloidal particles are charged, they will move to a certain pole under the action of an external electric field（ cathode or anode ）The phenomenon of directional movement is called electrophoresis.

other

have Coalescence precipitation ， Salting out ， Brownian motion Phenomenon, dialysis Function, etc.

classification

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1. It can be divided into:

aerosol ——Gas dispersant Dispersion system , which Dispersant It can be liquid or solid, such as smoke, fog, etc.

Hydrosol ——As liquid dispersant The dispersion can be gaseous, liquid or solid, such as Fe(OH) three Colloid.

Solid sol ——Dispersion system with solid as dispersant Dispersant It can be gaseous, liquid or solid, such as Tinted glass 、 Smoky crystal 。

2. It can be divided into: Particle colloid 、 Molecular colloid 。 For example, soil is particle colloid, starch colloid and protein colloid are molecular colloid.

Note: At the beginning of the 20th century, gelatin 、 protein The solution easy to form colloid with water is called hydrophile colloid. Hydrophilic colloids are now commonly referred to as macromolecules or (polymer) solutions and hydrophobic colloids as colloidal dispersion systems (often referred to as colloids) or sols^[1] 。 According to this, middle schools often simply think that protein solution is a colloid. Although it cannot be said wrong (similar in nature), it is also an outdated statement.

In addition to classifying according to the particle size of dispersions, they can also be classified according to the properties of dispersions and dispersion media. Table 1 lists eight types of colloidal dispersion systems^[2] 。

Table 1 Classification of heterogeneous dispersion systems according to aggregation state
Dispersion medium	dispersed phase	name	example
liquid	solid liquid gas	Sols, suspensions, ointments Emulsion foam	Gold sol, silver iodide sol, toothpaste Milk, margarine, oil-water emulsion Soap foam, cheese
gas	solid liquid	aerosol	Smoke and dust fog
solid	solid liquid gas	Solid suspension Solid emulsion Solid foam	Red glass and photographic film colored with gold Pearl, black phosphorus (P, Hg) Foam plastic

Common colloids

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Fe(OH) three Colloid Al(OH) three Colloid, silicic acid colloid, starch colloid Protein colloid , soymilk, fog, ink, paint, AgI colloid, Ag two S Colloid, As two S three Colloid, colored glass, jelly egg-white , blood, etc. For example, noodles are a common starch colloid, because of its solubility, water swells.

application

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1. Agricultural production: the role of soil in fertilizer conservation. Many substances in soil, such as clay humus, often exist in the form of colloid.

2. Medical health: hemodialysis, serum Paper electrophoresis Various amino acids and proteins are separated by electrophoresis. In medicine, more and more highly dispersed colloids are used to test or treat diseases. For example, colloidal magnetic fluid cancer treatment is to Magnetic material Colloidal particles are made and used as drug carriers to deliver drugs to the focus under the action of magnetic field, so as to improve the curative effect.

3. Daily life: the principle of making tofu, soymilk, milk and porridge (the agglomeration of colloid), alum Clean water.

4. Physical geography: delta is formed at the estuary of rivers, and its formation principle is electrolyte Make the river sediment form colloid and agglomerate.

5. Industrial production: manufacturing Tinted glass (solid sol). In metal ceramics 、 polymer The addition of solid colloidal particles into materials such as, can not only improve the impact strength, breaking strength, tensile strength and other mechanical properties of materials, but also improve the optical properties of materials. Colored glass is composed of some colloidal metallic oxide Made by dispersing in glass. Some gunpowder and explosives in the national defense industry must be made into colloids. Preparation of some nano materials, mineral processing in metallurgical industry, dehydration of petroleum crude oil, Plastic 、 rubber and Synthetic fibre Colloid will be used in the manufacturing process.

6. Wastewater treatment: some pollutants in wastewater exist in the form of colloid, so many advanced wastewater treatment equipment aim to study how to quickly and efficiently remove pollutants in the form of colloid in wastewater.

Principle of water purification

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Chemical interpretation

The diameter of colloidal particles is generally between 1nm-100nm, which determines that colloidal particles have huge specific surface area, strong adsorption force, and can adsorb suspended solids or pigments in water to form precipitation, thus Water purification This is the principle of colloidal water purification.

The substances that can naturally form colloid with high concentration in water and have no obvious side effects on water quality are KAl (SO four ) two ·12H two O( alum )、FeCl three ·6H two O, etc. (Note: long-term drinking of alum purified water may lead to the risk of Alzheimer's disease and other diseases). Such substances are called water purifying agents, and the chemical principle of forming colloid is to make it hydrolyze:

FeCl three + 3H two O

Fe(OH) three (Colloid)+3HCl

Note: Fe (OH) three Colloid is reddish brown, which is commonly used in tap water purification, and can also be used to purify the contaminated water heavy metal pollution High efficiency and low cost.

Al 3+ + 3H two O= Al(OH) three (Colloid)+3H +

Specific operation

Put a handful of soil into the water, and the large particles of sediment will sink quickly. The turbid small particles of soil will also settle at the bottom of the container due to the influence of gravity, and the salts in the soil will dissolve into true solutions. However, there are some extremely small soil particles mixed in the true solution that neither sink nor dissolve. People call these tiny particles that can not be observed even under the microscope colloidal particles, and the system containing colloidal particles is called colloidal system.

Colloid purification

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Usually, the colloid is purified by dialysis^[3] 。 dialysis also called dialysis , Principle: Use Semipermeable membrane The properties that can penetrate small molecules and small ions but cannot penetrate colloidal particles Sol The process of removing small molecules or ions as impurities^[4] 。

During dialysis, the colloidal solution is placed in the dialyzer composed of semi permeable membrane, and the colloidal solution is regularly replaced outside the dialyzer Dispersion medium (usually water). During dialysis, the applied DC electric field can often accelerate the diffusion of small ions from the membrane to the outside Electrodialysis (electrodialysis)。 Semipermeable Selectivity Separate different solute Method of the particle. stay electric field The dialysis of charged solute particles (such as ions, colloidal particles, etc.) in the solution under the action of electrodialysis is called electrodialysis. Electrodialysis is widely used in chemical industry, light industry, metallurgy, paper making, seawater desalination, environmental protection and other fields; In recent years, it is more widely used in the purification and research of amino acids, proteins, serum and other biological products. There are many kinds of electrodialysers, and W. Bowery's three chamber type is typical, and its structure is shown in the figure below. The electrodialysis device consists of anode chamber, intermediate chamber and cathode The chamber is composed of three chambers. The middle DD is a semi permeable membrane with good sealing, E is a sheet or rod electrode such as Pt, Ag, Cu, etc., F is a glass tube connected to the middle chamber for washing, and S is a pH meter. Electrodialysis is essentially a desalination technology. Positive, positive anion The exchange membrane has Selectivity Put salt containing solution into the device, and under the action of DC, positive and negative ions will migrate to the cathode and anode respectively through the membrane. Finally, in the intermediate chamber between the two membranes, the salt concentration decreases, and the anion and anode chambers are concentration chambers. Electrodialysis The method can dilute, concentrate, separate or prepare some electrolytic products for electrolyte solutes or some substances. In practical application, more than 100 pairs of exchange membranes are usually used to improve separation efficiency 。 During electrodialysis, Ion exchange membrane Permeability, ion concentration diffusion, water permeability, polarization ionization and other factors will affect the separation efficiency.

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