Thomas Graham (1861) divided soluble substances into two classes, ‘crystalloids’ and ‘colloids,’ according to their powers of diffusion across vegetable or animal membranes. Substances such as salt, sugar and urea which diffuse rapidly were termed Crystalloids on account of the fact that they are readily obtained in the crystalline form. The other class includes many amorphous substances like gelatin, starch and gum which exhibit little or no tendency to diffuse through the membrane and were, because of their gluey nature, called Colloids.
A colloid, or disperse phase, is a dispersion of small particles of one material in another that does not settle out under gravity. In this context, ‘small’ means that one dimension at least is smaller than about 500nm in diameter. Many colloids are suspensions of nanoparticles (particles of diameter up to about 100nm). In general, colloidal particles are aggregates of numerous atoms or molecules, but are commonly but not universally too small to be seen with an ordinary optical microscope. They pass through most filter papers but can be detected by light-scattering and sedimentation.
NATURE OF COLLOIDAL SOLUTIONS
In a true solution, the solute particles are present as molecules or ions giving a homogeneous mixture which consists of a single phase. In a colloidal solution, on the other hand, the unit particles of the dissolved substance are either very large, molecules (starch, for example, has m. wt. of about 32,000) or essentially aggregate of a large number of molecules. These particles even though they may consist of thousands of molecules are too small to be seen under the microscope. Thus to the naked eye, there could be no difference between a colloidal solution or an ordinary solution.
However, if colloidal particles grow in size further, they become visible under the microscope and then we get what we call a suspension. The particles of a coarse suspension are visible even with the naked eye. The colloidal state can thus be regarded as the intermediate stage between molecules and particles of a coarse suspension.
CLASSIFICATION AND TERMINOLOGY
So far we have referred only to colloidal solutions which are diphasic systems with a solid as the dispersed phase and a liquid as the dispersion medium. In a diphasic system, however, each one of the two phases can either be a gas, liquid or solid. In all, eight such systems are possible.
Colloidal systems of two gases cannot exist since the unit particles of a gas are simple molecules and are incapable of producing two phases. Colloidal solutions are frequently referred to as sols. Colloidal solutions in water are termed hydrosols. When the dispersion medium is alcohol or benzene, they are called alcosols and benzosols respectively.
Gelatin, gum and certain other organic substances which directly pass into colloidal solution when brought in contact with water are called hydrophilic (water-loving) colloids. When once precipitated from the colloidal form they can be directly reconverted into the colloidal form and are for this reason termed reversible colloids.
Insoluble substances like metals, metal sulphides, metal hydroxides and other inorganic substances which do not readily yield colloidal solutions when brought in contact with water are called hydrophobic (water-hating) colloids. When once precipitated, they can· not be directly obtained back into the colloidal form and are, therefore termed irreversible colloids. More general terms than hydrophilic and hydrophobic are lyophilic and lyophobic respectively, lyo meaning solvent.
PREPARATION OF COLLOIDAL SOLUTIONS
Practically all substances can form colloidal solutions although not with the same ease. Gelatin, gum, starch, etc., for example, yield colloidal solutions on simple warming or agitation with water. Metals and other inorganic substances, on the contrary, can be obtained in the colloidal form with difficulty and by the use of special devices.
To get a substance in colloidal form we can either start with, the material in bulk and break it down into fine particles of colloidal dimensions (Dispersion Methods), or we can bring about the union of a large number of atoms or molecules to form bigger particles of colloidal size (Condensation Methods). Thus we have the following methods for the preparation of colloidal solutions:-
(i) Mechanical Dispersion:
The substance to be dispersed is ground as finely as possible by the usual methods. It is then shaken with the dispersion medium and thus obtained in the form of a coarse suspension. This suspension is then passed through a Colloid Mill. The simplest type of Colloid Mill called Disc Mill consists of two metal discs nearly touching each other and rotating in opposite directions at a very high speed. The suspension passing through these rotating discs is exposed to a powerful shearing force and the suspended particles are torn apart to yield particles of colloidal size.
(ii) Electro-Dispersion Bredig’s Arc Method:
This method consists in striking an arc between the electrodes of a metal which is to be obtained in colloidal form, the electrodes being immersed in the dispersion medium, commonly water. The intense heat of the arc turns the metal into vapours which are immediately condensed by the surrounding ice-cold water to give particles of colloidal size. A slight trace of potassium hydroxide in water helps to stabilise the sol. The sol is then filtered to free it from a few relatively bigger suspension particles eventually formed. Gold, platinum, silver, copper and such other metals can thus be obtained in the colloidal form.
Peptization is the converse of coagulation. It is the process by which a stable colloidal solution can be produced from substances originally present in massive form when the colloidal particles pre-exist in the substance to be dispersed. In other e words, it is the redispersion of a coagulated sol. Freshly prepared ferric hydroxide on treatment with a small amount of ferric chloride solution (peptizing agent) at once forms a dark reddish brown sol. Another familiar example of peptization in qualitative analysis is the formation of colloidal solution of aluminium hydroxide by the addition of a small amount of dilute hydrochloric acid, not sufficient to form aluminium chloride solution.
Condensation or Aggregation Methods
(i) Excessive Cooling:
The colloidal solution of ice in an organic solvent like chloroform or ether is obtained by freezing a solution of water in the solvent. The molecules of water which can no longer be held in solution gather together to form particles of colloidal size.
(ii) Lowering of Solubility by Exchange of Solvent:
Substances like sulphur, rosin, etc., which are more soluble in alcohol than in water, give a hydrosol by pouring a small amount of their alcoholic solution in excess of water. The substance is present in the molecular state in alcohol but on transference to water, in which it is insoluble, molecules precipitate out to form particles of colloidal size. The indicator phenolphthalein is soluble in alcohol but not in water and is, therefore, supplied to the laboratory in alcoholic solution. If water is added to this solution, a milky liquid is produced which contains phenolphthalein in the colloidal solution.
(iii) Passing Vapour of an Element into a Liquid:
If the vapours of a boiling element are conducted into a liquid, condensation takes place, sometimes with the formation of a stable sol. Thus mercury sols result on passing a stream of vapours from the boiling element into cold water containing suitable stabilizing electrolytes such as ammonium salts or citrates. Sulphur sols can easily be prepared by a similar procedure.
(iv) Chemical Action:
When the solubility of the substance is sufficiently small, it may be obtained in the colloidal form by chemical precipitation. Before the actual precipitation commences the insoluble substance first of all appears in the molecular state but as the precipitation proceeds a large number of these molecules assemble and grow to the colloidal size. The various types of chemical reactions available for the purpose are:
(i) Double decomposition. Colloidal solution of arsenious sulphide may be obtained by the action of hydrogen sulphide on arsenious oxide (As2O3 + 3H2S —-> As2S3 + 3H2O).
(ii) Oxidation. A milky colloidal solution of sulphur is frequently formed during qualitative analysis by the action of hydrogen sulphide on the solution of an oxidising agent, say nitric acid (H2S + O —> H20 + S).
(iii) Reduction. A violet colloidal solution of gold can be obtained by the reduction of a gold chloride solution using stannous chloride solution (2AuCl3 + 3SnCl2 —-> 3SnCl4 + 2Au) as reducing agent. Colloidal silver may be obtained by reducing silver nitrate solution with tannic acid solution.
(iv) Hydrolysis. A deep red colloidal solution of ferric hydroxide can be prepared rapidly by the gradual addition of 2 or 3 c.c. of a 30% solution of ferric chloride to 500 C.c. of boiling water with constant stirring.
FeCl3 + 3H2O —-> Fe(OH)3 + 3HCl.
The neutral ferric chloride solution used as a reagent in the laboratory becomes dark red on standing. This change in colour is due to the formation of colloidal ferric hydroxide by partial hydrolysis.
PURIFICATION OF COLLOIDAL SOLUTIONS
Colloids are often purified by dialysis, the process of squeezing the solution through a membrane. The process of separating a ‘crystalloid’ from a ‘colloid’ by diffusion or ‘filtration’ through a membrane was named Dialysis by Graham and the apparatus employed to effect such a separation is called a Dialyser. Graham’s dialyser consists of a shallow cylinder open at both ends, over one end of which a membrane is tied. The colloidal solution to be dialysed is placed in this cylinder which is then suspended in a large dish containing distilled water. The distilled water is renewed from time to time, but it is preferable to use a continuous flow, as dialysis is greatly accelerated thereby. This may be done by allowing water to flow into the outer vessel and removing it by means of a syphon. The aim is to remove much of the ionic material that may have accompanied their formation. A membrane (for example, cellulose) is selected that is permeable to solvent and ions, but not to the colloid particles.
Dialysis is very slow, and is normally accelerated by applying an electric field and making use of the charges carried by many colloidal particles; the technique is then called electrodialysis.
In ultrafiltration, the liquid dispersion medium, as well as the substance in true solution, is removed from the colloidal material. Pressure is applied to the solution in a strong cylinder so as to force the liquid through the very small pores of a special membrane when the solid colloidal material is left behind. Ordinary filter papers can be used for ultrafiltration by impregnating them with a solution of collodion in acetic acid. Graded filters of different effective pore size can be made by varying the concentration of the acid. By using such graded filters pure colloidal solution with uniform required particle size can be obtained when coarse bigger particles are held back. Ultrafilters are useful in removing soluble impurities from colloids and they find extensive application in bacteriology as bacteria can generally be removed from solution by ultrafiltration.
PROPERTIES OF COLLOIDAL SOLUTIONS
1. Faraday-Tyndall Phenomenon or Tyndall Effect
When a strong beam of light is concentrated on a colloidal solution the path of the beam is illuminated by a bluish light (and becomes visible when observed from the side). This phenomenon discovered by Faraday and later studied by Tyndall has been named Faraday Tyndall Phenomenon or simply Tyndall Effect. The cause of this phenomenon is the scattering of light by the colloidal particles. This scattering of light cannot be due to simple reflection because the size of the particles is smaller than the wavelengths of visible light which are, therefore, unable to reflect light waves. This is why the colloidal particles cannot be seen directly under the microscope. In this phenomenon of scattering the particles themselves become self-luminous due to the absorption of light energy which is then scattered as light of shorter waves. This accounts very well for the bluish Tyndall Cone. Faraday-Tyndall phenomenon has been employed as the basic principle for the construction of ultramicroscope. It has been used to detect solid suspended impurities in solutions.
The colour of colloidal solutions is determined by the wavelength of the light scattered by the colloidal particles which again depends on the size and nature of the particles. This is fully borne out by the experimental data obtained in the case of silver sols.
3. Brownian Movement
Careful ultramicroscopic examination of a colloidal solution reveals that the suspended particles are in constant rapid zig-zag motion called the Brownian Movement, after the name of its discoverer Sir Robert Brown (1827). The phenomenon is very striking in colloidal solutions and becomes less intense as the particles increase in size. It is detectable even with particles which are so large that they are visible under an ordinary microscope. Brown himself first noted this movement while examining pollen grains suspended in water under a microscope.
Particles, of a colloidal solution, are charged and move relatively towards the dispersion medium under the influence of an electric field. It follows, therefore, that if the particles can be maintained stationary, the dispersion medium would move, This movement of the dispersion medium under the infl1lence of the electric field is known as Electro-osmosis This can be observed by fixing a porous diaphragm made out of the material of the dispersed phase in a U-tube containing the dispersion medium. When an electric field is set up across the ends of the tube, the dispersion medium is seen to move towards one or the other electrode. The direction of motion is opposite to that which the diaphragm would follow if it were free to move.
5. Precipitation or Coagulation
It is a remarkable fact that precipitation, or coagulation, of colloidal solutions, can readily be brought about by the addition of small amounts of electrolytes. When for example, a few drops of barium chloride solution are added to a colloidal solution of arsenious sulphide, it immediately becomes turbid and in a short time, the sulphide separates in the form of a precipitate.
Since it is the charge present on the ion which is responsible for the precipitation of the colloidal solution, it is not surprising that the power of an ion to coagulate a sol. depends on its valency. The higher the valency of the active ion, the greater is its precipitating action. This is known as the Hardy-Schulze Law. The precipitating action of the cations Na+, Mg2+, AI3+ on As2S3 sol in conformity with the Hardy-Schulze law has been found to be in the order Al3+ > Mg2+ > Na+
Colloidal solutions such as those of metals like gold and silver can normally be precipitated by small amounts of electrolytes. This can be prevented, or at least retarded, by the previous addition of a more stable hydrophilic colloid like gelatin or albumin. For example, if a little gelatin is added to a gold sol. It is no longer precipitated on the addition of sodium chloride. The process by which the sol. particles are protected from precipitation by electrolytes due to the previous addition of some hydrophilic colloid is termed Protection and the colloid which IS added to achieve this is commonly called a Protective colloid.
Gold number is the number of milligrams of protective colloid which just prevents the coagulation (accompanied by a change in the colour from red to blue) of 10 c.c. of a given gold sol. when 1 c.c. of a 10% solution of sodium chloride is added to it.
A dispersion of tiny droplets of one liquid in another liquid is known as an emulsion. Any two immiscible liquids can yield an emulsion. There are two types of emulsions recognised:
(1) Oil-in-Water emulsions, and
(2) Water-in-Oil emulsions
Water is usually one of the components and the other is an oil or a liquid insoluble in water which takes the place of oil. The emulsions composed entirely of water and oil are not stable. The dispersed droplets at once come together (coalesce) and form a separate layer. To stabilise an emulsion the addition of a third substance, known as an emulsifying agent or emulsifier, is essential. The emulsifier concentrates at the oil-water interface and forms a film sufficiently tough to prevent the coalescence of the droplets. Soaps, gelatin, and gum are useful emulsifying agents.
Preparation of Emulsions.
(1) The oily substance is mixed with the emulsifying agent, say soap, and thoroughly ground in a mortar. This is then agitated with water to form an emulsion.
(2) The emulsifier is mixed with a little water and thoroughly ground. The oil is then added to it bit by bit and the liquid shaken.
(3) Homogenizing. This method is often applied for stabilising an already available emulsion. The emulsion is forced through capillary tubes under high pressure and the stream is allowed to break against a hard surface. Homogenized cream and milk are prepared by this method.
Properties of Emulsions
Emulsions show all the properties of sols.
1. Cataphoresis. Like sols, droplets in an emulsion are electrically charged. They migrate to the oppositely charged electrode under the influence of electric field.
2. Dilution. An emulsion can be diluted with any amount of the dispersion medium while the dispersed liquid, when mixed with it, will at once form a separate layer. This property of emulsions is used to detect the type of a given emulsion.
Colloidal systems containing a liquid dispersed in a solid are called gels. A gel is obtained by the coagulation of certain colloidal solutions when the coagulated material encloses the entire liquid medium yielding a semi-solid mass.
In the process of gel formation or gelation, the sol particles come together and form bigger aggregates which finally grow so large that they touch each other. These aggregates continue to grow and form a continuous network which encloses the entire dispersion medium. Thus a gel is usually assumed to have a structure somewhat like a honey-comb.
Properties of Gels. On standing, an inorganic gel loses water and shrinks. The shrinking of gel with the simultaneous exudation of the solvent from it is termed Syneresis or weeping of the Gel. The gel structure offers little resistance to the diffusion of any substances dissolved in the sol before setting.
APPLICATIONS OF COLLOID CHEMISTRY
Most of the substances we come across in our everyday life are colloids and we are mainly colloids ourselves. Few important applications of Colloid Chemistry.
(I) Colloidal Medicines. Colloidal medicines are more effective on account of their easy assimilation and adsorption.
(i) Argyrol and protargol are protected colloidal solutions of silver. They are used as a cure for granulations.
(ii) Colloidal gold, manganese, calcium, etc., are used for intramuscular injections to raise the vitality of human system in diseases like tuberculosis and rickets.
(iii) Colloidal Sulphur is used as a germ killer, especially in plants.
(II) Purification of Water. Impure water contains clay particles and bacteria, etc., suspended in it. When alum is dissolved in such water, the coagulation of colloidal impurities takes place. Alum furnishes aluminium ions AI3+ in solution which discharge the negatively charged sol. particles and cause them to settle down. The clear water can then be decanted off.
(III) Formation of Delta. The river water carries with it sand particles and many other substances in the suspended state. Sea-water, on the other hand, contains a number of electrolytes dissolved in it. When the river water meets the seawater, the colloidal sand and other suspended materials present in the former are precipitated by the electrolytes of the
latter and delta is formed.
(IV) Cleansing Action of Soap. The action of soap is two-fold:-
(a) It forms a colloidal solution in water and removes dirt by simple adsorption at the surface of the sol particles, and
(b) It emulsifies the greasy or oily materials which are thereby detached from the body or cloth. Other substances which are usually sticking on account of the ‘grease are also automatically released when the latter is emulsified.
(V) Blue Colour of the Sky. Under the ultramicroscope, the colloidal particles appear to be bluish stars. Only blue light is scattered and the rest of it is absorbed. There are numerous dust and water particles floating about in the sky. They scatter blue light and make the sky look bluish. Without this scattering, the sky would have been all dark.
(VI) Blood. Blood is a colloidal solution of an albuminoid substance and is coagulated to a clot by trivalent aluminium ions of alum or ferric ions of ferric chloride. This explains the stoppage of bleeding by the application of alum or ferric chloride.