COLLOIDS

Mohammad Safiqul IslmaNSTU

• Classification Based on the State of the Dispersed Phase and Dispersion Medium

• The below table lists various types of colloids with various examples.

• Types of colloids• Dispersion Medium Dispersed Phase Type of

colloid Example Gas Liquid Aerosol Fog, mist Gas Solid Aerosol Smoke Liquid Gas Foam Whipped cream Liquid Liquid Emulsion Mayonnaise, hair cream Liquid Solid Sol Paints, cell fluids Solid Gas Foam Pumice, plastic foams Solid Liquid Gel Jelly, cheese Solid Solid Solid Sol Ruby glass (glass with dispersed metal)

Classification Based on the State of the Dispersed Phase and Dispersion Medium

Dispersion Medium Dispersed Phase Type of colloid Example

Gas Liquid Aerosol Fog, mist

Gas Solid Aerosol Smoke

Liquid Gas Foam Whipped cream

Liquid Liquid Emulsion Mayonnaise, hair cream

Liquid Solid Sol Paints, cell fluids

Solid Gas Foam Pumice, plastic foams

Solid Liquid Gel Jelly, cheese

Solid Solid Solid Sol Ruby glass (glass with dispersed metal)

• Fog and smoke are aerosol which are liquid droplets or solid particles dispersed throughout a gas. When liquid droplets are dispersed throughout another liquid phase this results in emulsion, as in the case of butterf at dispersed throughout homogenized milk. A sol consists of solid particles dispersed in a liquid. Foam consists of gas being dispersed in a liquid phase as in the case of whipped cream.

• Out of the various types of colloids, sols, gels and emulsions are very common. In the later sections 'sols' and 'emulsions' are discussed in some detail.

Classification Based on the Nature of Interaction Between Dispersed Phase and

Dispersion Medium• Colloidal systems, depending on the nature of

attraction between the dispersed phase and the dispersion medium are classified into lyophobic(solvent hating) and lyophilic (solvent loving). If water is the dispersion phase is water, then the colloids are either hydrophilic or hydrophobic.

• 1) Lyophilic colloids• In this type of colloids sols, the dispersed phase has

great attraction for the dispersion medium. In such colloids, the dispersed phase does not precipitate easily and the sols are quite stable. If the dispersion medium is separated from the dispersed phase, the sol can be reconstituted by simply remixing with the dispersion medium. Hence, these sols are called reversible sols.

• Examples of lyophilic sols include sols of gum, gelatine, starch, proteins and certain polymers in organic solvents

• 2) Lyophobic colloids• In this type of colloidal sols, the dispersed phase has

little affinity for the dispersion medium. These colloids are easily precipitated on the addition of small amounts of electrolytes, by heating or by shaking and therefore are not stable. Once precipitated, it is not easy to reconstitute the sol by simple mixing with the dispersion medium. Hence, these sols are called irreversible sols. Examples of lyophobic sols include sols of metals and their insoluble compounds like sulphides and oxides. Lyophobic sols need stabilizing agents to keep the dispersed phase from precipitating out.

• Hydrophobic sols are often formed when rapid crystallization takes place. With rapid crystallization, many centres of crystallization called nuclei are formed at once. Ions are attracted to these nuclei and very small crystals are formed. These small crystals are prevented from settling out by the random thermal motion of the water molecules.

Classification of Colloids Based on Type of Particles of the Dispersed Phase

• 1) Multimolecular colloids• 2) Macromolecular colloids 3) Associated colloids.

• Multimolecular colloidsIn this type of colloids the colloidal particles are aggregates of atoms or small molecules with molecular size less than one nanometer (1 nm). For e.g., gold sol consists of particles of various sizes which are clusters of several gold atoms. Similarly, sulphur sol consists of colloidal particles which are aggregates of S8 molecules. The molecules in the aggregates are held together by Van der Waal forces.

• Associated colloids (Micelles)Certain substances behave as strong electrolytes at low concentration but at higher concentrations these substances exhibit colloidal characteristics due to the formation of aggregated particles. These aggregated particles are called micelles. Micelles are called associated colloids. The formation of micelles takes place only above a particular temperature called Kraft Temperature (Tk) and above particular concentration called the Critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions. Surface active molecules such as soaps and synthetic detergents form associated colloids in water. For soaps, the CMC is about 10-4 to 10-3 mol L-1. Micelles have both a lyophilic and lyophobic parts. Micelles may consists of more than 100 molecules.

• Micelles are formed by specific molecules which have lyophilic as well as lyophobic ends. Ordinary soap which contains sodium stearate(C17H35COONa) forms micelle in water. The stearate ion has a long hydrocarbon end that is hydrophobic (because it is nonpolar) and a polar carboxyl group (COO-) that is hydrophilic.

Aggregation of RCOO- ions to form a micelle

• When the concentration of sodium stearate is below its CMC, then it behaves as a normal electrolyte and ionizes to give Na+ and C17H35COO- ions. As the concentration exceeds the CMC, the hydrophobic end starts receding away from the solvent and approach each other. However, the polar COO- part interacts with water. This leads to the formation of a cluster having the dimensions of a colloid particles. In each cluster a large number of stearate groups clump together in a spherical manner such that the hydrocarbon parts interact with one another and the COO-

groups remains projected in water.

• CHARACTERISTICS OF LYOPHlLIC AND LYOPHOBIC SOLS

• Some features of lyophilir and IYopho~ic sols are listed• (J) Ease of preparation• Lyophilic sols can be obtained straightaway by mixing the

material (starch, protein) with asuitahle solvent. The giant molecules of the material are of colloidal size and these at once pass into the colloidal form on account of interaction withthe sol vent.

• Lyophobic sols are not obtained by simply mixing the solid material with the solvent.

• (2) Charge on particles:• Particles of a hydrophilic sol may have Iitlle or no charge at

all.• Particles of a hydrophohic sol carry positive or negative

charge which gives them stability.

• (3) Solvation• Hydrophilic sol particles are, generally

solvated, TIlat is,they arc surrouned by an adsorbed layer

• of the dispersion medium which does not permit them to come together and coagulate. Hydration

of gelatin is an example,• There is no solvallon of the hydrophobic sol

particles for want of interaction wilh tbemedium

• (4) Viscosity• Lyophilic sols are viscous as the particle size

increases due to solvation. and the proportion of free medium decreases.

• Viscosity of hydrophobic sol is almost the same as of the dispersion medium itself.

• (5) Precipitation• Lyophilic sols are precipitated (or coagulated)

only by high concentration of the electrolytes when the sol particles are desolvated,

• Lyophobic sols are precipitated even by Iowconcentration of electolytes, the protective layer being absent.

• (6) Reversibility:• The dispersed phase of lyophilic sols when separated

by coagulation or by evaporation of the medium, can be reconverted into the colloidal form just on mixing with the dipersion medium, Therefore this type of sols are designated as Reversible sols.

• On the other hand, the lyophobic sols once precipit:ued cannot be re-formed merely by mixing with dispersion medium. These are terefore, called Irreversible sols.

(7) Tyndall effect• On account of relatively small panicle size, lyophilic

sols do not scatter light and show no Tyndall effect.• Lyophobic sol particles are large enough to exhibit

Tyndall effect.• (X) Migration in Electric field• Lyophilic sol particles(proteins) migrate to anode or

cathode, or not at all, when placed in electric field.• Lyophobic sol particles move either to anode or

cathode, according as they carry negative or positi vecharge.

• The differences

PREPARATION OF SOLS• Lyophilic Sols may be prepared by Simply

warming the solid with the liquid dispersion medium e.g .. starch with water. On the other hand lyophobic sols have to be prepared by special methods.

• These methods fall into two categories:• 1. Dispersion Methods in which larger macro-

sized particles are broken to colloidal size,• 2. Aggregation Methods in which colloidal

size particles arc built up by aggregating single ions or molecules.

AGGREGATION METHODS

• These methods consist of chemical reactions or change of solvent whereby the atoms or molecules of the dispersed phase appearing first, coalesce or aggregate to form colloidal particles. The conditions (temperature, concentration, cte.) used are such as permit the formation of sol particles but prevent the particles becoming too large and forming precipitate. The unwanted ions (spectator ions) present in the sol are removed by dialysis as these ions may eventually coagulate the sol.

• The more important methods for preparing hydrophobic sols are listed below:

• Excess hydrogen sulphide (electrolyte) is removed by passing in a stream of hydrogen.

• (2) Reduction: • Silver sols and gold sols can be obtained by

treating dilute solutions of silver nitrate or gold

chloride wilh organic reducing agent like tannic acid or ethanal (HCHO)

• AgNO + tannic acid Ag sol• AuCI) + tannic acid - Au sol

• (3) Oxidation• A sol of sulphur is produced by passing

hydrogen sulphide into a solution of sulphur dioxide.

• (4) Hydrolysis• Sols of the hydroxides of iron, chromium and

aluminium are readily prepared by the hydrolysis

of salts of the respective metals. In order to obtain a red sol of ferric hydroxide, a few mlsof 30%

• ferricchloride solution is added to a large volume of almost boiling water and stirred with a glassrod

• (5) Change of Solvent• When a solution of SlIlphur or resin in

ethanol is added to an excess of water, the sulphur or resin sol is formed owing to decrease in solubility. The Substance is present in molecular slate in ethanol but on transference to water, the molecules precipitate out to form colloidal particles.

Dispersion methods

• (I) Mechanical dispersion using Colloid millThe solid along with the liquid dispersion

medium is fed into a Colloid mill. The mill consists of

two steel plates nearly touching each other and rotating in opposite directions with high speed. The solid particles are ground down to colloidal size and are dispersed in the liquid to give the sol.

• 'Colloidal graphite' (a lubricant) and printing inks ' are made by this method.

• Recently, mercury sol has been prepared by disintegrating a layer of mercury into sol particles in water by means of ultrasonic vibration.

• (2) Bredig's Arc Method• It is used for preparing hydrosols of metals e.g .• silver, gold

and platinum. An arc is struck betwccn the two metal electrodes held close together beneath de-ionized water. The water is kept cold by immersing the container in ice/water bath and a trace of alkali (KOH) is added. The intense heat of the spark across the electrodes vaporises some of the metal and tile vapour condenses under

• water. Thus the atoms of tile metal present in the vapor aggregate to form colloidal particles in water. Since the metal has been ultimately converted into sol particles (via metal vapour), this

• method has been treated as of dispersion.

• (3) By Peptization• Some freshly precipitated ionic solids are

dispersed into colloidal solution in waler by the addition of small quantities of electrolytes, particularly those containing a common ion. The precipitate adsorbs the common ions and electrically charged particles then split from the precipitate as colloidal particles.

• The dispersal of a precipitated material into colloidal solution by the action of an electrolyte in solution, is termed peptization. The electrolyte used is called a peptizing agent. Peptization is the reverse of coagulation of a sol.

• Examples of preparation of sols by peptization

• (I) Silver Chloride, Ag-Cl, can be converted into a sol by adding hydrochloric acid (Clbeing common ion).

• (2) Ferric hydroxide, Fe(OH) , yields a sol by adding ferric chloride (Fe' being common ion)

PURIFICATION OF SOLS

In the methods of preparation stated above, the resulting sol frequently contains besides colloidal particles appreciable amounts of electrolytes. To obtain the pure sol,these electrolytes have to be removed. This purilication of sols can he accomplished by three methods:

• (a) Dialysis• (b) Electrodialysis• (e) Ultrafiltration

Dialysis

• Removal of soluble impurities from sols by the use of semipermeable membrane is known as dialysis.Solutes present in a true solution can pass through a semipermeable membrane such as parchment paper or cellophane. However, sol particles cannot pass through such membranes. When a bag made up of such a membrane is filled with the colloidal sol and then placed in fresh water, the soluble particles such as electrolytes pass through the membrane and go into the water leaving behind the colloidal sol.

• By using a continuous flow of fresh water, the concentration of the electrolyte outside the membrane tends to be zero. Thus diffusion of the ions into pure water remains brisk all the time. In this way, practically I the electrolyte present in the sol can be removed easily.

• The process of removing ions (or molecules) from a sol by diffusion through a permeable membrane is called Dialysis. The apparatus used for dialysis is called a Dialyser.

• The most important application of dialysis is in the purification of blood with the aid of an artificial kidney machine. The dialysismembrane permits excess ions and waste products like urea molecules to pass through and does not allow the colloidal particles of haemoglobin to pass through.

Electrodialysis

• In this process. dialysis is carried under the influence of electric field. Potential is applied between the metal screens supporting the membranes. This speeds up the migration of ions to the opposite electrode. Hence dialysis is greatly accelerated. Evidently electrodialysis is not meant for non·electrolyte impurities like sligar and urea.

Ultra-Filtration

• Sols pass through an ordinary filter paper. Its pores are too large to retain the colloidal particles. However, if the filter paper is impregnated with collodion or a regenerated cellulose such as cellophane or visking, the pore size is much reduced. Such a modified filter paper is called an ultrafilter. The separation of the sol particles from the liquid medium and electrolytes by filtrationthrough an ultrafilter is called ultrafiltration. Ultrafiltrationis a slow process. Gas pressure (or suction) is to be applied to speed it up. The colloidal particles are left on the ultrafilter in the form of slime. The slime may be stirred into fresh medium to get back the pure sol. By using graded ultrafilters, the technique of ultrafiltrationcan be employed to separate sol particles of different sizes.

PROPERTIES OF SOLS• COLOUR• The colour of a hydrophobic sol depends on the

wavelength of the light scallered by the dispersedparticles. The wavelength of the scattered light again depends on the size and the nature of theparticles. This is in the case of silver sols.

The colour changes produced by varying particles size have been observed in many other Cases

OPTICAL PROPERTIES OF SOI.SI) Sols exhibit Tyndall effect:

When a strong beam of light is passed through a sol and is viewed at right angle_" the path of light shows up as a hazy beam of cone. This is due to the fact that sol particles absorb light energy and then emit it in all directions in space.

The phenomenon of the scattering of light by the sol particles is called Tyndall effect.

The illuminated beam or cone formed by the scattering of light by the sol particles is often referred as Tyndall beam or Tyndall cone.

• True solutions do not show the tyndall effectt. Since ions or solute molecules arc too small to scatter light, the beam of light passing thuough a true solution is not visible when viewed from the side. Thus Tyndall effect can be used to distinguish a colloidal solution from a true solution.