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Essay: Colloids and their Preparation

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1: Introduction

Thomas Graham in 1861 studied the ability of those dissolved substances capable to diffuse into water across a permeable membrane. He observed that crystalline substances likeC6H12O6, CH4N2O, and NaCl passed through themembrane, while others like glue, gelatin and gum arabic did not. The former he called crystalloids and the latter colloids (Greek, kolla= glue ;eidos= like). He thought that the difference in the behavior of ‘crystalloids’ and ‘colloids’ was because of the particle size. Later on it was realised that any sort of substance, despite of its nature, could be transformed into a colloid by sub-classifying it intoatoms or molecules of colloidal size.

2:Definition

A colloid is a matter in which one substance of minutely scattered insoluble particles is draped throughout another substance. Sometimes the scattered substance itself is called the colloid. [1] In a true solution as sugar or salt in H2O, the solute particles are spreaded in the solvent as alone molecules or ions. Thus the diameter of the scattered particles ranges from 1Å to 10 Å. [2] A colloid is a mixture that has particles range of between 1 and 1000 nm in diameter, besides are still able to remain evenly distributed throughout the solution. [3] 3:Types

As we have seen in the above lines, a colloidal system is made of 2stages. The substance classified as the colloidal particles is the Dispersed phase. The other continuous phase in which the colloidal particles are dispersed is the Dispersion medium. E.g., for a colloidal solution of Cu in H2O, Cu particles formed the dispersed phase and water the dispersion medium. As mentioned above, a colloidal system is made of a dispersed phase and the dispersion medium. B/c either the dispersed phase or the dispersion medium can be a gas, liquid or solid, there are 8 sorts of colloidal systems possible. A colloidal dispersion of one gas in other is not possible since the 2 gases would give a same molecular mixture. In this chapter we will condemn our study strictly to the colloidal systems which consist of a solid substance dissolved in a liquid. These are often referred to as Sols or Colloidal solution. The colloidal solutions in H2O as the dispersion medium are termed Hydrosols or Aquasols. When the dispersions medium is alcohol or benzene, the sols are referred to as Alcosolsand Benzosols respectively 4:LYOPHILIC AND LYOPHOBIC SOLS OR COLLOIDS[4]

Sols are colloidal systems in which a solid is dipped in a liquid.

These can be sub-classified into 2classes :

(a) Lyophilic sols (solvent-loving)

(b) Lyophobic sols (solvent-hating)

Lyophilic sols are those in which the scattered phase exhibits a definite affinity for the mediumor the solvent.

The examples of lyophilic sols are dispersions of starch, gum, and protein in water.

Lyophobic sols are those in which the dispersed phase has no attraction for the medium or thesolvent.

The examples of lyophobic sols are dispersion of Au, Fe2O3 and S in H2O. The affinity or attraction of the sol particles for the medium, in a lyophilic sol, is due to hydrogen bonding with H2O. If the dispersed phase is a protein (as in egg) hydrogen bonding takes place between H2O molecules and the amino groups ( –NH–, –NH2) of the protein molecule. In spreading starch in H2O, hydrogen bonding occurs between H2O molecules and the – OH groups of the starch molecule. There are no similar forces of attraction when S or Au is mixed in water.

5:CHARACTERISTICS OF LYOPHILIC AND LYOPHOBIC SOLS[5] Some features of lyophilic and lyophobic sols are as follows (1) Ease of preparation

Lyophilic sols can be obtained easily by mixing the material (starch, protein) with an appropriate solvent. The macro molecules of the material are of colloidal size and these at once pass into the colloidal form on account of interconnection with the solvent.

Lyophobic sols are not getting able by simply mixing the solid material with the solvent.

(2) Charge on particles

Particles of a hydrophilic sol may have either small or no charge at all Particles of a hydrophobic sol carry +ve or -ve charge which gives them stability.

(3) Solvation

Hydrophilic sol particles are generally solvated. That is, they are bounded by an adsorbed layer of the dispersion medium which does not allow them to come, gather and coagulate. E.g. Hydration of gelatin .

There is no solvation of the hydrophobic sol particles for want of interaction with the medium.

(4) Viscosity

Lyophilic sols are thick as the particle size goes up due to solvation, and the proportion of free medium goes down. Warm solutions of the dispersed stage on cooling set to a gel E.g., preparation Of table jelly.

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 mixed.

Lyophobic sols are precipitated even by low concentration of electrolytes, the protective layer being not present.

(6) Reversibility

The dispersed phase of lyophilic sols when distinguished by coagulation or by evaporation of the medium, can be changed again into the colloidal form just on dissolving with the dispersion medium. Therefore this type of sols are designated as Reversible sols.

On the other hand, the lyophobic sols once precipitated cannot be changed again merely by mixing With dispersion medium. These are, therefore, called Irreversible sols.

(7) Tyndall effect Due to relatively tiny particle size, lyophilic sols do not scatter light and show no Tyndall effect.

Lyophobic sol particles are big enough to exhibit tyndall effect.

(8) Migration in electronic 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 -ve or +ve charge.

6:PREPARATION OF SOLS[6]

Lyophilic sols may be prepared by simply warming the solid with the liquid dispersion mediumE.g., starch with H2O. On the other hand, lyophobic sols have to be prepared by special methods.

These methods fall into 2categories :

(1) Dispersion Methods in which biggermacro-sized particles are split down to colloidal size.

(2) Gathering Methods in which colloidal size particles are made up by gathering single ions or molecules.

DISPERSION METHODS

In these methods, material in excess is dispersed in other medium.

(1) Mechanical dispersion By Using Colloid Mill

The solid along with the liquid dispersion medium is supplied into a Colloid mill. The mill containing two steel plates almost touching each other and rotating in anti directions with great speed. The solid particles are ground down to colloidal size and are then vanished in the liquid to give the sol.

Colloidal graphite’ (a lubricant) and printing inks are made by this method.Now a days, mercury sol has been manufactured by shattering a layer of mercury into sol particles inH2O by means of US(ultra sonic) vibration..

(2) Bredig’s Arc Method

It is used for making hydrosols of metals e.g., Ag, Au and Pt. An arc is struck in mid of the two metal electrodes held close together underde-ionized water. The H2O is kept cold by occupying the container in ice/water bath and a trace of alkali (KOH) is added. The high heat of the spark across the electrodes evaporates some of the metal and the haze condenses under H2O. Thus the atoms of the metal present in the hazes aggregate to form colloidal particles in H2O. Since the metal has been basically converted into sol particles (via metal vapour), thismethod has been treated as of dispersion.

Non-metal sols can be made by suspending coarse particles of the substance in the dispersionmedium and striking an arc between iron electrodes.

(3) By Peptization

Some newly precipitated ionic solids are dispersed into colloidal solution in H2O by thecombination of small quantities of electrolytes, eventually those containing a same ion. The pptsadsorbs the common ions and electrically charged particles then break from the precipitate as colloidalparticles.

The diffusal 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 alter of coagulation of a sol.

Examples of preparation of sols by peptization

(1) Silver chloride, Ag+Cl–, can be converted into a sol by combining hydrochloric acid (Cl– being common ion.)

(2) Ferric hydroxide, Fe(OH)3, give a sol by adding ferric chloride (Fe3+ being common ion).

AGGREGATION METHODS

These methods consists of chemical reactions or change of solvent whereby the atoms or molecules of the diffusal phase appearing 1st, coalesce to form colloidal particles.

The states (temperature, concentration, etc.) used are such as permit the making of sol particles but stops the particles becoming too huge and forming ppt. The ions which aren’t required (spectator ions) present in the sol are eradicated by dialysis as these ions may finally coagulate the sol.

The more chief methods for preparing hydrophobic sols are as follows : (1) Double Decomposition

An (As2S3) sol is prepared by passing a slow stream of H2S gas through a cold solution of (As2O3). This will prolong till the yellow colour of the sol gets max intensity.

As2O3 + 3H2S ⎯⎯→As2S3 (sol) + 3H2O

Too much H2S (electrolyte) is erased by passing in a stream of H2 gas.

(2) Reduction

Silver sols and gold sols can be achieved by the reaction of dilute solutions of AgNO3 or AuCl2 with organic reducing agents like tannic acid or methanal (HCHO) AgNO3 + tannic acid ⎯⎯→Ag sol

AuCl3 + tannic acid ⎯⎯→Au sol

(3) Oxidation

A sol of S is produced by passing H2S into a solution of SO2.

2H2S + SO2 ⎯⎯→2H2O + S↓

In qualitative analysis, S sol is frequently encountered when H2S is passed through the solution to make ppt of group 2 metals if an oxidizing agent (Chromate or ferric ions) happen to be present. It can be removed by boiling (to coagulate the sulphur) and filtering through two filter papers folded together.

(4) Hydrolysis

Sols of the hydroxides of Fe, Cr and Al are readily produced by the hydrolysis of salts of the respective metals. In order to obtain a red sol of Fe(OH)3, a few drops of 30% FeCl3 solution is added to a large volume of nearly boiling water and mixed with a glass rod.

FeCl3 + 3H2O ⎯⎯→Fe(OH)3 + 3HCl

red sol

(5) Change of Solvent

When a solution of Sor resin in C2H5OH is added to an excess of H2O, the S or resin sol is formed owing to goes down in solubility. The matter is present in molecular state in C2H5OH but on transference to water, the molecules precipitate out to form colloidal particles.

7:PURIFICATION OF SOLS[7]

In the methods of preparation as mentioned above, the obtained sol frequently contains besides colloidal particles appreciable amounts of electrolytes. To get the pure sol, these electrolytes have to beerased. This purification of sols can be achieved by 3methods : (a) Dialysis

(b) Electrodialysis

(c) Ultrafiltration

Dialysis

Animal membranes (bladder) or those made of parchment paper and cellophane sheet, have very fine holes. These holes allow ions (or small molecules) to pass through but not the large colloidal particles. When a sol containing vanished ions (electrolyte) or molecules is placed in a bag of permeable membrane immersed in pure water, the ions spread through the membrane. By using a continuous flow of fresh water, the concentration of the electrolyte which is not inside the membrane becomes almost zero. Thus diffusion of the ions into pure H2O remains brisk all the time. In this way, practically all the electrolyte present in the sol can be erased effortlessly.

The phenomena of erasing ions (or molecules) from a sol by spreading through a permeable membrane is called Dialysis. The device used for dialysis is called a Dialyser.

Example.A Fe(OH)3 sol (red) form by the hydrolysis of FeCl3 will be dissolved with someHCl acid. If the impure sol is placed in the dialysis bag for small time, the outside water will give a white ppt with AgNO3. After a bit of long time, it will be found that almost the whole of HCl acid has been erased and the pure red sol is left in the dialyser bag.

Electrodialysis

In this operation, dialysis is carried under the influence of electric field (Fig. 22.8). Potential is forced between the metal screens giving support to the membranes. This speeds up the transfer of ions to the opposite electrode. Hence dialysis is accelerated. Evidently electrodialysis is notmeant for nonelectrolyte impurities like sugar and urea.

Ultrafiltration

Sols pass through a simple filter paper, Its holes aretoo huge to maintain the colloidal particles. However, if thefilter paper is infused with collodion or a regeneratedcellulose such as cellophane or visking, the hole size ismuch minute. Such a modified filter paper is called anultrafilter.The dissociation of the sol particles from the liquidmedium and electrolytes by filtration with the help of an ultrafilteris called ultrafiltration.

Ultrafiltration is a steady process. Gas pressure (orsuction) has to be forced to speed it up. The colloidal particles are left on the ultrafilter in the form of slime. Theslime can be mixed into fresh medium to get back the puresol. By the help of graded ultrafilters, the technique ofultrafiltration can be employed to separate sol particles ofvarious sizes.

8:STABILITY OF SOLS[8]

A true colloidal solution is stable. Its particles don,t ever coalesce and separate out. The stability of sols is because of 2factors : (1) Presence of like charge on sol particles

The dispersed particles of a hydrophobic sol contains a like electrical charge (all +ve or all -ve) on their surface. Since same charges repel each other, the particles push away from each other and resist joining together. However, when an electrolyte is mixed to a hydrophobic sol, theparticles are discharged and ppt formed.

(2) Presence of Solvent layer around sol particle

The lyophilic sols are stable for 2 reasons. Their particles possess a charge and in additionhave a layer of the solvent bound on the surface. E.g, a sol particle of gelatin has a -vecharge and a water layer envelopes it. When NaCl is mixed with colloidal solution of gelatin,its particles are not precipitated. The H2O layer around the gelatin particle doesn,t permit the Na+ ions to run into it and discharge the particle. The gelatin sol isn,t precipitated by mixing ofNaCl solution. actually, lyophilic sols are more stable than lyophobic sols.

9:ASSOCIATED COLLOIDS[9]

The molecules of substances as soaps and artificial detergents are tinier than the colloidalparticles. Whereas in concentrated solutions these molecules form aggregates of colloidal size.Substances whose molecules combines spontaneously in a given solvent to form particles of colloidaldimensions are Associated or Association Colloids.

The colloidal aggregates of soap or detergent molecules formed in the solvent are referred to asmicelles.

Explanation.Soap or detergent molecule ionises in water to form an anion and Na ion.Thus sodium stearate (a typical soap) furnishes stearate anion and sodium ion in aqueoussolution.

C17H35COO– Na+ ⎯⎯→C17H35COO– + Na+

Sodium stearate Stearate ion

As many as seventy stearate ions aggregate to form a micelle of colloidal size. The stearate ion has along hydrocarbon chain (17 carbons) with a polar —COO– group at 1 end. The zigzag hydrocarbontail is given by a wavy line and the polar head by a hollow circle. In the micelle formation, the tails being insoluble in H2O are directed to the centre, while the soluble polar heads are on the surface in contact with H2OThe charge on the micelle because of the polar heads accountsfor the stability of the particle.

Cleansing Action of Soaps and Detergents

The cleansing action of soap is due to

(1) Solubilisation of grease into the micelle

(2) Emulsification of grease

Solubilisation.

In relatively strong solution the soap (or detergent) anions directlyform a micelle. The hydrocarbon tails are in the inner side of the micelle and COO– ions on the surface.The grease stain is then absorbed into the interior of the micelle which behaves like liquid hydrocarbons. As the stain is removed from the fabric, the dirt particles sticking to the stain are alsoerased.

Emulsification.

As already discussed the soap or detergent molecules are ionised in H2O. Theanions are made of oil-soluble hydrocarbon tails and water-soluble polar heads. Thus soap anionhas a long hydrocarbon tail with a polar head, —COO–. When soap solution is amixedwith a fabric, the tails of the soap anions are fixed into the grease stain. The polar heads started from the greasesurface and form a charged layer around it. Thus by mutual repulsions the grease droplets are suspended in H2O. The emulsified grease stains are cleaned away with soap solution.

10:EMULSIONS[10]

These are liquid-liquid colloidal systems. In other words, an emulsion is adispersion of finely divided liquid droplets in another liquid.

Generally 1 of the 2 liquids is H2Oand the other, which is immiscible with H2O, isdesignated as oil. Either liquid can make the dispersed phase.

Types of Emulsions

There are 2 types of emulsions.

(a) Oil-in-Water type (O/W type)

(b) Water-in-Oil type (W/O type)

Examples of Emulsions

(1) Milk is an emulsion of O/W type. Tiny droplets of liquid fat are added in H2O.

(2) Stiff greases are emulsions of W/O type, H2O being dispersed in lubricating oil.

Preparation of Emulsions

The dispersal of a liquid in the form of anemulsion is called emulsification. This can bedone by agitating a tiny amount of 1liquid with the bulk of the other. It is better achieved by passing a mixture of the 2liquid through a colloid mill known ashomogenizer.The emulsions can simply be obtain byshakingthe 2 liquids are unstable. The droplets ofthe dispersed phase coalesce and make a separate layer. To have a stable emulsion, tinyamount of a 3rd substance called theEmulsifier or Emulsifying agent is addedduring the preparation. This is usually a soap,synthetic detergent, or a hydrophilic colloid.

Role of Emulsifier

The emulsifier concentrates at the interface and decreases surface tension on the side of 1liquid which rolls into droplets. Soap, E.g., is made of a long hydrocarbon tail (oil soluble)with a polar head —COO–Na+ (water soluble). In O/W type emulsion the tail is pegged into the oildroplet, while the head prolongs into H2O. Thus the soap acts as go-between and the emulsifieddroplets are not permitted to coalesce.

Properties of Emulsions

(1) Demulsification.

Emulsions can be broken or ‘demulsified’ to get the constituent liquids byheating, freezing, centrifuging, or by addition of appreciable amounts of electrolytes. They are alsobroken by destroying the emulsifying agent. For example, an oil-water emulsion stabilized by soap isbroken by addition of a strong acid. The acid converts soap into insoluble free fatty acids.

(2) Dilution.

Emulsions can be diluted with any amount of the dispersion medium. On the other hand the dispersed liquid when mixed with it will at once form a separate layer. This property ofemulsions is used to detect the type of a given emulsion 11:GELS[11]

A gel is actually a jelly-like colloidal system inwhich a liquid is dissolved in a solid medium.E.g., when a warm sol of gelatin is cooled,it lays to a semisolid mass which is a gel. This operation of a gel making is known as Gelation.Explanation.Gelation may be consider aspartial coagulation of a sol. The coagulating solparticles first combine to make long thread-likechains. These chains are then interconnected to forma solid framework. The liquid dispersion mediumgets locked in the spaces of this framework.

The resulting semisolid porous mass has a gel likestructure. A sponge absorbed in water is example of gel structure.

Two sorts of Gels

(a) Elastic gels are those which contains elastic properties. They convert their form onapplying force and change back to original shape when the force is eraased. Gelatin, starch and soaps areillustrations of substances which make elastic gels.Elastic gels are formed by cooling highly concentrated lyophilic sols. The bondings or links between themolecules (particles) are due to electrical attraction and are not hard.

(b) Non-elastic gels are those which are harde.g., silica gel. These are formed by appropriatechemical action. Thus silica gel is made by adding concentrated HCl acid to sodiumsilicate solution of the accurate concentration. The resulting molecules of silicic acid polymerise to make silica gel. It has a network connected by covalent bonds which give a strong and hard structure.

Properties of Gels

(1) Hydration.

A fully dehydrated elastic gel can be reproduced by addition of H2O. But once a nonelastic gel is freed from moisture, addition of H2O will not bring about gelation.

(2) Swelling.

Partially dehydrate elastic gels imbibe H2O when dipped in the solvent. This causes increase in the volume of the gel and process is called Swelling.

(3) Syneresis.

Many inorganic gels on standing reduce in size which is achieved by exudationof solvent. This process is called Syneresis.

(4) Thixotropy.

Some gels are semisolid when at rest but revert to liquid sol on agitation. This reversible sol-gel conversion is referred to as Thixotropy. Iron oxide and silver oxide gels exhibit this property. The modern thixotropic paints are also an example.

12:EXAMPLES[12]

Some Examples of Colloids are as follows

Dispersion Medium Dispersed Phase Type of Colloid Example Solid Solid Solid sol Ruby glass

Solid Liquid Solid emulsion/gel Pearl, cheese

Solid Gas Solid foam Lava, pumice

Liquid Solid Sol Paints, cell fluids

Liquid Liquid Emulsion Milk, oil in water

Liquid Gas Foam Soap suds, whipped cream

Gas Solid Aerosol Smoke

Gas Liquid Aerosol Fog, mist

13:APPLICATIONS[13]

Colloids play an essential role in our daily life and industry. A knowledge of colloid chemistry isimportant to understand some of the different natural phenomena around us. Colloids make up some of our modern products. some of the important applications of colloids are as follows.

(1) Foods

Many of our foods are colloidal in nature. Milk is an emulsion of butterfat in H2O secured by a protein, casein. Salad dressing, gelatin deserts, fruit jellies and whipped cream are other examples.

Ice cream is a dispersion of ice in cream. Bread is a dispersion of air in baked dough.

(2) Medicines

Colloidal medicines being finely divided, are more useful and are easily absorbed in oursystem. Halibut-liver oil and cod-liver that we take are, actually, the emulsions of the respective oils in wH2O. Many ointments for application to skin consist of physiologically active parts mixedin oil and made into an emulsion with H2O. Antibiotics such as penicillin and streptomycin areformed in colloidal form suitable for injections.

(3) Non-drip or thixotropic paints

All paints are colloidal dispersions of solid pigments in a liquid medium. The modern nondrip orthixotropic paints also contain long-chain polymers. At rest, the chains of molecules are coiled andentrap much dispersion medium. Thus the paint is a semisolid gel structure. When shearing stress isapplied with a paint brush, the coiled molecules straighten and the entrapped medium is released. Assoon as the brush is removed, the liquid paint reverts to the semisolid form. This renders the paint‘non-drip’.

(4) Electrical precipitation of smoke

The smoke coming from industrial plants is a colloidal dispersion of solid particles (carbon, arsenic compounds, cement dust) in air. It is a nuisance and damges the atmosphere. So,beforepermitingthe smoke to escape into air, it is treated by Cottrell PrecipitatorThe smoke is let past a series of sharp points charged to a high potential (20,000 to 70,000 V). Thepoints discharge high velocity electrons that ionise molecules in air. Smoke particles adsorb these+ve ions and become charged. The charged particles are attracted to the oppositely chargedelectrodes and forms ppt. The gases that leave theCottrell precipitator are thus freed fromsmoke. In addition, valuable materials may be secured from the precipitated smoke. For example,arsenic oxide is mainly recovered from the smelter smoke by this method.

(5) Clarification of Municipal water

The municipal water obtained from natural sources often consists ofcolloidal particles. The phenomenaof coagulation is used to eradicae these. The sol particles carry a -ve charge. When aluminiumsulphate(alum) is added to H2O, a gelatinous precipitate of hydrated aluminium hydroxide (floc) isproduced Al3+ + 3H2O ⎯⎯→Al(OH)3 + 3H+

Al(OH)3+ + 4H2O + H+ ⎯⎯→Al(OH)3(H2O)4

+

+vely charged flocattracts to it -tive sol particles which are coagulated. The flocalong with the suspended matter comes down, leaving the H2O clear (6) Formation of Delta

The river water consists of colloidal particles of sand and clay which carry -ve charge. Thesea water, on the other hand, contains cations ions such as Na+, Mg2+, Ca2+. As the river water meets sea water, these ions discharge the sand or clay particles which formsppt as delta.

(7) Artificial Kidney machine

The human kidneys cleans the blood by dialysis through natural membranes. The toxic waste products such as urea and uric acid pass through the membranes, while colloidal-sized particles ofblood proteins (haemoglobin) are retained. Kidney failure, therefore, leads to death due to accumulation of poisonous waste products in blood . Now-a-days, the patient’s blood can be cleansed by shuntingit into an ‘artificial kidney machine’. Here the impure blood is form to pass through a series ofcellophane tubes surrounded by a washing solution in H2O. The toxic waste chemicals (urea, uricacid) diffuse across the tube walls into the washing solution. The cleaned blood is changed back to thepatient. The use of artificial kidney machine saves the life of 1000 of persons each year.

(8) Adsorption indicators

These indicators function by preferentialadsorption of ions onto sol particles. Fluorescein(Na+Fl) is an example of adsorption indicatorwhich is used for the titration of NaCl solution against AgNO3 solution.When AgNO3 solution is run into asolution of NaCl containing a littlefluorescein, a white precipitate of AgNO3 is first produced. At the end-point, the whiteprecipitate chnges sharply pink.

Explanation.

The indicator fluorescein is adye (Na+Fl–) which gives coloured anion Fl– inaqueous solution. The white precipitate of AgCl formed by running AgNO3 solution intoNaCl solution is partially colloidal in nature.

(a) Before the end-point,

Cl– ions are inexcess. The AgCl sol particles adsorb these ionsand become -vely charged. The -veAgCl/Cl– particles cannot adsorb the colouredfluorescein anions (Fl–) due to electrostatic repulsion. Thus the precipitate remains white.

(b) After the end-point,

Ag+ ions become in excess. AgCl sol particles adsorb these andacquire +ve charge. The +veAgCl/Ag+ particles now attract the coloured fluorescein anions(Fl–) and turn rose-red.

Thus the end-point is marked by white precipitate changing to pink.

(9) Blue colour of the sky

This is an function of Tyndall consequence. The higher ambiance contains colloidal dirt or frost particles discrete in atmosphere. As the sun energy enters the air (Fig. 22.33) these hit the colloidal particles. The particles soak up daylight and disperse glow of blue color (4600–5100Å). The light that is occurrence at earth’s surface is considerably reddened due to the removal of most of the blue light in the higher air.

14: References

[1] :http://www.britannica.com/science/colloid

[2] : essentials of physical chemistry ArunBahl , BsBahl , G.D. Tuli , S.Chand [3] :http://chemwiki.ucdavis.edu/Core/Physical_Chemistry/Physical_Properties_of_Matter/Solutions_and_Mixtures/Colloid [4] :http://www.chemistrylearning.com/lyophobic-colloid/ http://www.chemistrylearning.com/lyophilic-colloids/ [5] :http://www.chemistrylearning.com/difference-between-lyophobic-and-lyophilic/ [6] :http://chemistry-desk.blogspot.com/2013/08/preparation-of-colloids.html [7] :http://www.chemistrylearning.com/purification-of-colloids/ [8] :http://www.emedicalprep.com/study-material/chemistry/surface-chemistry/sols-stability.html [9] :http://encyclopedia2.thefreedictionary.com/Association+Colloid [10] :https://en.wikipedia.org/wiki/Emulsion

[11] :https://en.wikipedia.org/wiki/Gel

[12] :http://chemwiki.ucdavis.edu/Core/Physical_Chemistry/Physical_Properties_of_Matter/Solutions_and_Mixtures/Colloid [13] :http://www.chemistrylearning.com/applications-of-colloids/ 2016-3-14-1457969076

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