COLLOIDS, Preparation, Purification & Appli [Compatibility Mode]

COLLOIDS-Preparation and Purification

Definition of colloids

� A colloid is defined as a particle that has some linear dimension between 1 nm and 1 µm. nm and 1 µm.

� A dispersion of such particles is called a colloid dispersion.

� Dispersions of Microscopic/sub-microscopic particles in fluid medium

Difference b/w colloid and emulsion:

�

Colloid is a uniform dispersion of fine solid particles in a liquid medium. An emulsion is a uniform dispersion of one An emulsion is a uniform dispersion of one liquid in another liquid which are not miscible. Or

Whether emulsions are mixtures of two immiscible liquids, they can be categorized as colloids in which both the dispersed phase and the dispersion medium are liquids.

colloids

� If the dispersion medium or continuous phase is a gas term..the term aerosol is applied.

� If the dispersion medium is a liq. the term sol/hydrosol (when the medium is water) sol/hydrosol (when the medium is water) are used.

� colloid dispersion can found in many industrial areas…mafac. Food, cosmotics, paints, oil fileds, and many biological systems

� Much air and water pollution is also a colloidal in nature.

• Although the size of the particles is very small, a

colloid dispersion is quite different from a solution.

• For example, a true solution passe through parchment

or cellophane papers, but a colloid dispersion cannot;

only the continuous medium of the dispersion seeps

through. through.

• Some of the dispersions pass through an ordinary filter

paper like a true solution.

• The particles of the dispersed phase can be viewed

easily in an ultramicroscope or an electron microscope.

• They can be discerned by light scattering.

• A state of subdivision in which the particles, droplets, or bubbles dispersed in another phase have at least one dimension between 1 and

Colloidal systems

dimension between 1 and 1000 nm

• all combinations are possible between :

gas, liquid, and solid

W. Ostwald

Surface area of colloidal systems

• Cube (1cm; 1cm; 1cm)

after size reduction to an edge length of 500 nm:

� surface area of 60 m2

• Spinning dope (1 cm3) • Spinning dope (1 cm3)

after spinning to a fibre with diameter of 1000 nm:

� fiber length of 1273 km

• 1 liter of a 0.1 M surfactant solution:

� interfacial area of 40000 m2

Surface atoms [in %]

in dependence on the particle size [in nm]

70

80

90

100

0

10

20

30

40

50

60

70

20 10 5 2 1

part of surface

atoms in %%

nm

Size and shape of colloids:

� Particles lying in the colloidal size have large surfacearea when compared with the surface area of anequal volume of larger particles.

� Specific surface: the surface area per unit weight or� Specific surface: the surface area per unit weight orvolume of material.

� The possession of large specific surface results in:

1- platinium is effective as catalyst only when found in colloidalform due to large surface area which adsorb reactant on theirsurface.

2- The colour of colloidal dispersion is related to the size of thepaticles

e.g. red gold sol takes a blue colour when the particles increase insize

Size and shape of colloids:

- The shape of colloidal particles in dispersion is important:

The more extended the particle the greater its specificsurface the greater the attractive force between theparticles of the dispersed phase and the dispersionparticles of the dispersed phase and the dispersionmedium.

Flow, sedimentation and osmotic pressure of the colloidalsystem affected by the shape of colloidal particles.

� Particle shape may also influence the pharmacologicaction.

Different shapes of colloids Colloid shapes: dumbbell, hemisphere, disc, and sphero-cylinder shapes.

Colloid shapes: dumbbell, hemisphere, disc, and sphero-cylinder shapes.

shapes of colloidsColloid shapes: Prolate (egg shape), Oblate (pumpkin-shaped), Disc, Random coil and rod shapes.

albumen.conservation-us.org

Applications of colloidal solutions:

1- Therapy--- Colloidal system are used as therapeuticagents in different areas.

e.g- Silver colloid-germicidalCopper colloid-anticancerMercury colloid-Antisyphilis Copper colloid-anticancerMercury colloid-Antisyphilis (syphilis: A common venereal disease caused by the treponema pallidum spirochete);

2- Stability---e.g. lyophobic colloids prevent flocculation in suspensions.

e.g- Colloidal dispersion of gelatin is used in coating over tablets and granules which upon drying leaves a uniform dry film over them and protect them from adverse conditions of the atmosphere.


4- Absorption--- As colloidal dimensions are smallenough, they have a huge surface area.

� Hence, the drug constituted colloidal form is releasedin large amount.in large amount.

e.g- sulphur colloid gives a large quantity of sulphur andthis often leads to sulphur toxicity

5-Targeted Drug Delivery--- Liposomes are ofcolloidal dimensions and are preferentially taken up bythe liver and spleen.


6- Photography:

A colloidal solution of silver bromide in gelatine is appliedon glass plates or celluloid films to form sensitiveplates in photography.plates in photography.

7- Clotting of blood:

- Blood is a colloidal solution and is negatively charged.

- On applying a solution of Fecl3 bleeding stops and blood clotting occurs as Fe+3 ions neutralize the ion charges on the colloidal particles.


8- Sewage disposal:

Colloidal particles of the dirt, mud etc. carry electric charge, hence when sewage water is passed through the plates kept at a high potential, the colloidal particles are kept at a high potential, the colloidal particles are coagulated due to electrophoresis and the suspended matter gets removed.

9- Purification of water:

- The precipitation of colloidal impurities present in watercan be done by adding certain electrolytes like alum etc.the negatively charged colloidal particles of impurities getneutralized by the Al3+ ions and settle down and purewater can be decanted off.


10- Smoke Precipitation:Smoke particles are actually electrically charged colloidal particles of

carbon in the air. Precipitation of smoke particles is carried out by Cottrell precipitator which is based on the principle of electrophoresis. electrophoresis.

Smoke is allowed to pass through a chamber having a number of metal plates attached to a metal wire connected to a source of high potential. Charged particles of smoke get attracted by oppositely charge electrode get precipitated after losing their charge and the hot air passes out through the chimney.

The dust particles are also removed in this process. Thus the nuisance of smoke in big industrial cities can be avoided.


11- Artificial rain: Artificial rain can be caused by spraying oppositely charged colloidal dust or sand particles over a cloud. The colloidal water particles present in the cloud will be neutralized and coagulate to from bigger water drops causing artificial rain.from bigger water drops causing artificial rain.

12-Rubber industry:Latex is a colloidal solution of negatively charged rubber particles. From latex, rubber can be obtained by coagulation.Rubber plated articles are prepared by depositing negatively charged rubber particles over the article to be rubber plated by making that article an anode in a rubber plating bath.


13) Smoke screen:In warfare smoke screens are used which are nothing but colloidal dispersion of certain substances in the air.

14) Other applications:a) Blue color of the sky is due to the scattering of light by a) Blue color of the sky is due to the scattering of light by colloidal dust particles in air (Tyndall effect). Similarly, sea water looks blue due to scattering of light by the colloidal impurities present in sea water.b) Tail of comets is seen as a Tyndall cone due to the scattering of light by the tiny solid particles left by the comet in its path.c) Blood is a colloidal solution and the stoppage of bleeding on applying ferric chloride solution is due to coagulation of blood forming a clot.

Types of colloidsColloids are usually classified according to:

1- The original states of their constituent parts

Types of colloids:

2-The nature of interaction between dispersed phase anddispersion medium.

A-Lyophilic colloids (solvent attracting) (solventloving) – The particles in a lyophilic system have aloving) – The particles in a lyophilic system have agreat affinity for the solvent.

� If water is the dispersing medium, it is often known asa hydrosol or hydrophilic.

� readily solvated (combined chemically or physically,with the solvent) and dispersed, even at highconcentrations.

� More viscid

Types of colloids:� Examples of lyophilic sols include sols of gum, gelatin, starch,

proteins and certain polymers (rubber) in organic solvents.

� the dispersed phase does not precipitate easily

� the sols are quite stable as the solute particle surrounded by two� the sols are quite stable as the solute particle surrounded by twostability factors: a- negative or positive charge b- layer of solvent

� If the dispersion medium is separated from the dispersed phase, thesol can be reconstituted by simply remixing with the dispersionmedium. Hence, these sols are called reversible sols.

� Prepared simply by dissolving the material in the solvent being usede.g. dissolution of acacia in water.

Acacia: Any of various spiny trees or shrubs of the genus Acacia

Types of colloids:

� Lyophilic colloid dispersions are formed quite easily by the spontaneous dispersion of the colloid particles in the medium.

� For example, the swelling of gelatin in water indicates the � For example, the swelling of gelatin in water indicates the high affinity between the water and gelatin molecules.

� Classic examples of the lyophilic colloids are macromolecular proteins, micelles and liposomes.

Types of colloids:B-lyophobic (solvent repelling) (solvent hating) - The particles

resist solvation and dispersion in the solvent.- The concentration of particles is usually relatively low.- Less viscid

- These colloids are easily precipitated on the addition of small

charge

- These colloids are easily precipitated on the addition of smallamounts of electrolytes, by heating or by shaking

- Less stable as the particles surrounded only with a layer of positiveor negative charge

- Once precipitated, it is not easy to reconstitute the sol by simplemixing with the dispersion medium. Hence, these sols are calledirreversible sols.

- Examples of lyophobic sols include sols of metals and their insolublecompounds like sulphides and oxides.

e.g. gold in water

Types of colloids:

� Lyophobic colloids do not pass into the dispersed state spontaneously.

� They are generally produced by � They are generally produced by mechanical or chemical action.

� An example of lyophobicncolloid is latex paint

Types of colloids:

- At low concentration: amphiphiles exist separately(subcolloidal size)

- At high concentration: form aggregates or micelles (50or more monomers) (colloidal size)or more monomers) (colloidal size)

Association colloids

Types of colloids: Association colloids

Critical micelle concentration (C.M.C) : the concentrationat which micelle form

- The phenomenon of micelle formation can beexplained:explained:

1- below C.M.C: amphiphiles are adsorbed at theair/water interface

2- As amphiphile concentration is raised: both theinterphase and bulk phase become saturated withmonomers (C.M.C)

3- any further amphiphile added in excess: amphiphilesaggregate to form micelles

Types of colloids:Association colloids

- In water: the hydrocarbon chains face inwards intothe micelle forming hydrocarbon core and surroundedby the polar portions of the amphiphile associatedwith water molecules.with water molecules.

- In non-polar liquid: the polar heads facing inward andthe hydrocarbon chains are associated with non-polarliquid.

- At concentrations close to C.M.C sphericalmicelles

- At higher concentrations lamellarmicelles

C.M.C:Molecules with hydrophilic and hydrophobic components are said to be amphiphilic (Figure A).

An amphiphilic molecule can arrangeitself on the surface of water so thatthe polar part interacts with the waterand the non-polar part is held abovethe surface (either in the air or in anon-polar liquid) as shown in Figure B.

Molecules can form aggregates or micelles in which the hydrophobic portions are oriented within the cluster and the hydrophilic portions are exposed to the solvent. An example of a spherical micelle is shown in Figure C.

C.M.C: In colloidal and surface chemistry, the critical micelleconcentration(CMC) is defined as the concentration of surfactantsabove which micelles form and all additional surfactants added to thesystem go to micelles. The CMC is an important characteristic of asurfactant and can be found by measuring surface tension.

The graph shows the three phases: 1, at low concentrations of surfactant only slight change in surface tension is detected; 2, additional surfactant 2, additional surfactant decreases surface tension; and 3, the surface becomes fully loaded, with no further change in surface tension.

CMC is found as the point where the baseline of minimal surface tension and the slope where surface tensionshows linear decline intersect.

Association Colloids

Types of colloids:

� The formation of association colloids is spontaneous, provided the concentration of amphiphile in solution exceed C.M.C.

� For reversible colloids, the dispersed phase is spontaneously distributed in the surrounding medium by thermal energy.

� For example, a protein crystal dissolves in waternspontaneously.

� Such spontaneous dispersion leads to an equilibrium size distributioncorresponding to the minimum value of the thermodynamic potential.

� If the dispersed phase is thrown out of the colloid state, its redis-persion is achieved easily.

� The irreversible colloid dispersions, on the other hand, are thermodynamically unstable.

� To illustrate, a gold crystal, if brought in contact with water, will not � To illustrate, a gold crystal, if brought in contact with water, will not generate the sol spontaneously.

� The subdivision of the gold crystal into small particles can be performed only by supplying a considerable amount of energy.

� The total free energy of the gold-water interface is a positive quantity.

� The small entropic gain in the subdivision process is not sufficient to make the formation of sol spontaneous.

� This type of colloids has a natural tendency to be thrown out of the dispersion medium from the colloid state.

� Their stability in the colloid state is achieved with considerable difficulty.

Colloidal systems

� have large surface areas

� surface atoms become dominant� surface atoms become dominant

Colloidal systems� Coagulation and flocculation

The terms coagulation and flocculation are widely used in colloid chemistry to mean aggregation of the particles.

� The term coagulation to implies the formationof compact aggregates leading to the macroscopic separation of a coagulum, and

the term flocculation is used implying the formation of a � the term flocculation is used implying the formation of a loose or open network which may or may not separate macroscopically.

� Various chemicals (e.g. polymers) are used to induce flocculation or coagulation. Example: Brine sol purification for NaOH Prod.These are known as flocculants or coagulants.

� The intermolecular and surface forces play important roles in the stabilization and coagulation of colloids.

Critical coagulation concentration

� When the colloid particles are stabilized, the process is known as peptization.

� The reverse process, i.e., the destabilization of colloids is termed coagulation.termed coagulation. Certain ions are necessary to cause peptization or coagulation. The amount of electrolyte required to induce coagulation

depends upon the valence of the counterion in the salt.

� This concentration of electrolyte is known ascritical coagulation concentration (CCC) or flocculation value

� The valence of the counterion is a very important parameter for the coagulation of a sol.

� The amount of divalent counter ion required to coagulate the AgI colloid is much smaller than the amount of the monovalent counterion.

� The amount of trivalent counterion required is even less. The effect of valence of counterion on coagulation can be The effect of valence of counterion on coagulation can be explained by the SchulzeHardy rule.

� It states that the critical coagulation concentration varies with the inverse sixth-power of the valence of the counterion.

� The specific nature of these ions is less important. Also, the effect of the valence of the coions (i.e., the ions of the electrolyte having the same charge as the colloid particles) is less significant on coagulation.

Thermodynamic and kinetic stabilities of colloids

� Colloidal gold: stabilized against coagulation !

� Creme: stabilized against coagulation !

� Milk: stabilized against coagulation !

Causes for colloidal stability

Colloidal stability can be influenced by the

degree of affinity for the solvent in which

the colloid is suspended. the colloid is suspended.

This leads to the following colloid

classification .

Dispersed Systems:

Dispersed Systems:

� Dispersed systems consist of particulate matter (dispersedphase), distributed throughout a continuous phase(dispersion medium).

They are classified according to the particle diameter of the� They are classified according to the particle diameter of thedispersed material:

1- Molecular dispersions (less than 1 nm)

- Particles invisible in electron microscope

- Pass through semipermeable membranes and filter paper

- Particles do not settle down on standing

- Undergo rapid diffusion

- E.g. ordinary ions, glucose

Dispersed Systems:

2- Colloidal dispersions (1 nm - o.5 um)- Particles not resolved by ordinary microscope, can be

detected by electron microscope.- Pass through filter paper but not pass through

semipermeable membrane.Particles made to settle by centrifugation- Particles made to settle by centrifugation

- Diffuse very slowly- E.g. colloidal silver sols, naural and synthetic polymers

3- Coarse dispersions (> 0.5 um)- Particles are visible under ordinary microscope- Do not pass through filter paper or semipermeable

membrane.- Particles settle down under gravity- Do not diffuse- E.g. emulsions, suspensions, red blood cells

4) Peptization.

Colloidal mill� In a colloid mill, the droplets

are disrupted in a conical gap between the rotor and the stator, each being either smooth or toothed.

� The design of tooth varies. � Gap width, rotor radius,

rotational speed and flow rate

57

rotational speed and flow rate are very important as far as the disruptive stresses are concerned.

� Typical gap widths in these machines are 100–3000 µm and the peripheral speed usually varies in the range of 5 to 40 m/s

Colloidal mill rotor and stator

58

Ultrasonifiers – Principle of Pohlman whistle

Outlet

Vibrating blade

Intlet

59

Nozzle

� For the preparation of emulsions in small-scale (e.g., in laboratory applications), the use of ultrasound is a well-known method.

� This procedure can be employed to obtain finely-dispersed emulsions in continuous or batch units

Preparation of emulsions by use of ultrasound

� The ultrasonic power is adjustable and can be adapted to the desired products andemulsification requirements.

� Highly intensive ultrasound supplies the power needed to disperse one liquid into the other in the form of small droplets.

� In the dispersing zone, imploding cavitation bubbles cause intensive shock wavesin the surrounding liquid, which result in the in the surrounding liquid, which result in the formation of liquid jets of high velocity.

� As coalescence of the droplets after disruption influences the final droplet size distribution,

� efficient emulsifiers are required to maintain the final size distribution of the droplets at a level that is same as the distributionimmediately after the droplet disruption in the ultrasonic dispersing zone. 60

Probe sonicator emulsification system

4) Peptization: Dispersion Methods

Peptization is the method of breaking down the freshly prepared precipitate particles into the particles of colloidal size.

This is done by adding suitable electrolytes. The electrolyte added is called peptizing agent.

Preparation of Lyophobic colloids:

Prepared by: Under Dispersion Methods

I. Physical method (Bridge‘s arc method) or Electrical dispersion method

- This method is employed for obtaining colloidal solutions of metals e.g. - This method is employed for obtaining colloidal solutions of metals e.g. silver, gold, platinum

ice

Dispersion medium(Water + kOH)

I. Physical method (Bridge‘s arc method)

� An electric current is struck between twometallic electrodes placed in a container ofwater.

� The intense heat of the arc converts the metalinto vapours which condensed immediately inthe cold water bath.

� This results in the formation of particles ofcolloidal size.

Preparation of Lyophobic colloids:Condensation method

II. Chemical method: by oxidation

- Sulphur solution is obtained by bubbling H2S gas (Hydrogen sulfide) through the solution of an oxidizing agent like HNO3 (Nitric acid) or Br2 in water , according to agent like HNO3 (Nitric acid) or Br2 in water , according to the following equations:

- Br2 + H2S S + 2 HBr

- HNO3 + H2S H2O + NO2 + S

Purification of colloidal solutions:

� When a colloidal solution is prepared is often contains certainelectrolytes which tend to destabilize it. The following methods areused for purification:

1- Dialysis:

- Semipermeable cellophane membrane prevent the- Semipermeable cellophane membrane prevent thepassage of colloidal particles, yet allow the passageof small molecules or electrolytes.

The Graham dialyser consists of a hollow cylinderopen at both ends, over one end of which amembrane is tied.The colloidal solution to be dialysed is placed inthis cylinder which is then suspended in a largevessel containing flowing distilled water.

After several hours all the molecules or ions of the crystalloid pass though the membrane leaving behind pure colloidal solution.

Dialysis:

� Separation of substances in solution by means of their unequal diffusion through semipermeable membranes

� In medicine, Dialysis (from Greek dialusis,"διάλυσις", meaning dissolution, dia, meaning through, and lysis, meaning dissolution, dia, meaning through, and lysis, meaning loosening or splitting) is a process for removing waste and excess water from the blood and is used primarily as an artificial replacement for lost kidney function in people with kidney failure.

� Dialysis of the blood to remove toxic substances or metabolic wastes from the bloodstream; used in the case of kidney failure

� very slow process, can be quickened some what by using hot water or by applying an electric field

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Hemi dialysis: Here blood of such patients is cleaned by artificial machines whose kidneys are damaged or mal functioning.

Purification of colloidal solutions:

2- Electrodialysis:

- In the dialysis unit, the movement of ions across themembrane can be speeded up by applying an electriccurrent through the electrodes induced in the solution.current through the electrodes induced in the solution.

- The most important use of dialysis is the purification ofblood in artificial kidney machines.

- The dialysis membrane allows small particles (ions) topass through but the colloidal size particles(haemoglobin) do not pass through the membrane.

2-Electrodialysis

Electro Dialysis (ED) is a membrane process, during which ions are transported through semi permeable membrane, under the influence of an

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permeable membrane, under the influence of an electric potential. The membranes are cation- or anion-selective, which basically means that either positive ions or negative ions will flow through.

2-Electrodialysis

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2-Electrodialysis:

3-Ultra filtration:

� Here colloids are separated from crystalloids with the help of ultra filter papers. papers.

� Which are obtained by treating simple filter paper with gelatin or collodion so, that the pores become less or narrow in size through which only crystalloid particles can pass.

4-Centrifugation:

� Here impurities are removed from colloids with the help of a machine of high speed called centrifugal machine.high speed called centrifugal machine.

� Here colloids settle down while impurities remain in centrifuge.

� Also called as Ultra-centrifugation

4-Centrifugation:

� Centrifugation is a process which involves the use of the centrifugal force for the sedimentation of heterogeneous mixtures with a centrifuge, used in mixtures with a centrifuge, used in industry and in laboratory settings.

� This process is used to separate two immiscible liquids.

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COLLOIDS, Preparation, Purification & Appli [Compatibility Mode]

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