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MSc Final : BIOCHEMISTRY

NATURE OF STABILITY OF COLLOIDAL

SYSTEMSINCLUDES:1) Introduction to Colloidal Systems2) Classification of Colloids3) Stabilization Of Colloids

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INTRODUCTIONDispersed Phase = SoluteDispersion Medium = SolventColloid is a mixture in which one substance is divided

into minute particles (colloidal particles) and dispersed throughout a second substance(dispersion medium).

The mixture is also called colloidal system, colloidal solution, or colloidal dispersion.

Examples are : Milk, fog, jellies and much more.

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COLLOIDAL PARTICLES are smaller than SUSPENSION SOLUTES (<1000nm) and bigger than Solutes of a TRUE SOLUTION (>1nm).

So, for determining a mixture is a Colloid we can see the particle size of mixtures’ solute.

They cannot be seen by microscope directly but the size of its particles can be seen by an electron microscope.

Hence the size of colloidal particles ranges in between : 1-1000nm

NOTE: Colloids are not only for liquid mixtures. The medium could be solid, liquid or gas, and the dispersed substance can also be solid, liquid or gas.

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CLASSIFICATIONThe colloids are classified on the basis of:1. State of Dispersion.2. Nature of Interaction.

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1.State of Dispersiona) Dispersion medium is SOLID.b) Dispersion medium is LIQUID.c) Dispersion medium is GAS.

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a)Dispersion medium SOLID

When dispersion medium is solid, the dispersed phase can be solid, liquid or gas. Based on the state of dispersed phase, colloids are called Solid Sols or Gels.

Different types of colloids and examples of each type are given here:

Dispersion

Medium

Dispersed Phase

Names of

Colloids

Examples are:

SOLID SOLID SOLID SOL GEMSTONE, PEARLS, COLOURED GLASS.

SOLID LIQUID GEL CHEESE, BUTTER, JELLY, JAM, SHOE POLISH.

SOLID GAS SOLID SOL PUMICE STONE, FOAM, RUBBER.

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SOLID SOLS

SOLID SOLS

GELS

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b) Dispersion medium LIQUID

When dispersion medium is liquid, the dispersed phase can be solid, liquid or gas. Based on the state of dispersed phase, colloids are called Sols, Emulsions or Foam.

Different types of colloids and examples of each type are given here:

Dispersion Medium

Dispersed Phase

Name of Colloids

Examples are

LIQUID SOLID SOLS PAINTS, GUM, MUDDY WATER.

LIQUID LIQUID EMULSION MILK, HAIR CREAM, COD-LIVER OIL.

LIQUID GAS FOAM FROTH, SOAP LEATHER, WHIPPED CREAM.

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SOLS

EMULSIONS

FOAMS

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c) Dispersion medium GASWhen dispersion medium is gas, the dispersed phase can

be solid or liquid but not the gas.It is important to note that colloid is a heterogenous

mixture. When gas is mixed with another gas, it forms a completely miscible homogeneous mixture and not a colloidal solution

Colloids of gas with dispersed phase as solids or liquids are called Aerosols. Different types of colloids and examples of each type are given here:

Dispersion Medium

Dispersed Phase

Name of Colloids

Examples are

GAS SOLID AEROSOLS SMOKE, DUST.

GAS LIQUID AEROSOLS INSECTICIDE, FOG, CLOUDS.

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Aerosols with SOLIDS

Aerosols with LIQUIDS

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3. STABILITY OF COLLOIDAL SYSTEMS

The stability of colloids is very important as it helps our daily useful colloids to become much effective, also it is necessary to remove useless colloids from our environment.

This stability is achieved by: Balance of forces Electrical forces Interaction with Solvent Stabilization by Cloaking Steric Stabilization

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BALANCE OF FORCES It is achieved by weak forces called

Vander Waals Forces. And not total attraction by the thermal

motion which keep the very small atomic molecules apart.

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ELECTRIC FORCES Electrical forces keep the colloids dispersed. When particles of colloidal

dimension suspended in a liquid collide with each other, they do so with much smaller kinetic energies than is the case for gases, so in the absence of any compensating repulsion forces, we might expect van der Waals or dispersion attractions to win out. This would quickly result in the growth of aggregates sufficiently large to exceed colloidal size and to fall to the bottom of the container. This process is called coagulation.

Each particle with its double layer is more or less electrically neutral. However, when two particles approach each other, each one "sees" mainly the outer part [shown here in blue] of the double layer of the other. These will always have the same charge sign (which depends on the type of colloid and the nature of the medium), so there will be an electrostatic repulsive force that opposes the dispersion force attractions.

Electrostatic (columbic) forces have a strong advantage in this respect because they act over much greater distances do van der Waals forces.

But as we will see further on, electrostatic repulsion can lose its effectiveness if the ionic concentration of the medium is too great, or if the medium freezes. Under these conditions, there are other mechanisms that can stabilize colloidal dispersions.

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INTERACTION WITH THE SOLVENT

According to the interaction between particles of dispersed phase & those of dispersion medium:

1) Lyophilic (solvent loving).2) Lyophobic (solvent hating).3) Association (amphiphilic).

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1. LYOPHILIC (Solvent Loving)

Colloidal particles interact to an appreciable extent with the molecules of the dispersion medium (solvent loving).

Obtained simply by dissolving the material in the solvent ( due to the high affinity).

Hydrophilic

Solvent is WATER.

Examples: Acacia,

Insulin in water.

Lipophilic

Solvent is non-aqueous,

ORGANIC.

Examples: Polystyrene,

Rubber.

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NOTE : The material that form Lyophilic colloid in a certain solvent may not do so in another solvent,

e.g.; Acacia + water Lyophilic colloid

(hydrophilic type).Acacia + benzene NO Lyophilic

colloid formed.

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The dispersed phase does not precipitate easily

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

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

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2. LYOPHOBIC (Solvent Hating)

Colloidal particles have very little or no attraction for the dispersion medium (solvent hating).

Colloidal particles: inorganic particles (e.g. : Gold , silver, sulfur….) Dispersion medium: water.

These colloids are easily precipitated on the addition of small amounts of electrolytes, by heating or by shaking

Less stable as the particles surrounded only with a layer of positive or negative charge.

Once precipitated, it is not easy to reconstitute the sol by simple mixing with the dispersion medium. Hence, these sols are called irreversible sols.

Not obtained simply i.e.: Need special method for preparation.

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As LYOPHOBIC COLLOIDS are difficult to form, the special methods for their formation includes:

A) Condensation method.B) Dispersion method.

Condensation Method

Dispersion Method

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STABILIZATION BY CLOAKING

"Stabilization by stealth" has unwittingly been employed since ancient times through the use of natural gums to stabilize pigment particles in inks, paints, and pottery glazes.  These gums are also widely used to stabilize foods and personal care products.

A Lyophobic colloid can be made to masquerade as Lyophilic by coating it with something that itself possesses suitable Lyophilic properties.

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STERIC STABILIZATIONAlternatively, attaching a Lyophobic material to a

colloid of any type can surround the particles with a protective shield that physically prevents the particles from approaching close enough to join together.  This method usually employs synthetic polymers and is often referred to as Steric Stabilization.

Synthetic polymers, which can be tailor-made for specific applications, are now widely employed for both purposes. The polymer can be attached to the central particle either by simple adsorption or by chemical bond formation.

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SURFACTANTS AND MICELLE FORMATION

Surfactants are molecules consisting of a hydrophilic "head" connected to a hydrophobic chain.  Because such molecules can interact with both oil and water phases, they are often said to be amphiphilic. Typical of these is the well known cleaning detergent  sodium dodecyl sulfonate  ("sodium laurel sulfate“).

Amphiphiles possess the very important property of being able to span an oil-water interface. By doing so, they can stabilize emulsions of both the water-in-oil and oil-in-water types. Such molecules are essential components of the lipid bilayers that surround the cells and cellular organelles of living organisms.Emulsions are inherently unstable; left alone, they tend to separate into "oil" and "water" phases.

When a detergent-like molecule is employed to stabilize an emulsion, it is often referred to as an emulsifier. The resulting structure is known as a micelle.

Emulsifiers are essential components of many foods. They are widely employed in pharmaceuticals, consumer goods such as lotions and other personal care products, paints and printing inks, and numerous industrial processes.

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PROPERTIES OF COLLOIDS Brownian Movement Tyndall Effect Adsorption

BROWNIAN MOVEMENT:

If a colloid is viewed under a special microscope, the dispersed particles will be seen moving in a rapid, random, zigzag motion through the dispersion medium.

This motion in a colloid is one of the reasons why particles remain suspended indefinitely.

TYNDALL EFFECT

A beam of light passing through a transparent pure colourless solid, liquid or gas cannot be seen except at the point where it enters and leaves the object.

scattering of light

3. Adsorption

The binding of molecules to a surface

It is different from absorption.

(ex. Activated carbon – binds molecules to purify)

ADSORPTION Adsorption is the process of collecting a

thin layer of molecules or ions of a substance with which it is in contact.

Surfaces of solids and liquids can adsorb molecules from fluid phases (liquids and gases) onto their surface.Chemisorptions – fairly strong,

resembling chemical bonds, and difficult to break

ex: activated charcoalPhysisorption – weak, involves only

Van der Waals forces and the adsorbed material can usually removed again.

ex: silica gel

activated carbon

silica gel

Electric Charge A dispersed colloidal particle can adsorb

electrically charged particles on its surface. opposite attracts; same repel prevents coagulation and precipitation of

dispersed colloidal particles

OMG! You’re my

exact opposite!