Introduction for Definition of colloids substance:

       Colloid substance is a heterogeneous combination in which particles having size between 10^-7to 10^-4 cm are dispersed in a continuous dispersion medium. The continuous medium in a colloid is called the dispersion medium and the particles form the dispersed phase. The particles of the dispersed phase are generally called colloidal particle.

Classification of Colloids - Definition of Colloids Substance:

   The range of the colloidal particles is not visible to naked eye. Colloidal substance is also referred to as sols. If the dispersion medium is water they are called hydrosols and if it is alcohol, they are called alcosols. Colloids play an important part in our day life.

Example:

• Vegetables
• Milk
• Blood
• Toothpaste
• Fruit juices
• Bread
• Office paste (gum)
• Jelly

Classification of colloids:

   Depending on the physical status of the dispersed phase and dispersion medium the colloids can be classified into following types.

dispersed phase	dispersion medium	common name	example
gas	liquid	foam	forth, whipped cream, soap leather, fire extinguisher foam
gas	solid	foam	pumice, stone, foam, rubber
liquid	gas	aerosol	fog, liquid sprays, mist
liquid	liquid	emulsion	milk, hair cream, oil in water emulsion
liquid	solid	gel	cheese, butter, jam, jelly
solid	gas	solid aerosol	smoke, dust
solid	liquid	sol	paints, gold sol, cell fluids, toothpaste, milk of magnesia, mud
solid	solid	solid sol	coloured glasses, gems, precious stones, pigmented plastics

Colloids definition can also be classified on the origin of attraction or affinity of dispersed phase and dispersion medium for each other. On this basis colloids have been classified as

• Lyophilic colloids
• Lyophobic colloids

Definition of Lyophilic colloids:

These are solvent (dispersion medium) loving colloids. They directly pass into colloidal state when brought in contact with the solvent. Gelatin and starch are water loving (or hydrophilic) colloids.

Definition of Lyophobic colloids:

These are solvent hating colloids. They do not easily pass into colloidal state when brought in contact with the solvent,

Example:

Metals

Characteristics Properties of Colloids Substance - Definition of Colloids Substance:

• Heterogeneous nature
• Particle size
• Stability
• Filtration
• Brownian movement
• Tyndall effect
• Electrophoresis
• Coagulation

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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.

interaction of lyophobic particles(oil) with the solvent(water) through addition of an emulsifier(soap)

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

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

Multimolecular colloids

In 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.

Macromolecular colloids

Macromolecular colloidal particles are formed when on dissolution in a suitable solvent, the macromolecules have sizes which are in the colloidal range. Naturally occurring macromolecules are starch, proteins and cellulose. Man made macromolecules are polymers such as polyethylene, nylon and polystyrene. These colloids are quite stable and resemble true solutions in many respects.

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 (T_k) 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. 

Formation of Colloids

There are two basic methods of forming a colloid: reduction of larger particles to colloidal size, and condensation of smaller particles (e.g., molecules) into colloidal particles. Some substances (e.g., gelatin or glue) are easily dispersed (in the proper solvent) to form a colloid; this spontaneous dispersion is called peptization. A metal can be dispersed by evaporating it in an electric arc; if the electrodes are immersed in water, colloidal particles of the metal form as the metal vapor cools. A solid (e.g., paint pigment) can be reduced to colloidal particles in a colloid mill, a mechanical device that uses a shearing force to break apart the larger particles. An emulsion is often prepared by homogenization, usually with the addition of an emulsifying agent. The above methods involve breaking down a larger substance into colloidal particles. Condensation of smaller particles to form a colloid usually involves chemical reactions—typically displacement, hydrolysis, or oxidation and reduction.

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PROPERTIES OF COLLOIDALS

The main characteristic properties of colloidal solutions are as follows.

(1) Physical properties

(i) Heterogeneous nature : Colloidal sols are heterogeneousin nature. They consists of two phases; the dispersed phase and the dispersion medium.

(ii) Stable nature : The colloidal solutions are quite stable. Their particles are in a state of motion and do not settle down at the bottom of the container.

(iii) Filterability : Colloidal particles are readily passed through the ordinary filter papers. However they can be retained by special filters known as ultrafilters (parchment paper).                         

(2) Colligative properties

(i) Due to formation of associated molecules, observed values of colligative properties like relative decrease in vapour pressure, elevation in boiling point, depression in freezing point, osmotic pressure are smaller than expected.    

(ii) For a given colloidal sol the number of particles will be very small as compared to the true solution.

(3) Mechanical properties

(i) Brownian movement

Robert Brown

(a) Robert Brown, a botanist discovered in 1827that the pollen grains suspended in water do not remain at rest but move about continuously and randomly in all directions.

(b) Later on, it was observed that the colloidal particles are moving at random in a zig – zag motion. This type of motion is called Brownian movement.

(c) The molecules of the dispersion medium are constantly colloiding with the particles of the dispersed phase. It was stated by Wiener in 1863that the impacts of the dispersion medium particles are unequal, thus causing a zig-zag motion of the dispersed phase particles.

(d) The Brownian movement explains the force of gravity acting on colloidal particles. This helps in providing stability to colloidal sols by not allowing them to settle down.

(ii) Diffusion : The sol particles diffuse from higher concentration to lower concentration region. However, due to bigger size, they diffuse at a lesser speed.

(iii) Sedimentation : The colloidal particles settle down under the influence of gravity at a very slow rate. This phenomenon is used for determining the molecular mass of the macromolecules.

(4) Optical properties : Tyndall effect

In the investigations on radiant heat in air it had been necessary
to use air from which all traces of floating dust and other particulates had been removed.
 A very sensitive way to detect particulates is to bathe the air with intense light.
The scattering of light by particulate impurities in air and other gases,
 and in liquids, is known today as the Tyndall Effect or Tyndall Scattering.

(i) When light passes through a sol, its path becomes visible because of scattering of light by particles. It is called Tyndall effect. This phenomenon was studied for the first time by Tyndall. The illuminated path of the beam is called Tyndall cone. 

(ii) The intensity of the scattered light depends on the difference between the refractive indices of the dispersed phase and the dispersion medium.

(iii) In lyophobic colloids, the difference is appreciable and, therefore, the Tyndall effect is well - defined. But in lyophilic sols, the difference is very small and the Tyndall effect is very weak.

(iv) The Tyndall effect confirms the heterogeneous nature of the colloidal solution. 

(v) The Tyndall effect has also been observed by an instrument called ultra – microscope

Some example of Tyndall effect are as follows

(a) 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.

(b) Due to scattering the sky looks blue.   

(c) The blue colour of water in the sea is due to scattering of blue light by water molecules.

(d) Visibility of projector path and circus light.

(e) Visibility of sharp ray of sunlight passing through a slit in dark room.             

(5) Electrical properties 

(i) Electrophoresis

(a) The phenomenon of movement of colloidal particles under an applied electric field is calledelectrophoresis.

(b) If the particles accumulate near the negative electrode, the charge on the particles is positive.

(c) On the other hand, if the sol particles accumulate near the positive electrode, the charge on the particles is negative.

(d) The apparatus consists of a U-tube with two Pt-electrodes in each limb.

(e) When electrophoresis of a sol is carried out with out stirring, the bottom layer gradually becomes more concentrated while the top layer which contain pure and concentrated colloidal solution may be decanted. This is called electro decanation and is used for the purification as well as for concentrating the sol.  

(f) The reverse of electrophoresis is called Sedimentation potential or Dorn effect. The sedimentation potential is setup when a particle is forced to move in a resting liquid. This phenomenon was discovered by Dorn and is also called Dorn effect.

(ii) Electrical double layer theory

(a) The electrical properties of colloids can also be explained by electrical double layer theory. According to this theory a double layer of ions appear at the surface of solid.          

(b) The ion preferentially adsorbed is held in fixed part and imparts charge to colloidal particles.

(c) The second part consists of a diffuse mobile layer of ions. This second layer consists of both the type of charges. The net charge on the second layer is exactly equal to that on the fixed part.

(d) The existence of opposite sign on fixed and diffuse parts of double layer leads to appearance of a difference of potential, known as zeta potential or electrokinetic potential. Now when electric field is employed the particles move (electrophoresis)

(iii) Electro-osmosis

(a) In it the movement of the dispersed particles are prevented from moving by semipermeable membrane.

(b) Electro-osmosis is a phenomenon in which dispersion medium is allowed to move under the influence of an electrical field, whereas colloidal particles are not allowed to move.

(c) The existence of electro-osmosis has suggested that when liquid forced through a porous material or a capillary tube, a potential difference is setup between the two sides called as streaming potential. So the reverse of electro-osmosis is called streaming potential.       

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CLICK HERE TO EXPERIENCE FUN ACTIVITIES!>> 

Activities to do! :P

First Activity ;D - Ice Cream Making 

   What is a summer without ice cream cones, or a slice of apple pie without a melting scoop of vanilla ice cream? Though it looks simple – cream, milk, sugar and eggs – ice cream has some interesting science keeping it together.

Solutions, Emulsions and Colloids

A solution is the result of mixing a solid, liquid or gas into a liquid or gas where the combined substances mix together completely (miscible) and cannot easily be separated – for example, mixing sugar (a solid) into water (a liquid).

A colloid is the result of mixing solids, liquids or gases that do not normally mix together properly (immiscible), with very small particles (1 NM to 1 µam) of one substance becoming evenly spread throughout the other substance. A colloid is stable, so one substance does not settle out of the other. An emulsion is a type of colloid created by mixing two immiscible liquids.

Emulsifiers keep colloids and emulsions stable – for example, in mayonnaise, something called lecithin, which is found in egg yolks, acts as an emulsifier, keeping the oil spread through the watery vinegar. Try shaking together oil and water – the two will settle out from each other. Add a drop or two of washing up liquid (emulsifier) and shake together again – does the water settle below the oil? The detergent acts as an emulsifier, keeping the oil and water mixed together.

Other food colloids include:

• milk – liquid-liquid (milk fat spread through water from the milk)
• mayonnaise – liquid-liquid (oil spread through water from the vinegar)
• butter – liquid-liquid (water from the milk spread through milk fat)
• whipped cream – gas-liquid (air spread throughout milk fat)
• smoke (if you burn something!) – solid-gas (smoke particles spread through air).

Telling the Difference Between a Solution and a Colloid

Put the solution or colloid in a glass jar and shine a torch through it (if the colloid is too thick to allow light through, dilute it with water) – for example, a jar or sugar water, and a jar of milk diluted with water. With a solution (sugar-water), the light will shine through. With a colloid (milk-water), the light will reflect off the tiny particles and will not shine through. This is called the ‘Tyndall effect’.

Ice Cream

Ice cream is a colloid combining an emulsion and a foam, with milk fat, ice crystals and air (a liquid, a solid and a gas) spread throughout another liquid (the water from the milk). In traditional homemade ice cream, the lecithin in egg yolks acts as the emulsifier, keeping the colloid stable. Commercial ice creams may use other emulsifiers, and add stabilisers to stop the air bubbles in the foam disappear.

Once it starts to freeze, ice cream is whipped to add air, creating foam, another colloid (gas-liquid). This also helps the small fat particles join together into bigger ones, which keeps the air bubbles stable. Commercial ice creams may use stabilisers to stop the air bubbles disappearing and keep the ice crystals small.

Making Ice Cream in a Bag

Stir together half a cup of milk and half a cup of cream (or one cup of milk), and quarter of a cup of sugar, pour into a freezer bag, and seal. Put the bag in a bigger bag, and completely surround it with salt (the larger the salt crystals the better) and ice. The salt reduces the temperature even further. It should take about five minutes to freeze. Try shaking the bags while the ice cream freezes to add air – does this make the ice cream softer? Try adding different flavours, like chocolate syrup or jam.

Activity #2 - Getting Critical over Colloids

Getting Critical Over Colloids

What is a colloid? If you have made Oobleck out of corn starch and water, then you know that a colloid is a mixture that acts like a solid and a liquid at the same time! What is the critical factor in making a colloidal material? Will different sources of starch change the recipe?

Objective

The purpose of this project is to find the critical point for colloidal mixtures composed of different types of starches.

Introduction

What do ketchup, Oobleck, and quicksand have in common? They are all made up of tiny, solid particles suspended in water. Chemists call this type of mixture a colloidal suspension, and the amount of solid and water to use is called the critical concentration. The critical concentration for each colloidal material is unique and depends on many different factors.

Colloids have very interesting physical properties. One of the more interesting physical properties of colloidal materials is that sometimes they seem to be solid and other times they seem to be a liquid. Because of this odd behavior, colloids are called non-Newtonian fluids, because they break the rules of ideal fluids described by Isaac Newton in the 1700s.

Colloidal suspensions respond differently to different forces. A fast, hard force will cause the colloid to appear solid, but a slow, even force will cause the colloidal material to flow like a liquid. This can be dangerous if you live in an area with clay soil, because sideways forces during a flood or earthquake can cause the earth to suddenly become very unstable!

As it turns out, colloidal materials are very common. Even though they have such strange physical properties, these properties make them very useful products and materials. Foam, gel, glue, and clay are all examples of colloidal materials. There are many colloidal materials found in food products, like: marshmallows, mayonnaise, pudding, milk, butter, and jelly. Building materials such as cement, stucco, plaster, and paint are colloidal materials. Even our bodies and other living organisms are made of colloidal materials! They are everywhere!

In this experiment, you will learn about a very simple colloidal material: starch suspended in water. You will test starch from different plant sources (corn, potato, rice, tapioca) to see if the colloids share similar physical properties. You will measure the amount of water needed to make a colloid out of each type of starch. Will these colloidal suspensions be the same or different?

Activity 3- Cooking and Mixtures

Activity 4- Making a Gelatin! :D

 Buy some dry gelatin. Dissolve it in warm water and, with subsequent dilutions, determine what is the minimum concentration of dry gelatin necessary to obtain a normal gelatin at room temperature. Do not keep gelatins a long time because they easily become cultures of bacteria. Store them in a refrigerator and, after a day, throw them away. 

 Reversibility of the gelatin. By means of the temperature, make some gelatin pass from the gel to sol states and vice versa.

Activity 5 - Stability of Emulsions!

1 - Stability of the emulsions. Fill two plastic bottles halfway with water, then put 5 cc (about a spoonful) of vegetable oil in each. Only in one of these bottles, put 0.5 cc (about 20 drops) of liquid detergent for dishes. Close the bottles and shake them for a couple of minutes to emulsify the oil, then place them on a table and observe them. The drops of oil will try to reassemble and to surface. By comparing the two emulsions, you will see that the one with detergent will be much more stable (figure 28). In fact, even after a month, the white color of this emulsion indicates that there is a great deal of small oil drops in the liquid, while in the other bottle the liquid is become nearly transparent, this is a sign that near all the oil drops have fused together and surfaced. 2 - Vinegar and vegetable oil. Using a kitchen whisk, emulsify a teaspoon of vinegar with 125 cc of peanut oil or olive oil. The emulsion will result instable.

Figure 28 - The two emulsions of the experiment 1 after 24 hours of rest. In the right bottle, some detergent has produced a more stable emulsion.

3 - Mayonnaise. To the ingredients of the test 2, add an egg yolk and emulsify again. The emulsion will be much more stable. Add some salt and if you want some pepper and you will have obtained a good mayonnaise. If you prefer, you can replace the vinegar with lemon juice. Why is the emulsion stable with the egg yolk? This is due to the presence of lecithin in the egg yolk. Lecithin is a surfactant and the molecules spread on the surface of the oil drops with the hydrophilic head outward. As these heads are electrically charged, the oil drops will repel and their merging is prevented. Lecithin is a phospholipid and it has a structure like that of the phospholipids which form the membranes of cells. Another well known lecithin and which you can find on the market is soy lecithin.

THANKYOU! :D