What is Colloidal chemistry, Colloids ?

Colloidal chemistry, Colloids

 

Colloids

 

 Here you will learn about introduction of colloids, their classification and preparation methods.

Introduction of Colloids:

In a solid, liquid, or gaseous media, one or more components are dispersed as relatively large solid particles or liquid droplets to form a mixture known as a colloidal mixture. A colloid's particles are frequently electrically charged, remain scattered, and do not settle as a result of gravity.

Any substance, including thin films and fibers, with at least one dimension in this general size range, which covers a range of around 10⁷ to 10³ cm, is referred to as a colloid. This term also refers to any substance that contains particles that are significantly larger than atoms or regular molecules yet too small to be visible to the unassisted eye.

Simply we say that colloids are heterogeneous solutions in which one substance is broken down into very small size particle and disperse into second substance thoroughly. Their particle size ranges between 1 to 1000nm. Colloids' dimension, which falls between atomic and bulk dimensions, is the cause of many of their intriguing characteristics.

Surface and Interface

The word "surface" is used to refer to a phase boundary in chemistry. A surface does not have thickness geometrically, only area. However, chemically, it is an area where the properties of one phase and the phase next to it differ. This transformation happens over at least molecular-scale distances. Therefore, for us, a surface has a thickness that, when we want a purely geometric description, we may envision it reducing to zero. In this context, the word interface is sometimes employed. This phrase just emphasizes that the surface of interest is the zone that separates two phases.

Difference between solution, suspension, and colloids 

Difference

Colloids classification

There are four types of colloids and they further subdivided into different types.

Sols and gels can be change into one another and these are reversible this phenomenon is called as thixotrpy.

(A) Based on physical state

1.Emulsions

These are solutions in which dispersion medium and dispersed phase are both liquid. These can be two types,

i) Oil in water

Oil is dispersed phase and water is dispersion medium. For example, milk.

ii) Water in oil

Water act as dispersed phase and oil act as dispersion medium. For example, vanishing cream.

2. Gels

Solid act as dispersion medium and liquid act as dispersed phase like butter.

3. Aerosols

Air is present as dispersion medium in these colloids. Examples include, dust , smoke and clouds.

4. Solid solutions

In these types of solution solid is used as dispersion medium and other substance act as dispersed phase, like gemstone. 

(B) Colloid classification based on affinities to the carrier fluid

Lyophilic and Lyophobic are the terminologies used to describe colloidal "particles" based on their affinity to the fluid in which they are disseminated. Literally, these words imply "solvent loving" and "solvent fearing," respectively. The phrases hydrophilic or hydrophobic are frequently used when referring to a medium or solvent made of water. However, when used to categorize colloids, the distinction is not always clear-cut. This nomenclature is especially helpful when assessing surface activity such as the wettability of a surface.

1.Lyophilic colloids

The term "lyophilic colloids" is typically used to describe soluble macromolecules with colloidal-sized individual particles (macromolecules like proteins or synthetic polymer chains). However, there are colloidal macromolecules with both lyophilic and lyophobic components (such as proteins with hydrophobic (hydrocarbon) portions and hydrophilic (peptide and carboxyl groups) portions).

 Micelles are a type of spontaneously forming small molecule cluster that forms in aqueous solutions (mostly) of specific compounds (therefore these are often called association colloids). Micelle production resembles phase separation because micellization begins at a specific concentration, known as the critical micelle concentration. The micelles, however, allow the distinct tiny molecules to maintain their individuality.

2.Lyophobic colloids

Lyophobic sols are irreversible, cannot recycle. There particles are visualized by using ultra microscope. For example, Metallic sols like Ag, gold etc.

(C) Dispersion medium-based classification

1.Acrosols

Dispersed medium is particles of air like smoke

2.Hydrosols

Water is present as dispersion medium in these colloids, like starch.

3.Alcosols

In these types of colloids alcohol is used as dispersion medium.

D) Sol Particles Based Classification

These are of two kinds,

1. Multi-molecular colloids

These colloids are formed by using Bredig^,s method like metallic sols(multi molecular sols). Their particles have van der walls forces. Their separation occurs easily and these are lyophobic unstable colloids.

2. Macromolecular colloids

As the name suggest their molecules are of big size, they are formed by demolition of macromolecular substances which convert into colloids size rang. Dispersed phase particles are polymers having large size. Naturally occurring macromolecules are enzymes, proteins etc. These are lyophobic colloids.

 

Colloidal solution preparation

Lyophilic sols are stable sols because of strong interaction forces among the medium and phase. There preparation occurs by using different methods.

(A) Condensation method

Condensation method, in this method solute particles of small size condense and form dispersion phase.

1.Chemical method

i) Oxidation

Colloids of Sulphur are obtained by this method, oxygen is passed into the solution of hydrogen sulphide by using an oxidizing agent, HNO₃, H₃Br₂ etc.

 2H₂S + O₂ → 2H₂O + 2S (Sulphur sol)

ii) Double Decomposition

Arsenic sulphide sol is prepared by using arsenic oxide cold solution and passing hydrogen sulphide gas through this solution in water.

AS₂O₃ + 3H₂S → AS₂S₃ + 3H₂O Arsenic sulphide (sol)

iii) Reduction

Pt, Au and silver metal colloids are formed by this method. Reducing agents like phenyl hydrazine, hydrogen peroxide, formaldehyde etc. are used. Metals are converted into aqueous solution of their salts than reacted with reducing agent.

Gold sols are called as purple of Cassius because these are prepared by the reduction of gold chloride solution that has purple color.

2AuCl₃ + 3SnCl₂ → 3SnCl₄ + 2Au (gold sol)

2AuCl₃ + 3HCHO + 3H₂O → 2Au + 3HCOOH + 6HCl

iv) Hydrolysis

Different types of salt solutions are hydrolyzed rapidly by boiling. As an example, aluminum hydroxide and ferric hydroxide are formed by this method as,

 FeCl₃ + 3H₂O → Fe(OH)₃ + 3HCl colloidal sol

If sodium silicate is hydrolyzed than silicic acid sols are formed.

v) Colling

Excesses Colling method, in this method colloidal sol of ice are formed. Ice act as organic solvent in this method like chloroform ether and sol of ice is formed by Colling the water solution an solvent till its freezing point. Water molecules separate in solution and combine together to form colloidal size particle.

vi) Changing physical state and solvent

If the solvent in which solutions are normally formed and the state in which solutions exist as stable is changed then their true structure is changed because the forces disturbed their molecules and solutions convert into colloidal solutions. For example, Mercury and Sulphur sols are formed by using cold water which contains stabilizer like ammonium salt is passed through mercury and Sulphur (physical state change).

Sulphur and phosphorus solution are soluble in alcohol but not soluble in water their sol can be formed by passing their alcoholic solution into water (solvent change). Milky solution formed.

(B) Dispersion Method

These techniques involve the breaking up of a substance's (suspension) big particles into smaller ones. The strategies listed below are used.

1.Mechanical dispersion

This technique involves first grinding the material into large particles. To create a suspension, it is next combined with a dispersion medium. A colloidal mill is then used to grind the suspension.

It comprises of two metallic color’s moving at an extremely fast 7000 revolutions per minute in the opposing direction, almost touching each other. The mill's dyes are spaced apart in such a way as to subject a coarse suspension to a strong shearing force, producing particles of colloidal size. This technique can be used to create a colloidal solution of black ink, color, lacquers, and colors.

2. Electrical Dispersion or Bredig’s Arc Method

The preparation of gold, copper, silver, or platinum sols is done using this technique. The metal that needs to be turned into a sol is fashioned into a two-electrode device and submerged in a dispersion medium like water or another liquid.

Ice is used to keep the dispersion medium chilly. The electrodes are connected by an electric arc. A colloidal solute is produced by and given great heat. For stability and cooling purposes, electrolytes are utilized in this procedure.

3. Peptization

Peptization is the process by which a freshly made precipitate is changed into a colloidal solution. In this method, the Peptization agent, also known as the peptizing agent, is added in smaller amounts along with the electrolyte.

 

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