Water in starch or water in ink is an example of a heterogeneous mixture. Water is considered a dispersed medium in the above criteria, and starch or ink can be regarded as a dispersed phase. Two phases exist in this case. One step possesses finely differentiated particles suspended in a continuous medium. The stage with finely differentiated particles is called the dispersed phase. A dispersed medium consists of two media that do not mix. These two media can be different in properties altogether. Precisely, they can be a liquid, a solid or a gas. There are various everyday life examples of dispersed media. Milk, which we most probably use daily, is made up of oil drops dispersed in water. Shaving creams contain tiny airdrops of water dispersed in the air.
The dispersed phase of dispersion is the discontinuous phase that distributes throughout the dispersion medium. It is one of the two stages of a colloid. The dispersion medium is the continuous phase of a colloid, and it distributes throughout the dispersion medium. The dispersed phase is known as the internal phase, whereas the dispersion medium is called the external phase. Examples of the dispersed phase include dust in the air, whereas that of dispersion medium includes water in milk.
There are two types of dispersion systems, Molecular Dispersions and Coarse Dispersions. Molecular dispersions are dedicated solutions to a solute phase in the solvent. The dispersed phase is homogeneously distributed in the dispersion medium. Examples of molecular dispersion are air (consisting of various gasses like nitrogen and oxygen), electrolytes and alloys.
The second type of dispersion medium is of the coarse kind. These are heterogeneous dispersed systems. Fast sedimentation of the dispersed phase is observed owing to the coarse dispersions, mainly due to gravity.
Based on the type of dispersed phase, colloids can be classified into various types. These include sol, emulsion, foam and aerosol. A colloidal suspension of solid particles in a liquid is called a sol, e.g., Ruby glass. An emulsion is a colloidal suspension of two drinks, e.g., milk. Foam is formed when gas particles get trapped in a liquid or solid, e.g., soap in water. Aerosols are small particles of liquid or solid dispersed in a gas, e.g., smoke, fog, mist, etc.
Depending upon the minute particles' nature of the dispersed phase, the colloids are mainly differentiated into three types:
Multimolecular Colloids: When a substance gets dissolved in a dispersion medium, it gets separated into several smaller molecules of different sizes. Usually, their size varies from 1-1000nm. After separation of the particles, the colloid constitutes several atoms and molecules known as multimolecular colloids.
Example: Hundreds of Sulphur molecules are held together by van der Waals force and form Sulphur Solution.
Macromolecular Colloids: Its size is relatively more. However, in a suitable solvent, these form solutions whose size may or may not remain in the colloidal range. This arrangement is popular as Macromolecular Colloids.
Example: Enzymes, cellulose and proteins are naturally formed macromolecular colloids, but rubber and polythene are synthesized.
Associated Colloids: Basically, these are behaving like strong electrolytes. However, when a suitable condition like in higher concentration, they show the behaviour of colloidal particles. Because of this nature, these are known as Associated Colloids and also famous as a micelle.
Examples: Soaps and detergents
The agglomerated particles get separated from each other. A new line of interaction between the dispersion medium's inner surface and the outer surface of dispersed particles is created. This whole process is aided by molecular diffusion and convection. Through molecular diffusion phenomena, dispersion occurs through different concentrations of the media introduced throughout the bulk medium. The difference in engagement between dispersed material and the bulk medium creates a concentration gradient that drives the medium's dispersion. This results in an equal dispersion of particles in the medium. In convection, variation in velocity between paths of flow in the bulk medium facilitates the equal distribution of dispersed material in the medium.
Diffusion is the primary mechanism in dispersion, although it is driven by convection in some cases. In most cases, convection is what helps in accelerating the process of diffusion.
Q1. What is a Colloid, and How it Differs from a Solution?
Ans. A colloid is a substance in which minute, microscopically dispersed insoluble particles of one substance are suspended in another substance. The size of colloidal particles varies from 1-1000 nanometers. We will study more about colloids in another column. A solution exists in a single-phase only, and no visual interface exists. Whereas in a colloid, two different phases, namely the dispersed phase and dispersion medium, exist. An interface between them can be observed.
Q2. Can We Distinguish Between the Dispersed Phase and the Dispersed Medium?
Ans. Yes, we can distinguish between the dispersed phase and the dispersed medium. On adding dispersion medium, emulsions can be diluted to any extent. The disperse phase forms a separate layer if added in excess. There are various advanced techniques of separation of the dispersed phase and dispersion medium. One such method is micro-filtration. In colloids, both the stages cannot be separated by simple filtration but can be separated using centrifugation. For example, butter is separated from cream by centrifugation.
Q3. What are the Types of Dispersion Medium?
Ans. The three types of dispersion medium are solid, liquid and gas. The colloids are classified according to the interaction between the dispersed phase and dispersion medium. The dispersion mediums are again characterized owing to their interactions with the dispersed medium. However, depending upon the phase of dispersion, Colloidal Dispersion can be separated into eight types such as: