PHYSICAL CHEMISTRY OF MILK

Module 2 Colloidal chemistry

Lesson 2
CLASSIFICATION OF COLLOIDS

2.1 Introduction

About 87% of milk is water, in which the other constituents are distributed in various forms. Milk has various components in the form of emulsions, colloids, molecular and ionic solutions. Lactose in milk is present in form of true solution and milk fat as an emulsion whereas milk proteins are dispersed as a colloid. The term colloid is not applicable to a given substance but rather to a specific state of a matter endowed with certain characteristic properties.

2.2 Characteristic Features

A colloid is a substance microscopically dispersed evenly throughout another substance. A colloid system is one in which a substance is distributed throughout another substance in the form of finely divided particles. These systems are known as disperse systems or dispersions. They usually consist of at least two phases; a dispersed phase (or internal phase) and a continuous phase (or dispersion medium). The dispersed phase consists of the suspended particles and dispersion medium surrounding the suspended particles. As such all dispersions are heterogeneous systems. A colloidal system may be solid, liquid or gaseous condition. Colloids are specific form of disperse system in which the particle sizes ranges from 10 to 2000 Ǻ. In some cases, a colloid can be considered as a homogeneous mixture. This is because the distinction between "dissolved" and "particulate" matter can be sometimes a matter of approach, which affects whether or not it is homogeneous or heterogeneous.

Considering the degree of dispersion, colloids will be between the coarse dispersions and molecular dispersions or true solutions. In solutions, the solute is dispersed in the solvent (dispersion medium) in the form of separate molecules or ions. The colloids are micro heterogeneous in nature and in these colloids dispersed particles comprise a separate phase that is divided by an interface from the dispersion medium. In a colloid system, the particle size is larger than dissolved molecules but smaller than settleable solids since the particles are having fine dimensions and greater surface area. The size of the particles in a colloidal system ranges approximately 1 to 10⁵ nm. The dispersed particles are large enough to scatter the light and do not dissolve.

2.3 Classification

Colloids could be classified on the basis of the affinity of dispersed molecules with dispersing media. If the affinity on dispersion medium is more it is lyophillic or reversible sol. If the affinity on dispersion medium is less it is lyophobic or irreversible sol. Lyophilic colloidal systems with aqueous continuous phase are known as hydrophilic while the lyophobic systems with aqueous dispersion medium are known as hydrophobic. Reversible colloids are thermo dynamically stable. They are either macromolecules (such as proteins) or association of colloids (micelles). Irreversible colloids constitute a true phase in aqueous medium. The classification of their physical state of dispersed phase and dispersion s presented in Table 2.1

Table 2.1 Classification of colloids on the basis of the physical state of dispersed phase and dispersion medium

Dispersed phase	Dispersion medium	Name	Examples
Solid	Gas	Solid aerosol	Smoke dust, cloud
Solid	Liquid	Sol	Paints, blood
Solid	Solid	Solid sol	Coloured gem stones
Liquid	Solid	Gel	Jelly, Gelatin
Liquid	Liquid	Emulsion	Milk, hair cream
Liquid	Gas	Liquid Aerosol	Fog, mist
Gas	Solid	Solid sol	Pumice stone, foam rubber
Gas	Liquid	Foam	Whipped cream froth

2.4 Differences Between the Two Types of Colloids

The major differences between lyophilic and lyophobic colloids are presented in Table 2.2. The lyophilic (solvent attracting) colloid is one whose particles have a strong attraction for the molecules of dispersion medium and binding large number of them into so called solvent shells. On the contrary in lyophobic (solvent repelling) colloid, the particles do not interact so strongly with the molecules of the surrounding medium. The terms hydrophilic and hydrophobic are used in case of aqueous colloid solution. Lyophobic colloids are heterogeneous and highly dispersed colloid systems. Removal of the dispersion medium would result in the formation of powdery deposit by the dispersed phase and there will be insignificant amount of dispersion medium in it.

Table.2.2 Differences between the lyophyllic and lyophobic colloids

LYOPHYLLIC	LYOPHOBIC
Reversible	Irreversible
Can be prepared by simple methods of mixing	Needs special methods for preparation
More solvated because of which it is more stable	Not solvated hence less stable but can be made stable by adding small quantity of lyophillic colloid
Viscosity is more for sol than for dispersion medium	Viscosity of sol is similar to dispersion medium
Surface tension is less than dispersion medium	Surface tension of colloid sol is same for dispersion medium
Exhibit weak tyndall effect	Exhibit strong tyndall effect

2.5 Distinction Between the True and Colloidal Solutions

The distinguishing differences between colloid and true solution are given in Table 2.3 and depicted in fig. 2.1. A true solution is a homogeneous solution in which the solute particles have diameters less than 10^-7cm. i.e., the solute particles are of molecular dimensions. The particles are invisible even under powerful microscopes.

Table 2.3 Differences between true and colloidal solutions

Property	True solutions	Colloidal solutions
Particle size	Less than 10 - 7 cm	Between 10 - 5 and 10 - 7 cm
Visibility of particles	Invisible to naked eye and even not under powerful microscope	Invisible to naked eye. Visible under powerful microscope
Sedimentation of particles	Do not settle down	Settle down under high centrifugation
Filtration through filter paper	No residue is formed	Residue is formed

Colloidal particles are heterogeneous mixture of two phases namely the dispersed phase and dispersion phase and the particles remain in their own boundary surfaces which separate them from the dispersion medium. Colloid is a solution in which the particle size ranges between 10^-7 and 10^-5 cm and the particles can be seen under a powerful microscope.

The particles in true solution will not settle down but the particles in colloidal suspension can settle upon high speed centrifugation.

Fig. 2.2 Types of solutions

True solutions will not form any residue when filtered through a filter paper/ parchment paper or animal membranes whereas colloids can pass through ordinary filter paper but do not pass through animal membranes.

The composition and properties of a true solution are same throughout and is clear and transparent. True solution does not scatter the light whereas colloidal solution always reflects the light. The phenomena known as the "Tyndall Effect" is used to differentiate between the effects of colloids and true solutions.

The colloidal particles were observed to be in constant motion in a zig zag path in all possible directions. This motion is known a Brownian motion. It arises because of the impact of solvent molecules on the colloidal particles. The forces are unequal from different directions.