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A colloidal system is a homogeneous non-crystalline system consisting of large molecules or ultra-microscopic particles (1-1000 nm). A colloidal system has two phases. One substance is dispersed in another substance.

Two phases of a colloidal system
Figure 01: Two phases of a colloidal system

Colloidal systemDispersed phaseDispersion medium
FoamGasLiquid
Solid foamsGasSolid
AerosolLiquidGas
EmulsionLiquidLiquid
Solid emulsions (gels)LiquidSolid
SmokeSolidGas
SolSolidLiquid
Solid solSolidSolid
Table 01: Types of colloidal systems

Colloidal solution

In a colloidal solution, the dispersion medium is a liquid. Colloidal solutions have intermediate properties of true solutions and suspensions.

E.g.:–

True solution

A homogeneous mixture that consists of two or more substances, can be defined as a true solution. Those substances are dissolved in a solvent. The particle size of the substances is less than 1 nm. Particles in a true solution cannot be filtered.

E.g.:-

  • An aqueous solution of salt
  • An aqueous solution of sugar

Suspension

Suspension can be defined as a heterogeneous mixture of two or more substances. Those substances are dispersed in a solvent.  The particle size of the substances is higher than 1000 nm. Sometimes the particles will settle down at the bottom. In a suspension, dispersed particles are visible to the naked eye.

E.g.:-

  • Starch in water
  • Calcium carbonate (CaCO3) in water
Comparison of particle sizes in true solutions, colloids, and suspensions
Figure 02: Comparison of particle sizes in true solutions, colloids, and suspensions

Sol

The liquid in a liquid system, liquid in a solid system, and solid in a liquid system are the most common types of colloids. A colloidal solution where a solid in a liquid is called 'sol'.

The dispersion medium of a solType of the sol
WaterHydrosol
AlcoholAlco-sol
Table 02: Types of sols based on their dispersion medium

Sols can be categorized into two categories based on the affinity of the dispersed phase towards the dispersion medium.

  • Lyophilic
  • Lyophobic

Lyophilic sol is a colloid where the dispersed phase has a strong affinity to the dispersion medium.

Lyophobic sols are colloids where the dispersed phase has less affinity or no affinity to the dispersion medium.

PropertyLyophilicLyophobic
PreparationCan easily make by shaking or mixing.Needs special machines/ mills to prepare
Recovery after precipitationEasily recovered after precipitation.Precipitation is irreversible. Once it is precipitated, it cannot be bought back to colloidal form.
ChargeThe colloidal system can be positive, negative, or neutral.The system is positive or negative. Cannot be neutral.
StabilityVery stable. The system is self-stabilized.The system is unstable. It needs a little bit of electrolyte to make the system stable.
ElectrophoresisDispersed particles can migrate to anode, cathode, or not migrate.Dispersed particles migrate to the anode or cathode.
HydrationHave very strong hydration. Solvent lovingNot going to hydrate very well. Solvent hating.
The concentration of the dispersed phaseCan have a higher concentration of dispersed phase.Only low concentrations are available.
Table 03: Properties of lyophilic and lyophobic sols.

Properties of sols

PropertyExample
Optical propertiesTyndall effect
Kinetic propertiesBrownian motion
Electrical propertiesElectric charge
Electrophoresis
Precipitation
Coagulation
Table 04: Properties of sols

01. Tyndall effect

In a colloid or suspension, the light will be scattered by the dispersed particles. It will make beams of light visible. Therefore, the path of the light is visible. This effect is known as the 'Tyndall effect'.

The amount of scattering depends on the size of the dispersed particles and the wavelength of the light.

Tyndall effect
Figure 03: Tyndall effect

In true solutions, we cannot see the light path. There is no Tyndall effect in true solutions.

02. Brownian motion

The continuous random motion of particles is known as the 'Brownian Motion'. In a colloidal solution, dispersed particles travel several directions at a given time.

Brownian motion of colloidal particles
Figure 04: Brownian motion of colloidal particles

1D motion – particles are considered as moving only forward and backward in x direction.

Colloidal solution eq 01

2D motion – particles are considered as moving on a plane.

Colloidal solution eq 02

3D motion – particles are considered as moving in 3D space.

Colloidal solution eq 03

Where,

  • x2 – Mean square displacement (MSD)
  • D – Diffusive coefficient
  • t – time lag

The diffusive coefficient of the system is given by the “Einstein – Stokes equation.”

Colloidal solution eq 04

Where,

  • D = Diffusive coefficient
  • kB = Boltzmann constant
  • T = Absolute temperature
  • η = viscosity of the medium
  • a = radius of the colloidal particles

03. Electric charge

Most of the colloidal particles are carrying a positive or negative charge. If the wan-der Wal’s attractions of particles are much higher, particles are aggregated and settle down at the bottom. But in a colloidal system, there is another force to overcome the aggregation. It is the repulsive force between colloidal particles. This repulsive force is given by the charged particles in the colloidal system.

Charge of colloidal particles
Figure 05: Charge of colloidal particles

In an aqueous medium, FeCl3 will form a red-color colloidal solution of Fe(OH)3 as follows.

Colloidal solution eq 05

Fe(OH)3 will absorb Fe3+ ions in the system. Therefore, Fe(OH)3 is surrounded by positively charged ions. So, the colloidal particles are positively charged.

Fe(OH)3 (ferric hydroxide) Colloidal system
Figure 06: Fe(OH)3 (ferric hydroxide) Colloidal system

We can prepare positively or negatively charged colloidal particles by changing the way we prepare the colloid. Let’s consider the following colloidal system.

Silver iodide (AgI) Colloidal system
Figure 07: Silver iodide (AgI) Colloidal system

When we add AgNO3 solution dropwise to the KI solution, negatively charged colloidal particles will form. Because in the KI solution, there are excess amounts of negatively charged ions (I-). So, AgI will be surrounded by I- ions and form negatively charged colloidal particles.

But if we add KI solution dropwise to an AgNO3 solution, negatively charged colloidal particles will be created. Because there is an excess amount of positively charged ions (Ag+) in the AgNO3 solution. So, AgI is surrounded by Ag+ ions and forms positively charged colloidal particles.

Electrical double layer

When colloidal particles are surrounded by a layer of charged particles, that layer is also surrounded by an oppositely charged layer. Therefore, a colloid particle is surrounded by two charged layers. This double layer is known as the electrical double layer.

Electrical double layer of colloidal particle
Figure 08: Electrical double layer of colloidal particle

An electrical double layer is described in two models which are the “Helmholtz double layer” and “The stern double layer”. Helmholtz's double-layer model describes that there is a fixed charged layer surrounding the colloid particle. And there is an outer charged layer that is movable.

Helmholtz double layer
Figure 09: Helmholtz double layer

The stern double layer describes that there is a compact charged layer surrounding the colloid particle. Surrounding the compact layer there is a diffusive later. Both layers have positive and negatively charged particles. But the overall charge is negative or positive (not neutral).

The stern double layer
Figure 10: The stern double layer

04. Electrophoresis

Electrophoresis is the movement of sol particles under an applied electrical potential. According to the charge of the colloidal particle, they will move to the positive electrode or negative electrode. Positively charged particles will move to the negative electrode and negatively charged particles will move to the positive electrode.

Electrophoresis - Movement of sol particles under an applied electrical potential
Figure 11: Electrophoresis - Movement of sol particles under an applied electrical potential

  • Negatively charged colloids – Ag, Au, As2S3, Starch
  • Positively charged colloids – Fe(OH)3, Al(OH)3

05. Coagulation and precipitation of colloids

The aggregation and precipitation of discharged sol particles are known as coagulations or precipitation. To coagulate sol particles, the stability of the colloid should be broken down. The stability of colloids can be broken down by below four methods.

i. By adding an electrolyte

When a positively or negatively charged colloid is added to an electrolyte, charged particles of the electrolyte will neutralize the electrical double layer of the colloidal particles. Therefore, the stability of the colloid will be collapsed. So, particles will be aggregated and coagulated.

ii. By electrophoresis

A colloidal solution is inserted between positive and negative electrodes externally. Charged particles in the electric double layer travel to the electrodes according to their charge. Therefore, the stability of colloid collapse and particles will be coagulated.

iii. By mixing two oppositely charged sols

When mixing oppositely charged sols will neutralize each other. And the particles will be coagulated.

iv. By boiling

When boiling, the sol is supplied with energy. Therefore, the collision between the sol particles and water molecules is increased. So, the collisions remove adsorbed charged particles. Then, the particles will coagulate.


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References and Attributes

Figures:

The cover image was created using an image by Pixnio

Figure 03: Contains an image form PxHere


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