What are the three properties of colloids?

 Colloidal dispersion is derived from two words "colloidal" and 'dispersion"."colloidal" means "colloids" and "dispersion" means "spreading".

So colloidal dispersion is a system in which colloids particle or dispersed particles (also known as internal phase) are spreading or uniformly distributed in the dispersion medium (external phase or continuous phase).

Colloidal dispersion is a homogeneous and heterogeneous system, depending upon particle size time it may be homogeneous and sometimes it may be heterogeneous.

If the particle size of the dispersion phase is (<0.01 mm) examples are urea, oxygen gas, sucrose, and ions are these types of system known as homogeneous systems and it looks like a "true solution".

If the particle size of the dispersion phase is (0.5 to 1.0 mm) examples are insulin, acacia, albumin, and silver sols are these types of systems known as heterogeneous systems or phases and the appearance of this system looks translucent (claudy).

If the particle size of the dispersion phase is (10 to 1000 mm) examples are RBC, magnesium oxide, and calamine suspension are these types of systems known as heterogeneous systems or phases and the appearance of this system looks opaque (claudy).

Properties of colloidal dispersion (colloids):-

There are mostly three types of properties of colloidal dispersion they are:-

1. Optical properties

2. Kinetic properties

3. Electrical properties

1. Optical properties:-

Optical properties are performed by the source of light and the resolving power of the optical system mainly determines the precision and usefulness of such information.

The study of the optical properties helps in obtaining information regarding the shape, size, structure and molecular weight of colloids.

Optical properties are performed by various methods with the help of instruments and light sources they are:-

(a) Light microscope:-

•In the light microscope, its name suggests that light is used as a light source.

•The light microscope is an instrument used for visualising fine details of an object.

•The light microscope are creating a magnified image through the use of glass lenses, which firstly focus a beam of light and after light on an object or objectives and lenses help to enlarge the image formed.

• It is used for size separation purposes by about 2000A.

•The size of particles is in the range of colloids and hence light microscopes find little use.

•These types of particles, which are closer than 0.2 mm will be visible as a single blur.

(b) Turbidity Method:-

•The turbidity Method is used to approximate the concentration of dispersed particles and the molecular weight of the dispersed particle. 

•Turbidity methods are measured by spectrophotometer.

• Spectrophotometer:- In this method, we will measure the intensity of the transmitted light in the direction of the incident ray or light. 

Fig.Turbidity measurements by spectrophotometer (principle)

•You can see this figure, in this figure it represents the help of a light source incident light passing through the sample which is filled in the test tube sample absorb light and is unable to absorb light is transmitted as known as transmitted light and measure and indicates its intensity. 

•The relationship between turbidity and relative intensity of the transmitted light can be expressed as:-

(C) Ultramicroscope:- 

•Ultramicroscope is also known as "dark field microscope".

• By this method, we will be able to determine the size, shape, and structure of the colloid particle.

• Ultramicroscope has been used to observe the Tyndall beam.

• In this type of microscope, the dispersed particle sees or appears as bright spots and the background appear as dark so light focus only bright place which is the sample so easily we determine the size, shape, and structure of the colloid particle.

Principle:- The principal and working, are shown in the figure below.

(d) Light scattering:-

•Interaction of particles with light.

• This method is used to study proteins, polymers, association colloids and lyophobic sols.

• In this method, light is passing through colloids particle and absorbed light and when light is not absorbed, it will scattered and this is known as a scattering of light phenomenon. the working and principles are shown in the figure. 

Fig: Scattering of light 

(2) Kinetic properties of colloids:-

•The kinetic property of colloids is useful for studying colloidal systems in pharmacy because it describes the behaviour of particles when they collide with one another. 

•When two or more particles collide they interact, their motion can be influenced by various factors, such as their masses, velocities, and the forces acting upon them.

 • By examining these properties, we can gain a deeper understanding of the colloidal system.

• In colloids, kinetic energy refers to the energy possessed by an object due to its motion.

 • When objects collide, the total kinetic energy of the system may change. 

•This change can occur due to the transfer of

kinetic energy from one particle to another or the conversion of kinetic energy into other forms, such as potential energy or heat. 

•By analysing this kinetic energy we can determine whether the colloidal system is stable or not.

(a) Diffusion:-

•Diffusion is the process by which particles or molecules move from an area of higher 

concentration to an area of lower concentration. It is a fundamental concept in chemistry and plays a crucial role in various colloidal solutions. In other words, diffusion is the process by which particles or molecules spread out and move from areas of higher concentration to areas of lower concentration In the context of collision, diffusion occurs as a result of the Brownian motion or random motion of particles. 

•When particles collide, they exchange momentum and energy, causing them to change their direction and speed. 

•These collisions lead to a net movement of particles from regions of higher concentration to regions of lower concentration, resulting in diffusion.

•The behaviour of diffusion can be described using Fick's laws of diffusion, this is a mathematical equation that says that the rate of diffusion to is depended on various factors such as concentration gradients,  diffusion coefficients, and the properties of the medium in which diffusion occurs.

•  Fick's first law states that the rate of diffusion is directly proportional to the concentration gradient. 

• The more the difference between concentration between two areas the more rapid diffusion will happen.

 •  Fick's law helps us to determine the rate of diffusion by considering many other factors.

Formula

 𝑑𝑞 = −𝐷𝑆 𝑑𝑐×dx/dt

dq=amount of substance

dt=time taken for diffusion

S= surface plane area

dc= change in concentration

dx= distance travelled

(b) Brownian motion:-

•The Brownian motion refers to the random movement of microscopic particles suspended in a colloid due to the collision of molecules. 

•This phenomenon was first observed by the Scottish botanist Robert Brown in 1827 when noticed pollen grains moving in a random pattern under a microscope.

•In the context of collision, Brownian motion occurs when particles collide with each other or with other molecules in the fluid. 

•The collisions cause the particles to change their direction and speed, leading to their random movement. 

• This motion is a result of the collision of the colliding molecules and is influenced

by factors such as temperature, the viscosity of the fluid, and the size of the particles. Particles tend to show Brownian motion when their size is around 5micrometer.

• If the particle size is large the collision of

particles is less. So now the gravitation force comes into action and Brownian motion decreases as the size of particles increases.

•Brownian motion is found in collides in which the particles keep colliding with each other and

continuously change their directions.

 •To some extent, it slows down the stelling of particles. 

(c) Viscosity:-

Viscosity refers to the resistance of a fluid to flow. In the context of colloidal solutions,

viscosity plays a significant role in determining the behaviour and properties of these systems. viscosity in colloidal solutions refers to the resistance of the fluid to flow due to the interactions between suspended particles and the solvent.

Viscosity in colloidal solutions arises from the interactions between the suspended particles and the surrounding solvent. These interactions can be complex and depend on various factors such as particle size, shape, and surface chemistry, as well as the properties of the solvent.

 In colloidal solutions, the particles may interact through various mechanisms, including electrostatic repulsion, van der Waals forces, and steric hindrance. Due to these forces, some amount of attraction is found in all particles towards each other. Due to this, all particles experience frictional forces as they move through the fluid.

These forces arise due to the interactions between the particles themselves and between the particles and the solvent molecules. 

The greater the frictional forces, the higher the viscosity of the colloidal solution.

In addition to its impact on flow behaviour, viscosity in colloidal solutions also influences other properties such as sedimentation rate, stability, and the ability to disperse or mix with other substances.

Controlling viscosity is essential in many practical applications involving colloidal systems.

Formula =𝜂 = 𝜂°(1 + 2.5∅)

𝜂 Is the viscosity of the dispersion

𝜂° Is the viscosity of the dispersion medium

∅ Is the volume fraction of particles.

(d) Sedimentation:-

sedimentation in colloidal solutions refers to the process by which the colloidal particles settle down under the influence of gravity, leading to the formation of sediment or a precipitate. Colloidal solutions consist of tiny particles which due to gravity tend to move downwards

and settle at the bottom.

If a colloidal system shows sedimentation, then it is considered to be unstable. By controlling the

sedimentation we can enhance the stability of colloids. This stability of colloidal solutions is maintained by the repulsive forces between particles, which prevent them from aggregating and settling. However, under certain conditions, sedimentation can occur.

The process of sedimentation in colloidal solutions can be influenced by several factors:

Particle size: Smaller particles in the colloidal solution tend to stay suspended for longer periods due to their Brownian motion, which counteracts the settling effect of gravity. Larger particles may settle more rapidly.

Particle density: If the colloidal particles have a higher density than the surrounding solvent, they will settle more quickly. And if the particles have a lower density, they may float or remain suspended for a longer time.

Solvent viscosity: Higher viscosity of the solvent impedes the settling of particles. The resistance

provided by the viscous medium slows down the sedimentation process.

External forces: Apart from gravity, external forces like centrifugation or filtration can be applied to accelerate the sedimentation process. These forces increase the effective gravitational force acting on the particles, causing them to settle faster.

When sedimentation occurs, the colloidal particles gradually settle due to the force of gravity. Over time, the particles aggregate and form a layer of sediment at the bottom of the container. The rate of sedimentation depends on the aforementioned factors and can be described by Stokes' law, which relates the settling velocity of a particle to its size, density, and properties of the solvent. 

F = 6πηrv

Where,

F is the drag force or frictional force at the interface

η is the viscosity of a liquid

r is the radius of the spherical body

V is the velocity of the flow

To prevent or control sedimentation in colloidal solutions, various techniques can be employed. These include stirring or agitation to maintain particle dispersion, adding stabilizing agents or surfactants to enhance stability or adjusting the pH or temperature of the solution to alter the repulsive forces between particles. Sedimentation is undesired in a colloidal system but there is no technique now that can stop it we can only slow down it.