Disperse systems 4

Question 1

Optical properties

Answer 1

Although dispersed particles are often too small to be visible individually, they are large enough to interfere with the path of light travelling through
When light goes through a dispersion, some goes straight through but some is scattered by particles
The degree of light scattering depends on particle size: small particles bend light more than big particles
This property can be used to determine the molecular weight of colloids and give information on particle size and shape

Question 2

Turbidity measurement

Answer 2

Light scattering makes a dispersion appear turbid/ cloudy

The more concentrated the dispersion, the more light is scattered and the more turbid the dispersion looks

Question 3

Laser diffraction

Answer 3

Light from a laser is shone into a cloud of particles and the particles scatter the light
The amount by which the light is scattered depends on particle size

Question 4

Light scattering

Answer 4

Light scattering is also influenced by the wavelength of the light
White light is made of lights of different wavelengths and therefore different colours
The degree of light scattering depends on the colour of the light- blue light is scattered more than red light

Question 5

Brownian motion

Answer 5

Particles follow irregular paths because they collide with molecules of the continuous phase
The paths can be seen under a microscope
The velocity of the particles decreases with increasing size and vehicle viscosity
This leads to diffusion

Question 6

Diffusion

Answer 6

Particles move from regions of high to lower concentrations, to give a uniform concentration throughout the system
The rate is described by Fick’s first law which relates the amount of substance diffusing over time to the change in its concentration

Question 7

Osmotic pressure

Answer 7

When the presence of a semi-permeable membrane prevents particles from moving to achieve equal concentration throughout a system, the dispersant will move instead
Osmosis is the flow of the dispersant through the membrane from the region with a low particle concentration into the more concentrated region
Osmotic pressure is the pressure produced by this movement

Question 8

Sedimentation

Answer 8

Brownian motion keeps small particles moving in a colloidal dispersion but as the particles get bigger, the influence of gravity is greater
If the only force acting on small particles is gravity then they won’t sediment
But for particles larger than about 0.5-1 micrometres, gravity starts to become significant causing them to settle to the bottom of the formulation