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Dispersed Systems

Dispersed systems consist of :

Particulate matter (dispersed phase).

Dispersion medium (continuous medium).

Classification of dispersed systems (according to

particle size):

MOLECULAR DISPERSION

COLLOIDAL DISPERSION

COARSE DISPERSION

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Particle size: from 1nm to 0.5nm

Particles not resolved by ordinarymicroscope,can be detected by electronmicroscope.

Pass through filter paper but not pass throughsemipermeable membrane.

Particles made to settle by centrifugation

Diffuse very slowly

E.g. colloidal silver sols, natural and synthetic polymers , cheese, butter, milk

COLLOIDAL DISPERSION

Properties of colloids

Kinetic properties.

Optical properties.

Electrical properties.

Brownian motion.

Diffusion.

Sedimentation.

Osmotic pressure.

Viscosity.

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Colloidal particles are subjected to random collision with molecules of the dispersion medium so each particle move in irregular and complicated zigzag pathway.First observed by Robert Brown (1827) with pollen grains suspended in water.The velocity of particles increases with decreasing particle size and viscosity.Increasing the viscosity of dispersion medium (by glycerin) decrease then stop Brownian motion.

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Due to Brownian Motion particles move from region

of higher concentration to region of lower

concentration.

According to the Fick’s first law:

dm/dt = -DA dc/dx

D = Diffusing constant

dm = mass of substance diffusing in time dt across an area A under influence of concentration gradient dc/dx

The minus sign denotes that diffusion takes place in the direction of decreasing concentration.

The measured diffusion coeffecient can be used to determine the radius of particles or molecular weight.

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Sutherland-Einstein EquationThis equation is used to obtain radius of a spherical colloidal particles much larger than the soventmolecules.

D= diffusion coefficient

K= Boltzmann constant

T= Absolute temperature

η = viscosity of solvent

r= radius of spherical particle

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At small particle size (less than 0.5 um) Brownianmotion is significant & tend to prevent sedimentationdue to gravity & promote mixing instead.

So, we use an ultracentrifuge which provide strongerforce so promote sedimentation in a measurablemanner.

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The velocity of sedimentation is given by Stokes‘ Law:

V = 2r2( p-po) g / 9 η

V = rate of sedimentation

r= radius of particles

p & po = density of internal phase and external phase

g = gravitational constant

η = viscosity of medium

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Osmosis is a passage of particles across semi-

permeable membrane against concentration gradient.

The minimum pressure needed to nullify osmosis is

osmotic pressure.

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104 _ 106 Pascal

Larger particles have smaller osmotic pressure.

The method is based on Van's Hoff's law

= cRT

Can be used to determine the molecular weight of

colloid in dilute solution.

Replacing c by C / M (where C = the grams of solute / liter of solution, M = molecular weight)

/C = RT/M

= osmotic pressure

R= molar gas constant

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It is the resistance to flow of system under an applied

stress. The more viscous a liquid, the greater the

applied force required to make it flow at a particular

rate.

The viscosity of colloidal dispersion is affected by theshape of particles of the disperse phase:

Spherocolloids dispersions of low viscosity

Linear particles more viscous dispersions

If linear colloidal particles coil up into spheres thenthe viscosity of the system falls due to changing theshape.

Viscosity is used to obtain molecular weight ofmaterial comprising disperse phase

Shape of particles in solution

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Above is low viscous

solution

Below is high viscous

solution

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Einstein develpoed equation of flow applicable to

colloidal dispersion of spherical particles:

η= ηo ( 1+2.5φ)

ηo= viscosity of dispersion medium

η= viscosity of dispersion when volume fraction is φ

η. Can be measured by using viscometer

Relative viscosity= ηrel = η/ ηo= 1+2.5 φ

Specific viscosity= ηsp = (η/ ηo)-1= 2.5 φ

ηsp / φ = 2.5

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When you do not succeed in taking giant steps on the road to your goal, be satisfied with little steps,

and wait patiently till the time that you are able to run, or better still, to fly.

Be satisfied to be a little bee in the hive who will soon become a big bee capable of making

honey…

Thank you …

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