OIA1008 W2 DISPERSE SYSTEM: PROPERTIES OF COLLOIDS

Optical Properties of Colloids:

(*) Faraday-Tyndall Effect:

Strong beam of light passing through colloidal solution: path of light becomes visible when viewed from right angle to the incident light. Colloidal Particles absorb light energy & scatter in all directions

Sol particles scatter light, forming illuminated beam or cone known as Tyndall Effect.

Tyndall Effect not shown by true solution as ions or solute too small that they cannot reflect light, thus used to distinguish between true solution and colloidal solution.

Magnitude of turbidity/ opalescence depend on nature, size & conc. of particles -> examined by ultramicroscope

Lyophobic colloids - significant and well defined; Lyophilic solds - very small difference and weak

*Tyndall Effect confirms heterogenous nature of colloidal solution.

Turbidity measurements: light scattering describe in terms of turbidity

Turbidity or Tyndall effect of hydrophilic colloidal system << Lyophobic colloid dispersion

Turbidity measured by spectrophotometer/ photoelectric colorimeter & nephelometer

Turbidity - MW of colloid

Determine MW by turbidity method

Determine MW by light scattering method

Polarizable particles placed in oscillating electric field of light beam -> light scatters

Light pass through polymer solution, measure energy loss as absorption convert to heat & scattering

Intensity of light: conc., size & polarisability - proportional constant depends on MW

Not require calibration to obtain adsolute MW: provide info of shape & MW

Can perform rapidly w less sample & measure absolute MW

Average MW of scattering polymers relate their light scattering properties:

K = wave vector

C = concentration of solution

R (teta) = reduced Rayleigh ratio

P (teta) = particle scattering function

(teta) = scattering angle

A = osmotic viral coefficient

n0 = solvent refractive index

Lambda = wavelength of light

Na = Avagadro's number

Rz = radius of gyration

(*) Kinetic Properties of Colloids

Motion of particles wrt. dispersion medium

Thermally induced -> thermal motion (Brownian movement, diffusion, osmosis)

Brownian Movement - Robert Brown

Colloidal particles subjected to random collision w molecules -> erratic motion (Brownian motion) - complicated Zig-zag path

Increase viscosity of dispersion medium decrease Brownian motion

Decrease particle size increase velocity

Motion of molecules in dispersion medium cannot observed

Diffusion - caused by Brownian motion

Particles diffuse spontaneously from region higher conc. to region lower conc. until system uniform

Osmotic Pressure

Solution & solvent are separated by semi-permeable membrane -> tendency to equalize chemical potentiation (concentration) on either side

Pressure necessary to balance osmotic flow (osmotic pressure)

*van't Hoff equation: pi = cRT

Ideal solution: pi = (Cg/M) RT

Colloidal dispersion: pi/Cg = RT (1/M + BCg)

Ideal system (line I) & real system (line II & III)

(*) Determine MW by osmotic method

Line I: slope, B = 0, reflects dilute spherocolloidal system

Line II: B large liner, reflect linear colloid in solvent (poor affinity) -> linear lyophobic colloidal

Line III: B larger, reflect linear colloid in solvent (high affinity) -> linear lyophilic colloidal; nonlinear at higher conc. or marked interaction

Extrapolate intercept identical for both line II & III -> MW of colloid independent of solvent used

Gravitational induced -> gravitational motion,w or w/o F (sedimentation)

Velocity (v) of sedimentation of spherical particles (Stokes' law)

Lower size limit of particles obeying Stokes' Law is about 5um

Force > gravity must applied to cause sedimentation (centrifugal force)

Ultracentrifuge produce F of 10^6 g (sediment colloidal particles)

* External force (viscosity)

Expression of resistance to flow of system under applied stress

Affected by shape of dispersed phase (colloids)

Spherocolloids -> lower dispersion

Linier colloid particles -> more viscous dispersion

(*) Determine MW by viscosity method data