Colloids

Lecture 2

Mechanical Properties (Brownian movement)

When colloidal solutions have been observed through ultra microscope, the colloidal particles are seen in constant and rapid zigzag motion called Brownian movement. Sir Robert Brown first observed the phenomenon in 1827. Suspensions and true solutions do not exhibit Brownian movement.

(area/unit time)

Osmosis

In osmosis, the solvent water moves through a semipermeable membrane

Water flows from the side with the lower solute concentration into the side with the higher solute concentration

Eventually, the concentrations of the two solutions become equal.

Osmotic pressure

Equal to the pressure that would

prevent the flow of additional

water into the more concentrated

solution

Increases as the number of

dissolved particles increase

Particularly of dissolved macromolecular materials, as the turbidity depends on the size (molecular weight) of the colloidal material involved.

Optical Properties (Tyndall Effect)

When a strong beam of light is passed through a colloidal solution, the path of the light becomes visible when viewed from a direction at right angle to that of the incident light. This occurs because the colloidal particles absorb light energy and then scatter it in all directions. The phenomenon of scattering of light by sol particles to form illuminated beam or cone is called Tyndall effect or Tyndall beam or Tyndall cone.

solutions colloids suspensions

< 1 nm > 100 nm

Passage of light Scattering in beam Scattering inall directions

Absorption of light

C) Electrical properties

Electrical Properties (Electrophoresis)

Colloidal particles of a sol either carry positive or negative charge. Sols in, which the colloidal particles carry positive charge are called positive sols. When colloidal particles carry negative charge, the sols are called negative sols. The existence of charge on the colloidal particles can be demonstrated by a phenomenon called electrophoresis where the colloidal particles, when placed in an electric field, move towards either cathode or anode depending upon the charge on them. Sols of basic dyestuffs, ferric hydroxide, aluminium hydroxide etc., are some common examples of positive sols. Colloidal solutions of gums, starch, soap solution, metals (Ag, Cu, Au, Pt etc.), metal sulphides, and some acid dyestuffs are the examples of negative sols

Zeta potentialIs an abbreviation for electrokinetic potential in colloidal systems.Zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to dispersed particle.

- If all the particles have a large negative or positive zeta potential they willRepel each other and there is dispersion stability.

-If the particles have low zeta potential values then there is no force to prevent the Particles coming together and there is dispersion instability.

Zeta potential is not measurable directly but it can be calculated using theoritical models and an experimently-determined electrophoretic mobility or dynamic electrophoretic mobility.

electrophoresisInvolves the movement of a charged particle through a liquid under the influence of an applied potential difference.

An electrophoresis cell, fitted with two electrodes, contains the dispersion. When a potential is applied across the electrodes, the particles migrate to the oppositely charged electrode.

Electrophoresis: The movement of a charged particle relative to the liquid it suspended in under the influence of an applied electric field This technique finds application in Measurements of zeta potentials of model systems (like polystyrene latex dispersion),To test colloidal stability theory,To asses the stability of coarse dispersion,In identification of charge groups

The particles move with a characteristic velocity which is dependent on the strength of the electric field (measured by the instrument), the dielectric constant and the viscosity of the medium.The velocity of a particle in a unit electric field is referred to as its electrophoretic mobility