The Colloidal State (biochemistry)

1. The Colloidal State
Maryam Fida (o-1827)

2. Colloid & Crystalloids
• Colloid: Resembling Glue
• Term used initially for solutions of substances e.g. proteins, starch and gums that
do not diffuse through most membranes
• Depends on the size of the solute particles or also called Disperse phase.
• Solvent is called continuous dispersion medium
• Crystalloids: substance which diffuse readily through membranes

3. Classification of Particles
• True solution particles in a solution: < 1nm. When dissolved they
disintegrate into individual ions or molecules. Never settle out of solution.
Not visible with electron microscope
• Colloidal Particles: 1 – 100 nm. Do not settle out of solution
spontaneously but can be separated by various methods. Visible with
electron microscope
• Suspension Particles: Bigger than colloidal particles. They along with
solvent make suspensions. Can be seen with ultramicroscope and sometimes
with naked eye

4. Colligative Properties of Solutions
• Those properties which are simultaneously bestowed upon a solvent by its
content of solute particles
• As the number of particles increases in a solution, the following changes are
seen:
• The osmotic pressure is raised
• Boiling point is raised
• Freezing point is depressed
• Vapor pressure is decreased
• These changes are minimal in a colloidal solution as these properties depend
upon the number of the particles of the solution only

5. Types of colloidal solution
• Lyophobic (liquid-hating) colloids or Suspensoids: No affinity between
the particles of solute (disperse phase) and the solvent (dispersion medium). e.g.
colloid metals such as gold, platinum and silver in water
• Their colloid particles carry one type of charge only and are kept away from each
other
• Therefore they do not aggregate or spontaneously precipitate
• Charge can be neutralized by adding a dilute electrolyte solution which will
precipitate these particles which is irreversible
• Once lyophobic colloidal particles are precipitated, it is impossible to re-form the
colloidal solution

6. • Lyophilic (liquid-loving) colloid or emulsoids:
Quite stable. These particles are surrounded by two stability factors:
1. Negative or positive charge
2. Layer of solvent
• These two factors prevent particles from coming together, forming aggregates
and being precipitated
• If the dispersion medium is separated from the dispersed phase, the sol can be
reconstituted by simply remixing with the dispersion medium.

7. Separation of colloidal particles
• Aging of colloidal solution: Colloidal solution allowed to stand
undisturbed for a long time. Particles settle down.
• Electrophoresis: Pass electric current through a solution. The charged
particles will travel to oppositely charged electrodes
• Ultracentrifugation: Separation of colloidal particles by subjecting them to
a force of several thousand of gravity (G) in an ultracentrifuge. Lighter particles
come to the surface (creaming or floatation). Heavier particles sink to bottom
(sedimentation). Units used are Svedberg floatation (Sf units) and Svedberg
sedimentation (S units)

8. • Ultrafiltration: Filtration under pressure
• Special filters of unglazed porcelain with pores as small as 1nm used to
separate colloidal particles from solutions.
• Pressure applied to speed up the process. e.g. formation of tissue (interstitial)
fluid and urine in kidneys
• Dialysis: If we place a solution containing NaCl and a colloid (e.g. a protein) in
a bag made of cellophane and immerse this bag in water, NaCl will move out of
the membrane but protein molecules will be held back. This process is
determined by the size of the pores in the meshwork of the membrane. e.g
hemodialysis and peritoneal dialysis

9. • Precipitation with electrolytes: e.g. precipitation of serum albumin
and serum globulin by full and half saturarion with ammonium sulfate
• Adsorption: colloidal particles e.g. proteins can be separated from their
solutions

10. Properties of Colloidal Solutions
• Sols and Gels: A colloidal solution in liquid form is called sol. Many lyophilic
colloids can be changed into a semi-solid form called Gel
• e.g. fruit jellies, gelatin desserts and custards. Gelatin when dilute is a sol. On
higher concentrations jelly results. By warming this gel can be converted again
to sol.
• Thixotropy: Structure of a gel may be broken by shaking and stirring and on
standing a gel may be formed again. This phenomenon is called thixotropy

11. • Imbibition: Process by which dry proteins take up water and retain it.
Plasma albumin can hold 17ml water/gram and does not let plasma water to
leave lumen of capillaries
• Brownian movement: Zig-Zag motion of particles in a colloidal solution
seen through an ultramicroscope

12. • Tyndall effect:
• Pass an intense beam of light transversely through a colloidal solution
• View it at right angles by an ultramicroscope
• The colloid particles appear as bright spots against a dark background and
path of light can be clearly seen
• This is due to reflection and scattering of light by the particles present in a
solution
• The ultramicroscope does not make visible the particles themselves. The
particles absorb light energy and then become self – luminous as part of
the energy is given off.

13. • Mutual precipitation of colloids:
• If sufficient negatively charged colloid is added to a positively charged
colloid, their electric charges are decreased to critical value and
flocculation (precipitation) take place
• Protective colloids:
• When a gelatin solution is added to a gold sol, the particles of the
lyophilic gelatin are adsorbed by the particles of the lyophobic gold and
gold particles become more resistant to precipitation
• The resulting particles have properties similar to those of lyophilic colloids
• e.g.Bile salts and bile pigments keep the insoluble cholesterol and calcium
salt of bilirubin in colloidal suspension. In their absence gall stones are
formed