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colloids
A colloid is a dispersion of small particles of one material in
another, where the size of the particles is smaller than 500 nm
(the range of the wavelength of visible light, therefore they are
not visible with an ordinary optical microscope).
dispersed particles
(e.g. oil droplets,
clay particles, etc.)
continuous phase
(e.g. water)
colloidal system
(e.g. milk, muddy
water, etc.)
classification of colloids
dispersed particles
solid
continuous
phase
fluid
solid
fluid
(solid)
(solid)
(solid)
sol
foam
foam
sol,
(suspen- emulsion
sion), gel
aerosol,
gaseous smoke,
(dust)
Aerosol,
aerosol,
Nebel
fog
gaseous
Schaum
foam
dispersion of a
material into fine
particles is
connected to a
drastic increase in
surface:
original surface of
the complete cube:
A = 6 a²
after cube is split
into 10x10x10
subunits:
A = 1000x0.01x6 a²
= 60 a²
a
surface tension γ
F
γ = dw/dσ
γ = F/(2l)
l
fluid
film
liquid
surfaceinduced
pressure
P
P = 2γ /r
surface-induced pressure
∆P = 2γ/r
∆P = 2γ/r
∆P = 4γ/r
r
droplet
(gas-)bubble
foam bubble
distribution of hydrophobic material at an oilwater interface:
oil
β
water
σ
α
distribution of hydrophilic material at an oilwater interface:
oil
β
water
σ
α
distribution of amphiphilic material (surfactant)
at an oil-water interface:
oil
β
surfactant,
e.g. soap
water
σ
α
spreading of surfactant on an oil-water
interface:
σ
∗
γ
α
π = F/l
γ
water
movable
barrier
oil
β
an amphiphilic component in pure water may
form micelles:
surfactant,
e.g. soap
water
surfactants stabilize colloidal dispersions of
particles:
continuous phase
particle
10 Nanometer
surfactant
examples for colloidal systems:
- food (milk, yoghurt, mayonnaise, raw juices etc.)
- paint (liquid dispersions, spray, electrophoresis etc.)
- glues (white wood glue, glue stick etc.)
- lubricants (e.g. oil-water mixture for cooling)
- technical aerosols (e.g. air-gasoline mixture)
- pollutants (smoke, foam on natural water surfaces)
- pharmaceutical applications (e.g. Aspirine® in water)
- natural occurance of colloids:
clay in water, clouds, fog, dust,
planctonic microorganisms