The difference in physical properties of a substance
in “normal” dimensions and in colloidal dimensions is due to the increased
specific surface when large particles are subdivided into particles of
colloidal size. The specific surface Asp
is defined as the ratio of surface area A, to the volume V, or to the
mass
Asp = A/V = 4πr2 / 4/3πr3
= 3/r cm-1
Also Asp
= A/M = A/Vδ = 4πr2 / 4/3πr3δ
=
3/rδ
If a cube , 1 cm on edge is repeatedly subdivided into
smaller cubes until each of the latter is 1 μm on edge (of colloidal
dimension), the total surface area will increase from 6cm2 to 6000cm2, but the specific
surface will increase to 6x107 cm-1
As the surface area increases, there is a corresponding
increase in surface energy. In an attempt to decrease these energy, smaller
particles or droplets tend to regroup to decrease the surface area. Thus
colloidal dispersions are intrinsically unstable, with a tendency to flocculate
or agglomerate.
The reduction in particle size to colloidal dimensions may
lead to change in other physical properties which arte otherwise
considered constant. For example solubility and osmotic pressure may increase,
while vapor pressure and melting point may decrease. Color also
changes as the particle size is reduced. Thus large particles of antimony
trisulfide are red, changing to yellow in finely divided particles. Sols of
gold with large particles are purple or blue, but become brilliant red as
particles become fine.
The shape of the particles affects viscosity. Sols of
spherical particles (e.g. hemoglobin) have lower viscosity while threadlike
particles (e.g. fibrin) confer high viscosity to their sols.
