Introduction to Surfaces and Colloids - Surface and Colloid Chemistry: Principles and Applications - page 5

Colloid

Formation of Colloidal
Particles

FIGure 1.5 Formation of fine (colloidal) particles (schematic; size less than micrometer).

The surface chemistry of small particles is an important part of everyday life
(such as dust, talcum powder, sand, raindrops, emissions, etc.). The designation col-
loid is used for particles that are of small dimension and that cannot pass through
a membrane with a pore size ca. 10 −6 m (= micrometer [μm]) (Thomas Graham
described this about a century ago).
The nature and relevance of colloids is one of the main current research topics
(Birdi, 2002). They are an important class of materials, intermediate between bulk
and molecularly dispersed systems. Colloid particles may be spherical but, in some
cases, one dimension can be much larger than the other two (as in a needle-like
shape). The size of particles also determines whether they can be seen by the naked
eye. Colloids are not visible to the naked eye or under an ordinary optical micro-
scope. The scattering of light can be suitably used to see such colloidal particles
(such as dust particles, etc.). Their size then may range from 10 −4 to 10 −7 cm. The
units used are as follows:

1 μm = 10 −6 m
1 nm = 10 −9 m
1 Å (Angstrom) = 10 −8 cm = 0.1 nm = 10 −10 m

The unit Angstrom (Å) is related to the famous Swedish scientist, and currently nm
(10 −9 m) is mainly used. Nanosize (nanometer range) particles are currently of much
interest in different applied-science systems. The word nano is derived from Greek
and means dwarf . Nanotechnology has actually been getting a strong boost from
the last decade of innovation, as reported by the surface and colloid literature. In
fact, light scattering is generally used to study the size and size distribution of such
systems. Since colloidal systems consist of two or more phases and components, the
interfacial area-to-volume ratio becomes very significant. Colloidal particles have
a high surface-area-to-volume ratio compared with bulk materials. A significant
proportion of the colloidal molecules lie within, or close to, the interfacial region.
Hence, the interfacial region has significant control over the properties of colloids.
To understand why colloidal dispersions can either be stable or unstable, we need to
consider the following:

1. The effect of the high surface-area-to-volume ratio (e.g., 1000 m 2 surface
area per gram of solid [active charcoal])

2. The forces operating between the colloidal particles