Introduction to Surfaces and Colloids - Surface and Colloid Chemistry: Principles and Applications - page 6

There are some very special characteristics that must be considered as regards col-
loidal particle behavior: size and shape, surface area, and surface charge density. The
Brownian motion of particles is a much-studied field. The fractal nature of surface
roughness has recently been shown to be of importance (Birdi, 1993). Recent applica-
tions have been reported where nanocolloids have been employed. Therefore, some
terms are needed to be defined at this stage. The definitions generally employed are
as follows. Surface is a term used when one considers the dividing phase between
Gas-Liquid
Gas-Solid
Interface is the term used when one considers the dividing phase:
Solid-Liquid (Colloids)
Liquid 1 -Liquid 2 (Oil-Water: emulsion)
Solid 1 -Solid 2 (Adhesion: glue, cement)
In other words, surface tension may be considered to arise due to a degree of unsatu-
ration of bonds that occurs when a molecule resides at the surface and not in the
bulk. The term is used for solid-vapor or liquid-vapor interfaces. The term interfa-
cial tension is more generally used for the interface between two liquids (oil-water),
two solids, or a liquid and solid. It is, of course, obvious that, in a one-component
system, the fluid is uniform from the bulk phase to the surface. However, the orien-
tation of the surface molecules will be different from those in the bulk phase. For
instance, in the case of water, the orientation of molecules inside the bulk phase will
be different from those at the interface. Hydrogen bonding will orient the oxygen
atom toward the interface. The question one may ask, then, is: how sharply does the
density change from that of a fluid to that of gas (a change by a factor of 1000)? Is
this transition region a monolayer deep or many layers deep? Many reports are found
where this subject has been investigated.
The Gibbs adsorption theory (Birdi, 1989, 1999, 2002, 2008; Defay et al., 1966;
Chattoraj and Birdi, 1984) considers the surface of liquids to be monolayer. The sur-
face tension of water decreases appreciably on the addition of very small quantities
of soaps and detergents . The Gibbs adsorption theory relates the change in surface
tension to the change in soap concentration. The experiments that analyze the spread
monolayers are also based on one molecular layer. The latter data indeed conclu-
sively verifies the Gibbs assumption (as described later). Detergents (soaps, etc.) and
other similar kind of molecules are found to exhibit self-assembly characteristics.
The subject related to self-assembly monolayer (SAM) structures will be treated
extensively (Birdi, 1999). However, no procedure exists that can provide information
by direct measurement. The composition of the surface of a solution with two com-
ponents or more would require additional comments.
Colloids (Greek for glue-like) are a wide variety of systems consisting of finely
divided particles or macromolecules (such as glue, gelatin, proteins, etc.) that are
found in everyday life. In Table 1.1 examples of such typical colloidal suspensions
are given. Further, colloidal systems are widespread in their occurrence, and have