Chemistry Reference
In-Depth Information
Figure 10.11.
Adsorbed surfactant orientation as a function of surface coverage on a
nonpolar surface: (a) low coverage—primarily trains; (b) intermediate coverage—trains and
''L''s; (c) surface saturation—approximately vertical, close-packed, with near minimum area
per molecule, although some tilt may be present.
It is generally found that surface saturation is attained at or near the cmc for the
surfactant. In many cases the isotherm is continuous, while in others an inflection
point may be found. The existence of the inflection point is usually attributed to a
relatively sudden change in surfactant orientation—from train- or L-shaped to a
more perpendicular arrangement.
The adsorption of surface-active agents onto nonpolar surfaces from nonaqueous
solvents has been much less intensively studied than has that of aqueous systems.
Generally, work has been limited to various carbon black dispersions in hydro-
carbon solvents. The orientation of the adsorbed molecules in such systems appears
to remain more-or-less parallel to the surface, although the exact details are found
to depend greatly on the history of the carbon surface.
An important consequence of adsorption of surfactants onto a nonpolar surface
is that the net character of the surface is drastically changed. If the adsorbed species
is charged, the adsorbed layer imparts, to some extent, at least, the characteristics of
such a surface, with all the attending strengths (e.g., increased stability in dispersed
systems) and weaknesses (sensitivity to electrolyte). If the adsorbed material is non-
ionic, the same will generally hold true. More details on surface modification by
adsorbed species are given later.
10.4.3. Polar, Uncharged Surfaces
Polar, uncharged surfaces include many of the synthetic polymeric materials such
as polyesters, polyamides, and polyacrylates, as well as many natural materials
such as cotton. As a result, the mechanism and extent of surfactant adsorption
onto such materials have great potential technological importance. The mechanism
of adsorption onto these surfaces will be more complex than that of the nonpolar
case discussed above, since such factors as orientation will be determined by a
balance of several forces.
The potential forces operating at a polar surface include the ever-present disper-
sion forces, dipolar interactions, and hydrogen bonding and other acid-base inter-
actions. The relative balance between the dispersion forces and the uniquely polar
interactions is of importance in determining the mode of surfactant adsorption. If
