Chemistry Reference
In-Depth Information
Figure 10.14.
Surface charge reversal by surfactant adsorption: (a) native surface (charges
omitted for clarity); (b) complete surface charge neutralization; (c) in excess of surfactant,
charge reversal by bilayer adsorption.
In addition to the electrostatic consequences of specific charge-charge inter-
actions, surfactant adsorption by ion exchange or ion pairing results in orientation
of the molecules with their hydrophobic groups toward the aqueous phase; there-
fore the surface becomes more hydrophobic and less easily wetted by that phase.
Once the solid surface has become hydrophobic, it is possible for adsorption to con-
tinue by dispersion force interactions. When that occurs, the charge on the surface
will be reversed, acquiring a charge opposite in sign to that of the original surface,
because the hydrophilic group will now be oriented toward the aqueous phase
(Figure 10.14). In a system normally wetted by water, the adsorption process re-
duces the wettability of the solid surface, making its interaction with other less
polar phases (e.g., air) more favorable. Industrially, the production of a hydrophobic
surface by the adsorption of surfactant lies at the heart of the froth flotation process
for mineral ore separation. Because different minerals have different surface charge
characteristics, leading to differences in adsorption effectiveness and efficiency, it
becomes possible to obtain good separation by the proper choice of surfactant type
and concentration.
Charge reversal cannot, of course, occur on adsorption of nonionic surfactants.
However, the character of the surface can be altered significantly with respect to its
wettability by aqueous or nonpolar liquids.
The adsorption of surfactants onto a clean nonpolar surface must occur with the
hydrophilic group oriented outward into the aqueous phase. Adsorption, therefore,
will always result in an increase in the hydrophilic character of the surface. Such
action is responsible for the generally increased dispersibility of materials such as
carbon black in aqueous surfactant systems, and the stability of aqueous latex poly-
mers in paints. The action of surfactant adsorption onto colloidal surfaces can be
useful to destabilize as well as stabilize systems. It may be useful, for example, to
''break'' an aqueous dispersion, to isolate the dispersed material, or to facilitate
the process of separating dispersed solids in the sewage treatment process, although
polymers and polyvalent cation salts are most commonly employed in such
