Chemistry Reference
In-Depth Information
Native chain conformation Expanded coil
Figure 7.7.
A schematic illustration of polymer coil expansion due to dispersion (hydro-
phobic) surfactant-polymer interaction.
the hydrophobic effect. Dipolar and acid-base interactions may be present, depend-
ing on the exact nature of the system. Ionic interactions will be minimal or nonexis-
tent. For the polymer, it is reasonable to infer that the impact of the hydrophobic
effect will be related to the ability of the polymer to undergo hydrogen bonding
with the solvent (water), as well as the relative availability of nonpolar binding
sites along the polymer chain.
If the primary mechanism of ionic surfactant-nonionic polymer interaction is
hydrophobic, the adsorption of surfactant molecules will produce changes in the
polymer chain conformation that are due to repulsions between the adsorbed
ionic surfactant head groups (Figure 7.7). The properties of the solution will be
altered as a result of such changes. For example, the solution viscosity may be
found to increase substantially since the repulsion will force the polymer chain
to uncoil or expand in the solution. If neutral salt is then added to such a system,
repulsion between neighboring groups will be screened and the expanded coil will
contract or collapse, thus again affecting various macroscopic properties of the
solution such as viscosity. Such expansion and collapse of surfactant-polymer com-
plexes as a function of the extent of surfactant adsorption may be seen as being
analogous to the solution behavior of polyelectrolytes as a function of the degree
of dissociation and electrolyte content. In fact, the surfactant-polymer complex
may be viewed as a noncovalent polyelectrolyte.
The bulk of the work on cationic surfactant-nonionic polymer interactions has
utilized long-chain alkylammonium surfactants. It has been found that the interac-
tions between such species become stronger as the chain length of the surfactant
is increased, reflecting the greater drive to substitute surfactant-polymer for
surfactant-water and polymer-water interactions. The nature of the cationic head
group seems to have a significant effect on polymer-surfactant interactions. It has
been found that the reduced viscosity of aqueous solutions of dodecylpyridinium
thiocyanate/PVAc changed very little with variations in the surfactant concen-
tration, whereas
solutions of dodecylammonium thiocyanate/PVAc showed
