Chemistry Reference
In-Depth Information
potential for confusion. The dominating forces can be broken down into the cate-
gories of electrostatic attractions and repulsions, dipolar interactions, including
hydrogen bonding or acid-base chemistry, dispersion and van der Waals forces,
and the overall hydrophobic effect. In most cases, combinations of those forces
are involved, thereby adding to the fun of interpreting the experimental results.
While the electrostatic processes are fairly straightforward, involving the interac-
tion of charged species on the polymer with those on the surfactant molecule,
the remaining interactions are less easily quantified and can be quite complex. Poly-
mers in particular add their own new twists, since in solution they will have second-
ary and tertiary structures that may be altered during the binding process in order
to accommodate the bound surfactant molecules. The nature of the surfactant-
polymer complex may significantly alter the overall energy of the system so that
major changes in polymer chain conformation will result. Any and all of those
changes may result in major alterations in the microscopic and macroscopic proper-
ties of the system.
Forces opposing the association of surfactant molecules with polymer chains
include thermal energy, entropic considerations, solvent effects, and repulsive inter-
actions among electrical charges of the same sign. It is clear that the strength and
character of surfactant-polymer interactions depend on the properties of both com-
ponents and the medium in which the interactions occur. However, even in systems
where identical mechanisms are active for different surfactant and/or polymer
types, the macroscopic symptoms of those interactions may be manifested in
such a way that entirely different conclusions could easily be drawn.
As in the case of surfactants, four general types of polymer can be defined with
respect to the electronic nature of the species: anionics, cationics, nonionics, and
amphoterics. Not surprisingly, each polymer type will exhibit characteristic inter-
actions with each surfactant class, with variations occurring within each group. It is
little wonder, then, that surfactant-polymer interactions can produce some very
''interesting'' effects and become the subject of some lively discussions.
With the understanding that a great deal remains to be learned about the subject
as a whole, the following comments will introduce a few of the observed facts about
this field of study.
7.4.2. Nonionic Polymers
Probably the largest volume of published work in the field of surfactant-polymer
interactions has involved surfactants and nonionic polymers such as polyvinylpyr-
rolidone (PVP), polyvinyl alcohol (PVA), polyvinyl acetate (PVAc), polypropylene
glycol (PPG), methyl cellulose (MC), and polyethylene oxide (POE). The preferred
surfactant has been (of course!) the classic—sodium dodecylsulfate (SDS). The
results of most studies with SDS and similar surfactants indicate that the more
hydrophobic the polymer, the greater is the interaction with anionic surfactants.
For a given anionic surfactant, it has been found that adsorption progresses in
the order PVP
PVA. In such systems, the primary
driving force for surfactant-polymer interaction will be van der Waals forces and
PPG
PVAc
MC
PEG
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