Chemistry Reference
In-Depth Information
Most of the published work on the phase behavior of anionic surfactants has
dealt with the simple carboxylic acid soaps, with less information available on
the sulfates, sulfonates, and similar compounds. It is generally found, however,
that certain trends hold over a wide range of products so that one can predict events
with some degree of confidence.
For a typical anionic surfactant the micelles remain approximately spherical or
ellipsoidal over a substantial concentration range above the cmc, but ultimately they
become rodlike as the concentration continues to increase. At concentrations in the
range of 20-30% by weight, a new phase normally appears that is birefringent and
quite viscous. X-Ray diffraction studies show that this phase consists of many long,
parallel, rodlike aggregates arranged as illustrated in Figure 5.3b. The aggregate
interiors are apparently rather fluid, resembling liquid hydrocarbon in many
respects. This phase is what is classically referred to as a ''liquid crystal'' in that
it possesses a substantial degree of order in at least one dimension while not being
truly crystalline. As indicated previously, the usual terminology for such a structure
is the normal hexagonal or simply the hexagonal phase. In the soap industry it is
traditionally referred to as the ''middle phase.''
As the surfactant concentration continues to increase, it may become energeti-
cally favorable for the surfactant molecules to arrange themselves into a bilayer
structure, the lamellar phase (the ''neat'' phase to the soapmaker) (Figure 5.3c).
Continuation of the process of increasing the surfactant concentration may lead
to the formation of the reversed or inverted hexagonal phase, which again involves
an array of rodlike aggregates, but now with the rod interiors as the aqueous phase
(Figure 5.3d). It is in the region of transition between the normal and the reversed
hexagonal phases that the cubic bicontinuous phase appears.
In terms of surfactant structure, one can expect that a more hydrophilic
head group will tend to delay the formation of the hexagonal and subsequent
mesophases as a result of charge repulsion among adjacent molecules in the
micelle. While the effect of changes in counterion will usually be small, if a
significant degree of ion binding occurs for a given head group, thereby reducing
its hydrophilicity, one can expect the appearance of the hexagonal phase at
lower surfactant concentration. Within an homologous series, it is generally
found that the appearance of the liquid crystalline phases will occur at lower con-
centrations for higher members of the series, paralleling the normally observed
decrease in cmc.
Much less work has been reported on the phase behavior of cationic surfactants.
In general, the phase diagrams for simple quaternary ammonium halides closely
resemble those of anionic surfactants. In some cases, it has been found that the
phase behavior of such materials is much more sensitive to counterion effects
than is that of anionic species.
Dodecyltrimethylammonium chloride in water, for example, shows the presence
of two distinct viscous isotropic cubic phases at room temperature. Similar results
are found for the decyl and tetradecyl analogs, while the hexadecyl and octadecyl
members of the series do not exhibit these phases. The cubic phases are apparently
absent in the corresponding bromides of all members of the series.
