Chemistry Reference
In-Depth Information
given concentration, reducing the effective size of the POE group, and facilitating
the formation of the lamellar aggregate. The cloud point for the nonionic surfactant
will lie at the upper boundary of the liquid isotropic phase for a given surfactant
concentration.
It may be noted that the changes in a surfactant system that lead to liquid crystal
transformation appear to go hand in hand with changes that affect the size or hydro-
philicity of the head group: namely, changes in hydration, changes in the degree of
counterion binding, electrical charge screening, and so on. It will be remembered
from Chapter 4 that the same factors are found to effect changes in simple micellar
solutions. In summary, changes that reduce the effective area of the head group
cause the system to be oriented away from spherical aggregates and toward cylin-
ders, lamellar structures, and reversed micelles.
5.3. TEMPERATURE AND ADDITIVE EFFECTS ON PHASE
BEHAVIOR
The particular aggregate phase of an amphiphilic material present at a particular
concentration will depend on molecular structure, temperature, ionic strength of
the aqueous phase, and the presence or absence of other solutes, especially other
weakly surface-active materials such as esters, amides, and alcohols. For a typical
anionic surfactant, an increase in temperature will usually increase the surfactant
concentration at which the hexagonal phase appears. This reflects the increased
solubility of the monomeric surfactant, the higher cmc, and the greater thermal
mobility and solubility of the hydrophobic tail. An increase in ionic strength of
the solution will generally have the opposite effect; that is, the hexagonal phase
will appear at lower concentration.
A simplified explanation for those observations is that the increased ionic
strength reduces the charge repulsion between adjacent head groups, facilitating
the closer packing of molecules into the rodlike aggregates. In effect, the surface
area requirements of the head group are reduced, leading to the effect predicted by
geometric considerations discussed in Chapter 4. A similar explanation has been
invoked for the effect of low concentrations of short-chain alcohols. The nonionic
species can pack between the larger surfactant molecules, decreasing the effective
dielectric constant of solution in the area of the charged groups and serving to insu-
late somewhat the neighboring charges, permitting higher packing densities and the
transition to aggregate structures with a smaller radius of curvature.
More hydrophobic additives such as free fatty acids and their esters and amides,
long-chain monohydric alcohols, and nitriles may have an even more dramatic
effect on the phase behavior of a surfactant due to solubilization phenomena.
The general subject of solubilization in micellar systems is discussed in Chapter 6.
For now, we will focus on the effects that the presence of solubilized materials may
have on liquid crystal phases.
Four main classes of solubilizates can be defined on the basis of the overall
nature of the additive. These are the completely hydrophobic materials, such as
