Chemistry Reference
In-Depth Information
unfavorable energetic situation; and (2) the formation of an infinitely extended
structure is unfavorable from an entropic standpoint. The formation of spherical
closed vesicles, then, addresses both those factors: the edge effect is removed by
the formation of a closed system, and the formation of structures of finite size over-
comes much of the entropic barrier. As long as the curvature of the vesicle is gentle
enough to allow the packed molecules to maintain close to their optimum surface
area, vesicles will represent viable structures for the association of surfactants and
related materials.
Over the years it has been confirmed that geometric factors control the packing
of surfactants and lipids into association structures. The concept has already been
introduced, but warrants repetition in the current context for clarity. The packing
propensity of a given amphiphilic structure can be conveniently given by the critical
packing parameter, denoted here as P
c
, and given by
v
a
o
l
c
P
c
¼
ð
5
:
1
Þ
where v is the volume of the hydrophobic portion of the molecule, a
o
is the optimum
head group area, and l
c
is the critical length of the hydrophobic tail, effectively the
maximum extent to which the chain can be stretched out, subject to the restrictions
of bond lengths and bond angles. The value of P
c
will determine the type of asso-
ciation structure formed in each case. The structures to be expected from molecules
falling into various ''critical packing'' categories are summarized in Table 5.1.
Examples of materials that do not fit neatly into such a scheme may be found,
but
the general concepts are usually found to be valid. For surfactants and
TABLE 5.1. Expected Aggregate Characteristics of Amphiphiles as Determined
by Their Molecular Structure and Packing Parameter P
c
General Surfactant Type
P
c
Expected Structure
Simple surfactants with single
chains and relatively large head
groups
<
0.33
Spherical or ellipsoidal micelles
Simple surfactants with relatively
small head groups, or ionic
materials in the presence of
large amounts of electrolyte
0.33-0.5
Relatively large cylindrical or
rod-shaped micelles
Double-chain surfactants with
large head groups and flexible
chains
0.5-1.0
Vesicles and flexible bilayer
structures
Double-chain surfactants with
small head groups or rigid,
immobile chains
1.0
Planar extended bilayers and
cubic bicontinuous phases
Double-chain surfactants with
small head groups, very large and
bulky hydrophobic groups
>
1.0
Inverted micelles
