Chemistry Reference
In-Depth Information
applications may be different as well. The hydrocarbon sulfonates are generally
more chemically stable than the sulfate esters, but the economics of their prepara-
tion is an impediment to their widespread use.
If the original dodecane molecule were terminally chlorinated (path 6) and
reacted with trimethylamine (path 7), the resulting compound would be dodecyltri-
methylammonium chloride, a water-soluble compound exhibiting some surfactant
properties, but not generally as useful as the anionic analogs:
CH
3
Þ
3
Cl
n-C
12
H
26
þ
Cl
2
!
n-C
12
H
25
Cl
þ
N
ð
CH
3
Þ
3
!
n-C
12
H
25
N
ð
The utility of such compounds is limited not so much by their surface activity as
by their interaction with various oppositely charged components found in practical
systems (see Chapters 9 and 10 ).
Up to this point we have covered three of the four general classes of surfactants
defined so far: anionic, nonionic, and cationic. To produce an example of the fourth
class, an amphoteric or zwitterionic surfactant, it is only necessary to react the
dodecylchloride prepared as described above with a difunctional material such as
N,N-dimethyl-3-aminopropane-1-sulfonic acid (path 8):
C
12
H
25
N
þ
CH
2
CH
2
CH
2
SO
3
HCl
C
12
H
25
Cl
þð
CH
3
Þ
2
NCH
2
CH
2
CH
2
SO
3
H
!
C
12
H
25
N
þ
CH
2
CH
2
CH
2
SO
3
þ
M
þ
þ
Cl
þ
!
MOH
The result is just one of several possible chemical types that possess the amphoteric
or zwitterionic character of this class of materials. Under acidic conditions, the
molecule carries a net positive charge. Under basic conditions, the acid is neutra-
lized and the molecule carries both a positive charge and negative charge. In this
context, we are talking about the electrical nature of the surface-active portion of the
molecule and not any associated, but non-surface-active ions such as Cl
or M
þ
.
The number of chemical modifications of the dodecane or similar simple hydro-
carbon molecules that can lead to materials with good surfactant characteristics is
impressive. When hydrocarbons containing aromatic groups, unsatruration, branch-
ing, heteroatom substitution, polymers, or other interesting functionalities are con-
sidered, the synthetic possibilities seem almost unlimited. Only imagination, time,
and money seem to limit our indulgence in creative molecular architecture.
In each example discussed above, an aqueous ''solubilizing group'' has been
added to the basic hydrophobe to produce materials with varying amounts of useful
surfactant characteristics. When one considers the wide variety of hydrophobic
groups that can be coupled with the relatively simple hydrophiles discussed so
far and add in more complex and novel structures, the number of combinations
becomes impressive. When viewed in that light, the existence of thousands of dis-
tinct surfactant structures doesn't seem surprising.
Using the evolution of dodecane-based surfactant structures as a jumping-off
point, the discussion will now turn to more specific examples of surfactant building
blocks. As noted, the chemical possibilities for surfactant synthesis seem almost
