Chemistry Reference
In-Depth Information
limitless. The following discussions, on the other hand, will be limited by space and
time, and are designed as guides rather than a comprehensive compilation.
2.2.2. Surfactant-Solubilizing Groups
The solubilizing groups of modern surfactants fall into two general categories:
those that ionize in aqueous solution (or highly polar solvents) and those that do
not. Obviously, the definition of what part of a molecule is the solubilizing group
depends on the solvent system being employed. For example, in water the solubility
will be determined by the presence of a highly polar or ionic group, while in
organic systems the solubilizing functionality will be the organic portion of the
molecule. It is important, therefore, to define the complete system under consi-
deration before discussing surfactant types. As noted before, because the majority
of surfactant work is concerned with aqueous environments, the terminology
employed will generally be that applicable to such systems. Generality will be
implied, however.
The functionality of ionizing hydrophiles derives from a strongly acidic or basic
character, which leads to the formation of highly ionizing salts on neutralization
with appropriate bases or acids. In this context, the carboxylic acid group, while
seldom considered as such in acid-base theory, is classified as a strong acid. The
nonionizing, or nonionic hydrophilic, groups, on the other hand, have functional-
ities or groups that are individually rather weak hydrophiles (alcohols, ethers,
esters, etc.) but have an additive effect so that increasing their number in a molecule
increases the magnitude of their solubilizing effect.
The most common hydrophilic groups encountered in surfactants today are
illustrated in Table 2.1, where, as already noted, R designates some suitable
TABLE 2.1. The Most Commonly Encountered Hydrophilic Groups in Commercially
Available Surfactants
General Class Name
General Solubilizing Structure
R—SO
3
M
þ
Sulfonate
R-OSO
3
M
þ
Sulfate
R-COO
M
þ
Carboxylate
R-OPO
3
M
þ
Phosphate
R-N
þ
R
0
x
H
y
X
(x
¼
1-3, y
¼
3-1)
Ammonium
R-N
þ
R
0
3
X
Quaternary ammonium
R-N
þ
(CH
3
)
2
CH
2
COO
Betaines
R-N
þ
(CH
3
)
2
CH
2
CH
2
SO
3
Sulfobetaines
Polyoxyethylene (POE)
R-OCH
2
CH
2
(OCH
2
CH
2
)
n
OH
R-OCH
2
CH
2
(OCH
2
CH
2
)
n
OSO
3
M
þ
Polyoxyethylene sulfates
Polyols
R-OCH
2
-CH(OH)-CH
2
OH
Sucrose esters
R-O-C
6
H
7
O(OH)
3
-O-C
6
H
7
(OH)
4
Polyglycidyl esters
R-(OCH
2
CH[CH
2
OH]CH
2
)
n
-
-OCH
2
CH[CH
2
OH]CH
2
OH
