Chemistry Reference
In-Depth Information
butyric, and possibly slightly longer acids are not sufficiently hydrophobic to be
surface-active. However, if the free carboxylic acid is esterified with a fatty alcohol
of proper length, usually C
8
-C
18
, the resultant materials will generally perform well
as surfactants. A typical example of such a material is sodium dodecylsulfoacetate:
C
12
H
25
O
2
CCH
2
SO
3
Na
þ
The esters of sulfoacetic,
a
-sulfopropionic, and
a
-sulfobutyric acids with long-
chain alcohols are highly surface-active but have found little general application
because of their relatively high cost. They have several specialty applications, espe-
cially in personal care products such as toothpastes, shampoos, and cosmetics.
Some have also been approved for use in conjunction with mono- and diglycerides
in food applications.
The
a
-sulfocarboxylic acids of higher molecular weight, especially lauric, pal-
mitic, and stearic acids, are highly surface-active. It is generally found that the
water solubility of the unesterified
a
-sulfo acids decreases as the length of the car-
bon chain increases, as might be expected. Less obvious are the observed effects of
the state of neutralization of the carboxylic acid and the nature of the counterion on
solubility. In the case of the free acid sulfonate, solubility is found to increase in the
order Li
K, while the solubility of the neutralized dialkali salt increases
in the opposite order. The use of alkylammonium counterions is also found to in-
crease the solubility of a specific-chain-length material.
The presence of the free acid or carboxylate salt combined with the sulfonate
group has suggested the use of such materials as corrosion inhibitors when applied
to metal surfaces, which can form strong salts or complexes with carboxylic acid
groups. Related applications would be in ore flotation. The materials have also
found some utility as viscosity reducers in liquid detergent formulations.
The presence of the free carboxyl group in these surface-active materials sug-
gests the possibility of additional derivatization, such as esterification or amidation.
Esterification of the carboxyl group has usually been found to increase the water
solubility of a given acid, as long as short-chain alcohols are employed. Unlike
many esters, those of the
a
-sulfocarboxylic acids are generally resistant to hydro-
lysis, although they are readily biodegradable under both aerobic and anaerobic
conditions:
<
Na
<
R
2
OOCCHR
1
SO
3
M
þ
The surface properties of a series of
a
-sulfocarboxylic acid esters, in which the
chain length of the acid (R
1
) was varied from C
2
to C
18
and that of the alcohol
(R
2
) from C
1
to C
16
, were found to vary considerably with the location of the sul-
fonate group along the chain. Esters of short-chain alcohols and long-chain acids
exhibit critical micelle concentrations roughly equivalent to those of the analogous
alkyl sulfate surfactant of the same hydrocarbon chain length. Such materials show
good detergent properties, while materials with equal carbon numbers, but with
longer alcohol and shorter acid groups, were superior wetting agents with larger
