Chemistry Reference
In-Depth Information
Thus for, the discussion has dealt primarily with the effects of changes in the
hydrophobic group on the ability of a surfactant to reduce the surface tension of
a solution. It was stated earlier that an ionic head group usually plays a relatively
minor role in determining the efficiency and effectiveness of a surfactant. While that
may be true for groups closely related in size and electrostatic character, alterations
in those factors can produce significant changes in their activity at the solution-air
interface. A class of surfactants well suited to the study of such effects is that of the
quaternary ammonium salts in which three of the alkyl groups are short-chain units
such as methyl, ethyl, and propyl. The substitution of propyl for methyl groups in
n
tetradecyltrimethylammonium bromide resulted in a significant reduction in the
efficiency of adsorption, while not affecting the minimum surface tension obtained.
Presumably, the presence of the bulkier propyl substituents on the head group
greatly increases its area per molecule and therefore reduces its adsorption effi-
ciency.
In the case of anionic head groups, there appear to be only relatively minor
variations in effect from one group to another. The difference in cross-sectional area
between sulfate and sulfonate groups does not appear to influence greatly the acti-
vity of surfactants in lowering surface tensions, although some difference can be
noted when differences in cmc are taken into consideration. The role of the counter-
ion can be important when changes result in significant alterations in the ion bind-
ing properties of the molecule. Tight ion binding will reduce the extent of
electrostatic repulsion between adsorbed molecules, allowing for tighter packing
of surfactant at the interface and, in general, increases in both the efficiency and
the effectiveness of the surfactant. A similar result is obtained by the addition of
neutral electrolyte.
An interesting class of surfactants that has found practical application because
of tight ion binding and its effects on surface activity are those in which both
the anion and the cation of the pair are, individually, surface-active. Materials
such as n-decyltrimethylammonium n-decylsulfate (written in reverse to illustrate
the ion binding)
CH
3
Þ
3
N
þ
O
4
SC
10
H
21
C
10
H
21
ð
have been found to be much more efficient and effective at surface tension reduc-
tion than either simple material alone. The very strong ion pairing nature of their
association results in a high packing density at the interface and large values for
s
20
and
s
min
. Related materials were found to lower the surface tension of a solution
to approximately 24 mN/m, one of the lowest surface tensions yet reported for
totally hydrocarbon surfactant systems.
As has already been mentioned, the effect of changes in the hydrophobic chain
length on the effectiveness of surface tension reduction in nonionic surfactants is
relatively minimal. Increases in the length of the polyoxyethylene chain, on the
other hand, lead to significant reductions in the effectiveness of a given surfactant
hydrophobic group. It appears, then, that the primary factor involved in the
efficiency and effectiveness of nonionics in this application is the length of the
