Chemistry Reference
In-Depth Information
TABLE 4.16. Effects of Oxyethylene (OE) Introduction
between Hydrocarbon Chain and Sulfate Group in
C
12
H
25
(OCH
2
CH
2
)
x
OSO
3
Na Surfactants at 25
C
Number of EO (x)
cmc (mM)
0
8.0
1
0.5
2
0.3
4
0.2
solutions of ionic surfactants results in a decrease in the cmc, while nonionic mate-
rials are only slightly affected. In the presence of 0.1 N NaCl, the materials in Table
4.16 show a shift in cmc even greater than that found for the analogous dodecylsul-
fate salt. It seems clear, then, that the combined effects of oxyethylene groups and
ionic sulfates cannot be analyzed by simple analogy to either simple class of sur-
factants. In the absence of a clear-cut explanation for the unusual micellization
properties of these materials, it has been suggested that the results might stem
from either a reduction of the degree of hydration of the OE groups due to the pre-
sence of the sulfate or to a decrease in the work required to bring the ionic groups
into close proximity during micelle formation as a result of the greater space
requirements of the combination of hydrophiles. Data on the adsorption of alkyl
ether sulfates onto solid surfaces and at the liquid-vapor interface, and data on
micellar aggregation numbers, tend to support the importance of head group size
and sulfate spacing to the micellization of these materials, especially for x
>
2.
4.5.5. Counterion Effects on Micellization
As indicated by Eq. (4.12), the free energy of micelle formation for ionic surfac-
tants contains a term related to the interactions of solvent, in most cases water, with
the ionic head group. The degree of ionization of the ionic group, in terms of tight
ion binding, solvent-separated ion pairing, or complete ionization, might be
expected to greatly influence the magnitude of
G
mw
and consequently the cmc
and aggregation number of the system. Since electrostatic repulsions among the
ionic groups would be greatest for complete ionization, it is not surprising to find
that the cmc of surfactants in aqueous solution decreases as the degree of ion bind-
ing increases.
From regular solution theory it is found that the extent of ion pairing in a system
will increase as the polarizability and valence of the counterion increase. Conversely,
a larger radius of hydration will result in greater ion separation. It has been found
that, for a given hydrophobic tail and anionic head group, the cmc generally decreases
in the order Li
þ
Ca
2
þ
&
Mg
2
þ
.
In the case of cationic surfactants such as dodecyltrimethylammonium halides,
the cmc's are found to decrease in the order F
Na
þ
K
þ
Cs
þ
N(CH
3
)
þ
N(CH
2
CH
3
)
þ
>
>
>
>
>
>
Cl
Br
I
.
>
>
>
