Chemistry Reference
In-Depth Information
water molecules through the action of the added ions, the introduction of the mono-
meric surfactant molecule will require an additional amount of work to overcome
that added structural energy. The net result will be a ''salting out'' of the surfactant
and a decrease in the cmc. If, on the other hand, the added electrolyte acts as a
structure breaker, the required work will be less, the surfactant will be ''salted in,''
and the cmc will increase.
Nonionic surfactants that have highly hydrogen bonding interactions in water
such as POE ethers and sugar esters will seldom show significant salt effects
until the electrolyte concentration reaches the level at which the activity of the
water becomes affected. At that point, the competition between the dissolved salts
and the hydrophilic group for the available water becomes intense and the cmc will
be found to decrease. The same might be expected for the addition of non-ionized
additives that have a similar interaction with water such as sugars and other poly-
hydroxy materials, organic acids and amines, and water-soluble polymers.
In the case of the POE nonionic surfactants, there exists an additional possible
phenomenon that may help to explain the effect of certain cations on their proper-
ties. It is well known, for example, that the cyclic polyoxyethylene or ''crown''
ethers can form very strong complexes with many appropriately sized ions such
as Na
þ
and K
þ
. When the ionic radius of the ion is properly matched to the size
of the ''basket'' formed by the cyclic ether, interactions between the ether oxygen
atoms and the ion produce complexes with exceedingly large stability constants. It
seems reasonable to expect that linear POE chains of intermediate length, relatively
free to assume various configurations in solution, could do so in such a way as to
form a ''pseudocrown'' ether capable of forming complexes with cationic ions.
Should such a phenomenon occur with either the monomer or the micelle, we
would expect the overall thermodynamics of the system to be affected, including
that of the micellization process. Such a scenario is presently somewhat speculative,
but it represents an interesting potential field for further research.
The effectiveness of a given ion at altering the micellization process can be
qualitatively related to the radius of hydration of the added ions, and the contribu-
tion of the cations and anions will be approximately additive. In general, the smaller
the radius of hydration of the ion, the greater is its effect on the cmc. The approx-
imate order of effectiveness of anions at decreasing the cmc is the following:
1
2
SO
2
F
BrO
3
>
Cl
Br
NO
3
>
I
CNS
>
>
>
>
>
4
For cations, the order is
1
2
Ca
2
þ
NH
4
>
K
þ
Na
þ
Li
þ
>
>
>
It has been found that the tetraalkylammonium salts of surfactants exhibit an in-
crease in the cmc in the order
ð
C
3
H
7
Þ
4
N
þ
> ð
C
2
H
5
Þ
4
N
þ
> ð
CH
3
Þ
4
N
þ
