Chemistry Reference
In-Depth Information
If a surfactant possesses two polar groups, as do, for example, the taurines or
sulfoesters
SO
3
M
þ
R
X
ð
CH
2
Þ
n
where R is a normal surfactant hydrophobe and X is a polar amide or ester linkage,
the methylene groups lying between the two polar groups will contribute an effect
equivalent to approximately half that found for such groups in R.
Although usually regarded as hydrophilic, the first oxyethylene group attached to
the hydrophobic chain in surfactants of the type
OCH
2
CH
2
Þ
n
OSO
3
Na
þ
R
ð
where n
4, actually appears to behave in a manner suggestive of the addition of
approximately 2.5 methylene groups to the hydrophobic chain. Such anomalous
behavior might be attributed to changes in the solvation of the ether linkage brought
on by the close proximity of the highly solvated sulfate group. Succeeding OE
groups appear to have little or no significant effect on the hydrophobicity of the
molecule. Sulfated polyoxyethylene surfactants having n
<
>
4, as do most of the
commercially important members of this class, are usually composed of several
POE chain lengths and do not lend themselves to easy analysis of the effects of
the POE units.
In nitrogen-based cationic surfactants, it has generally been found that the pre-
sence of short-chain alkyl groups (fewer than four carbon atoms) attached to the
nitrogen has little apparent effect on the efficiency of adsorption of the molecule.
The dominant factor will always be the length of the primary hydrophobic chain.
That effect is true whether the alkyl groups are attached to a quaternary ammonium
group, an amine oxide, or a heterocyclic nucleus such as pyridine.
The nature of the charge on an ionic surfactant has a small effect on the effi-
ciency of surfactant adsorption. It is primarily the hydrophobic group that domi-
nates that characteristic. Some effect will, however, be seen if the counterion to
the primary charge is one that is highly ion-paired: that is, one that is not highly
solvated in the system and therefore produces a lower net electrical charge as the
molecules are adsorbed at the interface. The addition of neutral electrolyte to a
surfactant solution will produce a similar result in increasing the efficiency of
adsorption of a given ionic surfactant.
When one considers the efficiency of adsorption of nonionic surfactants, it must
be remembered that significant differences in the electrical nature of the hydrophilic
group can be expected to result in considerable change in the manner in which they
adsorb at a S/V interface. For POE surfactants with the same hydrophobic group
and an average of 7-30 OE units, the efficiency of adsorption at the solution-
vapor interface has been found to adhere to an approximately linear relationship
of the form
pC
20
¼
A
tr
þ
nB
ð
3
:
19
Þ
