‘‘adsorb,’’ albeit at some distance from the interface. However, the
theory is intricate and there is no complete consensus. Since the
potential error is generally small, except at very low ionic strength,
we will not discuss the point.
Another variable of a surfactant is itsHLB value, which is a measure
of the balance of hydrophobic and hydrophilic parts of the molecule. Some
values are given in Table 10.2. If HLB is large, the substance is well soluble
in water; if small, it is well soluble in oil. For HLB&7, the solubility is
about equal in both phases and generally not very high. The HLB value is
smaller for a longer or a more saturated aliphatic chain (compare the
various Spans in Table 10.2), and it is larger for a more polar or bigger polar
head group. Ionic surfactants always have HLB values>7. A head group
with poly(oxyethylene) chains also causes a high HLB value, although it
markedly decreases with increasing temperature. This is because the
oxyethylene groups become dehydrated at high temperature.
Surfactants also differ in the lowestinterfacial tensionthey can give.
For most amphiphiles, finalgat the A–W interface ranges between 35 and
42 mN?m^1. At the triglyceride oil–water interface the variation is relatively
larger, becauseg 0 is smaller andPis, in first approximation, the same as for
an A–W interface. Typical results are between 3 and 5 mN?m^1 , but smaller
values can be obtained for some mixtures of surfactants. An equimolar
mixture of a Span and a Tween, for instance, can give a higher surface excess
than either of them, and this then leads to a lowerg. Addition of salt tends
to give a lowergfor ionic surfactants, as mentioned above; a similar but
smaller effect has been observed for several nonionics.
An important function of amphiphiles isdetergency. Most detergents
are ionic surfactants, because these readily form micelles. Micelles can
accommodate hydrophobic molecules in their interior, and this is what
happens during washing processes. They can thus ‘‘solubilize’’ oil, though to
a limited extent; the amount is proportional to and of the same order as the
concentration of micelles. In such a way, oil-soluble vitamins or flavor
substances can be dispersed in water. Something similar can happen in an oil
that contains surfactants, where reverse micelles can form; see Figure 10.9a.
These contain in general fewer surfactant molecules than do ‘‘regular’’
micelles. The water inside can ‘‘solubilize’’ water-soluble substances, such as
proteins, in the oil. The latter has been used to achieve enzyme action on
substances dissolved in oil.
singke
(singke)
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