O–W than at the A–W interface, the reason thatPtends to be higher at the
O–W interface. See Figure 10.14 for proteins.
- At highP, the relation becomes intricate. Many small-molecule
surfactants tend to give near vertical curves, for Na-stearate up tillP¼
37 mN?m^1 at an A–W interface.
When increasing thetemperature, Eq. (10.4) would predict thatP
increases withT. However, a temperature increase will generally cause a
decrease in G, as mentioned in Section 10.2.2, hence a decrease ofP.
Moreover, the interaction energy between molecules in the monolayer may
change. The net result cannot be easily predicted. Nevertheless,gtends to
decrease with increasing temperature, becauseg 0 will decrease.
10.3 SURFACTANTS
Surfactants can fulfil a wide variety of functions. Some of the most
important can be categorized as follows.
- They are essential in the formation of foams and emulsions
(Chapter 11). Some surfactants can also be used to destabilize emulsions or
foams. - Because they adsorb onto particles, they can alter the interaction
forces between these (Chapter 12). - By adsorption onto interfaces, they can greatly alter wetting
properties (Section 10.6). - Small-molecule surfactants can give rise to a series of association
colloids, including micelles, mesomorphic structures, vesicles, and micro-
emulsions. - They can promote ‘‘dissolution’’ of substances, either by uptake in
micelles of the surfactant (Section 10.3.1) or by adsorption onto small
particles. - Some small-molecule surfactants interact with proteins in solution
and cause partial unfolding of the latter.
A very important function is detergency, which is generally due to a
combination of functions 3 and 5.
The suitability of a surfactant for the various functions is greatly
dependent on its chemical structure. A crucial difference is that between the
following two types.
Fairly small amphiphilic molecules, sometimes called soaps (although
strictly speaking a soap is a salt of a fatty acid) or emulsifiers
(although they are used for a number of other purposes as well). In
this chapter we will use the word ‘‘amphiphile’’ for short. The molar
mass is generally<1 kDa. They are discussed in Section 10.3.1.