11 · SOLUTIONS AND SOLUBILITYEthanoic acid is completely miscible with water in all proportions. This means that
ethanoic acid will never form two layers with water no matter how much acid is present.
However, hexanoic acid is virtually insoluble in water. Although both acids contain the
highly polar C=O and –O–H groups, the long carbon chain of the hexanoic acid
molecule increases the size of the London dispersion forces between the acid molecules;
this causes the acid molecules to stick together. Even the possibility of hydrogen bond-
ing between acid and water molecules is not enough to make hexanoic acid soluble in
water, and the only hydrogen bonding present occurs between acid molecules.174
Effect of chain length upon solubility
Look at this list of alcohols and their formulae:
butan-1-ol CH 3 CH 2 CH 2 CH 2 OH
hexan-1-ol CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 OH
nonan-1-ol CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 OH
octan-1-ol CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 OH
Which of these alcohols is the most soluble in water?Exercise 11C
Hydrophilic and hydrophobic ends of large organic
molecules
We can look at large organic molecules in an even simpler way, by thinking of
organic molecules such as hexanoic acid as consisting of a polar ‘head’ (the –COOH
part) and a zigzag hydrocarbon ‘tail’ (the –CH 2 – chain), as shown in Fig. 11.2. The
polar heads are attracted to water and are said to be hydrophilic(water loving),
whereas the tails attract each other by London dispersion forces (and so pull away
from water molecules) and are said to be hydrophobic(water hating).
The application of these ideas to soap action (detergency) is discussed in Box 11.1.
Detergency is an example of emulsification(orsolubilization) in which fats are
prevented from forming two layers in water-based mixtures.Fig. 11.2The arrangement of
molecules in a mixture
consisting of a small quantity
of hexanoic acid mixed with
water. Two distinct phases are
produced, with the less dense
acid floating on top.