Transient foaming behavior of aqueous alcohol solutions as related to their dilational surface properties

The foamability of aqueous alcohol solutions can be measured by continuously sparging air in a closed loop through the solution under standard conditions. The amount of foam produced in the steady state is taken as a measure for the foamability. Aqueous alcohol solutions show transient foaming behavior. This can be explained by assuming that, due to lack of disjoining pressure, thin aqueous alcohol films collapse when due to drainage a critical film thickness has been reached. This means that the drainage rate of these films determines to a large extent the lifetime of these films. Film drainage can be slowed down by surface tension gradients opposing the liquid flow in the film. The ability of aqueous alcohol solutions to generate a surface tension gradient can be measured by means of the overflowing cylinder technique. By changing the alcohol concentration for ethanol, 1-butanol, 1-hexanol, and 1-octanol it is found that both the foamability and the ability to generate a surface tension gradient pass over a maximum value at an alcohol concentration which is characteristic for the alcohol considered. This alcohol concentration coincides with the alcohol content at which a maximum in the ability to generate a surface tension gradient is found. It is concluded that the transient foaming behavior of these aqueous alcohol solutions is mainly determined by their ability to generate a surface tension gradient