The role of surfactant structure on foam behaviour

The foamability of polypropylene glycol (PPG) surfactants was investigated as a function of PPG concentration and molecular weight and compared with their surface activity. It was found that foamability increases with the amount of PPG adsorbed at the air–water interface up to monolayer coverage, but decreases at high concentrations due to the formation of PPG droplets, which destabilise the foam. Although high molecular weight PPGs have higher surface activity, foamability decreases with increasing PPG molecular weight because of their slower diffusion to, or at the air–water interface. The effect of the hydrophobicity or polarity of the end groups of PPG on foamability was also investigated, and it was found that foamability increases only if the PPG molecule is charged. The presence of salt destabilises the PPG foams.