Anti-foam action of oil vapours

Alkane vapours adsorb onto surfactant monolayers at the air–water surface to form mixed alkane–surfactant films. We have determined the effects of alkane vapours on the foamability and foam stability for aqueous foams stabilised by non-ionic alkyl ethoxylate surfactants of different head and tail chain lengths. Foamability was determined by measuring the volume of foam generated by bubbling nitrogen gas, with and without oil vapour, through the surfactant solution as a function of surfactant concentration. In the absence of oil vapour, the surfactant concentration C1/2 (corresponding to the transition from non-foaming to foaming behaviour) is less than the critical micelle concentration (cmc) for short chain surfactants and greater than the cmc for long chain surfactants. The addition of alkane vapours within the gas stream during foam formation increases C1/2 to concentrations in excess of the cmc, i.e. oil vapours inhibit foamability. In addition to the effects on foamability, oil vapours also accelerate the decay rates of foams derived from plots of foam volume versus time.