Foamability of fatty acid solutions and surfactant transfer between foam and solution phases

The foaming properties of aqueous solutions of fatty acids with different hydrocarbon chain lengths - hexanoate, laurate, and oleate - were assessed through measurements of foam volume and growth rate under continuous aeration. Simultaneous measurements of the partition of each surfactant between the bulk solution and foam phases were also carried out. Although bubble size measurements in bulk solution pointed towards quite similar gas dispersing abilities of the tested surfactants, their foamabilities under the same conditions were remarkably enhanced with increasing chain length of the hydrocarbon tail. Shorter chain surfactants - hexanoate and laurate - generated wet foams and the accumulation of these surfactants in the foam phase was very low, indicative of a low adsorption density of the reagents at the gas-liquid interface. In contrast, sodium oleate produced stable dry foams, and the amount of oleate transferred to the foam was much higher. The formation of these different types of foam was attributed to the higher adsorption of oleate at the gas-liquid interface compared to the lower adsorption of hexanoate or laurate, and to different mechanisms of foam stabilization. Regardless of the interfacial mechanism of the foaming action of the tested surfactants, it was generally found that strong foaming was possible only from surfactant solutions whose bulk concentration was higher than the critical coalescence concentration of a given surfactant. It was concluded that bubbles entering the foam phase from the bulk solution phase must already be stable against coalescence to induce foam formation and impart stability to the entire foam column. Foaming was observed only in true solutions of the tested surfactants, while precipitation of colloidal acid species completely depressed foam formation.