Effect of formation of calcium carbonate on foam stability in Neodol 25-3S anionic surfactant systems

The effect of oils and hardness on stability of foams made from solutions containing 0.01 wt% of three EO alcohol ethoxysulfate sodium salt of commercial anionic surfactant Neodol 25-3S was investigated. When dissolved calcium was present under alkaline conditions using Na2CO3 instead of NaOH as a pH regulator, solid CaCO3 precipitated. It was found in the absence of oil that CaCO3 particles did not destabilize foam, in contrast to those of the more hydrophobic calcium oleate. The rate of collapse of a foam column was also measured for foams generated from alkaline solutions of an anionic surfactant Neodol 25-3S containing dispersed drops of n-hexadecane, triolein, or mixtures of these oils with small amounts of oleic acid. Oils were added in increments up to oil to surfactant ratio of 1 on a weight basis and the hardness and pH of the aqueous solutions were fixed at 300 ppm and 9 respectively. It was found that triolein has almost no defoaming effect, but the defoaming effect of hexadecane was evident. The same trend was found with mixtures of n-hexadecane/triolein with addition of small amounts of oleic acid. The results of foam stability measurement in the presence of oil could be understood in terms of entry, spreading and bridging coefficients, i.e., ESB analysis. Moreover, as the initial concentration of carbonate ions increased, foam stability was greatly improved for both hexadecane/oleic acid and triolein/oleic acid oils. Evidently, CaCO3 precipitated preferentially, reducing the number of calcium ions available to form calcium oleate.