Inorganic Salts as Chemical Foam Suppressors

Can the foamability of surfactant aqueous solutions be controlled chemically? Well-known antifoams can prevent foaming by inducing the coalescence of the bubbles, but can the surfactants be deactivated chemically? If yes, how does this affect the surface tension of their aqueous solutions and their foaming capacity? To shed a light on these fundamental questions, we chose a well-known surfactant containing in its molecule a sulfate group (Sodium dodecyl sulfate, SDS) and mixed it with BaCl2, (the solubility of BaSO4 is 0.245 mg/100 mL water, T = 20 °C), Pb(NO3)2 (the solubility of PbSO4 is 40.4 mg/100 mL water, T = 25 °C) and FeCl3 (the solubility of Fe2(SO4)3 is 25.6 g/100 mL water, T = 20 °C) at different molar ratios (MXn/SDS): 1/2, 1/1, 2/1, 4/1. The results were surprising: in the case of BaCl2, despite being in stoichiometric molar ratio with SDS (BaCl2 + 2SDS -> Ba(DS)2 + 2 NaCl), or in excess of BaCl2, which should convert the whole amount of SDS into a sediment, the surface tension value remained significantly lower than that of the single surfactant. At the same time, foamability was either low or absent. It therefore appears that all of the surfactants should be converted into a sediment with very small solubility, but the low surface tension indicates the opposite. The lack of foamability indicated the opposite of that opposite. With Pb(NO3)2 and FeCl3, the results are even stranger. The surface tension values are substantially smaller than those of the single surfactants, and at the same time, low foamability or lack of foamability was observed. It appears that the surfactant exists and at the same time does not exist in the aqueous solution. Where is the truth? Future studies will shed a light.