Switchable materials in general and CO2-switchable materials in particular are of great interest in environmental research. Inspired by this, the present work deals with the surface and foaming properties of (a) aqueous solutions of a non-switchable surfactant where a CO2-switchable additive is added and (b) aqueous solutions containing CO2-switchable tail surfactants which are a subclass of CO2-switchable surfactants.
For studying (a) CO2-switchable additives, a 1:1 and a 1:5 (molar ratios) mixture of the non-switchable surfactant C14TAB (tetradecyltrimethyl-ammonium bromide) and the CO2-switchable additive TMBDA (N,N,N,N-tetramethyl-1,4-butanediamine) were used. It was found that surface properties, foamability, and foam stability can be changed by switching the additive with CO2 as a trigger. This is due to the surface activity of TMBDA in its neutral, i.e. unprotonated, form which disturbs the tight packing of the surfactant molecules and thus increases the average head group area. As a consequence, foams generated in the presence of N2 are less stable than the TMBDA-free counterparts. The diprotonated form of TMBDA, on the other hand, is a 2:1 valuable electrolyte with hardly any surface activity which affects surface but not foam properties.
For studying (b) CO2-switchable tail surfactants, four potential surfactants were originally planned to be investigated, however, only the anionic surfactant C8N(Me)C4SSn (sodium 4-(methyl(octyl)amino)butane-1-sulfonate) shows promising switchable surface and foaming properties. In the presence of N2, the unprotonated surfactant is able to stabilize foams. By protonating the CO2-responsive trialkylamine group in the surfactants hydrocarbon chain, the amphiphilic nature of the surfactant is reduced, i.e. its ability to stabilize foams is reduced.