CO2-Switchable Additive and Surfactants

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.