초록
<P>Switchable materials in general and CO<SUB>2</SUB>-switchable materials in particular are of great interest in environmental research. The replacement of common non-switchable materials (solutions, solvents, surfactants, <I>etc.</I>) with their switchable counterparts has a great potential to make processes more environmentally friendly by enhancing reusability and circularity and thus reducing energy costs and material consumption. Inspired by this, the present work deals with the surface and foaming properties of aqueous solutions of a non-switchable surfactant in presence of a CO<SUB>2</SUB>-switchable additive. A 1 : 1 and a 1 : 5 (molar ratios) mixture of the non-switchable surfactant C<SUB>14</SUB>TAB (tetradecyltrimethylammonium bromide) and the CO<SUB>2</SUB>-switchable additive TMBDA (<I>N</I>,<I>N</I>,<I>N</I>,<I>N</I>-tetramethyl-1,4-butanediamine) were investigated. It was found that surface properties, foamability, and foam stability can be changed by switching the additive with CO<SUB>2</SUB> as a trigger. This observation can be explained by the fact that TMBDA is surface active in its unprotonated, <I>i.e.</I> neutral form, which disturbs the tight packing of the surfactant molecules on the surface. As a consequence, foams generated with surfactant solutions containing the neutral TMBDA are less stable than their TMBDA-free counterparts. On the other hand, the switched diprotonated additive is a 2 : 1 electrolyte with hardly any surface activity and thus does not affect surface and foam properties.</P><P>Graphic Abstract</P><P>This study overturns the prevailing understanding of the effect of CO<SUB>2</SUB>-switchable water additives: the switching does not only alter the ionic strength of the solution but also the composition of the surface thus affecting foam stability.<BR/><IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=d3sm00273j'/><BR/></P>