Foams stabilized by superchaotropic ions: The birth, life, and collapse of unconventional systems

The adsorption of macro ions onto interfaces covered by neutral surfactants can provide electrostatic stabilization of liquid foams, analogous to conventional ionic surfactants. Here, we investigate how the adsorption of superchaotropic Keggin nano-ions affects both the macroscopic and microscopic structure of liquid foams and their stability.

Foams were prepared using a nonionic surfactant supplemented with either ionic surfactants or nanometer-sized superchaotropic ions at varying molar ratios. Multi-scale foam structural features, including foam height, liquid fraction, bubble size distribution, and thin film thickness, were monitored using an advanced experimental setup combining time-resolved small-angle neutron scattering (SANS) and optical imaging. This approach provides a comprehensive structural picture of the foam and its ageing mechanisms.

Superchaotropic ions stabilize foams as effectively as ionic surfactants, but produce foams with larger initial bubbles due to delayed adsorption at the interface; the resulting quicker liquid flow results in an overall dryer foam. Despite differences in valency and chemistry, the equilibrium foam film thickness converges around 30 nm. Comparison with isolated thin-film measurements suggests that the equilibrium of foam films in three-dimensional foams is governed by additional factors beyond those captured in conventional thin-film experiments. The study demonstrates that nano-ions provide a novel route toward stimuli-responsive foam stabilization, highlights the critical role of interfacial adsorption dynamics in controlling foam properties at all relevant stages, from initial formation to evolution and ultimately collapse, and reveals subtle yet fundamental differences between the behavior of isolated thin films and foam films embedded within three-dimensional foams.