Repulsive, but sticky - Insights into the non-ionic foam stabilization mechanism by superchaotropic nano-ions

Hypothesis: The superchaotropic Keggin polyoxometalate alpha-SiW12O404 (SiW) was recently shown to stabilize non-ionic surfactant (C18:1E10) foams owing to electrostatic repulsion that arises from the adsorption of SiW-ions to the foam interfaces. The precise mechanism of foam stabilization by SiW however remained unsolved. Experiments: Imaging and conductimetry were used on macroscopic foams to monitor the foam collapse under free drainage and small angle neutron scattering (SANS) at a given foam height allowed for the tracking of the evolution of film thickness under quasi-stationary conditions. Thin film pressure balance (TFPB) measurements enabled to quantify the resistance of single foam films to external pressure and to identify intra-film forces. Findings: At low SiW/surfactant ratios, the adsorption of SiW induces electrostatic repulsion within foam films. Above a concentration threshold corresponding to an adsorption saturation, excess of SiW screens the electrostatic repulsion that leads to thinner foam films. Despite screened electrostatics, the foam and single foam films remain very stable caused by an additional steric stabilizing force consistent with the presence of trapped micelles inside the foam films that bridge between the interfaces. These trapped micelles can serve as a surfactant reservoir, which promotes self-healing of the interface leading to much more resilient foam films in comparison to bare surfactant foams/films. (c) 2023 Elsevier Inc. All rights reserved.

Automated summary

In recent research, it was found that the superchaotropic Keggin polyoxometalate alpha-SiW12O404 (SiW) can stabilize non-ionic surfactant foams through electrostatic repulsion. The precise mechanism of foam stabilization by SiW remained unresolved. Imaging and conductimetry were used to monitor the foam collapse, while small angle neutron scattering (SANS) tracked the evolution of film thickness. Thin film pressure balance (TFPB) measurements quantified the resistance of single foam films to external pressure and identified intra-film forces. The findings showed that SiW induces electrostatic repulsion within foam films at low SiW/surfactant ratios, and above a concentration threshold, excess SiW screens the electrostatic repulsion, resulting in thinner foam films. Despite this, the foam and single foam films remained stable due to an additional steric stabilizing force from trapped micelles inside the foam films, which acted as a surfactant reservoir and promoted self-healing of the interface, leading to more resilient foam films compared to bare surfactant foams/films.