Correlating molecular structure and surface forces in confined liquid films

Thin films are central structural elements of foams and valuable model systems for probing liquids under confinement. Classical methods relate film thickness to disjoining pressure but rarely resolve molecular origins such as film composition, adsorbed amounts, ion-specific effects, and interfacial structure. Combining complementary spectroscopic techniques with thin film pressure control should provide direct molecular-level insights into confined film structure and the forces governing film stability.

A Thin Film Pressure Balance capable of measuring surface forces was coupled with UV/Vis, infrared (IR), and Raman spectroscopies to characterise foam films under controlled confinement. IR spectroscopy enabled direct, quantitative, model-free determination of water core thickness. Raman spectroscopy probed both the surfactant and aqueous adlayers, revealing adsorbed amounts and structural changes upon confinement. Anion-specific effects and the influence of the alkyl chain length of films stabilised by alkyltrimethylammonium surfactants were systematically investigated by varying the halide counterions (Br^-, Cl^-, F^-) and chain length (C₁₂, C₁₄, C₁₆).

Surfactant surface excess and molecular orientation were independent of disjoining pressure across the studied range, whereas water structural features varied with confinement. Counterion identity strongly influenced monolayer dissociation, following the order Br^- < Cl^- < F^-, consistent with headgroup binding affinities. The combined spectroscopic approach resolved both core water and surfactant layer thicknesses without reliance on model assumptions. This methodology provides a powerful new route to interrogate molecular structure in confined films, extending the scope of foam film studies beyond macroscopic stability to the fundamental chemistry of interfaces.