Correlating molecular structure and surface forces in confined liquid films

Eric Weißenborn; Michael Hardt; Björn Braunschweig; Eric Tyrode

doi:10.1016/j.jcis.2025.139419

Correlating molecular structure and surface forces in confined liquid films

J Colloid Interface Sci. 2026 Feb 15;704(Pt 2):139419. doi: 10.1016/j.jcis.2025.139419. Epub 2025 Nov 7.

Authors

Eric Weißenborn¹, Michael Hardt¹, Björn Braunschweig², Eric Tyrode³

Affiliations

¹ Institute of Physical Chemistry and Center for Soft Nanoscience, University of Münster, Corrensstraße 28/30, 48149 Münster, Germany.
² Institute of Physical Chemistry and Center for Soft Nanoscience, University of Münster, Corrensstraße 28/30, 48149 Münster, Germany. Electronic address: braunschweig@uni-muenster.de.
³ Department of Chemistry, KTH Royal Institute of Technology, Teknikringen 30, 11428 Stockholm, Sweden. Electronic address: tyrode@kth.se.

PMID: 41253078
DOI: 10.1016/j.jcis.2025.139419

Abstract

Hypothesis: 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.

Experiments: 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).

Findings: 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.

Keywords: Alkyl chain dependence; Anion specific effects; Confinement; Disjoining pressure; Foam films; IR spectroscopy; Ion specific effects; Molecular structure; Raman spectroscopy; Surface forces.