Hydrophilic nano-porous model surfaces suitable for direct force measurements

• Hydrophilic PPFs exhibit high stability under different hydration conditions. • Surface forces of PPFs are analyzed using direct measurement techniques. • DPE-PPFs show expected force behavior in water and KNO3 of varying concentrations. • NPC-PPFs show complex repulsive force in liquids. • DPE-PPFs serve as a model system for direct force measurements. Molecular-level understanding of liquid–solid interfaces is key to the development of many physical, biological and chemical systems. In this context, functional plasma polymer films (PPFs) offer tunable surfaces ranging from hydrophilic to hydrophobic behavior. In this study, smooth hydrophilic PPFs with average surface roughness below 0.3 nm were fabricated from hexamethyldisiloxane (HMDSO) precursor by varying the substrate-plasma separation. The reference method consisted of direct plasma exposure (DPE) of mica and silicon substrates, while an alternative near-plasma chemistry (NPC) approach allowed film formation at a controlled distance, filtering out high-energy ions. Secondary ion mass spectroscopy revealed chemical and structural differences between the PPFs. Despite these, both films exhibited similar negative zeta potential in aqueous solution at neutral pH. The interfacial interactions at the PPF-liquid interface were accessed at different length scales using atomic force microscopy, colloidal probe microscopy, and the extended surface force apparatus. Both deposited films exhibited a degree of porosity and exceptionally low adhesion in water. DPE-PPF interactions were consistent with electrostatic double-layer theory, supporting its suitability as a model system for fundamental surface force studies. In contrast, NPC-PPF surfaces exhibited additional repulsive forces at 20 ± 10 nm in water, suggesting an additional mechanism beyond ordinary double-layer repulsion.