Effect of Subphase Ions on Multilayer Deposition of an Anionic Gemini Surfactant on Solid Substrates

ABSTRACT A recently reported anionic Gemini surfactant belonging to the “minimal linker” family has been shown to form closely packed crystalline monolayers at the air–water interface, like a simple fatty acid, but its ability to deposit onto solid substrates as multilayer films remains entirely unexplored. In this study, we have investigated multilayer deposition of this new surfactant, dubbed C 18 ‐0‐C 18 , onto solid substrates and explored the impact of various subphase cations, including Na + (aq) , Mg 2+ (aq) , and Zn 2+ (aq) on multilayer film formation. Multilayer deposition was assessed using a combination of approaches, including evaluation of surface pressure‐area isotherms, surface pressure kinetic stability, transfer ratio (TR) measurements, X‐ray Reflectivity (XRR), and Atomic Force Microscopy (AFM). Measurements of deposited films were further supported by measuring film structures directly at the air‐water interface using Brewster Angle Microscopy (BAM). TR, XRR, and AFM results indicated poor multilayer deposition from both a pure water and Mg 2+ enriched subphase, but effective multilayer formation from Zn 2+ and Na + subphases. The mechanism of multilayer deposition (or lack thereof) is discussed in the context of ion‐head group interactions and compared with results of film deposition from corresponding monomeric fatty acids described in the literature.