SAMs under Water: The Impact of Ions on the Behavior of Water at Soft Hydrophobic Surfaces

Understanding the behavior of water at hydrophobic surfaces has been a topic of much interest for many decades. In most areas of biological, environmental, or technological relevance, the aqueous phase is not pure water, but comprises a host of ions including those associated with the acidity or basicity of the solution. The notion that ions, including hydroxide and/or hydronium, accrue at hydrophobic interfaces is increasingly invoked as a possible explanation for the behavior of water adjacent to soft hydrophobic interfaces such as liquids and monolayers. The focus of this study is on exploring the behavior of aqueous solutions of salts, acids and bases in contact with hydrocarbon and fluorocarbon self-assembled monolayers (SAMs) using vibrational sum frequency spectroscopy (VSFS). The studies take a systematic approach to understanding how each component of the SAMs’ interfaces contribute to the overall observed behavior of ions and water in the overall boundary region. To achieve this, the spectroscopy of the SAM/water interface in the presence and absence of aqueous phase ions, acids and bases is compared with similar measurements taken at the substrate (SiO2)/water interface and the hydrophobic liquid/water interface. The results show that the behavior of water and ions at the SAM/aqueous interface is significantly influenced by the substrate surface for both hydrocarbon and fluorocarbon SAM systems. Conditions where water and ions near a SAM interface mimic that of a liquid hydrophobic surface are identified.