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

Since hydrophobic surfaces are by definition “water hating”, one might assume from a simplistic perspective that solvated ions in the water would avoid a hydrophobic surface. While this might be true for idealized solid hydrophobic surfaces, 3 it is certainly not the case for solvated ions near more complex biological assemblies where ions not only approach but also transport across these boundary layers. Unraveling the various factors that influence ion behavior in these complex biological systems relative to their idealized solid systems is not an easy task. For example, what roles do the polar groups of a biological assembly play, or that the more fluid nature of the hydrophobic boundary layer plays? At the air/ water and solid/liquid interfaces, the polarity of adsorbates such as proteins andmacromolecules alters the structure of interfacial water and affects ion adsorption behavior. 6 Recent studies of aqueous ionic solutions near simple hydrophobic liquids are shedding light on the effect of ions on water at an extended hydrophobic fluid interface. 11 From these spectroscopic and computational studies 11 it is clear that inorganic ions do penetrate into the aqueous/hydrophobic liquid interface. Experimental support for these conclusions comes from spectroscopically observed changes of the molecular orientation and hydrogen bonding of interfacial water molecules when ions are present at the liquid/liquid interface. These studies show that the weak interactions between water and the hydrophobic liquid assist in the formation of an interfacial potential that creates an environment that can draw ions into the interfacial region. The behavior is found to be quite distinct from what has been observed for interfacial water in similar experimental 19 and computational studies conducted at the air/water interface. 30 Comparison of the experimental results obtained at the air/water and CCl4/water interface using similar ions and techniques show that there is a greater tendency for the ions to go to the organic liquid/water interface than the air/water interface. The case has recently been made that other ions, such as OH , are also attracted to hydrophobic liquid interfaces because of the stabilization afforded by the approach of the OH ions, which reduces the dielectric constant at the interface and the dipole fluctuations. These ions are estimated to have a density maximum some 6 7 Å from the Gibbs dividing surface at the liquid/ liquid and air/water interfaces. Recently, there has been an increased interest in using hydrophobic self-assembled monolayers (SAMs) chemically attached to a silica substrate as model systems for understanding water and ion behavior at more complex soft hydrophobic surfaces. If the behavior of water molecules at the SAM/water junction is largely determined by the nature of the water/ hydrophobic interactions at the terminus of the monolayer, then one can use such SAM/water systems as a model for exploring a