From Franklin to Today: Toward a Molecular Level Understanding of Bonding and Adsorption at the Oil−Water Interface

With Benjamin Franklin’s oil on water experiments as a historical example, people have long been fascinated with the physical characteristics of the interface between water and an organic liquid and the unique chemistry that can occur at that interface. In this paper, we present our current understanding of the structure, orientation, and bonding characteristics of this fluid and dynamic interfacial region based on the efforts in our laboratory over the past decade and the important research of others in this field. In our studies, in which we have used a combination of surface specific nonlinear vibrational spectroscopy in conjunction with molecular dynamics simulations, we find that a general feature of organic−water interfaces is that of weak bonding interactions between adjacent water molecules and between the water and organic molecules. These weak water−organic interactions, present at all of the interfaces that we have studied, result in significant interfacial structuring and molecular orientation on both sides of the interface. How the structuring of both the interfacial water and organic molecules is dependent on the nature of the organic media is discussed as well as how this interfacial structuring can facilitate molecular and ion transport across the interface. The discussion of the neat organic−water interface is followed by a summary of how this picture is changed with the addition of ions, surfactants, and biomolecules, and how the presence of the organic media plays a role in the adsorption and conformation of adsorbates relative to the vapor−water interface. Examples of recent applications for the oil−water interface to synthesis and future perspectives are discussed as well.