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

Abstract

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. † 2008 marked the Centennial of the American Chemical Society's Division of Physical Chemistry. To celebrate and to highlight the field of physical chemistry from both historical and future perspectives, The Journal of Physical Chemistry is publishing a special series of Centennial Feature Articles. These articles are invited contributions from current and former officers and members of the Physical Chemistry Division Executive Committee and from J. Phys. Chem.

Assembly and molecular order of two-dimensional peptoid nanosheets through the oil–water interface

“…This is demonstrated by the lack of a free-OH oscillator peak near 3,670 cm −1 , indicating that water molecules in the topmost interfacial layer are not present (29). There is also strong evidence from the VSF spectrum that the interfacial peptoid layer is highly ordered.…”

Is it the shape of the cavity, or the shape of the water in the cavity?

“…The structure of molecules of water at an interface with air, [86,95] a non-polar liquid, [96][97][98] or a solid surface presenting hydrophobic functional groups [99][100][101] share a commonality: the layer of water in direct contact with the non-water surface is xenophobic, and the water molecules it contains participate in fewer hydrogen bonds, on average, than water in the bulk; this layer of water is ~40% less dense [102] than water in the bulk. Molecules of water one layer away from the non-water surface have a structure similar to bulk water, and participate in the DDAA ( Figure 4A) pattern of hydrogen bonding.…”

Protein electron transfer: Dynamics and statistics

“…The result is an electrostatically patchy surface of the protein, [7][8][9] producing, in turn, a patchwork of polarized and unpolarized domains of water facing it. 10,11 Water is known to orient its dipoles parallel to the dividing surface at hydrophobic patches 12,13 and flip its dipoles inward or outward depending on the surface charged group. [14][15][16] These heterogeneously polarized, heterogeneously compressed, [17][18][19] and potentially mutually frustrated interfacial water domains merge into an interfacial sub-ensemble involving several hundreds of water molecules, with its properties quite distinct from the bulk.…”