The Unique Molecular Behavior of Water at the Chloroform—Water Interface

Abstract

The molecular bonding and orientation of water at the chloroform-water interface has been examined in this study using vibrational sum-frequency spectroscopy (VSFS). The results provide a key puzzle piece towards our understanding of the systematic changes in the interfacial bonding and orientation of water that occur with variations in the polarity of the organic phase, especially when compared with previous studies of different liquid-liquid interfacial systems. In these VSFS studies the OH spectral responses of interfacial water molecules are used to characterize the interactions between water and the organic phase. The spectral analysis, aided by isotopic dilution studies, shows that the moderate polarity of the chloroform phase results in a mixed interfacial region with stronger organic-water bonding and fewer bonding interactions between adjacent water molecules than was previously found for studies of non-polar organic liquid-water interfaces. Even with the more mixed interfacial region and stronger organic-water interactions, interfacial water retains a significant amount of orientational ordering. These results are compared with recent predictions from molecular dynamics simulations about how molecules behave at the chloroform-water interface.

The Water-Hydrophobic Interface: Neutral and Charged Solute Adsorption at Fluorocarbon and Hydrocarbon Self-Assembled Monolayers (SAMs)

“…For these water molecules, one OH bond is oriented toward the FDS surface (free OH), with which it weakly interacts, and the companion OH protrudes into the aqueous phase and is red shifted from the free OH due to its bonding interactions with other interfacial water molecules. Such sharp peaks were previously reported at the air-water 46,60 , organic liquid-water, 61,–64 and solid-water interfaces. 10,65,66 The frequency of this peak is a measure of the strength of the interaction between water and the hydrophobic phase.…”

Role(s) of adsorbed water in the surface chemistry of environmental interfaces

“…Isotope scrambling data suggested that CO 2 reacted with adsorbed water to yield adsorbed carbonate and protonated surface groups, M-OH 2 + , via a proposed carbonic acid intermediate, H 2 CO 3 , 88 a rather elusive chemical species albeit an important one (M -metal atoms on the surface). The proposed reaction mechanism is shown in eqn ( 18) through (21). Quantum chemical calculations supported the formation of solvated carbonate ions on the surface.…”

“…These differences further control the thermodynamics of solvation, stability, permeability, reactivity and other molecular properties of water at the interface. 14 Recent studies by Shamay et al and others 20,21 have highlighted the influence of hydrogen bonding on the orientation of surface water molecules in the presence of adsorbed gases (e.g. CO 2 and SO 2 ) and organic molecules (e.g.…”

“…Sum frequency generation (SFG) is an important non-linear, laser-based tool that can be used to investigate environmental interfaces, even so-called buried interfaces, due to the fact that SFG intensity arises from noncentrosymmetric environments, such as at interfaces, and vanishes in the bulk phases where molecules experience a centrosymmetric environment. 21,33,34 The published literature is rich with numerous such studies where vibrational SFG is employed to probe structure and dynamics at aqueous interfaces. 35,36 In a recent study, Allen and co-workers were able to confine a unique type of water bond, the dangling O-H bond of surface adsorbed water, in a twodimensional environment, using vibrational SFG spectroscopy at the air/H 2 O/monolayer interface.…”

Ordered polyelectrolyte assembly at the oil–water interface

“…We know, for example, that weak bonding interactions between interfacial water and various hydrophobic oils is a general trait for these oil-water systems, resulting in significant molecular orientational ordering and structuring of both phases near the interfacial region (13)(14)(15). The existence of an interfacial electric field created by this structuring has been shown to facilitate the adsorption of simple electrolyte ions at the interface (15), surfactant adsorption, and also the penetration of oriented water into the organic phase (16).…”