Comparisons of The Structure of Water at Neat Oil/Water and Air/Water Interfaces as Determined by Vibrational Sum Frequency Generation

We have employed vibrational sum frequency generation (VSFG) to investigate the structure of water at neat oil/water and air/water interfaces through the OH stretching modes of the interfacial water molecules. We find that at the oil/water interface the prevailing structure of the water molecules is a tetrahedral arrangement much like the structure of ice while at the air/water interface we observe an equal distribution between an ice-like and a less ordered water-like arrangement. The relationship between the structure of the interfacial water molecules and the properties of the other fluid (air or oil) is discussed in terms of hydrogen bonding at the interface. We also compare our results to previous VSFG studies at similar interfaces and find a difference in the spectra obtained from the oil/water interface. Introduction There are numerous examples of processes that are affected by the structural features of interfacial water molecules. 1 Among the most important are solvent extraction, charge transfer across liquid/liquid interfaces, stability of membranes and the activity of proteins in an aqueous environment. In recent years an increasing amount of experimental^-^ and theoretical^ H work has been performed in an effort to better understand the structure of water molecules at a variety of interfaces. We have employed the interface sensitive technique of vibrational sum frequency generation (VSFG) to probe the structure of water molecules at the air/water and oil/water interface. Our studies show that there is a striking difference between the VSFG spectra obtained from these interfaces. From this difference we infer that nominally all the water molecules at the oil/water interface are in a tetrahedral arrangement much like the structure of ice whereas at the air/water interface only about 60% of the water molecules are in the same arrangement with the remaining water molecules being in a more random and bond-disordered arrangement presumed to be more water-like. This observation is explained in terms of the structural changes liquid water must make in order to solvate non-polar oil molecules. The VSFG process has been explained in detail elsewhere4>12,13 thus the only the important components will be discussed here. First, under the dipole approximation VSFG is inherently interface selective due to the fact that it is a second order nonlinear process. Secondly, VSFG allows us to obtain a vibrational spectrum of the interfacial molecules. This ability arises from the fact that the sum frequency response is proportional to the square of the material dependent nonlinear susceptibility %. % can be separated into a non-resonant, XNR' ^ resonant, XRV' portion, the former being independent of IR frequency and the latter dependent on IR frequency as follows sfg~ V %NR + ^ (2) %R e^v Wir CD where Isfg is the sum frequency intensity generated at cosfg, yv is the relative phase of the v th vibrational mode, Ivis and Iir are the visible and IR intensities, and the sum is taken over all vibrational modes v of the molecules at the interface. Since the susceptibility is in general complex the resonant terms in the summation are associated with a relative phase yv which is used to account for any interference between two modes which overlap in energy. The resonant portion of % can be expressed such that the dependence on the IR frequency is