Session P31: Water at Interfaces: From Spectroscopy Techniques to Computer Simulations

Water at surfaces with tunable surface chemistries and the chiral imprint of water around DNA.

Aqueous interfaces are ubiquitous in atmospheric chemistry and biological systems but are notoriously hard to probe experimentally. Surface-specific vibrational spectroscopy offers an avenue to directly probe the vibrational modes of the water OH stretching band but this method is challenging to implement to buried surfaces. Here we present results from sum-frequency generation (SFG) spectroscopy probing the buried interface between a functionalized surface and aqueous solutions. Studying such buried surfaces offers the advantage of being able to systematically tune the surface chemistry using self-assembled monolayers, i.e. the hydrophobic and hydrophilic character, and examine the effect on the interfacial water. In addition to water at these controlled surfaces, we have initiated studying water at biological surfaces. This includes the solvation structure around DNA. X-ray experiments at cryogenic temperatures have found crystallographic water in the minor grove of DNA giving rise to the notion of a spine of hydration surrounding DNA. Such structured water should exhibit a chiral structure adapted from DNA. We investigate if such a chiral water structure exist around DNA at room temperature using chiral SFG.   

Molecular Views of Water at the Water/Air and Water/Lipid Interface

At the surface of water, the water hydrogen-bonded network is interrupted, conferring properties on interfacial water different from bulk water. We elucidate, using a combined experimental and computational molecular dynamics approach, how the water hydrogen bond network is terminated at a phospholipid interface, and how this is different from conventional surfactant interface [1]. Moreover, for the water/air interface, we show that the evaporation of water -- i.e. the release of individual water molecules from the bulk into the gas phase -- is not a purely stochastic event. Rather, the evaporation follows one specific pathway, involving a delicately timed, concerted motion of several water molecules to `launch' a single molecule from the surface [2]. [1] Lipid Carbonyl Groups Terminate the Hydrogen-Bond Network of Membrane-Bound Water, T. Ohto, E.H.G. Backus, C. Hsieh, M. Sulpizi, M. Bonn, Y. Nagata, J. Phys. Chem. Lett. J. Phys. Chem. Lett. 6, 4499$-$4503 (2015). [2] Molecular Mechanism of Water Evaporation, Nagata, Y.; Usui, K.; Bonn, M., Phys. Rev. Lett. 2015, in print.   

Correlation between Pyroelectricity and Alignment of Interfacial Water.

In this work we investigate the connection between arrangement of water and pyroelectricity. Before the current work, pyroelectricity was attributed only to polar materials. Nevertheless, nonpolar Amino acid crystals and Yttrium doped Barium Zirconate ceramics exhibit pyroelectricity. Experimental results with MD simulation suggest that the source of pyroelectricity is polar arrangement of water molecules at the crystal surface, which leads to the formation of a deformed polar layer in the crystal. This makes the surface pyroelectricity an important surface characterization tool. Another phenomenon suggests that the converse effect to surface pyroelectricity is also exists i.e. alignment of water by pyroelectricity. We demonstrated that polar crystals in general and specifically positive pyroelectric charge can catalyze the freezing of supercooled water (SCW). Our studies show that pyroelectric effect increases the freezing point of SCW by 2 to 8 degrees. The fact that the freezing point is correlated to the amount of the surface charge together with the relative low electric field, implying that the surface charge aligns the interfacial water molecules or stabilizes sub-critical ice nuclei.   

High-resolution imaging and spectroscopy of interfacial water at single bond limit.

Hydrogen bond is one of the most important weak interactions in nature and plays an essential role in a broad spectrum of physics, chemistry, biology, energy and material sciences. The conventional methods for studying hydrogen-bonding interaction are all based on spectroscopic or diffraction techniques. However, those techniques have poor spatial resolution and only measure the average properties of many hydrogen bonds, which are susceptible to the structural inhomogeneity and local environments, especially when interfacial systems are concerned. The spatial variation and inter-bond coupling of the hydrogen bonds leads to significant spectral broadening, which prohibits the accurate understanding of the experimental data. In this talk, I will present our recent progress on the development of new-generation scanning probe microscopy/spectroscopy (SPM/S) with unprecedentedly high sensitivity and resolution [1,2], for addressing weak inter- and intra-molecular interactions, such as hydrogen bonds and van der Waals force. Based on a qPlus sensor, we have succeeded to push the real-space study of a prototypical hydrogen-bonded system, i.e. water, down to single bond limit. Combined with state-of-the-arts quantum simulations, we have discovered exotic nuclear quantum effects (NQEs) in interfacial water and revealed the quantum nature of the hydrogen bond from a completely new perspective [3]. [1] J. Guo et al., Nature Materials 13, 184 (2014). [2] J. Chen et al., Nature Communications 5, 4056 (2014). [3] X. Meng et al., Nature Physics 11, 235 (2015).   

Water at lipid and surfactant interfaces---structure, dynamics, and spectroscopy

I will consider water confined by reverse micelles, multi-bilayers, and gyroid phases of a number of lipids and surfactants. Theoretical and simulation results will be compared with several non-linear IR experiments, including 2DIR and pump-probe anisotropy. In addition, I will discuss water near lipid and surfactant monolayers, especially making the connection with SFG and 2DSFG experiments.   

Orientational dynamics of water at an extended hydrophobic interface

Aqueous interfaces are central to many physical processes, but the dynamics of interfacial water molecules have been little studied. We have measured the orientational dynamics of water at its interface with a self-assembled monolayer of octadecylsilane on fused silica. A surface-sensitive sum-frequency probe generated by mixing a visible and a vibrationally resonant infrared (IR) pulse is used to monitor the dangling (non-hydrogen-bonded) OH stretch vibration after excitation with a resonant IR pump pulse. By measuring pure and isotopically diluted water with orthogonal pump polarizations, we find that relaxation of the dangling OH stretch excitation is dominated by the out-of-plane jump from a dangling to a hydrogen-bonded configuration and the subsequent redistribution of energy from the surface hydrogen-bonded OH stretch excitation. The out-of-plane jump time is 1.5(1)ps, 30\% slower than that reported for the air-water interface and twice as short as the jump time between hydrogen bonded configurations in the bulk. Molecular dynamics simulations indicate that the slower dynamics at the hydrophobic interface compared to the water-air interface are due to the hydrogen bonds at the hydrophobic interface being stronger than those at the water-air interface.   

Water Adsorption on the LaMnO$_3$ Surface

Studying the adsorption of water on the metallic LaMnO$_3$ surface can provide insight into this complicated surface-adsorbate interaction. Using density functional theory, we investigated the adsorption of a water monomer, dimer, trimer and a monolayer on the surface. The electronic structure of ground state configurations is explored using analysis of density of states, charge density, and crystal orbital overlap populations. We found that the interaction between the surface and water molecules is stronger than hydrogen bonding between molecules, which facilitates wetting of the surface. Adsorbed water molecules form very strong hydrogen bonds, with substantially shifted OH stretch modes. For the monolayer of adsorbed water, a hint of a bilayer is observed with a height separation of only 0.2 \r{A}. However, simulated scanning tunneling microscopy (STM) images and vibrational spectra suggest a significant difference between the two layers due to intermolecular bonding and interaction with the substrate.