Session A28: Focus Session: Confined and Biological Water I

Liquid Water, the ``Most Complex'' Liquid: New Results in Bulk, Nanoconfined, and Biological Environments

We will introduce some of the 63 anomalies of the most complex of liquids, water. We will demonstrate some recent progress in understanding these anomalies by combining information provided by recent experiments and simulations on water in bulk, nanoconfined, and biological environments. We will interpret evidence from recent experiments designed to test the hypothesis that liquid water may display ``polymorphism'' in that it can exist in two different phases---and discuss recent work on water's transport anomalies [1] as well as the unusual behavior of water in biological environments [2]. Finally, we will discuss how the general concept of liquid polymorphism [3] is proving useful in understanding anomalies in other liquids, such as silicon, silica, and carbon, as well as metallic glasses, which have in common that they are characterized by two characteristic length scales in their interactions. This work was supported by NSF Chemistry Division, and carried out in collaboration with a number of colleagues, chief among whom are C. A. Angell, M. C. Barbosa, M. C. Bellissent, L. Bosio, F. Bruni, S. V. Buldyrev, M. Canpolat, S. -H. Chen, P. G. Debenedetti, U. Essmann,G. Franzese, A. Geiger, N. Giovambattista, S. Han, P. Kumar, E. La Nave,G. Malescio, F. Mallamace, M. G. Mazza, O. Mishima, P. Netz, P. H. Poole, P. J. Rossky, R. Sadr,S. Sastry, A. Scala, F. Sciortino, A. Skibinsky, F. W. Starr, K. C. Stokely J. Teixeira, L. Xu, and Z. Yan.\\[4pt] [1] L. Xu, F. Mallamace, Z. Yan, F. W. Starr, S. V. Buldyrev, and H. E. Stanley, ``Appearance of a Fractional Stokes-Einstein Relation in Water and a Structural Interpretation of Its Onset,'' Nature Physics 5, 565--569 (2009). \\[0pt] [2] P. Kumar, Z. Yan, L. Xu, M. G. Mazza, S. V. Buldyrev, S. -H. Chen. S. Sastry, and H. E. Stanley, ``Glass Transition in Biomolecules and the Liquid-Liquid Critical Point of Water,'' Phys. Rev. Lett. 97, 177802 (2006). \\[0pt] [3] H. E. Stanley, ed. , Liquid Polymorphism [Advances in Chemical Physics], series edited by S. A. Rice (Wiley, New York, 2010).   

Faster water dissociation fluctuations on the surface of Cassiterite

We investigated water dissociation dynamics on rutile (110) surface (titanium dioxide) and cassiterite (110) surface (tin dioxide) using molecular dynamics simulation. We find that dissociation events occur around five times more frequently on cassiterite compared to rutile. The water on cassiterite surface is found to be closer to the surface due to stronger hydrogen bond formation between water and the surface. The increase in the strength of hydrogen bond is confirmed by a red shift of frequency OH vibrations at the surface. We will provide evidence that the origin of the stronger hydrogen bond on the surface of cassiterite resides in higher covalency between water and the bridging oxygen atoms at the surface.   

Cross-over of hydrophobic effects observed in amphiphilic aqueous solutions

The hydrophobic effect is important in nature and responsible for phenomena in many fields. Changes in structure and dynamics of water induced by apolar groups are believed to play a key role in protein folding, ligand binding and formation of biological membranes. The length scale dependence of hydrophobic and associated crossover length has been discussed intensively, but has not been observed directly by experimental techniques sensitive to the microscopic structure. Here we report experimental evidences for the cross over behavior of hydrophobic effects in amphiphilic aqueous solutions based on x-ray adsorption and small angle x-ray scattering data. Tetraalkyl ammonium (C$_{n}$H$_{2n+1})_{4}$N$^{+}$ (TAA) cations show hydrophobic effects on the structure of water other than ionic effects. For cations with shorter side hydrocarbon chains, hydrophobic hydration enhances the hydrogen bonds (HBs) in liquid water and separates the solute with solvents; whereas for cations with longer chains ($\sim $ 5 A) we find that the HBs are weakened and the solutes and solvent are attracted to each other. The temperature dependence of the crossover length is also investigated.   

Nonequilibrium Structural Phase Transformation of Interfacial Water Assemblies Observed by Ultrafast Electron Crystallography

Interfacial water, when compared with bulk phases, exhibits unique properties in various systems. By using ultrafast electron crystallography with atomic-scale spatiotemporal resolution, we report studies of structures and dynamics of interfacial ice assemblies on different surfaces. Structurally, ordered ice may be formed not only due to energetics of the water-surface interaction (commonly addressed as hydrophobicity and hydrophilicity), but also as a result of the surface morphology of the underlying substrate. The ultrafast dynamics also reveals new properties of a hydrogen-bond network. Following the photoexcitation of the substrate by a femtosecond infrared pulse, the interfacial ice structure undergoes, in few tens of picoseconds, nonequilibrium phase transformation identified through the observation of a structural isosbestic point in diffraction. This noncontinuous lattice expansion, from the unperturbed state to a highly expanded, nonthermal one, is caused by the structural vibration in the substrate and energy transfer across the interface. In addition, the cooperativeness of the hydrogen-bond network prevents a substantial evaporation lost of water molecules during repeated extreme expansion and recovery of the lattice. We provide the time scales involved and discuss the nature and implications of ice-substrate structural dynamics.   

Femtosecond movies of water near interfaces at sub-Angstrom resolution

The behavior of liquid water near interfaces with nanoscopic variations in chemistry influences a broad range of phenomena in biology. Using inelastic x-ray scattering (IXS) data from 3rd-generation synchrotron x-ray sources, we reconstruct the Greens function of liquid water, which describes the {\AA}-scale spatial and femtosecond-scale temporal evolution of density fluctuations.~ We extend this response function formalism to reconstruct the evolution of hydration structures near dynamic surfaces with different charge distributions, in order to define more precisely the molecular signature of hydrophilicity and hydrophobicity.~ Moreover, we investigate modifications to surface hydration structures and dynamics as the size of hydrophilic and hydrophobic patches are varied.   

Studies of Water Diffusion on Single-Supported Bilayer Lipid Membranes by Quasielastic Neutron Scattering

Bilayer lipid membranes supported on a solid surface are attractive model systems for understanding the structure and dynamics of more complex biological membranes that form the outer boundary of living cells. We have recently demonstrated the feasibility of using quasielastic neutron scattering to study on a $\sim $1 ns time scale the diffusion of water bound to single-supported bilayer lipid membranes. Two different membrane samples characterized by AFM were investigated: protonated DMPC + D$_{2}$O and tail-deuterated DMPC + H$_{2}$O. Both fully hydrated membranes were deposited onto SiO$_{2}$-coated Si(100) substrates. Measurements of elastic neutron intensity as a function of temperature on the High Flux Backscattering Spectrometer at NIST reveal features in the diffusive motion of water that have not been observed previously using multilayer membrane stacks. On slow cooling, the elastic intensity shows sharp step-like increases in the temperature range 265 to 272 K that we tentatively interpret as successive mobile-to-immobile transitions of water bound to the membrane.   

Structure, Dynamics and Thermodynamics of Confined Water: a Computational Perspective

The behavior of water in confining geometries with characteristic dimensions in the nm range is of interest in a wide range of scientific fields and technical applications, such as biological self-assembly and tribology. Recent computational work on water in nano-scale confinement by inorganic and biological surfaces sheds new light on the roles of surface chemistry and geometry on water structure, water phase behavior in hydrophobic confinement over broad ranges of temperature and pressure, the evolution from surface-influenced to bulk-like dynamics, the dependence of dynamics on surface polarity, the relationship between the mechanical properties of glassy water thin films and surface polarity, water structure in confinement by heterogeneous surfaces, and the control of surface hydrophobicity by coupling polarity and topography. Such studies help develop the physical understanding needed for the rational design of surfaces for practical applications.   

How water meets a very hydrophobic surface

Recent X-ray and neutron scattering experiments [1] have provided evidence both for and against the existence of low-density regions between water and hydrophobic surfaces. We have studied the interface between water and fluoroalkylsilane self-assembled monolayers using synchrotron X-ray reflectivity. These surfaces are significantly more hydrophobic than methyl-terminated monolayers. We find that the width of the depleted region increases with aqueous contact angle and becomes larger than the experimental resolution. When the advancing contact angle is 120$^{\circ}$, the effective depletion layer thickness is $\sim $5{\AA}. We interpret the apparent contradictions in previous studies as scatter in measurements of sub-resolution effects. The water-gap interface is expected to undergo large fluctuations [2]; our data place an upper limit of 3.5{\AA} on the RMS width caused by fluctuations. \newline [Supported by NSF grant no. DMR-0705137] \newline \newline [1] e.g., A. Poynor et al., \textit{PRL}\textbf{ 97}, 266101, 2006; M. Mezger et al., \textit{PNAS }\textbf{103}, 18401, 2006; M. Maccarini et al., \textit{Langmuir} \textbf{23}, 598, 2007 \newline [2] e.g. X. Zhang et al., \textit{Science} \textbf{295}, 663 (2002); D. Chandler, \textit{Nature} 445, 831 (2007)   

Time resolved lateral dynamic force microscopy for exploring nanoscopic water bridge

Lateral dynamic force microscopy based on time-resolved scheme is employed for a good understanding of dynamics of nanoscopic water bridge connecting a sharp tip with a flat sample. In its formation and stepped compression at which the tip and the sample in a true non-contact, the nanoscopic water bridge under oscillatory shear stress shows a transient response behavior for a long time ($ \ge 10^{2} $ ms). This observation obviously demonstrates that an inadequate fast measurement in dynamic force microscopy can lead a misunderstanding of dynamic physical properties of the nanoscopic water.   

Structure and dynamics of water confined within reverse micelles

The structure and dynamics of water confined within reverse micelles (RMs) of varying water content ([water]/[surfactant]) formed by the surfactant Aerosol-OT in iso-octane are studied using molecular dynamics simulations. The intrinsic density profile of water in the RM is constructed with respect to the surface formed by the surfactant sulfur atoms and reveals a high density shell at the surfactant interface, a core region which becomes more bulk-like as RM size increases and an intermediate region between the interface and core. Water diffusion in the presence of partially absorbing boundaries (compatible with the intrinsic profile) provides a simple picture for describing confined diffusion within the RM. Water reorientation is strongly perturbed with respect to bulk water due to the presence of surfactant head groups and counterions. Our results for water dynamics in RMs are compared with the results of time-resolved IR and quasielastic neutron scattering experiments.   

Enhancement of Hydrophobic Solvation by Hydrophilic Functional Groups: Trehalose and Kojibiose in Water

The structure and dynamics of water around biomacromolecules differs significantly from that of water in bulk in ways critical for biological function. The manner in which water structure differs is a function of both chemical and topological heterogeneity. Attempts to disentangle these effects have generally focussed on solvation of large molecules at either particular locations or in an averaged sense. In either case, understanding how chemical and topological heterogeneity combine can be difficult. Here we circumvent this problem by examining, in all atom simulations, water structure around the disaccharides Trehalose and Kojibiose. Taken together water structure around these molecules provides a series of internal control experiments for disentangling topological and chemical effects and allows us to conclude that, in the case of Trehalose, topological effects can lead to slow down of water reorientation by a factor of 2 relative to a chemically equivalent system.