Session T27: Focus Session: Solvation of Ions and Electrons III

Solvation and Organization of Ions at the Air-Water Interface

The air-water interface has been the focus of research in the Allen Lab at Ohio State for more than a decade. We utilize nonlinear and linear optical spectroscopic methods to understand the local intermolecular interactions and organization of water itself with various solutes and monolayers. Motivated by atmospheric aerosol chemistry of marine and urban regions, and biophysical applications related to lung lining and biomembranes, monovalent and divalent cations and anions continue to be investigated by our group using conventional and heterodyne-detection vibrational sum frequency generation (VSFG) spectroscopy. Interest is in the surface propensity and availability for reaction at water surfaces. Ion valency, polarizability, size, shape, and identity of the counterion are critical factors in considering ion organization and subsequent changes in interfacial electric field at the air water interface. The hydrating water molecules play a key role in the interfacial organization of other species in the solution, and is studied directly as it reveals the details of ion interfacial distributions. Phospholipids and fatty acids are also investigated using both VSFG and Brewster angle microscopy (BAM). Head group differences, especially with regard to hydrogen bonding capability and extent, are discerning factors for surface organization and shape distinction at the water surface.\\[4pt] In collaboration with Wei Hua and Dominique Verreault, Ohio State University.   

Sensitivity of ultrafast vibrational dynamics of interfacial water to cations at silica/water interfaces

Interfacial water structure is a key player in the chemistry of environmentally relevant processes, e.g. the dissolution of mineral surfaces. Since the properties of water are dominated by its hydrogen bonding network, understanding the local changes in this structure at the mineral/water interface is important. Vibrational sum-frequency generation (vSFG) spectroscopy is a tool that can probe the hydrogen bonding environment at buried interfaces. Time-resolved vSFG of the OH stretch is further sensitive to the changes in hydrogen bonding at surfaces. vSFG has demonstrated that interfacial water structure at silica is most sensitive to cations at neat water pH 6-8. Further, at this pH, increasing the ionic strength slows the vibrational relaxation of O-H stretch of water from $\sim$200 to $\sim$600 fs. Here, we show that the slowing down of O-H dynamics on addition of cations is more pronounced at pH 6 than at pH 12, even though the surface is negatively charged in both cases. This suggests that, in addition to the salt screening the surface charge, the cations actually change the local H-bonding environment of water differently at distinct bulk pH. Our results shed light on the ongoing debate on the role of ions in altering the structure of water at an interface.   

Vibrational Spectroscopy of Hydrated MOH$^{+}$ Clusters

Hydrated metal ions have importance in a number of fields of chemistry including environmental chemistry, biological chemistry, and catalysis. Although the aqua complexes of transition metals are well studied there is a dearth of information on hydroxide containing metal clusters which have been implicated in catalytic mechanisms of water oxidation. In this work we use cryogenic ion vibrational spectroscopy (CIVS) to interrogate clusters of the form MOH(H$_{2}$O)$_{\mathrm{n}}^{+}$(M$=$ Fe, Co, Ni, Cu, Zn) in the hydride stretching region (2400-3800 cm$^{-1})$. Comparison of the spectral data to density functional theory calculations allow for unambiguous assignment of the observed spectral features. The resulting spectra show both a metal dependent and size dependent shift of the hydroxide stretching frequency in all clusters. The spectra also reveal that the first solvation shells of Fe, Co, Cu, and Zn containing clusters are composed of 4 ligands while the first solvation shell in Ni clusters is at least 6 ligands. This is markedly different from previous work on Ni(H$_{2}$O)$_{\mathrm{n}}^{+}$, Cu(H$_{2}$O)$_{\mathrm{n}}^{+}$, and Zn(H$_{2}$O)$_{\mathrm{n}}^{+}$ cluster in which the first solvation shells are 4 ligands, 2 ligands, and 3 ligands respectively. Aspects of hydration in the second solvation shell of these clusters will also be discussed.   

Exploring Specific Ion Effects on Ion Hydration at Aqueous Interfaces

We used sum frequency generation vibrational spectroscopy to probe the interactions of ions with self-assembled monolayers and proteins at aqueous interfaces. I will discuss the behavior of cations and anions as they relate to the Hofmeister series, which is a rank ordering of the efficacy of these species to influence the physical behavior of colloidal and interfacial systems in solution. Ion specific effects at these interfaces were found to be determined by several factors. These include the sign and magnitude of the surface potential, ion pairing effects in the double layer, as well as the presence of polar and nonpolar interfacial moieties. At negatively charged, hydrophilic surfaces, we found that Na$^{\mathrm{+}}$ adsorption and double layer formation were modulated by the type of the counterion in solution. The same ordering was observed for the anions whether this interface was relatively hydrophobic or hydrophilic. Changing the sign of the charge at the interface also led to a similar Hofmeister ordering. Moreover, at negatively charged hydrophilic surfaces, the smallest and best hydrated cations were mostly favored over more poorly hydrated cations. By contrast, well hydrated cations were repelled from more apolar surfaces.   

Structural properties of simple aqueous solutions from ab initio simulations

Although water and salt solutions have been studied for many decades, several aspects of their microscopic and electronic properties remain uncertain. One of the open questions is whether simple ions have long-range effects on the structure of liquid water. We performed extensive first-principles molecular dynamics simulations [1] of solutions of a simple salt (NaCl) and found that while Na$^+$ does not significantly change the structure of water beyond the first solvation shell, Cl$^-$ has a far-reaching hydrogen-bond-breaking effect. We present an analysis of the structural modifications in terms of molecular polarizabilities and dipole angular correlations. Our results are in agreement with the traditional classification of Cl$^-$ as a structure-breaker, but at variance with several theoretical and experimental studies which did not observe significant modifications of the structure of water outside the first solvation shell of ions [2]. We also present an analysis of the electronic properties of the solutions [3]. \\[4pt] [1] \emph{Qbox} code, http://eslab.ucdavis.edu/software/qbox/index.htm;\\[0pt] [2] Y. Marcus, \emph{Chem.\ Rev.} \textbf{109}, 1346 (2009);\\[0pt] [3] A.~P. Gaiduk, C.~Zhang, F.~Gygi, G.~Galli, \emph{Chem.\ Phys.\ Lett.} \textbf{604}, 89 (2014).   

Ultrafast vibrational spectroscopy (2D-IR) of anions in ionic liquids

Ionic liquids hold promise for applications in energy storage as the electrolyte in electric double-layer capacitors and other devices. Further optimizing device performance through their physical and chemical properties requires an improved understanding of the detailed interactions between cation and anion and how they reorganize in the presence of charge. Here we probe the dynamics of thiocyanate ions in several bulk imidazolium bis(trifluoromethylsulfonyl)amide ionic liquids from femtoseconds to 100 ps using ultrafast vibrational spectroscopy. Two-dimensional infrared (2D-IR) spectroscopy of thiocyanate ions detect both inertial motion (on the hundreds of femtosecond timescale) as well as slower, diffusive motions (on the tens of picosecond timescale). The 2D-IR experiments show that the rate of fluctuation of the electrostatic environment around the thiocyanate is sensitive to hydrogen bonding at the 2-position of the imidazolium ring. The correspondence of time-scales with molecular dynamics simulations suggests that the picosecond motions we observe are fluctuations around a local free-energy minimum while the tens of picoseconds timescales correspond to the rearrangement of those local intermolecular arrangements. Our measurements implicate the break-up of local ion cages as the activating event for the onset of translational motions which are responsible for viscosity and conductivity. We additionally test our hypothesis that the SCN anion is sensitive to the local environment by introducing co-solutes -- counter-cations such as K+ and water. We observe contact-ion pairs, i.e., SCN- anions with a K+ counter charge in the first solvation shell through a static component in the frequency correlation function. We observe water-bound SCN- through changes in linewidth, population relaxation rates, and dynamics. The SCN- is an eloquent reporter of local structure and dynamics in these complex fluids.   

Structural dynamics of solvated OH$^{-}$ and H$_{3}$O$^{+}$ in liquid water: an \textit{ab initio} molecular dynamics study using PBE0 hybrid functional with van der Waals' interaction

The nature of solvation structure of hydroxide (OH$^{-}$) and hydronium (H$_{3}$O$^{+}$) solvated in liquid water is of fundamental interest, as it is the prerequisite to understand the autoprotolysis process, which is often a crucial step in chemical and biological activities. It has been revealed that the proton transfer (PT) process, especially in hydroxide case, is of great controversy and complexity compared to traditional textbook description. One major source of inaccuracy originates from the delocalization error and lack of dispersion force in the conventional adopted electron exchange correlation function in GGA functional leading to an over-structured H-bond (HB) structure. Now with state-of-the-art PBE0 hybrid density functional, and the inclusion of long-range van der Waals dispersion force, we are able to perform \textit{ab initio} molecular dynamics with higher accuracy. Through the analysis of mean square displacement of ion, HB geometrical distribution and lifetime, and the strength change of HB, we are presenting a quantitatively accurate picture of proton transfer structural mechanism.   

Solvation of a Ruthenium Water Oxidation Catalyst, [Ru(tpy)(bpy)(H$_{2}$O)]$^{2+}$

The splitting of H$_{2}$O into H$_{2}$ and O$_{2}$ is an important reaction for alternative energy sources, but it is hindered by the water oxidation step due to its unfavorable thermodynamics. Production of a suitable water oxidation catalyst (WOC) has posed a challenge to researchers for decades, and the reaction mechanism is not well understood. One of the most well-known and extensively studied WOCs is [Ru(tpy)(bpy)(H$_{2}$O)]$^{2+}$ (tpy $=$ 2,2':6,2''-terpyridine, bpy $=$ 2,2'-bipyridine) and its derivatives. In this talk, infrared vibrational spectra (2400-3800 cm$^{-1})$ obtained by Cryogenic Ion Vibrational Spectroscopy (CIVS) of [Ru(tpy)(bpy)(H$_{2}$O)]$^{2+}$ and its water clusters, [Ru(tpy)(bpy)(H$_{2}$O)]$^{2+}$\textbullet $n$H$_{2}$O, are presented. In particular, the OH stretches are used as a probe of solvation strength, and trends in their spectral shifts are examined as a function of cluster size. Determination of water binding geometries are facilitated by comparison with density functional theory (DFT) calculations. Additionally, IR and mass spectral data of electrochemically-produced intermediates of the water oxidation cycle are shown, which provide evidence of proton-coupled electron transfer (PCET) events, in agreement with proposed mechanisms.   

Ion Induced Hydrophilicity at Hexane Vapor Water Interface

How ions occupy themselves at hydrophobic interface plays a key role in physical, chemical, atmospheric, and biological processes. However, hydrophobic molecules often gather together on water surface, thus few experiment taking on the adsorption of hydrophobic molecules on aqueous solutions from vapor phase. In this work, we report unexpected mutually promoted adsorption of hydrophobic molecules and ions through unification of contact angle method and sum frequency vibrational spectroscopy (SFVS). We observe oil tends to spread on solutions with higher pH, implying interaction exists between hydrophobic molecules and ions. SFVS suggests that this phenomenon stems from relatively strong Coulomb interaction between hydrophobic molecule and hydroxide ions, the initial hydroxide ion adsorption free energy is calculated. The surface charge density induced at the interface is plotted to show the variation of surface charge density with both hydroxide ion molar fraction and hexane vapor pressure. In addition, we propose a new perspective on explanation of intensity decay of icelike band at high pH (\textgreater 10) aqueous solution$\backslash $hydrophobic interfaces, in which the relative relation between Debye length and coherence length plays an essential role.