Iron(III) Speciation Observed at Aqueous and Glycerol Surfaces: Vibrational Sum Frequency and X-ray.

Aqueous solutions of FeCl3 have been widely studied to shed light on a number of processes from dissolution, mineralization, biology, electrocatalysis, corrosion, to microbial biomineralization. Yet there are little to no molecular level studies of the air-liquid FeCl3 interface. Here, both aqueous and glycerol FeCl3 solution surfaces are investigated with polarized vibrational sum frequency generation (SFG) spectroscopy. We also present the first ever extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy measurements of solvated ions and complexes at a solution interface, and observe with both X-ray photoelectron spectroscopy (XPS) and XUV-RA the existence of Fe(III) at the surface and in the near surface regions of glycerol FeCl3 solutions, where glycerol is used as a high vacuum compatible proxy for water. XPS showed Cl- and Fe(III) species with significant Fe(III) interfacial enrichment. In aqueous solutions, an electrical double layer (EDL) of Cl- and Fe(III) species at 0.5 m FeCl3 concentration is observed as evidenced from an enhancement of molecular ordering of water dipoles, consistent with the observed behavior at the glycerol surface. At higher concentrations in water, the EDL appears to be substantially repressed, indicative of further Fe(III) complex enrichment and dominance of a centrosymmetric Fe(III) species that is surface active. In addition, a significant vibrational red-shift of the dangling OH from the water molecules that straddle the air-water interface reveals that the second solvation shell of the surface active Fe(III) complex permeates the topmost layer of the aqueous interface.

Hydration of ferric chloride and nitrate in aqueous solutions: water-mediated ion pairing revealed by Raman spectroscopy.

Iron is the most abundant transition metal in the earth's crust and is important for the proper functioning of many technological and natural processes. Despite the importance, a complete microscopic understanding of the hydration of ferric ions and water mediated ion pairing has not been realized. Hydrated Fe(iii) is difficult to study due to the process of complexation to the anion and hydrolysis of the hydrating water molecules leading to a heterogeneous solution with diverse speciation. Here, ferric chloride and nitrate aqueous solutions are studied using polarized Raman spectroscopy as a function of concentration and referenced to their respective sodium salt or mineral acid. Perturbed water spectra (PWS) were generated using multivariate curve resolution-alternating least squares (MCR-ALS) to show the residual spectral response uniquely attributable to the hydration of ferric speciation. The hydrogen bonding network associated with the hydrating water molecules in ferric chloride solutions are found to be more similar to hydrochloric acid solutions, whereas in ferric nitrate solutions, the network behaves more similar to sodium nitrate, despite increased acidity. Thus, in the FeNO3 and FeCl3 solutions, ion pairing and coordination, respectively, are significantly influencing the hydration spectra signature. These results further reveal concentration dependent changes to the hydrogen bonding network, hydrating water symmetry, and changes to the relative abundance of solvent shared ion pairs that are governed primarily by the ferric salt identity.

The Influence of Metal Ions on the Dynamics of Supported Phospholipid Langmuir Films.

The translational diffusion dynamics of the zwitterionic lipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) at a planar phosphorylated support surface containing metal ions (Mg2+, Ca2+, Ba2+, Ni2+, Zn2+, Cd2+, Zr4+) was investigated using X-ray photoelectron spectroscopy (XPS) and fluorescence recovery after photobleaching (FRAP). Fluorescence recovery curves yielded diffusion constants on the order of 2-5 µm2/s for the chromophore-tagged 12:0 NBD-Lyso-PC. Ionic interactions between the zwitterionic headgroup and metal ions were found to play a secondary role in mediating lipid fluidity. This work provides quantitative insight into the extent to which the fluidity of a supported lipid film is mediated by the ionic interactions between headgroup and surface versus that of the lipid-lipid tailgroup interactions.

Identification of Ion Pairs in Aqueous NaCl and KCl Solutions in Combination with Raman Spectroscopy, Molecular Dynamics, and Quantum Chemical Calculations.

This work summarizes a theoretical analysis of the perturbation on Raman spectra in aqueous NaCl and KCl solutions with the aim to detect ion pairs. The experimental Raman spectra, both polarized and depolarized, are perturbed by these ions to a comparable extent or somewhat less by KCl than NaCl. This result appears to be contrary to the molecular dynamics (MD) simulation showing that the isolated and separated ions of KCl should have a larger perturbation than NaCl, as the solvation shell of K+ is larger than that of Na+. The apparent discrepancy signifies the ion pair formation which is more substantial for KCl than NaCl. The MD simulations and quantum chemical calculations revealed that KCl forms ion pairs more than NaCl and that the ion pair formation reduces the perturbation on the Raman spectra more for KCl. The present analysis shows that the perturbed Raman spectra provide a useful sign to evaluate the ion pair formation in aqueous solutions.

Comparing Rotational and Translational Diffusion to Evaluate Heterogeneity in Binary Solvent Systems.

We report on the rotational and translational diffusion dynamics of two monovalent fluorescent probe molecules, cationic oxazine 118 and anionic resorufin, in the glycerol-water solvent system using time-resolved fluorescence anisotropy (TRFA) and fluorescence recovery after photobleaching (FRAP) measurements. Experimental measurements of the chromophores are compared to well-established hydrodynamic models of rotational diffusion by the modified Debye-Stokes-Einstein (DSE) equation and of translational diffusion by the Stokes-Einstein-Sutherland (SES) equation. Other quasi-hydrodynamic models by Geirer-Wirtz (GW) and Dote-Kivelson-Schwartz (DKS) are compared to the modified DSE and SES models to better understand their utility to these systems. Deviations from the theoretically predicted diffusion constants are attributed to local solvation differences between the cationic and anionic chromophores and heterogeneity within the glycerol-water solvent system. Direct comparison between the modified DSE and SES models allows for empirical determination of the solvent-solute frictional interaction factor.

Surface Charge and Overlayer pH Influence the Dynamics of Supported Phospholipid Films.

Understanding the thermodynamics and kinetics of interactions between model lipid bilayers and planar supports is of critical importance in the furtherance of biosensing and the creation of biomimetic devices. Evaluating these properties can be accomplished through understanding the diffusional properties of the bilayer constituents. In this report, the dynamics of a model DMPC bilayer supported on a phosphorylated silica surface are studied in the presence and absence of interfacial Ca2+ as a function of pH of the aqueous overlayer. The data for this system reveal the importance of the balance of ionic interactions between the interfacial species, and the dependence of the diffusional, kinetic and thermodynamic properties of the system on pH. The thermodynamic data suggest that interactions between the bilayer and surface are mediated enthalpically rather than entropically.