Electrolytes induce long-range orientational order and free energy changes in the H-bond network of bulk water

Electrolytes interact with water in many ways: changing dipole orientation, inducing charge transfer, and distorting the hydrogen-bond network in the bulk and at interfaces. Numerous experiments and computations have detected short-range perturbations that extend up to three hydration shells around individual ions. We report a multiscale investigation of the bulk and surface of aqueous electrolyte solutions that extends from the atomic scale (using atomistic modeling) to nanoscopic length scales (using bulk and interfacial femtosecond second harmonic measurements) to the macroscopic scale (using surface tension experiments). Electrolytes induce orientational order at concentrations starting at 10 μM that causes nonspecific changes in the surface tension of dilute electrolyte solutions. Aside from ion-dipole interactions, collective hydrogen-bond interactions are crucial and explain the observed difference of a factor of 6 between light water and heavy water.

Orientational ordering of water in extended hydration shells of cations is ion-specific and is correlated directly with viscosity and hydration free energy

Yixing Chen, Chung-Wen Liang, Halil Ibrahim Okur, Sylvie Roke

Specific ion effects in aqueous solutions are investigated at the molecular, nanoscopic and macroscopic levels. Femtosecond elastic second harmonic scattering (fs-ESHS) is used here to assess the chemical effects of ions on molecular and nanoscopic length scales of water, probing changes in the charge distribution around ions as well as structural orientational order of water molecules in extended hydration shells. We measured >0.05 M electrolyte solutions with a series of chloride salts (LiCl, NaCl, KCl, CsCl, RbCl, NH4Cl, MgCl2, CaCl2, and SrCl2). Ion specificity is observed in both the local electronic anisotropy and the nanoscopic orientational ordering of water. Both observables are influenced more by cations with larger valencies and smaller sizes and follow a direct Hofmeister trend. These ion-induced structural changes in the hydrogen-bond network of water are strongly correlated with the viscosity B-coefficient and the Gibbs free energy of hydration of ions. Such a connection between the nanoscopic and macroscopic changes provides a possibility to construct a molecular model for specific ion effects in aqueous solutions.

2017

Molecular insights in aqueous systems

Yixing Chen

The unique structure and dynamics of the water hydrogen (H)-bond network enable a multitude of structures and chemical reactions in both bulk solutions and at interfaces. The underlying molecular interactions between water and dissolved electrolytes, organic molecules, and nanoscale interfaces are difficult to study and hence not fully understood, especially when it involves interactions of length scales larger than one nanometer. In this thesis, we perform second order nonlinear light scattering experiments to investigate extended hydration shells (over one nanometer) and interfacial structures of buried aqueous interfaces that surround nanodroplets. We investigate the spatial range of ion-water interactions by probing the molecular structure of the water network in dilute electrolyte solutions. We find that electrolytes induce long-range, non-ion-specific orientational order in the H-bond network of water (over distances up to 70 hydration shells or 20 nm). These modifications in the water network start at electrolyte concentrations as low as 10 micromolar. The amount of induced orientational order and the concentration at which it occurs is different between light and heavy water. We link the observed ion induced changes in the bulk H-bond network of water to ion induced surface tension anomalies that occur in the same concentration range. We also study the same interactions in solutions with higher electrolyte concentrations focusing on cations. We observe specific ion effects in both the local charge distribution and water ordering in extended hydration shells of ions. Cations with larger charge densities induce larger changes in these two observables that follow a direct Hofmeister trend. The ion-induced structural changes in the water network are strongly correlated with the viscosity B-coefficient and hydration free energy of cations. These correlations facilitate constructing a better molecular model for specific ion effects in aqueous solutions. Next, we study the nanoscale hydrophobe/water interface and related interactions by probing the molecular structure of the interface formed with amphiphilic alkanols. We find that alkanols form a fluid film at the oil nanodroplet surface. Long chain alkanols show similar molecular structures and number densities at the interface. With increasing alkanol density, interfacial water loses its initial orientational alignment. The structure of interfacial oil molecules is more distorted for shorter alkanols. This interfacial structure differs significantly from those of macroscopic planar alkanol/water and alkanol/air interfaces and charged surfactant/oil/water interface, which can be explained by the balance between dispersive and H-bonding interactions. Finally, we develop a new membrane model system: 3D-lipid monolayers with tunable molecular structure on oil nanodroplets in water. This in-situ prepared system mimics the structure of lipid droplets in cells. The interfacial structure of lipids can be tuned from a tightly packed monolayer (liquid condensed phase) to a dilute one (liquid condensed/liquid expanded coexistence phase) by varying the lipid/oil/water ratio. The tunability of the chemical structure, the high surface-to-volume ratio, and the small sample volume make this system ideal for studies of molecular interactions of lipids in membranes.

EPFL2017

Self-assembly of organometallic macrocycles and cages

Thomas Brasey

Neutral metallamacrocycles with four or ten metal centers were obtained by reaction of [(R3P)PdCl2]2 with 2,3-dihydroxypyridine and derivatives or 2-hydroxynicotinic acid in the presence of base. In all these assemblies, the two O-donor atoms of the ligand bind to one metal center to form a chelate ring, while the N-donor atom binds to another metal center, linking the subunits together. The tetrameric complexes showed a low affinity for small aromatic diols. The procedure was adapted for the synthesis of water-soluble macrocycles: ligands which contain both the 2,3-dihydroxypyridine motive and a tertiary amine function that can be protonated were reacted in water with the acetato-bridged complex [(Et3P)Pd(CH3COO)2]2 to give tetrameric macrocycles in over 90% yields. Scrambling experiments showed that the Pd(II) macrocycles are highly inert. The reaction of ReBr3(CO)32 with 3-hydroxy-1,2,3-benzotriazine-4(3H)-one in the presence of base gave the ionic rhenium complex [ReBr(C7H4N3O2)(CO)3][NEt4], from which a neutral metallamacrocycle was obtained upon abstraction of (NEt4)Br. Its connectivity is comparable to that of the Pd(II) macrocycles. In coordinating solvents, this macrocycle disrupts and the monomeric solvent adducts are formed. In the presence of BF4-, it undergoes an anion-templated rearrangement to give a new, trimeric macrocycle which cocrystallizes with an unusual C3-symmetric [Ag(η2-benzene)3(OH2)][BF4] complex, the tetrafluoroborate anion being encapsulated in the cavity of the macrocycle. By reacting [(cymene)RuCl2]2 with 3,5-pyridinedicarboxylic acid, a trifunctional ligand in which the three donor atoms are unable to coordinate to the same metal center, an hexameric coordination cage was obtained. This complex has a trigonal antiprism geometry. Half of the carbonyl O-atoms of the bridging ligands are positioned in close proximity to each other, forming two binding sites on the surface. The cage can thus act as an exo-receptor for alkali metal cations (Na+, K+, Cs+). 1H and 13C NMR investigations as well as a single crystal X-ray analysis revealed that upon addition of alkali metal salts, the hexanuclear assembly partly rearranges to form a dodecameric coordination cage with icosahedral geometry. Although entropically disfavored, this process takes place because in this new complex all carbonyl O-atoms are part of a binding site (eight metal cations can be accommodated on the surface of the cage). Furthermore, due to the reduced surface curvature of the dodecanuclear assembly, these O-atoms are closer together, forming more suited binding sites for the alkali guest cations. To best of our knowledge, a coordination cage with an icosahedral geometry has not been reported so far.