Ionic bonding | EPFL Graph Search

Ionic bonding is a type of chemical bonding that involves the electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, and is the primary interaction occurring in ionic compounds. It is one of the main types of bonding, along with covalent bonding and metallic bonding. Ions are atoms (or groups of atoms) with an electrostatic charge. Atoms that gain electrons make negatively charged ions (called anions). Atoms that lose electrons make positively charged ions (called cations). This transfer of electrons is known as electrovalence in contrast to covalence. In the simplest case, the cation is a metal atom and the anion is a nonmetal atom, but these ions can be more complex, e.g. molecular ions like NH4+ or SO42−. In simpler words, an ionic bond results from the transfer of electrons from a metal to a non-metal to obtain a full valence shell for both atoms. It is important to recognize that clean ionic bonding — in which o

Mechanistic Insight into Supramolecular Polymerization in Water Tunable by Molecular Geometry

Lukas Pfeifer, Fan Xu

Supramolecular self-assembly in water based on non-covalent bonding is attracting major attention due to the potential of hydrogels and aqueous polymers in biomedical applications. Although supramolecular polymerization in organic solvents is well established, the key design features, the assembly mechanisms in water and achieving control over the aggregate structures remain challenging. Here, we present the assembly and disassembly of geometrical isomers of a stiff-stilbene bis-urea amphiphile (SA) in pure water. A remarkable feature of this system is that the (E)-isomer forms supramolecular polymers in both pure water and organic solvents. Taking advantage of this unique property, the hydrophobic effect was studied by comparing the supramolecular assembly in both systems. The assembly process in water follows an enthalpy-driven nucleation-elongation (cooperative) supramolecular polymerization mechanism with a standard Gibbs free energy (Delta G degrees = -53 kJ mol(-1)) double the value of the one found in toluene. We attributed this distinctive feature to the hydrophobic effect in water. Furthermore, we discovered an isomer-dependent assembly process, which can be used to control aggregation in aqueous media. Due to the substantial geometric difference between (E)-SA and (Z)-SA, we compared their assembly in water to study the influence of different driving forces involved in the process. The supramolecular polymerization of (E)-SA was cooperatively influenced by hydrogen bonding, pi-stacking, and hydrophobic effects, whereas the assembly of (Z)-SA was mainly driven by hydrophobic effects. As a result, the fiber length of (E)-SA in water is much longer than that of (Z)-SA, presenting opportunities for geometrical control of aggregation in aqueous media. [GRAPHICS]

CHINESE CHEMICAL SOC2022

Soft Self-Healing Nanocomposites

Yves Leterrier, Véronique Michaud, Sravendra Rana

This review article provides an overview of the research and applications of soft self-healing polymers and their nanocomposites. A number of concepts based on physical and chemical interactions have been explored to create dynamic and reversible gel and elastomer networks, each strategy presenting its own advantages and drawbacks. Physical interactions include supramolecular interactions, ionic bonding, hydrophobic interactions, and multiple intermolecular interactions. Such networks do not require external stimulus and are capable of multiple self-healing cycles. They are generally characterized by a rapid but limited healing efficiency. The addition of nanofillers enhances the mechanical strength of the soft networks as in conventional gels and elastomers, and do not compromise the network healing dynamics. In certain cases, nanofillers moreover trigger the healing process through e.g., multi-complexation processes between each component. Chemical interactions include Diels-Alder reactions and disulphide, imine, boronate ester, or acylhydrazones bonding, and are usually triggered with an external stimulus. The resulting healing is efficient, leading to good mechanical properties, but is generally slow at ambient temperatures, and dynamic chemical interactions are only reversible at higher temperatures. Conductive nanofillers were reported to speed up the healing process in such systems owing to their energy absorption properties. The challenges with nanofillers remain their functionalization and dispersion within the self-healing formulations. Soft self-healing gels and nanocomposites find applications in engineering such as coatings, sensors, actuators and soft robotics, and in the bio-medical field, including drug delivery, adhesives, tissue engineering and wound healing.

FRONTIERS MEDIA SA2019

Synthesis and Properties of Molecular Nanostructures on Surfaces

Nasiba Abdurakhmanova

One central aim of nanotechnology is the creation and exploitation of nanostructured materials with pre-programmed architecture and properties. The fabrication of functional nanostructures relies on versatile protocols of synthetic chemistry enabling the precise control of reactions at the molecular level. The surface-supported nanostructures investigated in this thesis have been synthesized using extended concepts of synthetic chemistry. To this end, mainly two chemical bonding types were employed: metal-organic coordination and covalent bonding. The resulting structures were characterized by scanning tunneling microscopy (STM) under ultra-high vacuum (UHV) conditions. Furthermore, the electronic and magnetic properties of the coordination structures were investigated by x-ray absorption spectroscopy (XAS) at the synchrotron radiation facility in Grenoble, France. The thesis is organized into the following three topics: - Organizational chirality of metal-organic coordination networks on a metal surface. The metal coordination networks formed by 7,7,8,8-tetracyanoquinodimethane (TCNQ) and Mn as well as Cs adatoms on Ag(100) are presented in chapter 3. The metal atoms form very similar chiral complexes with TCNQ. The complexes organize into densely packed and highly ordered domains whose organizational chirality depends on the metal center. This issue is addressed by theoretical modeling. It is shown that the exibility of the metal-organic coordination bond plays a key role in the supramolecular chirality. Mn forms rigid and directional bonds resulting in a heterochiral packing of the complexes. This organization avoids electrostatic repulsion between adjacent cyano groups of TCNQ. In the Cs-TCNQ4 structure this is achieved by an umbrella-like folding of four TCNQs around the central Cs atom, which yields a denser homochiral packing. This is enabled by the flexible nature of the ionic bonds. Moreover, the alkali atoms allow a mean to modify the electrostatic properties of the organic adlayer, which is important for the design of metal-organic interfaces in electronic devices. - Electronic and magnetic properties of the Me-TCNQ structures on metal surfaces. Previous investigations based on x-ray photoemission spectroscopy and density functional theory calculations for the Me-TCNQx systems on Cu(100) and Ag(100) surfaces show that two charge transfer (CT) channels are present, i.e. metal adatom-TCNQ and substrate-TCNQ. The choice of the substrate significantly influences CT, thereby controlling the electronic properties. In chapter 4 the magnetic properties of Ni-TCNQ and Mn-TCNQ structures on Ag(100) and Au(111) is presented. It is shown that the magnetic properties of Mn-TCNQ structures are essentially the same on Ag(100) and Au(111) surfaces, whereas for Ni-TCNQ the influence of the substrate is very pronounced. Both Ni-TCNQ/Au(111) and Ni-TCNQ/Ag(100) show ferromagnetic (FM) behavior, with stronger coupling observed on Au(111). This fact is especially interesting since the Ni impurities are found to be non-magnetic on both substrates. The observations are linked to the different CT channels and possible coupling mechanisms are identified. - The covalent bond formation reactions. Chapter 5 discusses the on-surface controlled synthesis of carbon-based nanostructures. The investigation of the properties of carbon-based nanostructures requires a better control over the synthesis process. To this end, one of the strategies proposed is the use of well-defined precursor molecules, which are deposited and activated on the surface to obtain the final structure. The surface catalyzed cyclodehydrogenation (SCDDH) reaction was used for the synthesis of fullerenes and nanotube caps. The selectivity of the C-C bond formation process is shown. Based on this result the isomeric pure C84 higher fullerene was synthesized. The same method was applied to the synthesis of nanotube caps. The precursor-based approach was also applied to the synthesis of N=9 and N=15 graphene nanoribbons (GNRs). Polyphenyl precursor molecules have been employed, which allow a synthetically simple mean to adjust the width of the ribbons. However, the resulting GNRs comprise many defects and the coupling mechanism is not very effective. The aspects of surface supported reactions are discussed and possible improvement strategies are proposed.

EPFL2012

Synthesis and Properties of Molecular Nanostructures on Surfaces

Nasiba Abdurakhmanova

EPFL2012