Noncovalent dispersive attractions

In a solution of a solute in a solvent there can exist noncovalent intermolecular interactions of solvent-solvent, solvent-solute, and solute—solute pairs. The noncovalent attractive forces are of three types, namely, electrostatic, induction, and dispersion forces. We speak of forces, but physical theories make use of intermolecular energies. Let V(r) be the potential energy of interaction of two particles and F(r) be the force of interaction, where r is the interparticle distance of separation. Then these quantities are related by... 

Attractive forces (sometimes also referred to as dispersion forces) between apolar molecules arising from the mutual polarizability of the interacting molecules. London forces also contribute to the interactive forces between polar molecules. See also van der Waals Forces Noncovalent Interactions... 

On the weak end of noncovalent interactions, we find van der Waals forces (<5 kJ mol ) which arise from the interaction of an electron cloud polarized by adjacent nuclei. Van der Waals forces are a superposition of attractive dispersion interactions, which decrease with the distance r in a dependence, and exchange repulsion decreasing with... 

In addition to electrostatic, exchange, and induction (a.k.a. deformation) energy, the fourth principal contributor to the interaction energy is the so-called dispersion energy [39]. This quantity is closely related to the London forces that are well known from freshman chemistry texts that originate from instantaneous fluctuations of the electron density of one molecule, which cause a sympathetic series of instantaneous density fluctuations in its partner. Dispersion, by its very nature, is attractive. In terms of ab initio molecular orbital theory, the dispersion energy is not present at the SCF level, but is a byproduct of the inclusion of electron correlation into the calculation. The reader is hence alerted to the fact that calculations that do not include electron correlation (and there are many such, particularly in the early literature) cannot be expected to include this fourth, and sometimes very important, component of the noncovalent force. 

Noncovalent functionalization is a method to improve solubility and processability without compromising the physical properties of the carbon nanomaterials [45]. This functionalization mainly involves surfactants, biomacromolecules or wrapping with polymers [41,45,46]. This technique is based on interactions of adsorbed molecules with the carbon nanomaterial surface through Van der Waals, 7t-7t, CH-tt, and other interactions [47]. Furthermore, the adsorbed surfactants provide repulsive and attractive forces creating a thermodynamically stable dispersion [33]. 

Lipid bilayers (Section 23.6A) A two-layer noncovalent molecular assembly comprised primarily of phospholipids. The hydrophobic phospholipid tail groups of each layer orient toward each other in the center of the two-layered structure due to attractive dispersion forces. The hydrophilic head groups of the lipids orient toward the aqueous exterior of the bilayer. Lipid bilayers are important in biological systems such as cell membranes. 

The sol-gel process performed in low concentrated polymer-solvent solutions is another attractive route to develop hybrid membranes because it allows an in situ dispersion of metal-based nanoparticles within the polymeric matrix, achieving a suitable interfacial morphology between the continuous and the dispersed phase. Silica particles and polyimide have been frequently used to produce these hybrid membranes [107,108]. In general, hydrolysis and condensation reactions are involved in the sol-gel process, when alkoxides are involved in the formation of the dispersed phase. The advantage of using this method is the formation of an inorganic network largely interconnected with the polymeric materials mainly with noncovalent interactions [109]. In Fig. 7.10 a... 

There has been an effort to employ DNA in nonbiolog-ical application. In particular, the ability of its aromatic nucleotide bases to m-stack has made DNA an attractive material to noncovalently functionalize CNTs. Zheng et al. discovered that DNA has a strong affinity for SWNTs, which enabled the dispersion of nanotubes in aqueous environments, and employing ion-exchange chromatography, DNA-dispersed nanotubes can be separated on the basis of their electronic structure (i.e semiconducting vs metal-lic). ... 

It will become evident in later sections that the nature of the weak noncovalent interactions in a cluster dictate which computational methods will produce accurate results. In particular, it is far more difficult to compute reliable properties for weakly bound clusters in which dispersion is the dominant attractive component of the interaction. For example, Hartree-Fock supermolecule computations are able to provide qualitatively correct data for hydrogen-bonded systems like (Fi20)2 even with very small basis sets, but this approach does not even bind Ne2- What is the origin of this inconsistency Dispersion is the dominant attractive force in rare gas clusters while the electrostatic component tends to be the most important attractive contribution near the equilibrium structure (H20)2- As London s work demonstrated,dispersion interactions are inherently an electron correlation problem and, consequently, cannot be described by Flartree-Fock computations. To this day, dispersion interactions continue to pose a significant challenge in the field of computational chemistry, particularly those involving systems of delocalized n electrons." ... 

Van der Waals Forces np [Johannes D. van der Waals 1923 Dutch physicist] (1926) (secondary valence force, intermolecular force) An attractive force, much weaker than primary covalence bonds, between molecules of a substance in which all the primary valences are saturated. They are believed to arise mainly from the dispersion effect, in which temporary dipoles induce other dipoles in phase with themselves. The primary van der Waals forces are dipole-dipole (polar molecules) and London forces (nonpolar molecules). These forces are attributed to the attractions between molecules and from noncovalent bonds (Goldberg, D. E., Fundamentals of Chemistry, McGraw-Hill Science/Engineering/ Math, New York, 2003). 

The techniques used to disperse nanofillers within a polymer matrix can be broadly categorized into kinetic and thermodynamic approaches. In the kinetic approaches, an external energy sotuce, such as shear forces or ultrasoimd vibrations, is used to temporarily disperse the filler followed by a method to trap this state. The thermodynamic approaches, on the other hand, involve the use of covalently or noncovalently bonded chemical additives to mediate the interfacial energies and thereby improve the filler-polymer compatibility and/or reduce the attractive interactions between the fillets. In many cases, combinations of both kinetic and thermodynamic approaches are adopted for optimal results. 

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