Weak dispersion forces

This type of liquid is characterized by direction independent, relatively weak dispersion forces decreasing with r-6, when r is the distance between neighbouring molecules. A simple model for this type of liquid, which accounts for many properties, was given by Luck 1 2> it is represented by a slightly blurred lattice-like structure, containing hole defects which increase with temperature and a concentration equal to the vapor concentration. Solute molecules are trapped within the holes of the liquid thus reducing their vapor pressure when the latter is negligible. 

The structure of graphite. Graphite has a two-dimensional layer structure with weak dispersion forces between the layers. 

Why is polyethylene a solid at room temperature when the only intermolecular forces it experiences are the weak dispersion forces ... 

HCl would have a higher value for a" because HCl is a polar molecule and therefore has stronger intermolecular forces than Hj. H2 is nonpolar with only weak dispersion forces to contend with. 1 point for correct answer and 1 point for supporting the choice with valid reasoning. 

What about substances made of non-polar molecules You learned in Chapter 3 that weak dispersion forces form between non-polar molecules. As temporary dipoles form, they cause molecules to move closer together. However, these attractions are temporary and weak. Thus, most small non-polar molecules do not hold together long enough to maintain their solid or liquid forms. As a result, most small non-polar molecules exist as gases at room temperature. For example, carbon dioxide (C02) is a gas at normal temperatures. 

The gap between the predictions and experimental results arises from imperfect dispersion of carbon nanotubes and poor load transfer from the matrix to the nanotubes. Even modest nanotube agglomeration impacts the diameter and length distributions of the nanofillers and overall is likely to decrease the aspect ratio. In addition, nanotube agglomeration reduces the modulus of the nanofillers relative to that of isolated nanotubes because there are only weak dispersive forces between the nanotubes. Schadler et al. (71) and Ajayan et al. (72) concluded from Raman spectra that slippage occurs between the shells of MWNTs and within SWNT ropes and may limit stress transfer in nanotube/polymer composites. Thus, good dispersion of CNTs and strong interfacial interactions between CNTs and PU chains contribute to the dramatic improvement of the mechanical properties of the... 

In common with other polar solutes, peptide-nonpolar stationary phase interactions can be discussed in terms of a solvophobic model. In this treatment solute retention is considered to arise due to the exclusion of the solute molecules from a more polar mobile phase with concomitant adsorption to the hydrocarbonaceous bonded ligand, where they are held by relatively weak dispersion forces until an appropriate decrease in mobile-phase polarity occurs. This process can be regarded as being en-tropically driven and endothermic, i.e., both AS and AH are positive. 

Physisorption or physical adsorption is the mechanism by which hydrogen is stored in the molecular form, that is, without dissociating, on the surface of a solid material. Responsible for the molecular adsorption of H2 are weak dispersive forces, called van der Waals forces, between the gas molecules and the atoms on the surface of the solid. These intermolecular forces derive from the interaction between temporary dipoles which are formed due to the fluctuations in the charge distribution in molecules and atoms. The combination of attractive van der Waals forces and short range repulsive interactions between a gas molecule and an atom on the surface of the adsorbent results in a potential energy curve which can be well described by the Lennard-Jones Eq. (2.1). 

When temporary dipoles are close together, a weak dispersion force exists between oppositely charged regions of the dipoles, as shown in Figure 13-8. 

Crystalline solids can be classified into five categories based on the types of particles they contain atomic solids, molecular solids, covalent network solids, ionic solids, and metallic solids. Table 13-4 summarizes the general characteristics of each category and provides examples. The only atomic solids are noble gases. Their properties reflect the weak dispersion forces between the atoms. 

The noble gases are colorless, tasteless, and odorless. In the liquid and solid states the only forces of attraction among the atoms are very weak dispersion forces. Polarizability and interatomic interactions increase with increasing atomic size, and so melting and boiling points increase with increasing atomic number. The attractive forces among He atoms are so small that He remains liquid at 1 atmosphere pressure even at a temperature of 0.001 K. 

It was soon realized that the error introduced by the Hartree-Fock model, which is the so-called correlation energy A = E i — E , is small for closed-shell systems (of the order of a few percent) but decisive for chemical reaction energetics. Moreover, weak interactions of the van der Waals type cannot be described with such a single-determinant Hartree-Fock model. Consequently, Hartree-Fock calculations on supramolecular assemblies, whose interaction is governed by weak dispersion forces, cannot provide accurate quantitative results and are likely to yield a wrong qualitative picture. However, in certain cases Hartree-Fock results may be of value. For instance, Houk and coworker have investigated the role of [C-H O] interactions in supramolecular complexes by means of dynamic and static Hartree-Fock calculations [59]. 

---