Graphite dispersion forces

Graphite is a soft, black, crystalline form of carbon that is a fair conductor of electricity. The carbon atoms in graphite are bonded together in layers. Within each layer, each atom is bonded to three other carbon atoms. But because adjacent layers are held together only by very weak London dispersion forces, graphite is very soft... 

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

This structure gives stability to the layers. The fourth, non-bonding pair of electrons is delocalized and free to move within the layers. This gives graphite the ability to conduct an electric current. There are no intramolecular forces between the layers. Dispersion forces attract one layer to another, enabling layers to slide by one another. Graphite feels slippery as a result of this characteristic. In fact, many industrial processes use graphite as a lubricant. 

Heats of adsorption measurements do not lead to very specific interpretation since the isosteric heat of adsorption (AH) arises from both nonspecific interactions, which occur in all cases of adsorption, and from specific interactions with the hydroxy groups nevertheless, valuable conclusions about the binding forces can be deduced. Saturated hydrocarbons, e.g., n-pentane, have a value of — AH of 8.0 kcal/mole, while saturated ethers have values of around 16 kcal/mole.14 Probably dispersion forces only are involved in the former case and additional specific interaction with the silanol-OH occurs in the second case. On graphite, where there is no specific interaction, the heats of adsorption of hydrocarbons and ethers are very similar.17 The heat of adsorption of furan (11 kcal/mole) is 5 kcal/mole less than that of tetrahydrofuran this again indicates the effect that delocalization of electrons by the double bonds has on the binding forces.14... 

The carbon sheets in graphite are separated by a distance of 335 pm and are held together by only London dispersion forces. Atmospheric gases can be absorbed between the sheets, thus enabling the sheets to easily slide over one another. As a result, graphite has a slippery feel and can be used as a lubricant. Because the sheets are so far apart, it s relatively difficult for an electron to hop from one sheet to the next and the electrical conductivity in the direction perpendicular to the sheets is therefore about 104 times smaller than the conductivity parallel to the sheets. 

However, since SCC-DFTB is derived from DFT, it inherits the DFT failures and shortcomings. On the one hand, there is the deficiency of DFT for the description of van der Waals bonded complexes. Here, we extended SCC-DFTB by an explicit treatment of attractive dispersion forces [36], an extension called hereafter SCC-DFTB-D, which has been added to DFT methods in the same way later on as well [37,38], We have shown that this term is crucial not only for the interaction of DNA bases [36,39,40] or DNA intercalators [41,42], but also, for example, for the structure and stability of water on a graphite surface [43] and certain peptide configurations [21,23,44],... 

Intramolecular forces do not account for all attractions between particles. There are forces of attraction called intermolecular forces. The prefix inter-means "between" or "among." For example, an interview is a conversation between two people. Intermolecular forces can hold together identical particles, such as water molecules in a drop of water, or two different types of particles, such as carbon atoms in graphite and the cellulose particles in paper. The three intermolecular forces that will be discussed in this section are dispersion forces, dipole-dipole forces, and hydrogen bonds. Although some intermolecular forces are stronger than others, all intermolecular forces are weaker than intramolecular, or bonding, forces. 

The physically active sites of carbon black should produce an increase in interfacial adhesion and hence an increase in reinforcement. Furthermore, this increase in reinforcement should be non-specific for different non-polar polymers, since it still reflects a dispersion force interaction. While its existence cannot be questioned, it clearly cannot be assumed to be responsible for the entire observed increase in reinforcement over and above that displayed by graphitized carbon black if other, more energetic bonds are also formed. 

A useful example is graphite. The contact angle of water on graphite was found to be 85.7° and was 19 dynes per cm. [8,13]. Kwe assume that graphite and water interact entirely by dispersion forces, we can use Equation 4, rearranged as... 

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