Forces between molecules dispersion

Intermolecular Forces Forces Between Molecules Dispersion Forces Dipole — Dipole Forces Flydrogen Bonding... 

For crystallographically non-equivalent atoms the corresponding components of the electric field gradient (EFG) at the respective sites differ from each other in magnitude and direction due to the crystal field effect. This generally includes a contribution to the EFG of electrostatic forces between molecules, dispersion forces, intermolecular bonding and short-range repulsion forces. Physically non-equivalent sites differ from each other only in the direction of... 

McLachlan A D 1963 Retarded dispersion forces between molecules Proc. R. Soc. A 271 387... 

Fig. 4, 33 The Drude model for dispersive interactions. (Figure adapted from Rigby M, E B Smith, W A Wakeham and G C Maitland 1986. The Forces Between Molecules. Oxford, Clarendon Press.)...

The weaker the intermolecular forces between molecules in the liquid phase, the higher is the vapor pressure. Generally, hydrogen bonding and the cumulative dispersion forces in larger molecules are the most significant factors (see Table 13-3). In order of decreasing vapor pressure,... 

Drawing a Conclusion Based on your data, in which liquid(s) are the attractive forces between molecules most likely to be dispersion forces ... 

The major forms of van der Waals forces between molecules that are not bonded together are the permanent dipole-dipole interaction, the dispersion-induced temporary dipole interaction, and the hydrogen bond. They are short-range forces that operate only when two atoms or molecules are in close proximity. The Lennard-Jones potential of 6-12 is a model of this potential field ... 

F2 is nonpolar the only intermolecular attraction present is due to dispersion forces. 1 point for stating that Fj only has dispersion forces between molecules. 

Relatively weak forces of attraction that exist between nonpolar molecules are called van der Waals forces or London dispersion forces. Dispersion forces between molecules are much weaker than the covalent bonds within molecules. Electrons move continuously within bonds and molecules, so at any time one side of the molecule can have more electron density than the other side, which gives rise to a temporary dipole. Because the dipoles in the molecules are induced, the interactions between the molecules are also called induced dipole-induced dipole interactions. 

Keesom, Debye, and London contributed much to our understanding of forces between molecules [111-113]. For this reason the three dipole interactions are named after them. The van der Waals4 force is the Keesom plus the Debye plus the London dispersion interaction, thus, all the terms which consider dipole-dipole interactions Ctotai = Corient+Cind- -Cdisp. All three terms contain the same distance dependency the potential energy decreases with l/D6. Usually the London dispersion term is dominating. Please note that polar molecules not only interact via the Debye and Keesom force, but dispersion forces are also present. In Table 6.1 the contributions of the individual terms for some gases are listed. 

Also, in the 1930 s London (9) indicated the quantum mechanical origin of dispersion forces between apolar molecules and in subsequent work extended these ideas to interaction between particles (10). It was shown that whereas the force between molecules varied inversely as the seventh power of the separation distance, that between thick flat plates varied inversely as the third power of the distance of surface separation. These ideas lead directly to the concept of a "long range van der Waals attractive force. A similar relationship was found for interaction between spheres (10). 

In a thick film, the molecules located at its free surface do not sense the presence of the substrate. In contrast, in a thin film they do interact with the substrate. For the majority of the molecules of a thick film, the range of the interaction forces is smaller than the thickness of the film. In contrast, it is larger for the molecules of a thin film. As a result, the free energy of a thin film depends on its thickness. Considering, for illustrative purposes, London dispersion forces between molecules, the following expression is obtained for the interaction... 

An attractive interaction arises due to the van der Waals forces between molecules of colloidal particles. Depending on the nature of dispersed particles, the Keesom forces (or the dipole-dipole interaction), the Debye forces (or dipole-induced dipole interaction), and the London forces (or induced dipole-induced dipole interaction) may contribute to the van der Waals interaction. First, the van der Waals interaction was theoretically computed using a method of the pairwise summation of interactions between different pairs of molecules of the two macroscopic particles. This method called the microscopic approximation neglects collective effects, and, as a consequence, misrepresents the Hamaker constant. For many problems of a practical use, however, specific features of the total interaction are determined by a repulsive part, and such an effective, gross description of the van der Waals interaction may often be accepted [3]. The collective effects in the van der Waals interaction have been taken into account in the calculations of Lifshitz et al. [4], and their method is known in the literature as the macroscopic approach. 

McLachlan AD (1963) Retarded dispersion forces between molecules. Proc Roy Soc (London) Ser A 271 387-401... 

Van der Waals forces are relatively short-range forces between molecules with permanent dipoles or molecules with induced dipoles, i.e. they almost occur between all molecules. These interactions include dipole-dipole interactions, dipole-induced dipole interactions and dispersion energy. 

Casimir and Polder also showed that retardation effects weaken the dispersion force at separations of the order of the wavelength of the electronic absorption bands of the interacting molecules, which is typically 10 m. The retarded dispersion energy varies as R at large R and is determined by the static polarizabilities of the interacting molecules. At very large separations the forces between molecules are weak but for colloidal particles and macroscopic objects they may add and their effects are measurable. Fluctuations in particle position occur more slowly for nuclei than for electrons, so the intermolecular forces that are due to nuclear motion are effectively unretarded. A general theory of the interaction of macroscopic bodies in terms of the bulk static and dynamic dielectric properties... 

Physical adsorption is a universal phenomena, producing some, if not the major, contribution to almost every adhesive contact. It is dependent for its strength upon the van der Waals attraction between individual molecules of the adhesive and those of the substrate. Van der Waals attraction quantitatively expresses the London dispersion force between molecules that is brought about by the rapidly fluctuating dipole moment within an individual molecule polarizing, and thus attracting, other molecules. Grimley (1973) has treated the current quantum mechanical theories involved in simplified mathematical terms as they apply to adhesive interactions. 

All the important contributions to the forces between molecules arise ultimately from the electrostatic interactions between the particles that make up the two molecules. Thus our main theoretical insight into the nature of intermolecular forces comes from perturbation theory, using these interactions as the perturbation operator H = Z e, /(4jtSor/y), where is the charge on particle i in one molecule, is the distance between particles i and / in different molecules, and 8q is permittivity of a vacuum. The definitions of the contributions, such as the repulsion, dispersion, and electrostatic terms, which are normally included in model potentials, correspond to different terms in the perturbation series expansion. 

Dipole-dipole forces are generally stronger than London dispersion forces. However, both of these forces between molecules are usually much weaker than ionic forces in crystals. There are exceptions. One major factor is the size of the atoms, ions, or molecules. The larger the particles are, the farther apart they are and the smaller the effects of the attraction are. If an ionic substance has very large ions—especially if the ions are not symmetrical—the ionic substance s melting point can be very low. A few ionic compounds are even liquid at room temperature, such as 1-butylpyridinium nitrate, shown in Figure 15. 

Are the London dispersion forces between water molecules weaker or stronger than the London dispersion forces between molecules of hydrogen sulfide, H2S ... 

What causes dispersion forces What factors determine the strengths of dispersion forces between molecules ... 

London dispersion forces Attractive forces between molecules of all kinds, which result from the formation of instantaneous dipoles in the molecules. The short-lived dipoles are caused by the unsymmetrical motion of electrons about the nuclear framework of the molecule. 

In addition to dispersion forces between molecules, there can be an additional force of attraction between the molecules if they have permanent dipoles whose strength and direction are fixed as a result of their molecular structure. The simplified expression that accounts for permanent dipole-dipole potential energy is... 

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