Molecules nonpolar

While vapor-phase corrections may be small for nonpolar molecules at low pressure, such corrections are usually not negligible for mixtures containing polar molecules. Vapor-phase corrections are extremely important for mixtures containing one or more carboxylic acids. 

An interesting alternative method for formulating f/(jt) was proposed in 1929 by de Boer and Zwikker [80], who suggested that the adsorption of nonpolar molecules be explained by assuming that the polar adsorbent surface induces dipoles in the first adsorbed layer and that these in turn induce dipoles in the next layer, and so on. As shown in Section VI-8, this approach leads to... 

Lsc th e force fields th at have dern on strated accuracy for particu lar molecules or simulations. For example, CiPLS reproduces physical properties in liquid simulations extremely well. MM+ reproduces the structure and thermodynamic properties of small, nonpolar molecules better than AMBER, BIO+, and OPLS. 

Hydrophobic effects include two distinct processes hydrophobic hydration and hydrophobic interaction. Hydrophobic hydration denotes the way in which nonpolar solutes affect the organisation of the water molecules in their immediate vicinity. The hydrophobic interaction describes the tendency of nonpolar molecules or parts thereof to stick together in aqueous media " . A related frequently encountered term is hydrophobicity . This term is essentially not correct since overall attractive interactions exist between water and compounds commonly referred to as... 

As is suggested frequently , this term might well result from the restriction of the hydrogen bonding possibilities experienced by the water molecules in the first hydration shell. For each individual water molecule this is probably a relatively small effect, but due to the small size of the water molecules, a large number of them are entangled in the first hydration shell, so that the overall effect is appreciable. This theory is in perfect agreement with the observation that the entropy of hydration of a nonpolar molecule depends linearly on the number of water molecules in the first hydration shell ". ... 

If one would ask a chemist not burdened with any knowledge about the peculiar thermodynamics that characterise hydrophobic hydration, what would happen upon transfer of a nonpolar molecule from the gas phase to water, he or she would probably predict that this process is entropy driven and enthalpically highly unfavourable. This opinion, he or she wo ild support with the suggestion that in order to create room for the nonpolar solute in the aqueous solution, hydrogen bonds between water molecules would have to be sacrificed. 

Filling of these cavities with a nonpolar molecules is enthalpy driven. See Diederich, F. Smithrud, D. B. Sanford, E. M. Wyman, T. B. Ferguson, S. B. Carcanague, D. R. Chao, I. Houk, K. N. Acta Chem. Scand. 1992, 46, 205 and references cited therein See (a) Blokzijl, W. Ph. D. Thesis, University of Groningen, 1991 (b) Streefland, L. Ph. D. TTzewis, University of Gronirigen, 1998 and references cited therein. 

The type of enforced hydrophobic effect that is operative in the retro Diels-Alder reaction cannot be referred to an enforced hydrophobic interaction, since there is no coming together, but rather a separation of nonpolar molecules during the reaction. It is better to refer to this process as an enforced hydrophobic effect. 

Induced dipole/mduced dipole attractions are very weak forces individually but a typical organic substance can participate m so many of them that they are collectively the most important of all the contributors to mtermolecular attraction m the liquid state They are the only forces of attraction possible between nonpolar molecules such as alkanes... 

Induced dipole/induced dipole forces are the only intermolecular attractive forces available to nonpolar molecules such as alkanes In addition to these forces polar molecules engage m dipole-dipole and dipole/mduced dipole attractions The dipole-dipole attractive force is easiest to visualize and is illustrated m Figure 4 3 Two molecules of a polar substance experience a mutual attraction between the positively polarized region of one molecule and the negatively polarized region of the other As its name implies the dipole/induced dipole force combines features of both the induced dipole/mduced dipole and dipole-dipole attractive forces A polar region of one mole cule alters the electron distribution m a nonpolar region of another m a direction that produces an attractive force between them... 

Another spherical, nonpolar molecule is methane its isotherms on both graphite and molybdenite at 77 K have a step-like character. Ethane, whilst slightly less symmetrical, is still nonpolar and it gives two distinct steps on cadmium at 97-4 K, the second step being nonhorizontal. ... 

The (dispersion plus repulsion) terms are known as the London or van der Waals forces. Spherical, nonpolar molecules are well described... 

The dissolution of polar molecules in water is favored by dipole—dipole interactions. The solvation of the polar molecules stabilizes them in solution. Nonpolar molecules are soluble in water only with difficulty because the relatively high energy cost associated with dismpting and reforming the hydrogen-bonded water is unfavorable to the former occurring. 

Binary Mixtures—Low Pressure—Polar Components The Brokaw correlation was based on the Chapman-Enskog equation, but 0 g and were evaluated with a modified Stockmayer potential for polar molecules. Hence, slightly different symbols are used. That potential model reduces to the Lennard-Jones 6-12 potential for interactions between nonpolar molecules. As a result, the method should yield accurate predictions for polar as well as nonpolar gas mixtures. Brokaw presented data for 9 relatively polar pairs along with the prediction. The agreement was good an average absolute error of 6.4 percent, considering the complexity of some of... 

What are the essential features of surfactant systems An important ingredient is obviously the repulsion between water and nonpolar molecules or molecule parts, the hydrophobic force. This interaction is however highly nontrivial, and its analysis is still an active field of research [4,22,23]. Qualitatively, it is usually attributed to the strong orientational and positional correlations between nonpolar molecules in solution and the surrounding water molecules. The origin of the interaction is therefore entropic free water forms a network of hydrogen bonds. In the neighborhood... 

Neutral hydrocarbons are generally nonpolar molecules. This is to be expected since carbon-carbon and carbon-hydrogen bonds are relatively nonpolar. Resonance effects can alter this picture, however, by redistributing electrons in novel ways. 

Nonpolar molecules such as H, N, O, I, and Cl have zero dipole moments, because e = 0. On the other hand, hydrogen fluoride, HF, has a large dipole moment of 1.75 Debye and so is strongly polar. Simple carbon compounds with symmetric arrangement of like atoms (e.g., methane, CH, and carbon tetrachloride,CCl.,) have zero dipole moments and so are nonpolar. 

Figure 2.8 Attractive dispersion forces in nonpolar molecules are caused by temporary dipoles, as shown in these models of pentane, C5H12-...

The extent to which molecules tend to orient themselves in an electrical field is a measure of their dipole moment. A polar molecule such as HF has a dipole moment a nonpolar molecule such as H2 or F2 has a dipole moment of zero. 

Orientation of polar molecules in an electric field. With the field off. polar molecules are randomly orientad. With the field on, polar molecules such as HF align their positive and negative ends toward the negative and positive poles of tire field, respectively. Nonpolar molecules such as H2 do not line up. 

In nonpolar molecules, dispersion is the only intermolecular force. 

Polar molecules, like nonpolar molecules, are attracted to one another by dispersion forces. In addition, they experience dipole forces as illustrated in Figure 9.9, which shows the orientation of polar molecules, such as Id, in a crystal. Adjacent molecules line up so that the negative pole of one molecule (small Q atom) is as dose as possible to the positive pole (large I atom) of its neighbor. Under these conditions, there is an electrical attractive force, referred to as a dipole force, between adjacent polar molecules. 

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