Molecular attraction

We have two interaction potential energies between uncharged molecules that vary with distance to the minus sixth power as found in the Lennard-Jones potential. Thus far, none of these interactions accounts for the general attraction between atoms and molecules that are neither charged nor possess a dipole moment. After all, CO and Nj are similarly sized, and have roughly comparable heats of vaporization and hence molecular attraction, although only the former has a dipole moment. 

CapillaTjflow is Hquid flow through the pores, interstices, and over the surfaces of soHds which is caused by Hquid—soHd molecular attraction and Hquid surface tension. 

Capillary flow is the flow of hquid through the interstices and over the surface of a solid, caused by liquid-solid molecular attraction. 

Adsorption on solids is a process in which molecules in a fluid phase are concentrated by molecular attraction at the interface with a solid. The attraction arises from van der Waals forces, which are physical interactions between the electronic fields of molecules, and which also lead to such behavior as condensation. Attraction to the surface is etihanced because the foreign molecules tend to satisfy an imbalance of forces on the atoms in the surface of a solid compared to atoms within the solid where they are surrounded by atoms of the... 

Uric acid is odourless in spite of three carbonyl groups, four trivalent nitrogen atoms and a double bond, and that it is similarly colourless in spite of four chromophores. Measurements of its refractive and dispersive properties indicate that it is a saturated body which suggests that molecular attraction exists between the various groups. 

If the surface is plane, r = oo. . P = 0. It is probable that a pressure K is exerted on the liquid by a plane surface, due to molecular attraction, so that equation (1) should be written... 

Molecular attraction (surface tension, adsorptive, diffusive, and osmotic forces)... 

Nevertheless, this molecular attraction exists and shows itself, when the gas is strongly compressed —and the distance between the molecules is greatly reduced—by causing deviations from Boyle s law. If we consider two layers of molecules, the distance between which is, of course, smaller than the radius of molecular attraction (Fig. 3), we see that their mutual attraction, or, in other words, the intrinsic pressure is proportional to the number of attracting molecules and to the number of attracted molecules, that is, proportional to the square... 

To connect the internal latent heat with the intrinsic pressure let us consider the forces to which a molecule of the liquid is subject. As long as it is in the interior of the liquid these are obviously equal in all directions, but the case is different when the molecule approaches the surface nearer than the radius of molecular attraction. Let O (Fig. 4) be such a molecule and describe round it a sphere with the radius C of molecular attraction then only the liquid within that sphere will have any effect on O. In the position shown the molecule is attracted downwards by the liquid contained in the segment ab (equal to AB), as the downward... 

In view of the fundamental importance of the Gibbs-Thomson formula, and the magnitude of the discrepancies between the figures calculated from it and the experimental results, it is of obvious interest to inquire to What causes the deviations may be due. The first point to be noticed is that the complex substances which exhibit them most markedly form, at least at higher concentrations, colloidal and not true solutions. It is, therefore, very probable that they may form gelatinous or semi-solid skins on the adsorbent surface, in which the concentration may be very great. There is a considerable amount of evidence to support this view. Thus Lewis finds that, if the thickness of the surface layer be taken as equal to the radius of molecular attraction, say 2 X io 7 cms., and the concentration calculated from the observed adsorption, it is found, for instance, for methyl orange, to be about 39%, whereas the solubility of the substance is only about 078%. The surface layer, therefore, cannot possibly consist of a more concentrated solution of the dye, which is the only case that can be dealt with theoretically, but must be formed of a semi-solid deposit. 

Structural influences such as conjugation or configuration and intra and inter molecular attractions hydrogen bonding in particular shift the IR bands of the groups involved. 

In either equations (1) or (2) the non-ideality parameter w (sometimes written w/RT) arises from the difference between the inter-molecular attraction of unlike species as compared to the mean of the intermolecular attraction for pairs of like species. The second parameter in equation (1), is sometimes ascribed... 

LlFSHITZ, E. M. Soviet Physics. J.E.T.P. 2 (1956) 73. Theory of molecular attractive forces between solids. 

The rate of diffusion in liquids is much slower than in gases, and mixtures of liquids may take a long time to reach equilibrium unless agitated. This is partly due to the much closer spacing of the molecules, as a result of which the molecular attractions are more important. 

Equation (3) represents the reduction in interfacial tension resulting from molecular attraction between liquid and solid. The term ([) is defined by Eq. (4),... 

Surface tension occurs when two fluids are in contact with each other. This is caused by molecular attractions between the molecules of two liquids at the surface of separation. It is expressed as dynes/cm or ergs/cm. ... 

Under the prevailing temperature and pressure conditions (Ref 3, p 190), the weak molecular attractions must be negligible compared to the mean molecular energies. This accords with the employment by Kihara and Hikita, in their equation of state. [See under DETONATION (AND EXPLOSION), EQUATIONS OF STATE IN (AND SOME OTHER EQUATIONS)]... 

The solubility parameter 6 is a measure of the cohesion of a material, or of the strength of molecular attractive forces between like molecules. The relationship between the solubility parameter and enthalpy of mixing is given by the Hildebrand equation (1) ... 

P (r) is the attractive contribution important at large separations. (This same notion is used in the van der Waals equation of state, in which the constant a accounts for molecular attraction and b for molecular repulsion.)... 

J. W. Capstick further explains the effect of temp, on the colour of compounds by assuming that (i) the molecules vibrate about certain mean positions, and that (ii) a rise of temp, produces a greater amplitude of vibration, but not a greater period, so that if the vibration be not quite harmonic, a greater amplitude may, as with a pendulum, require a longer period, (iii) A rise of temp, is also supposed to weaken the cohesion or inter-molecular attraction between the molecules, and thus lessen the force of restitution, so that the molecules vibrate more slowly and thus produce the same sequence of colour changes with rise of temp, as are observed when the mass of the molecule is increased. 

Figure 2.4 Schematic representation of Van der Waals corrections (i) Dotted line showing the spherical excluded region (of volume ra/3) surrounding a probe molecule (heavy circle) that is inaccessible to the center of mass of another molecule of diameter d. (ii) Wavy lines (molecular attractions) depicting the net pulling effect of attractions by surrounding molecules on a given probe molecule (heavy circle) about to strike the wall, thereby reducing the impact of collision and resulting pressure of wall collisions.

Surface tension is caused by hydrogen bonds. As shown in Figure 8.14, beneath the surface, each water molecule is attracted in every direction by neighboring molecules, with the result that there is no tendency to be pulled in any preferred direction. A water molecule on the surface, however, is pulled only by neighbors to each side and those below there is no pull upward. The combined effect of these molecular attractions is thus to pull the molecule from the surface into the liquid. This tendency to pull surface mol-... 

Differences in the strength of molecular attractions explain why different hydrocarbons condense at different temperatures. As discussed in Section 7.1, in our comparison of induced dipole—induced dipole attractions in methane and octane, larger hydrocarbons experience many more of these attractions than smaller hydrocarbons do. For this reason, the larger hydrocarbons condense readily at high temperatures and so are found at the bottom of the tower. Smaller molecules, because they experience fewer attractions to neighbors, condense only at the cooler temperatures found at the top of the tower. 

Ethers are not very soluble in water because, without the hydroxyl group, they are unable to form strong hydrogen bonds with water (Section 7.1). Furthermore, without the polar hydroxyl group, the molecular attractions among ether molecules are relatively weak. As a result, it does not take much energy to separate ether molecules from one another. This is why ethers have relatively low boiling points and evaporate so readily. 

The fluorine atoms in polytetra-fluoroethylene tend not to experience molecular attractions, which is why this addition polymer is used as a nonstick coating and lubricant. 

The structure of proteins determines their function and can be described on four levels, illustrated on page 447. The primary structure is the sequence of amino acids in the polypeptide chain. The secondary structure describes how various short portions of a chain are either wrapped into a coil called an alpha helix or folded into a thin pleated sheet. The tertiary structure is the way in which an entire polypeptide chain may either twist into a long fiber or bend into a globular clump. The quaternary structure describes how separate proteins may join to form one larger complex. Each level of structure is determined by the level before it, which means that ultimately it is the sequence of amino acids that creates the overall protein shape. Fhis final shape is maintained both by chemical bonds and by weaker molecular attractions between amino acid side groups. 

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