Interaction molecular attraction

DLVO theory explained major principles of coagulation of hydrosols by electrolytes and brought to common grounds all previous observations (primarily of qualitative nature) that related to individual cases and often seemed to be contradictory. In years that followed further extensions of DLVO theory that took into account the possibility of reversible particle aggregation were developed. At very small distances between particles in addition to the usual long-range interaction, molecular attraction and electrostatic repulsion, one must account for other factors that play role at a direct particle contact. The formation of peculiarly structured hydration layers in the vicinity of solid surface, the appearance of elastic forces that are responsible for the Born repulsion between surface atoms at the point of contact, the repulsion between the adsorbed surfactant molecules in contact zone between two particles, all represent the so-called non-DLVO stability factors . This means that more or less deep primary minimum remains finite. 

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. 

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... 

The internal forces of a pair of liquids are seldom so nearly alike as to permit their mixture to obey Raoult s law very closely throughout the whole range of composition. In the absence of chemical interaction, the attraction between two different molecular species, provided their dipole moments are zero or small, is approximately the geometric mean of the attractions between the like molecules, Since a geometric mean is less than an arithmetic mean, the mixing is accompanied by expansion and absorption of heat. The partial molal heat of transfer per mole from pure liquid to solution is given with fair accuracy for many systems by the equation,... 

The forces of attraction between neutral, chemically saturated molecules, postulated by van der Waals to explain non-ideal gas behaviour, also originate from electrical interactions. Three types of such inter molecular attraction are recognised ... 

The unusual solvent properties of supercritical fluids (SCFs) have been known for over a century (1). Just above the critical temperature, Tc, forces of molecular attraction are balanced by kinetic energy and fluid properties, including solvent power, exhibit a substantial pressure dependence. Many complex organic materials are soluble at moderate pressures (80 to 100 atmospheres) and SCF solvent power increases dramatically when the pressure is increased to 300 atmospheres. The pressure responsive range of solvent properties thus attainable provides a tool for investigating the fundamental nature of molecular interactions and is also being exploited in important areas of applied research (2,3). 

B. V. Derjaguin, I. I. Abrikosova, and E. M. Lifshitz, "Direct measurement of molecular attraction between solids separated by a narrow gap," Q. Rev. (London), 10, 295-329 (1956) see also W. Arnold, S. Hunklinger, and K. Dransfeld, "Influence of optical absorption on the van der Waals interaction between solids," Phys. Rev. B, 19, 6049-56 (1979), for more recent measurements with glasses. 

The value of 1.003 atm is slightly larger than that of He, owing to the fact that the larger S02 molecules occupy more total space, which means they will experience more collisions with the container than He atoms. The much higher correction factor for molecular attractions (0.0134 atm) is higher than He because the much larger molecule is more capable of intermolecular interactions than He. 

With respect to the molecular interactions the simplest asymmetric films are these from saturated hydrocarbons on a water surface. Electrostatic interaction is absent in them (or is negligible). Hence, of all possible interactions only the van der Waals molecular attraction forces (molecular component of disjoining pressure) can be considered in the explanation of the stability of these films. For films of thickness less than 15-20 nm, the retardation effect can be neglected and the disjoining pressure can be expressed with Eq. (3.76) where n = 3. When Hamaker s constants are negative the condition of stability is fulfilled within the whole range of thicknesses. 

The physical meaning of this equation can be discerned by noting the positive signs on the DMSI and HBMSI terms. Increased dispersion interactions increase molecular attractions and, hence, increase the boiling point. Similarly, an increase in H-bonding molecular interactions would have the same result. 

In contrast to dipole-dipole forces, London Dispersion interactions are much weaker in nature since they involve nonpolar molecules that do not possess permanent dipole moments. The only modes for molecular attraction are through polarization of electrons, which leads to the creation of small dipole-dipole interactions and mutual attractive forces. Since electron polarization occurs much more readily for electrons farther from the nucleus, this effect is more pronounced for molecules that are larger with a greater number of electrons, especially positioned on atoms with a high atomic number, consisting of more diffuse orbitals. These induced dipole forces are responsible for the liquefaction of gases such as He and Ar at low temperatures and pressures. The relative strength of London Dispersion forces is described by Eq. 3 ... 

Jobling treated viscosity as due to the interaction of thermal molecular motions and molecular attraction Parshad attributed it to molecular potential barriers Girard and Abadie related it to dielectric properties. Bosworth, assuming the transmission of phonons , analogous to Debye waves ( 6.K N), found ... 

Of the different types of forces responsible for intermolecular attraction, the foremost are the London or dispersion forces that act between all atoms and account for virtually all of the molecular attraction or cohesion in all molecules except the very polar molecules (described later). Dispersion forces are short-range interactions, effective at about 4 A, and rapidly decrease with the sixth power of the distance between molecules. Therefore, the adhesive polymer molecule must be flexible enough to come within this range of interaction with the rigid adherent surface under the conditions of bond formation. 

Two broad explanations are possible for the interaction of cellulose and caustic soda. For the first one, it is assumed that cellulose combines with caustic soda to form alkali-cellulose and swelling is due to the molecular attraction with associated... 

Adsorption of DPM from seawater by humic acid was recorded and the value of Kads was determined by a simple least squares analysis of the linear portion of the isotherm to be 1.11 for a 150 ppm suspension of humic acid in seawater (Table II). This low value of Kads is not unexpected, since one would expect the interaction between the neutral nonpolar DPM molecule and the suspended humic acid to be due to a rather weak molecular attraction. 

It would be expected that the trend described above should also be verified for proteins. Goklen and Hatton confirmed this hypothesis (38, 39), at least for low molecular weight proteins. They studied the solubilization of three proteins, cytochrome C, ribonuclease and lysozyme of similar size but with distinct pi, from a 1 mg/ml aqueous protein solution into a 50 mM AOT/isooctane organic phase (figure 2) and showed that no extraction occurs for pH > pi, i.e. for pH values where the net charge of the protein is negative. As soon as the pH decreases below the pi value, there is an abrupt enhancement of the protein solubilization the surfactant and the protein bear opposite charges and electrostatic interactions become attractive. The decrease of protein solubilization at low pH values is interpreted by the authors in terms of protein precipitation at the interface due to denaturation. 

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