Nonpolar attraction

Silica gel modified with carbon chains, including the most popular, -Cig (octadecylsilica), in which alkyl groups have 18 atoms of carbon, are usually applied in reversed phase separations. Different alky or aryl groups are used for modification of silica, such as -C2, -C4, -Cg, -CN, and -NH2. All these functional groups are hydrophobic therefore, retention of analytes is a result of nonpolar-nonpolar attractive forces or dispersion forces. Sorbents of silica gel modified with carbon chains are stable over a narrow pH range. 

This nonpolar character results in the attraction of nonpolar solutes. Since the solute has little polar character, it is more strongly immobilized on the membrane surface than is water. Thus, the solvent passes through the membrane relatively fast and positive separation is observed. The strength of this nonpolar attraction, quantitatively represented by the modified Small s number (1,20), will, in part, determine the extent of separation. 

The secondary bonds are presumably due to the same nonspecific attractive forces which cause precipitation of globulins in the absence of salt or gelation of denatured proteins under certain controlled conditions —a combination of coulombic, dipole, hydrogen bond, and nonpolar attractions depending on the amino acid distribution concerned (Section c, page 63). 

Much of chemistry is concerned with the short-range wave-mechanical force responsible for the chemical bond. Our emphasis here is on the less chemically specific attractions, often called van der Waals forces, that cause condensation of a vapor to a liquid. An important component of such forces is the dispersion force, another wave-mechanical force acting between both polar and nonpolar materials. Recent developments in this area include the ability to measure... 

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... 

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... 

Alkanes and cycloalkanes are nonpolar and insoluble m water The forces of attraction between alkane molecules are induced dipole/induced dipole attractive forces The boiling points of alkanes increase as the number of carbon atoms increases Branched alkanes have lower boiling points than their unbranched isomers There is a limit to how closely two molecules can approach each other which is given by the sum of their van der Waals radii... 

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... 

Both polar compounds ethanol and fluoroethane have higher boiling points than the nonpolar propane We attribute this to a combination of dipole/mduced dipole and dipole-dipole attractive forces that are present m the liquid states of ethanol and fluo roethane but absent m propane... 

In general arenes resemble other hydrocarbons in their physical properties They are nonpolar insoluble in water and less dense than water In the absence of polar sub stituents mtermolecular forces are weak and limited to van der Waals attractions of the induced dipole/mduced dipole type... 

Critical micelle concentration (Section 19 5) Concentration above which substances such as salts of fatty acids aggre gate to form micelles in aqueous solution Crown ether (Section 16 4) A cyclic polyether that via lon-dipole attractive forces forms stable complexes with metal 10ns Such complexes along with their accompany mg anion are soluble in nonpolar solvents C terminus (Section 27 7) The amino acid at the end of a pep tide or protein chain that has its carboxyl group intact—that IS in which the carboxyl group is not part of a peptide bond Cumulated diene (Section 10 5) Diene of the type C=C=C in which a single carbon atom participates in double bonds with two others... 

Solubility Properties. Fats and oils are characterized by virtually complete lack of miscibility with water. However, they are miscible in all proportions with many nonpolar organic solvents. Tme solubiHty depends on the thermal properties of the solute and solvent and the relative attractive forces between like and unlike molecules. Ideal solubiHties can be calculated from thermal properties. Most real solutions of fats and oils in organic solvents show positive deviation from ideaHty, particularly at higher concentrations. Determination of solubiHties of components of fat and oil mixtures is critical when designing separations of mixtures by fractional crystallization. 

The extremely nonpolar character of PFCs and very low forces of attraction between PFC molecules account for their special properties. Perfluorocarbons bod only slightly higher than noble gases of similar molecular weight, and their solvent properties are much more like those of argon and krypton than hydrocarbons (2). The physical properties of some PFCs are Hsted in Table 1. 

Sorption of nonionic, nonpolar hydrophobic compounds occurs by weak attractive interactions such as van der Waals forces. Net attraction is the result of dispersion forces the strength of these weak forces is about 4 to 8 kj/mol ( 1 2 kcal/mol). Electrostatic interactions can also be important, especially when a molecule is polar in nature. Attraction potential can develop between polar molecules and the heterogeneous sod surface that has ionic and polar sites, resulting in stronger sorption. 

Internal surfactant antistats ate physically mixed with the plastic resin prior to processing. When the resin is melted, the antistat distributes evenly in the polymer matrix. The antistat usually has some degree of solubiUty in the molten polymer. However, when the polymer is processed (extmded, molded, etc) into its final form and allowed to cool, the antistat migrates to the surface of the finished article due to its limited solubiUty in the solidified resin. The molecule of a surface-active agent is composed of a polar hydrophilic portion and a nonpolar hydrophobic portion. The hydrophilic portion of the surfactant at the surface attracts moisture from the atmosphere it is the moisture that has the static dissipative effect. 

Attractive and Repulsive Forces. The force that causes small particles to stick together after colliding is van der Waals attraction. There are three van der Waals forces (/) Keesom-van der Waals, due to dipole—dipole interactions that have higher probabiUty of attractive orientations than nonattractive (2) Debye-van der Waals, due to dipole-induced dipole interactions (ie, uneven charge distribution is induced in a nonpolar material) and (J) London dispersion forces, which occur between two nonpolar substances. 

If a surface is polar, its resulting electric field will induce a dipole moment in a molecule with no permanent dipole and, through this polarization, increase the extent of adsorption. Similarly, a molecule with a permanent dipole moment will polarize an otherwise nonpolar surface, thereby increasing the attraction. 

Modem understanding of the hydrophobic effect attributes it primarily to a decrease in the number of hydrogen bonds that can be achieved by the water molecules when they are near a nonpolar surface. This view is confirmed by computer simulations of nonpolar solutes in water [15]. To a first approximation, the magnimde of the free energy associated with the nonpolar contribution can thus be considered to be proportional to the number of solvent molecules in the first solvation shell. This idea leads to a convenient and attractive approximation that is used extensively in biophysical applications [9,16-18]. It consists in assuming that the nonpolar free energy contribution is directly related to the SASA [9],... 

A statistical mechanical fonnulation of implicit solvent representations provides a robust theoretical framework for understanding the influence of solvation biomolecular systems. A decomposition of the free energy in tenns of nonpolar and electrostatic contributions, AVF = AVF " + AVF ° , is central to many approximate treatments. An attractive and widely used treatment consists in representing the nonpolar contribution AVF " by a SASA surface tension term with Eq. (15) and the electrostatic contribution by using the... 

These unusual properties of fluorocarbons reflect theu nonpolar character, low polarizability, and overall relatively weak intermolecular attractions Saturated perfluoro-terr-ammes and -dialkyl ethers also closely resemble fluorocarbons rather than typical amines or ethers in their physical properties [4,... 

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