Weak polar forces

But the aromatic side chains are different. The geometric pattern of their interactions, both with each other and with polar groups in the interior, are governed by weakly polar forces. These interactions are not random, and the random packing of aliphatic residues does not take precedence over them. We shall now treat these interactions in detail. 

It s a bit more complicated than this, in that the number of arrangements will also depend upon the strength of the interactions relative to thermal energy, RT, a measure of the average kinetic energy of the molecules (per mole). In other words, if the interaction energies between the molecules are relatively weak, say, just dispersion forces and weak polar forces (see Chapter 8, page 208), then at ambient temperatures one would... 

Because most of you are probably too lazy to go and review stuff, we ll briefly mention a couple of pertinent points. First, the interaction between two molecules is described in terms of the potential energy (P.E.). When the molecules are too close they strongly repel, when they are too far apart they don t feel each other. There is some optimum distance apart where their interaction is a maximum, hence P.E. is a minimum, or has the largest negative or attractive value. Second, there are advanced theoretical models that deal with potential functions like this, which we will not consider, but for dispersion and weak polar forces the attractive energy varies as 1/r6, where r is the distance between the molecules. 

If you re thinking about this you might see where we re going. We have an expression where we either know the parameters (composition terms) or can measure them (cohesive energy densities of the pure components). But we don t know and cannot measure C. However, it has been shown that for dispersion forces and perhaps weak polar forces a geometric mean assumption is a good approximation (Equation 11-30) ... 

However the size of this term gets smaller as the size of the molecules (mA, mB) gets larger. On the other hand, the x term is always positive and unfavorable to mixing in systems where the molecules interact through dispersion and weak polar forces only, as can more readily be seen if we write x in terms of solubility parameters (neglecting any fudge factors)—see Equation 11-37 above. 

A rough rule of thumb would then be that if a polymer has a solubility parameter of say, 9, it should dissolve in solvents with solubility parameters between 8 and 10. Keep in mind that such considerations, approximate as they are, only apply to those systems that interact through dispersion and weak polar forces. 

Molecular Solids. These are substances such as solid argon or solid CO2 held together by relatively weak Van der Waals forces or weak polar forces. They are not expected to have significant attractions for gaseous or liquid reactants and thus no important catalytic properties. 

When iodine chloride is heated to 27°C, the weak intermolecular forces are unable to keep the molecules rigidly aligned, and the solid melts. Dipole forces are still important in the liquid state, because the polar molecules remain close to one another. Only in the gas, where the molecules are far apart, do the effects of dipole forces become negligible. Hence boiling points as well as melting points of polar compounds such as Id are somewhat higher than those of nonpolar substances of comparable molar mass. This effect is shown in Table 9.3. 

A bar of talc feels like a bar of soap which is why it is often called soapstone. Its exceptional softness (it is the softest of the Mohs minerals) is a direct result of its unusual crystal structure. This consists of sheets of silicate tetrahedra without metal ions between the sheets. Thus the sheets are bonded only by London polarization forces. The latter are particularly weak because silicate tetrahedra have relatively small polarizabilities. 

Permanent-dipole/permanent-dipole forces are weak attractive forces between permanently polar molecules. S+ atoms in one molecule attract S atoms in another molecule. They act in addition to the induced-dipole/induced-dipole forces. 

This conclusion falls in line with the fact that the anion radical could neither be detected after collision of the parent halide with alkali metal atoms in the gas phase (Compton et ai, 1978) nor upon y-irradiation in apolar or weakly polar solid matrixes at 77 K by esr spectroscopy (Symons, 1981). However, these observations are not absolute proofs that the anion radicals do not exist they might exist and be too short lived to be detectable. On the other hand, the reaction medium and the driving force conditions are quite different from those in the electrochemical experiments, which rendered necessary an independent investigation of the problem in the latter. 

HCl would have a higher value for a" because HCl is a polar molecule and therefore has stronger intermolecular forces than Hj. H2 is nonpolar with only weak dispersion forces to contend with. 1 point for correct answer and 1 point for supporting the choice with valid reasoning. 

The forces involved in the interaction al a good release interface must be as weak as possible. They cannot be the strong primary bonds associated with ionic, covalent, and metallic bonding neither arc they the stronger of the electrostatic and polarization forces that contribute to secondary van der Waals interactions. Rather, they are the weakest of these types of forces, the so-called London or dispersion forces that arise from interactions of temporary dipoles caused by fluctuations in electron density. They are common to all matter. The surfaces that are solid at room temperature and have the lowest dispersion-force interactions are those comprised of aliphatic hydrocarbons and fluorocarbons. 

These unusual properties can be explained by hydrogen bonding. This is a weak intermolecular force (bond) which occurs between water molecules because the bonds within the molecules are polar. 

Actual values of solubility parameters show the same trends for both solvents and polymers. Non-polar molecules and repeat units have weak intermolecular forces, small energies of vaporization, and therefore small solubility parameters. As might be expected, increased polarity increases the solubility parameter, and hydrogen bonding gives the largest values of all. 

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