Intermolecular forces of attraction

Application of pharmaceutical principles to drug dosage forms is illust rated when drug dosage forms are categorized according to their physical state, degree of heterogeneity, and chemical composition. The usual relevant states of matter are gases, liquids, and solids. Intermolecular forces of attraction are weakest in gases and strongest in solids. Conversions from one physical state to another can involve simply overcoming intermolecular forces of attraction by adding energy (heat) . Chemical composition can have a dramatic effect on physicochemical properties and behavior. For this reason, it is necessary to distinguish between polymers, or macromolecules, and more conventional ( i .e., smaller) molecules, or micromolecules.
check-tools.com

Intermolecular forces of attraction.
Because atoms vary in their electronegativity, electron sharing between different atoms is likely to be unequal . This asymmetric electron distribution causes a shift in the overall electron cloud in the molecule. As a result , the molecule tends to behave as a dipole ( i .e. , as if it had a positive and a negative pole) . The dipole associated with each covalent bond has a cor responding dipole moment (μ) defined as the product of the distance of charge separation (d) and the charge (q) :
μ = q × d
The molecular dipole moment may be viewed as the vector summary of the individual bond moments.

1. Nonpolar molecules that have perfect symmetry
(e.g. , carbon tetrachloride) have dipole moments of zero.

2. Polar molecules are asymmetric and have nonzero dipole moments.

3. When dipolar molecules approach one another close enough—“positive to positive” or “negative to negative”—so that their electron clouds interpenet rate, intermolecular repulsive forces arise.

Types of intermolecular forces of attract ion include the following:

1. Nonpolar molecules do not have permanent dipoles.
However, the instantaneous electron distribution in a molecule can be asymmetric.
The resultant transient dipole moment can induce a dipole in an adjacent molecule. This induced dipole- induced dipole interaction (London dispersion force), with a force of 0.5-1 kcal /mol, is sufficient to facilitate
order in a molecular array. These relatively weak electrostatic forces are responsible for the liquefaction of nonpolar gases.

2. The transient dipole induced by a permanent dipole, or dipole-induced
dipole interaction (Debye induction force) , is a stronger interaction, with a force of 1-3 kcal/mol .

3. Permanent dipole interactions (Keesom orientation forces),
with a force of 1-7 kcal /mol, together with Debye and London forces, constitute van der Waals forces. Collectively, they are responsible for the more substantive structure and molecular ordering found in liquids.

4. Hydrogen bonds.
Because they are small and have a large electrostatic field, hydrogen atoms can approach highly electronegative atoms (e.g., fluorine, oxygen, nitrogen, chlorine, sulfur) and interact electrostatically to
form a hydrogen bond. Depending on the electronegativity of the second atom and the molecular environment in which hydrogen bonding occurs, hydrogen bond energy varies from approximately 1 to 8 kcal/mol .

5. Ion- ion, ion-dipole, and ion- induced dipole forces.
Positive-negative ion interact ions in the solid state involve forces of 100-200 kcal/mol .
Ionic interact ions are reduced considerably in liquid systems in the presence of
other electrolytes. Ion-dipole interaction, or dipole induction by an ion, can also affect molecular aggregation, or ordering, in a system.