- Intermolecular electrical interactions
- Interaction between permanent dipoles
- Interaction between a permanent dipole and an induced dipole
- London forces or dispersion
- Van der Waals radios
- Forces and energy of the electrical interaction between atoms and between molecules
- References
The Van der Waals force are intermolecular forces electrical in nature that can be attractive or repulsive. There is an interaction between the surfaces of molecules or atoms, different in essence from the ionic, covalent and metallic bonds that form inside the molecules.
Although weak, these forces are capable of attracting gas molecules; also that of liquefied and solidified gases and those of all organic liquids and solids. Johannes Van der Waals (1873) was the one who developed a theory to explain the behavior of real gases.
In the so-called Van der Waals equation for real gases - (P + an 2 / V 2) (V - nb)) = nRT- two constants are introduced: the constant b (that is, the volume occupied by the molecules of the gas) and "a", which is an empirical constant.
The constant "a" corrects the deviation from the expected behavior of ideal gases at low temperatures, precisely where the force of attraction between the gas molecules is expressed. The ability of an atom to polarize in the periodic table increases from the top of a group to the bottom of it, and from right to left over a period.
As the atomic number increases - and therefore the number of electrons - those that are located in the outer shells are easier to move to form polar elements.
Intermolecular electrical interactions
Interaction between permanent dipoles
There are electrically neutral molecules, which are permanent dipoles. This is due to a disturbance in the electronic distribution that produces a spatial separation of the positive and negative charges towards the ends of the molecule, constituting a dipole (as if it were a magnet).
Water is made up of 2 hydrogen atoms at one end of the molecule and an oxygen atom at the other end. Oxygen has a higher affinity for electrons than hydrogen and attracts them.
This produces a displacement of the electrons towards the oxygen, leaving this negatively charged and the hydrogen positively charged.
The negative charge of a water molecule can interact electrostatically with the positive charge of another water molecule causing an electrical attraction. Thus, this type of electrostatic interaction is called Keesom forces.
Interaction between a permanent dipole and an induced dipole
The permanent dipole exhibits what is called a dipole moment (µ). The magnitude of the dipole moment is given by the mathematical expression:
µ = qx
q = electric charge.
x = spatial distance between the poles.
The dipole moment is a vector that, by convention, is represented oriented from the negative pole to the positive pole. The magnitude of µ hurts to express in debye (3.34 × 10 -30 Cm
The permanent dipole can interact with a neutral molecule causing an alteration in its electronic distribution, resulting in an induced dipole in this molecule.
The permanent dipole and the induced dipole can interact electrically, producing an electrical force. This type of interaction is known as induction, and the forces acting on it are called Debye forces.
London forces or dispersion
The nature of these attractive forces is explained by quantum mechanics. London postulated that, in an instant, in electrically neutral molecules the center of the negative charges of the electrons and the center of the positive charges of the nuclei might not coincide.
So the fluctuation of electron density allows the molecules to behave as temporary dipoles.
This is not by itself an explanation for attractive forces, but temporary dipoles can induce properly aligned polarization of adjacent molecules, resulting in the generation of an attractive force. The attractive forces generated by electronic fluctuations are called London forces or dispersion.
Van der Waals forces show anisotropy, which is why they are influenced by the orientation of the molecules. However, dispersion-type interactions are always predominantly attractive.
The London forces get stronger as the size of the molecules or atoms increases.
In halogens, the low atomic number F 2 and Cl 2 molecules are gases. The Br 2 with the highest atomic number is a liquid and the I 2, the halogen with the highest atomic number, is a solid at room temperature.
As the atomic number increases, the number of electrons present increases, which facilitates the polarization of the atoms and, therefore, the interactions between them. This determines the physical state of the halogens.
Van der Waals radios
The interactions between molecules and between atoms can be attractive or repulsive, depending on a critical distance between their centers, which is called r v.
At distances between molecules or atoms larger than r v, the attraction between the nuclei of one molecule and the electrons of the other predominates over the repulsions between the nuclei and the electrons of the two molecules.
In the case described, the interaction is attractive, but what happens if the molecules approach at a distance between their centers less than rv? Then the repulsive force predominates over the attractive one, which opposes a closer approach between the atoms.
The value of r v is given by the so-called Van der Waals radii (R). For spherical and identical molecules r v is equal to 2R. For two different molecules of radii R 1 and R 2: r v is equal to R 1 + R 2. The values of the Van der Waals radii are given in Table 1.
The value given in Table 1 indicates a Van der Waals radius of 0.12 nm (10 -9 m) for hydrogen. So the value of r v for this atom is 0.24 nm. For a value of r v less than 0.24 nm there will be a repulsion between the hydrogen atoms.
Table 1. Van der Waals radii of some atoms and groups of atoms.
Forces and energy of the electrical interaction between atoms and between molecules
The force between a pair of charges q 1 and q 2, separated in a vacuum by the distance r, is given by Coulomb's law.
F = k. q 1.q 2 / r 2
In this expression k is a constant whose value depends on the units used. If the value of the force - given by the application of Coulomb's law - is negative, it indicates an attractive force. On the contrary, if the value given for the force is positive, it is indicative of a repulsive force.
As the molecules are usually in an aqueous medium that shields the electrical forces exerted, it is necessary to introduce the term dielectric constant (ε). Thus, this constant corrects the value given for the electric forces by the application of Coulomb's law.
F = kq 1.q 2 /ε.r 2
Similarly, the energy for the electrical interaction (U) is given by the expression:
U = k. q 1.q 2 /ε.r
References
- The Editors of Encyclopaedia Britannica. (2018). Van der Waals forces. Retrieved on May 27, 2018, from: britannica.com
- Wikipedia. (2017). Van der Waals forces. Retrieved on May 27, 2018, from: es.wikipedia.org
- Kathryn Rashe, Lisa Peterson, Seila Buth, Irene Ly. Van der Waals Forces. Retrieved on May 27, 2018, from: chem.libretexts.org
- Morris, JG (1974) A Biologist`s Physical Chemistry. 2and edition. Edward Arnold (Publishers) Limited.
- Mathews, CK, Van Holde, KE and Ahern, KG (2002) Biochemistry. Third edition. Addison Wesley Longman, Inc.