Chapter 3: Interactions on a molecular scale
Origin and characteristic length scales of interactions
Molecular scale: balance between repulsive and attractive interactions governs the formation of
condensed phases. Gas-liquid coexistence is not possible without attractive interactions.
Macromolecular scale: Interaction between building blocks of the macromolecule chain determines
how swollen the macromolecule is (amongst other things)
Colloidal scale: Interactions affect the stability of emulsions, foams and dispersions. When repulsive
interactions between emulsion droplets are strong and attractive forces are weak the emulsion
droplets will not aggregate, and the emulsion is stable.
- Origin of all interactions on the molecular and colloidal scale is the electromagnetic and
gravitational force.
- Electromagnetic force: force between charged particles with a range between 10-10 and 10-6
meters.
- Gravitational force: attractive interaction between masses
- The electromagnetic forces are more important than the gravitational force, since masses
are usually small on these scales.
We can classify the interactions by the types of charges interacting;
1. Permanent charges
o Particles with a net positive or net negative charge
o Electrons, protons, ions
2. Permanent dipoles
o Particles with a net neutral charge but a center of mass of the positive charges does
not coincide with the center of mass of the negative charges
o A permanent dipole is formed when the constituent atoms of a molecule vary
significantly in electronegativity.
o Electronegativity: tendency of an atom to attract electrons
o The strongly electronegative atom will be slightly negative (𝛿 − ) and the weakly
electronegative atom will be slightly positive (𝛿 + )
o Net charge: 𝛿 − + 𝛿 + = 0
o Water molecule
3. Induced dipoles
o Dipole caused by the presence of another charge
o The presence of a permanent charge disturbs the charge distribution of an
uncharged molecule, inducing a dipole moment in the molecule
o Inducing charge need not be a permanent charge but can also be a permanent
dipole or another induced dipole.
o Two uncharged molecules can also induce a dipole in each other
o Important to realize: the electron charge distribution around a molecule fluctuates
very rapidly
, *orange section = Van der Waal’s forces
Among the interactions between permanent charges we can distinguish;
1. Covalent interactions (short-range attractions)
o When two atoms form a chemical bond in which electrons are being shared
between the atoms
2. Short-range repulsive interactions
o When two atoms or molecules approach each other to very short distances such the
electron clouds start to overlap and repel each other
o Charged molecules/ions: Born repulsion
o Uncharged molecules: Hardcore repulsion or steric repulsion
3. Longer-range Coulomb forces
o Interactions between permanent charges and permanent dipoles: Induction or Van der
Debye interaction Waal’s
o interactions between two permanent dipoles: Orientation interactions
o Interactions between two induced dipoles: dispersion interactions
Strength of an interaction
- Interaction can either be attractive or repulsive
- In both cases the strength of the interaction increases for decreasing separation distances
and vice versa
- We can characterize this strength by the force F(r) or the energy potential V(r) needed to
hold two molecules at a certain distance from each other r
- Force F(r) is positive when interactions are repulsive and negative when interactions are
attractive
- Energy potential V(r) is defined as the negative of the work performed in displacing two
interacting molecules from an infinite separation distance to a finite distance r
Origin and characteristic length scales of interactions
Molecular scale: balance between repulsive and attractive interactions governs the formation of
condensed phases. Gas-liquid coexistence is not possible without attractive interactions.
Macromolecular scale: Interaction between building blocks of the macromolecule chain determines
how swollen the macromolecule is (amongst other things)
Colloidal scale: Interactions affect the stability of emulsions, foams and dispersions. When repulsive
interactions between emulsion droplets are strong and attractive forces are weak the emulsion
droplets will not aggregate, and the emulsion is stable.
- Origin of all interactions on the molecular and colloidal scale is the electromagnetic and
gravitational force.
- Electromagnetic force: force between charged particles with a range between 10-10 and 10-6
meters.
- Gravitational force: attractive interaction between masses
- The electromagnetic forces are more important than the gravitational force, since masses
are usually small on these scales.
We can classify the interactions by the types of charges interacting;
1. Permanent charges
o Particles with a net positive or net negative charge
o Electrons, protons, ions
2. Permanent dipoles
o Particles with a net neutral charge but a center of mass of the positive charges does
not coincide with the center of mass of the negative charges
o A permanent dipole is formed when the constituent atoms of a molecule vary
significantly in electronegativity.
o Electronegativity: tendency of an atom to attract electrons
o The strongly electronegative atom will be slightly negative (𝛿 − ) and the weakly
electronegative atom will be slightly positive (𝛿 + )
o Net charge: 𝛿 − + 𝛿 + = 0
o Water molecule
3. Induced dipoles
o Dipole caused by the presence of another charge
o The presence of a permanent charge disturbs the charge distribution of an
uncharged molecule, inducing a dipole moment in the molecule
o Inducing charge need not be a permanent charge but can also be a permanent
dipole or another induced dipole.
o Two uncharged molecules can also induce a dipole in each other
o Important to realize: the electron charge distribution around a molecule fluctuates
very rapidly
, *orange section = Van der Waal’s forces
Among the interactions between permanent charges we can distinguish;
1. Covalent interactions (short-range attractions)
o When two atoms form a chemical bond in which electrons are being shared
between the atoms
2. Short-range repulsive interactions
o When two atoms or molecules approach each other to very short distances such the
electron clouds start to overlap and repel each other
o Charged molecules/ions: Born repulsion
o Uncharged molecules: Hardcore repulsion or steric repulsion
3. Longer-range Coulomb forces
o Interactions between permanent charges and permanent dipoles: Induction or Van der
Debye interaction Waal’s
o interactions between two permanent dipoles: Orientation interactions
o Interactions between two induced dipoles: dispersion interactions
Strength of an interaction
- Interaction can either be attractive or repulsive
- In both cases the strength of the interaction increases for decreasing separation distances
and vice versa
- We can characterize this strength by the force F(r) or the energy potential V(r) needed to
hold two molecules at a certain distance from each other r
- Force F(r) is positive when interactions are repulsive and negative when interactions are
attractive
- Energy potential V(r) is defined as the negative of the work performed in displacing two
interacting molecules from an infinite separation distance to a finite distance r
