Molecular interactions

Levente NovákIstván BányaiZoltán Nagy

Department of Physical Chemistry

Characterization of colloidal systems

● Degree of dispersion (=size)● Size and size distribution

● Morphology (shape and internal structure)● Important as even the same size distribution can lead to very

diferent properties● Spatial distribution of the dispersed particles

● Concepts of homogeneity and inhomogeneity● Interactions between particles

● Can influence the former properties

Particle interactions

● Interparticle interactions arise from interactions between individual molecules, atoms, or ions

● These interactions influence or determine the size, shape of the resulting particles, the stability of the system, as well as the

● particle/particle interactions● particle/medium interactions● medium/medium interactions

● Pair interactions: interactions between two isolated atoms, ions, or molecules

● Particle: cluster of molecules (or ions, atoms) forming a kinetic entity (possesses individual thermal translational movement and moves as a single entity)

Ionic and molecular interactions

● Coulomb● Ion–ion● Ion–permanent dipole● Ion-induced dipole

● Van der Waals atraction● Permanent dipole–permanent dipole● Permanent dipole–induced dipole● Induced dipole—induced dipole

● Repulsion (Pauli repulsion)● Hydrogen bonding → Lyophilic and lyophobic interactions (special case:

hydrophilic and hydrophobic interactions).● Dipolar molecules possess a dipole moment µ (in C×m) which is the measure

of the dipolarity and increases with increasing charge magnitude and charge separation distance.

Sign of the interaction:

● positive → repulsion● negative → atraction

Coulomb interactions

E i 1 i 2=

(ze)1(ze)2

4 πε×

1r

E i 2d 1=−

(ze )2μ1cosθ4 πε

×1r 2

Ion—ion interaction

Ion—dipole interaction

Range ≈ 50 nmE≈200-250 kJ/mol

Range ≈ 1.5 nmE≈15 kJ/mol

q : charge (C), q=zeµ : dipole moment (C×m or Debye)l : dipole length (m)r : distance between molecule and ion (m)θ : dipole angle (° or rad)ε : dielectric permitivity (F/m) ε=εr×ε0

Ion-dipole interactions

htp://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Mater/Intermolecular_Forces/Ion_-_Dipole_Interactions

Gary L. Bertrand, Professor of Chemistry, University of Missouri-Rolla

Ion—permanent dipole interaction Ion—induced dipole interaction

Ion-dipole interaction: example

Hydration of ions

Hydration of ions by water molecules is an ion–dipole interaction taking place between the charged species and dipolar water molecules

Dipole-dipole interactions

E d 1d 2=

(1−3 cosθ)μ1μ2

4 πε×

1r 3

E d 1d 2=−

23

μ12μ2

2

(4 πε)2k B T

×1r 6

a) When the temperature is low, dipoles are oriented. If they are parallel, (1-3 cos θ )= +2 and there is repulsion. If the dipoles are antiparallel, (1-3 cos θ )= -2 and there is atraction:

b) When the temperature is high, there is always atraction:

Range ≈ 1.5 nmE≈2 kJ/mol

Range ≈ 1-2 nmE≈0.3 kJ/mol

Dipole—dipole interaction

T : temperature (K)kB : Boltzmann constant (J/K)

Dipole moments (μ) and structure

Molecule Dipole moment

(Debye)

HF 1.91

HCl 1.05

HBr 0.79

H2O 1.85

H2S 0.93

NH3 1.46

Molecule Dipole moment

(Debye)

SO2 1.6

CO 0.1

CO2 0

Molecule Dipole moment

(Debye)

Methanol 1.7

Ethanol 1.7

Acetone 2.86

Phenol 1.45

1 Debye=3,33×10-30 C×m

There is an induction efect → dipoles make polarization → other dipoles form

Polarizability (α) and structure

Molecule Polariza-bility

He 0.2

H2 0.81

Ar 1.63

Xe 4

NH3 2.3

CH4 2.6

Molecule Polariza-bility

CO 1.65

H2O 1.44

O2 1.6

Cl2 4.6

CCl4 10.5

Molecule Polariza-bility

CH2=CH2 4.3

C2H6 4.5

C6H6 10.3

Polarizability increases with atom/molecule volume (or mass) but shape plays also an important role.

Induced dipole—induced dipole

Permanent dipole-induced dipole

Induction efect → always atraction with induced dipoles

E d 1d 2∼−

(μ12α2)

4 πε×

1r 6

μ : dipole molent (C×m or Debye)α : polarizability (without unit)ε : dielectric permitivity (F/m)

Induced dipole—induced dipole(London dispersion) interaction

● London dispersion interaction is universal● Always atractive● London forces are additive

● Increase with molecular weight ● Several physical properties of the liquids change proportionally with

the molecular weight (e.g. melting and boiling points, vapor pressure, surface tension, viscosity)Saturated vapor pressure: n-pentane > n-hexane > n-heptane > n-octane

● London dispersion forces increase with polarizability (α)● London forces depend on the molecule's shape

Evaporation heat: n-pentane > isopentane > neopentane

London dispersion interactions

E i 1 i 2∼−

32ℏ

I 1 I2

I1+I 2

α1α2

(4πε0)2

1r 6

I : ionization energy (J)α : polarizability (without unit)ħ : reduced Planck-constant (J×s)ε : dielectric permitivity (F/m)

Range ≈ 0.4 nmE≈2 kJ/mol

Examples of van der Waals interactions

London's dispersion interaction is of general nature, it is additive for particles composed of molecules and it depends on the size and the shape of the molecule or particle.

Contracted form of the van der Waals interactions

EA∼−β 11r−6 A∼q2

β 11

EA: Atractive energy (J)A: Hamaker constant (J)q: number of atoms per unit volumeβ11: pair interaction energy (J×m6)

Molecule Dipolemom. (D)

Pol(α)

*. Orien-tation%

Induc-tion%

Disper-sion%

CCl4 0 10.7 0 0 100

Ethanol 1.73 5.49 42.6 9.7 47.6

Benzene 0 10.5 0 0 100

Water** 1.82 1.44 84.8 4.5 10.5

β11 (Jm6×1077)CCl4 4.41Ethanol 3.4Benzene 4.29Cl-benzene 7.57F-benzene 5.09Toluene 5.16Water 1.82

** Without H-bonding

α4π ε 0

×10−30(m3)

Molecule

*

Pair interaction energy (β): sum of inductive, orientation, and dispersion efects for two molecules

Atraction and repulsion

Pauli repulsion

Etot∼const .r12 −

β 11

r6

Van der Waals atraction

Lennard-Jones (12-6) potential

Hydrophilic interaction: hydrogen bonding

● Hydrogen bond is the strongest secondary (physical) bond → hydrogen atom of a molecule bonds to the nonbinding electron pair of the other molecule

● Requisites for hydrogen bonding● Hydrogen atom bound to a strongly electronegative atom (F, O, N) → e.g. C-H

groups do not give hydrogen bonds while C-OH do● Presence of a nonbinding electron pair around the highly electronegative atom

F−H…:F (161.5 kJ/mol or 38.6 kcal/mol)

O−H…:N (29 kJ/mol or 6.9 kcal/mol)

O−H…:O (21 kJ/mol or 5.0 kcal/mol)

N−H…:N (13 kJ/mol or 3.1 kcal/mol)

N−H…:O (8 kJ/mol or 1.9 kcal/mol)

Hydrogen bonding in water

Hydrogen bonding: exampleDNA → hydrogen bonding between matching base

pairs

Polyaramide (Kevlar) Cellulose

Hydrophobic interaction

● Hydrophobic interaction● An unusually strong interaction between hydrophobic molecules in water or in

hydrophilic liquids (the interaction is stronger than without the hydrophilic medium)● Formation

1) Hydratation (solvation) of the hydrophobic molecule.

2) Structure of water molecules is broken → decrease of the kinetic degree of freedom and of entropy

3) Association of hydrophobic molecules minimizes such breakdown of water structure → entropy increases again

● Importance● e.g. proteins composed of hydrophobic domains have interactions between these

domains → tertiary structure is (partly) defined by these interactions

Hydrophobic interaction: example

There exists a chain length over which hydrophobic properties of organic compounds increase strongly → the structure of water due to hydrogen bonds is perturbed. E.g. alcohols having chains longer than this critical length (C4) are no longer freely miscible in water.

Examples

● Phospholipids in water arrange in two sheets (bilayer), each with a hydrophilic and a hydrophobic face

● The hydrophobic faces of the two sheets are in contact

● By disturbing the lamellar bilayer (e.g. shearing), liposomes can form

● In contrast micelles formed from soaps are monolayers (there is no internal hole)

Shape of large molecules

Relation between the primary and secondary structures:

● folded secondary structure depends on the primary structure

● secondary structure is stabilized by hydrogen bonds

Tertiary structure of proteins

Polypeptides are composed of hydrohilic and hydrophobic domains. Hydrophobic domains turn away from water → stabilized by dispersive interactions between more densely packed hydrophobic domains.

*Crowe, J.:Chemistry for the Biosciences Oxford UP. ISBN 0-19-928097-5, 2006

Efect of the medium

Quaternary structure

● Haemoglobin is an oxygen-transporting metalloprotein (with iron-containing heme as prosthetic groups)

● Composed of 4 subunits● Inter- and intramolecular

forces stabilize the structure of haemoglobin

α subunitsβ subunits

heme