Molecular interactions. Levente Novák István Bányai Zoltán Nagy Department of Physical Chemistry

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1 Molecular interactions Levente Novák István Bányai Zoltán Nagy Department of Physical Chemistry

2 Characterization of colloidal systems Degree of dispersion (=size) Morphology (shape and internal structure) Important as even the same size distribution can lead to very diferent properties Spatial distribution of the dispersed particles Size and size distribution Concepts of homogeneity and inhomogeneity Interactions between particles Can influence the former properties

3 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)

4 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 Sign of the interaction: positive repulsion negative atraction 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.

5 Coulomb interactions Ion ion interaction (ze)1 (ze)2 1 Ei i = 4πε r Ion dipole interaction (ze )2 μ 1 cos θ 1 E i d = 2 4 πε r q µ l r θ ε : : : : : : Range 50 nm E kj/mol charge (C), q=ze dipole moment (C m or Debye) dipole length (m) distance between molecule and ion (m) dipole angle ( or rad) dielectric permitivity (F/m) ε=ε ε r 0 Range 1.5 nm E 15 kj/mol

6 Ion-dipole interactions Ion permanent dipole interaction Ion induced dipole interaction htp://chemwiki.ucdavis.edu/physical_chemistry/physical_properties_of_mater/intermole cular_forces/ion_-_dipole_interactions Gary L. Bertrand, Professor of Chemistry, University of Missouri-Rolla

7 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

8 Dipole-dipole interactions Dipole dipole interaction a) When the temperature is low, dipoles are oriented. If they are parallel, (13 cos θ )= +2 and there is repulsion. If the dipoles are antiparallel, (1-3 cos θ )= -2 and there is atraction: Ed 1 d2 (1 3 cos θ ) μ 1 μ 2 1 = 3 4 πε r Range 1.5 nm E 2 kj/mol b) When the temperature is high, there is always atraction: E d d = 1 2 T : kb : μ21 μ (4 π ε )2 k B T r 6 Range 1-2 nm E 0.3 kj/mol temperature (K) Boltzmann constant (J/K)

9 Dipole moments (μ) and structure Molecule Dipole moment Molecule (Debye) Dipole moment Molecule (Debye) Dipole moment (Debye) HF 1.91 SO2 1.6 Methanol 1.7 HCl 1.05 CO 0.1 Ethanol 1.7 HBr Acetone H2O CO2 Phenol 1.45 H2S 0.93 NH Debye=3, C m There is an induction efect dipoles make polarization other dipoles form

10 Polarizability (α) and structure Molecule Polarizability Molecule Polarizability Molecule Polarizability He 0.2 CO 1.65 CH2=CH2 4.3 H H2O 1.44 C2H6 4.5 Ar 1.63 O Xe 4 C6H6 Cl2 4.6 NH3 2.3 CCl CH4 2.6 Polarizability increases with atom/molecule volume (or mass) but shape plays also an important role.

11 Induced dipole induced dipole

12 Permanent dipole-induced dipole Induction efect always atraction with induced dipoles Ed 1 d2 ( μ21 α2 ) 1 6 4πε r μ α ε : : : dipole molent (C m or Debye) polarizability (without unit) dielectric permitivity (F/m)

13 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

14 London dispersion interactions 3 I 1 I 2 α1 α2 1 E i i ℏ 2 I 1 +I 2 (4 π ε0 )2 r I α ħ ε : : : : Range 0.4 nm E 2 kj/mol ionization energy (J) polarizability (without unit) reduced Planck-constant (J s) dielectric permitivity (F/m)

15 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.

16 Contracted form of the van der Waals interactions Pair interaction energy (β): sum of inductive, orientation, and dispersion efects for two molecules E A β 11 r 6 Molecule CCl4 Ethanol Benzene Cl-benzene F-benzene Toluene Water β11 (Jm6 1077) EA: Atractive energy (J) A: Hamaker constant (J) q: number of atoms per unit volume β11: pair interaction energy (J m6) 2 A q β 11 Molecule Dipole mom. (D) Pol*. (α) Ethanol 1.73 Benzene Water** CCl4 Induction% Dispersion% * Orientation% α (m 3) 4π ε0 ** Without H-bonding

17 Atraction and repulsion Pauli repulsion const. β 11 E tot 12 6 r r Van der Waals atraction Lennard-Jones (12-6) potential

18 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)

19 Hydrogen bonding in water

20 Hydrogen bonding: example DNA hydrogen bonding between matching base pairs Polyaramide (Kevlar) Cellulose

21 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

22 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.

23 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)

24 Shape of large molecules Relation between the primary and secondary structures: folded secondary structure depends on the primary structure secondary stabilized bonds structure is by hydrogen

25 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 , 2006

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27 Efect of the medium

28 Quaternary structure Haemoglobin is an oxygentransporting metalloprotein (with ironcontaining heme as prosthetic groups) Composed of 4 subunits Inter- and intramolecular forces stabilize the structure of haemoglobin α subunits β subunits heme