ChE 413 213 Molecular SelfAssembly: A New EngineeringTool

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ChE 413/ 213Molecular Self-AssemblyA New
Engineering-Tool

• Instructor Mitchell Anthamatten
• Chemical Engineering Department
• University of Rochester

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Course Objectives

• To understand the role of molecular interactions
  in forming self-assembled structures.
• To synthesize previous coursework in chemistry,
  physics, and thermodynamics to enable new
  technologies to be understood
• To demonstrate creative and rational design of
  new materials and technologies using the concepts
  of molecular self-assembly.

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Self Assembly (SA) The action of independent
entities under distributed (non-central) control
to produce a larger structure or achieve a group
effect.
Self assembly can occur on any length scale. A
few examples

• Crystallization of water (2-3 Å)
• Host-guest crown ether coordination complex
  (10-15 Å)
• Organization of rod-shaped liquid crystals (30
  Å)
• Segregation of block copolymers (10 nm)
• Micelle/ emulsion formation (10 nm )
• Biological tissues (1 um)
• Capillary-assembled structures (O 10 um) see
  e.g. Whitesides, PNAS, v99 (8)
• Animal Behavior (O m)
• Astronomy, Eagle Nebula (light years)

molecular SA
Srinivasarao et. al Science 6 April 2001
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Why study self-assembly?
Frequency of Self-assembly in titles (SCI),
MRS Bulletin, October 2001

• valuable tool for nano- and micro- technology
• enables bottom-up route to designing materials
• will lead to better understanding of free
  energy landscape

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Thermodynamics Primer (1/4)
process change of state physical (e.g.
melting, freezing, expansion) chemical
(new substance formed) work ability to change
height of mass in surroundings i.e. transfer of
energy that involves organized motion system
volume that we are interested in surroundings
everything else, where we make observations energy
capacity to do work heat thermal form of
energy transfer of energy that makes use of
chaotic motion of molecules
endothermic describes a process that absorbs
energy exothermic describes a process that
releases energy
internal energy U total energy of a system
1st Law DU q w
gt the change in U is equal to the energy that
passes through a systems boundary as heat or
work
gt the total energy of an isolated system is
constant
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Thermodynamics Primer (2/4)
enthalpy H U PV Heat supplied at constant
pressure (so long as the system does no
additional work)
heat capacity _at_ const. pressure cp (?H/?T)P
Release of heat gt a decrease in enthalpy of a
system at const. pressure DH lt 0 exothermic
DH gt 0 endothermic
2nd Law of Thermo You cant convert heat into
work without some loss.
direction of spontaneous change toward a more
disordered form
(work not required)
DStot gt 0
(total entropy of an isolated system)
gt irreversible changes generate entropy gt
reversible changes do not generate entropy
of ways a system can be arranged to achieve
same energy lvl
S k ln W
k 1.38 10-23 J/K
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Thermodynamics Primer (3/4)
Thermodynamic Definition of entropy dS dq / T
gt DS ?dq / T Heat supplied at constant
pressure (so long as the system does no
additional work)
3rd law of thermo at T 0 K, all thermal
motions cease gt since S k ln W , S 0 !
Gibbs Free Energy GH-TS dGdH-TdS-SdT H
UPV dHdUPdVVdP dGdUPdVVdP-TdS-SdT
dUTdS-PdV dGVdP-SdT
Gibbs-Helmholz Eqn.
(T-dependence of GFE)
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Thermodynamics Primer (4/4)
chemical potential of a substance j
(n all other components)
fundamental eqn. of thermodynamics
Always try to think in terms of G.F.E. to explain
phenomena!
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Solution mixing of polycation polyanion
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Consider DG for this process Entropy gain due
to freeing of small ions (decreases DG) Entropy
gain due to formationof more random polymer
coils(decrease DG) Enthalpy gain due to
polymer-polymer interactions ( increase DG)
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Vorn Overbeek in J. Cellular Comparative
Physiology 1957, 49, 7
coacervation is a complex function of pH, cnc,
mixing ratio, etc.
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Shape site-specific interactions
determinefinal structure.
tobaccomosaic virus
DNA
phospholipid bilayer

• Increasingly complex molecular entities generally
  result in more complex, hierarchal structures.

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Self-assembly involves delicate balance among
non-covalent interactions and entropy.
DG DH - TDS

• Key intermolecular interactions
• electrostatic (Coulombic)
• dipole-dipole, induced dipole
• H-bonding
• van der Waals

Sc lt Sn lt Si
shape (VDW) nematic ordering
T1
T2
site-specific interactions lipid bilayer
T3
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Supramolecular structures exhibit error
correction.

• inherent reversibility
• self-healing of structures

- use covalent bonding to stabilize non-covalent
states
J. Org. Chem., 1995, v 60, 1033
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Examples of assisted and directed
self-assembly.
Self-assembly is influenced by,

• temperature
• nature of solvent (if any)
• surfaces (boundaries)
• gradients
• events

Hammond, P.T. 1994, Macromolecules
lthttp//pmm08.physik.hu-berlin.de/kirstein/films.h
tmgt