nano9-Self-Assembly

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Published on December 4, 2008

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Slide 1: Self-Assembly University of Tehran Faculty of Eng. Y. Mortazavi & A. Khodadadi Self-Assembled NanostructuresSynthetic Self-Assembled Materials: Principles and Practice : Self-Assembled NanostructuresSynthetic Self-Assembled Materials: Principles and Practice 2.1 Microscopic and Macroscopic Interactions Atoms, molecules, & particles organize themselves into functional structures as driven by the energetics of the system 2.1.1 Molecular Interaction Energies 2.1.1.1 Coulomb Interactions: Electrostatic effects from permanent charges on particles: ion-ion ion-permanent dipole permanent dipole-permanent dipole Self-Assembled NanostructuresSynthetic Self-Assembled Materials: Principles and Practice : Self-Assembled NanostructuresSynthetic Self-Assembled Materials: Principles and Practice 2.1.1.2 van der Waals Interactions (attractions α x-6) Permanent dipole – induced dipole attractions (Debye) Permanent dipole – permanent dipole attractions (Keesom) Induced dipole – induced dipole attractions (London) 2.1.1.3 Short range repulsions α x-12 2.1.1.4 Total Interaction: Lennard – Jones potential Self-Assembled NanostructuresSynthetic Self-Assembled Materials: Principles and Practice : Self-Assembled NanostructuresSynthetic Self-Assembled Materials: Principles and Practice 2.1.2 Macroscopic Interaction Energies 2.1.2.1 Van der Waals Attraction between two spherical particles with different sizes of R1 & R2 2.1.2.2. Electrostatic Repulsive Energy Double layer: charged surface, adsorbed species, diffuse region of charged species Electrical surface potential = φ = φ0 exp(-x/k), NA: Avogadro's const. ci = conc. of charged species, ε = permittivity k-1 = double – layer thickness high φ → repulsion low φ → aggregation of particles to clusters: T↑ ci↓ Self-Assembled NanostructuresSynthetic Self-Assembled Materials: Principles and Practice : Self-Assembled NanostructuresSynthetic Self-Assembled Materials: Principles and Practice Electrical double layer at interface between a positively charged metal oxide surface and an aqueous solution containing dissolved cations and anions. Negatively charged anions are adsorbed directly onto the positively charged metal oxide surface. Note the difference in orientation of water molecules (blue) around anions (green) compared with cations (red). 2.1.3 Hydrogen Bonding: Hydrophobic, and Hydrophilic Interactions : 2.1.3 Hydrogen Bonding: Hydrophobic, and Hydrophilic Interactions Hydrogen Bond: electrostatic H of water with O, N, F, Cl containing molecules Inter- & intra-molecular Hydrophobic interactions: When water molecules approach an inert surface (e.g. alkanes, hydrocarbon, and fluorocarbons) that cannot form hydrogen bond, arrange themselves to hydrogen bond with other molecules, while minimize contact with inert surface. Water molecules near the surface become more ordered as compared to free water molecules. Hydrophobic attraction of nonpolar molecules and surfaces 2.1.3 Hydrogen Bonding: Hydrophobic, and Hydrophilic Interactions : 2.1.3 Hydrogen Bonding: Hydrophobic, and Hydrophilic Interactions Hydrophilic Effect Hydrophilic molecules and groups (charged ions, polar molecules & groups, hydrogen bonding molecules and groups) are water soluble and repel each other in water. Hydrophilic molecules and groups prefer to be in contact with water 2.2 Surfactants and Amphiphilic Molecules : 2.2 Surfactants and Amphiphilic Molecules Amphiphilic molecules contain both hydrophilic and hydrophobic groups: surfactants, copolymers, and proteins; play a critical role in a wide range of self-assembly phenomena. Hydrophobic interactions between the tail groups and hydrophilic and/or electrostatic interactions between head groups Surfactants (even at low conc.) adsorb to surfaces or interfaces to reduce surface energy cationic, anionic, zwitterionic, and nonionic 2.2 Surfactants and Amphiphilic Molecules : 2.2 Surfactants and Amphiphilic Molecules Surfactant characteristics: Head group charge, chain length, head group size Cationic: long chain amines or ammonium salts are positively charges at pH<7, not charged at pH >7 thus not active Quaternary ammonium salts are charged and active at all pH’s Anionic: Long chain carboxylic or solfonic acids, are negatively charges at pH > 7, (at pH < 7 not ionized, not active) Surfactant Activity depends on: pH, ionic strength of the solution and by counter ions Counter ions neutralize charges on surfactant head groups, and even cause its precipitation. Non-ionic surfactants have the advantage in that pH, counter ions, or solvents do not affect their activity. 2.3 Transition from dispersed state to condensed state: a universal phenomenonBeginning of self-assembly of molecules, polymers, or microscopic particles : 2.3 Transition from dispersed state to condensed state: a universal phenomenonBeginning of self-assembly of molecules, polymers, or microscopic particles (a) Gas (dispersed) → Liquid (condensed) under van der Waals attraction forces (b) Polymer gels (dispersed)  Collapsed polymer gel (condensed) under rubber elasticity (entropy change), counter ions osmotic pressure, and electrostatic repulsive forces (c) Colloidal particles (dispersed)  self-assembled colloidal crystals and superlattice structures 2.4 Packing Geometry: Attaining the Desired Self-Assembled Structures : 2.4 Packing Geometry: Attaining the Desired Self-Assembled Structures Structure depends: on pH, T, electrolyte conc. Major driving forces for amphiphiles to form well-defined aggregates are hydrophobic attraction at hydrocarbon-water interfaces and hydrophilic ionic or steric repulsion between head groups. Geometric packing parameter (shape factor) v: hydrocarbon chain volume a0 = optimal head group area lc = critical chain length 2.4 Packing Geometry: Attaining the Desired Self-Assembled Structures : 2.4 Packing Geometry: Attaining the Desired Self-Assembled Structures Increase Surfactant Conc.: spherical micelles rod-like micelles  cubic  lamellar 2.4 Packing Geometry: Attaining the Desired Self-Assembled Structures : 2.4 Packing Geometry: Attaining the Desired Self-Assembled Structures 2.4.1 Effect of surfactant conc. Surfactant conc ↑: lower hydration of head groups → lower a0 → higher R → Lamellar structure 2.4.2 Effect of surfactant chain length lc increases, v increases, however R increases, spherical → Lamellar Structures 2.4.3 Effect of co-solvent Polar solvents, like alcohols, tend to associate with head groups and reduce the tendency for the surfactant molecules to associate; CM may disappear, nonpolar solvents associate with hydrophobic chains of the surfactant increase v → increase R → less curved structures 2.4.4 Effect of salts and ionic species on ionic surfactants, reduces repulsive energy between head groups, reducing a0 → larger R → less curved structures 2.4 Packing Geometry: Attaining the Desired Self-Assembled Structures : 2.4 Packing Geometry: Attaining the Desired Self-Assembled Structures 2.5 Self-assembled Block Copolymer Nanostructures : 2.5 Self-assembled Block Copolymer Nanostructures Block copolymers are amphiphilic molecules containing distinctively different polymer segments (blocks): polystyrene (H (C6H5 – CH2 – CH) - ) + poly isoprene (-CH2 – H3 (CH=CH2)C - ) block polystyrene and poly butadiene (- CH-CH=CH-CH) - ) block copolymer (-CH2-CH2-) polyethylene and poly propylene (-(CH2-CH3CH)-) block copolymer polyUrethane (-(CH2-CH2-O2C-NH-C6H10CH2-C6H10-NH-OC)-)+polyUrethane block copolymer Different blocks in a single polymer chain are covalentely bonded. The phase separation occurs on the nanometer scale, as determined by the dimension of the blocks. Factors: monomer type, composition and molecular size, and molecular configuration. 2.5 Self-assembled Block Copolymer Nanostructures : 2.5 Self-assembled Block Copolymer Nanostructures 2.5 Self-assembled Block Copolymer Nanostructures : 2.5 Self-assembled Block Copolymer Nanostructures Flory-Huggins segment-segment interaction parameter X: phase separation is favored E: Interaction Energy f = volume fraction fA = fB → straight cylinder → lamellar phase fA > fB → cone structure → B phase dispersed in A phase as spherical micelles fB > fA → cone structure → A phase dispersed in B phase as spherical micelles 2.6 Co-Assembly of Liquid Structures and Inorganic Materials : 2.6 Co-Assembly of Liquid Structures and Inorganic Materials Self-assembled liquid crystals from surfactants act as a template to support the growth of ceramic materials. These ordered structures are cross-linked together through the condensation of the aluminosilicate ions. Subsequently, the surfactant molecules can be removed by thermal or chemical treatment without collapsing the ordered structure (2-50 nm tunable). self-assembled periodic structures can be further used as a structural frame work to develop new materials: by incorporating functional molecules and active sites into the porous channels (e.g. nanoporous silica) by using the periodic nanoporous structures as a template (e.g. for carbon or metals) by physically confining a new material in the organized nanostructures. 2.6 Co-Assembly of Liquid Structures and Inorganic Materials : 2.6 Co-Assembly of Liquid Structures and Inorganic Materials 2.6.1 Interactions between cationic surfactants and Anionic Silicates Mesoporous materials: mixing aluminosilicate precursors (e.g. sodium aluminate, tetramethyl ammonium silicate, and silica) in a surfactant solution (e.g. ≥ 1 wt% cetyltrimethyl ammonium bromide (CTAB)) Inorganic ions play an important role in the self assembly process

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