Modulating The Self-assembly Of Ionic Aromatic Oligomers In Aqurous Solution

The driving forces for supramolecular self-assemblies are non-bonded and weak interaction, which can be sensitive to environmental conditions, such as ionic strength, pH and so on. External simulation modulated the organization of the self-assemblies not only contributes to understand the mechanism of aggregation, but also is very important in easily and effectively obtaining the functional materials, such as supramolecular hydrogel, nano-scaled photoelectric devices, supramolecular liquid crystal and so on. The current studies are concentrated on the nano-scaled morphology affected by ions and pH. However, salt and pH modulated organization of the self-assemblies remains a topic relatively unexplored with obscure underlying mechanism, mostly due to scarcity of experimental systems. Therefore, a series of rigid three-armed ionic aromatic oligoamide molecules were prepared in our group. Those provide novel model systems to investigate the effects of ionic strength and pH on arrangements and stabilities of organized1D nanostructures. In addition, the BTA-compounds with peripheral carboxyl are analogous to amyloid fibrils but more simple and controllable, this study may contributes to understand the aggregation behaviors of bio-macromolecules. Two aspects of work are included in our text as follows:(1) The effects of two classes of monovalent salts (Anion effects:NaCl, NaBr, NaNO3; Cation effects:NaCl, KC1, CsC1) on the aggregation behaviors of P7(COONa)3in aqueous solution have been investigated by SAXS, TEM and so on. The arrangement of nanotubes with increasing of NaCl concentration undergoes structural transition sequences from rectangular phase to hexagonal packing, then to lamellar gel, finally to disordered structure. The observed transitions are understood by ionic effects on electrostatic interaction between charged nanotubes and osmotic pressure due to ion partitioning. The study demonstrates rich phase behaviors of ordered, charged1D nanostructure by tuning ionic strength and underlying key physical principles. No specific anion effect is observed for NaBr/NaNO3, which alsoexhibit similar transition sequences; while KC1, CsCl/P7(COONa)3systems show specific cation effect.(2) The effects of pH on the self-assembly of ionic aromatic oligoamides are discussed. Due to the decrease of attraction, the rectangular arrangement of P7(COONa)3in aqueous solution would expand with increasing or decreasing pH before it disappears. Compared with P7(COONa)3, the highly charged P6(COONa)3, P5(COONa)3, P4(COONa)3with the less length of hydrophobic arms would be molecularly dissolved due to electrostatic repulsion. It is possible that carboxylate gets protonated after the hydrolysis of GdL and the partially protonated molecules start aggregating. First, the three molecules would self-assembly into nano fibrils with adding GdL, but with different arrangements. GdL/P6(COONa)3system arranges into hexagonal phase and GdL/P5(COONa)3system shows less order arrangement without high order peaks in SAXS curves. Disordered structure would be found in GdL/P4(COONa)3system. The effects of length of molecules are from various aspects: More GdL would be needed to make carboxylate protonated and start to aggregate due to the larger electrostatic repulsion with less volume of molecules, which is also an adverse factor to form an ordered arrangement Meanwhile, the introduced defect also results in the orderless arrays due to the asymmetry of molecules.