Active Roles of Water in Aqueous Assembly of Macromolecules

Aqueous self-assembly customarily focuses on the molecular interactions of assembling building blocks; the role of water is barely studied. The hydration of hydrophobic P+X- (P+: macromolecular phosphonium cation, X-: anion) is dependent on the ionic end groups, which is responsible for the consequent assembling behavior. The water interaction with the backbone was analyzed by FT-IR, and the dynamics were measured by low field-NMR spectroscopy. The combination of these two techniques reveals the effect of X- on hydration. When X- is I-, the ionic end group ordered water molecules that exerted a detectable long-range effect de-hydrating the backbone. The consequent hydrophobic interaction drove the aqueous assembly of P+I- into micelle-like aggregates with the ionic group exposed to water. In contrast, the ion pair with a hydrophobic anion of [BPh4]- was not able to hold water and did not deplete the hydration water. The hydrated backbone of P+[BPh4]- assembled into vesicles that were driven by hydration interactions. This elucidation at the molecular level is craved to progress aqueous supramolecular chemistry.