H-1 NMR studies on intermolecular association of amphiphilic cationic polyelectrolyte micelles induced by hydrophobic counteranions in water

摘要

Interactions of a series of amphiphilic cationic polyelectrolytes with various kinds of organic counteranions have been investigated in water by one- and two-dimensional H-1 NMR spectroscopy at 20 degreesC. The cationic polyelectrolytes were prepared by micellar homopolymerization of tail-type cationic surface-active monomers with a cationic charge with omega-end, (ST - C-m - AB, m = 5, 7, and 9, where ST is a styrenic group, C-m, an alkylene chain at the 4-position of styrene, and AB, alkyltrimethylammonium bromide). Aliphatic monosodium salt of maleic acid (MAS) and its stereoisomer, fumaric acid (FAS), sodium benzoate (NaB), potassium hydrogen phthalate (PHK), and sodium salicylate (NaSal) were added to a salt-free aqueous solution of the polyelectrolytes and H-1 NMR measurements were carried out. Amphiphilic P(ST - C-m - AB) polyelectrolytes act as efficient hosts to strongly capture the hydrophobic counteranions B-, PH-, and Sal(-), but not MA(-) and FA(-). The H-1 NMR signals of these hydrophobic counteranions remarkably shift upfield and broaden in water in the presence of the amphiphilic polyelectrolytes. The nuclear Overhauser effect ( NOE) signals between the cationic group of the polymer and aromatic benzoate counteranion protons are clearly observed to imply cation - pi interaction. The capturing of hydrophobic counterions by the polyelectrolytes is likely due to electrostatic, hydrophobic, and cation - pi interactions between them. The reduced viscosity, eta(sp)/C-p, for the solution at [PHK]/[P(ST - C-7 - AB)] = 1.0 steeply increases with increasing polymer concentration (C-p) above ca. 0.9 g/dL to show pronounced viscoelasticity.