Anion-Specific Effects on the Behavior of pH-Sensitive Polybasic Brushes

Abstract

The anion-specific solvation and conformational behavior of weakly basic poly(2-dimethylamino)ethyl methacrylate (poly(DMA)), poly(2-diethylamino)ethyl methacrylate (poly(DEA)), and poly(2-diisopropylamino)ethyl methacrylate (poly(DPA)) brushes, with correspondingly increasing inherent hydrophobicity, have been investigated using in situ ellipsometric and quartz crystal microbalance with dissipation (QCM-D) measurements. In the osmotic brush regime, as the initial low concentration of salt is increased, the brushes osmotically swell by the uptake of solvent as they become charged and the attractive hydrophobic inter- and intrachain interactions are overcome. With increased ionic strength, the brushes move into the salted brush regime where they desolvate and collapse as their electrostatic charge is screened. Here, as the brushes collapse, they transition to more uniform and rigid conformations, which dissipate less energy, than similarly solvated brushes at lower ionic strength. Significantly, in these distinct regimes brush behavior is not only ionic strength dependent but is also influenced by the nature of the added salt based on its position in the well-known Hofmeister or lyotropic series, with potassium acetate, nitrate, and thiocyanate investigated. The strongly kosmotropic acetate anions display low affinity for the hydrophobic polymers, and largely unscreened electrosteric repulsions allow the brushes to remain highly solvated at higher acetate concentrations. The mildly chaotropic nitrate and strongly chaotropic thiocyanate anions exhibit a polymer hydrophobicity-dependent affinity for the brushes. Increasing thiocyanate concentration causes the brushes to collapse at lower ionic strength than for the other two anions. This study of weak polybasic brushes demonstrates the importance of all ion, solvent, and polymer interactions.

Behavior of Weak Polyelectrolyte Brushes in Mixed Salt Solutions

“…At higher pH values, closer to and above the p K a , the percentage of charged monomer decreases. 10 For both the hydrophilic and hydrophobic polymers, the variation in overall brush thickness is nonmonotonic, as expected from existing theories 11 , 19 and experiments 3 , 5 for weak polyelectrolyte brushes.…”

Specific ion modulated thermoresponse of poly(N-isopropylacrylamide) brushes

“…Cleaned wafers and sensors (see ESI †) were amine-functionalised at room temperature via exposure to APTES vapour at <5 mbar for 30 min before being annealed for a further 30 min in air at 110 °C. [39][40][41] Bromine functionalisation of the APTES layer was then performed by immersion of the wafers and QCM sensors in a solution of BIBB (0.26 mL) and anhydrous triethylamine (0.30 mL) in anhydrous THF (10 mL) under a N2 atmosphere for 60 min. [39][40][41] Brush Polymerisation via ARGET ATRP…”

Combined Experimental and Theoretical Study of Weak Polyelectrolyte Brushes in Salt Mixtures

“…In the presence of monovalent salt solutions, the swelling of weak polybasic brushes (and gels 26 ) is high for strongly hydrated anions, like CH 3 COO – and Cl – , whereas weakly hydrated anions, like NO 3 – and SCN – , result in much less swelling and even complete collapse of the brush layer. 24,27,28…”