Chaotropic or Hydrophobic Effect: Distinct binding signatures of nano-ions to a non-ionic polymer

Abstract

The nanometric ions (nano-ions) SiW12O404- (SiW) and B(C6H5)4- (BPh4) are considered as a superchaotropic and a hydro-phobic ion, respectively, in extension to the chaotropic side of the Hofmeister series. A distinction between chaotropic, superchaotropic and hydrophobic ions, however, has not been presented so far. Herein, we show by measurement of the viscosity B-coefficient of SiW (and other nano-ions) and of ion binding to the non-ionic polymer hydroxypropylcellulose (HPC), how chaotropic, superchaotropic and hydrophobic ions can be unambiguously distinguished. The viscosity B-coefficient of the superchaotropic SiW is positive as for hydrophobic ions, and distinct from classical chaotropic ions with a negative B-coefficient. In HPC-solution, BPh4 and SiW bind to the polymer, dramatically increasing the viscosity and the cloud point. Heating induces characteristically distinct responses for the two nano-ions: The viscosity rises for BPh4 and decreases for SiW. These effects are related to nano-ion induced aggregation and electric charging of HPC, which, upon heating, become stronger for BPh4 and weaker for SiW as shown by Small Angle X-ray and Neutron Scattering. 1H-Nuclear Magnetic Resonance and Isothermal Titration Calorimetry showed that the structural effects are linked to binding thermodynamics. Upon heating, the binding constant decreases for SiW and increases for BPh4 arising respectively from an enthalpically favorable, exothermic, chaotropic driving force or an enthalpically unfavorable, endothermic, hydropho-bic driving force. Combining the viscosity B-coefficient and sign of the binding enthalpy enables distinguishing cha-otropic, superchaotropic and hydrophobic ions. Importantly, superchaotropic binding can be stronger or weaker than hydrophobic binding depending on the temperature. Ion hydration and binding are demonstrated as powerful tools to tune polymer solution properties.