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 hydrophobic ion, respectively, extending the chaotropic side of the Hofmeister series. We show by measurement of the viscosity B-coefficient of SiW (and other nano-ions such as dodecaborates and Keggin POMs) and of ion binding to the non-ionic polymer hydroxypropylcellulose (HPC), how chaotropic, superchaotropic and hydrophobic ions can be unambiguously distinguished. The B-coefficient of the superchaotropic SiW is positive as for hydrophobic ions, and distinct from chaotropic ions with negative B. In HPC-solution, SiW or BPh₄ bind to the polymer, dramatically increasing the viscosity and the cloud point. Heating induces characteristically distinct responses for the two nano-ions: The viscosity decreases for SiW and rises for BPh₄. These effects arise from nano-ion induced aggregation and electric charging of HPC, which, upon heating, become weaker for SiW and stronger for BPh₄ as shown by Small Angle X-ray Scattering. Isothermal Titration Calorimetry revealed striking thermochemical differences between SiW and BPh₄. Heating decreases the binding constant for SiW and increases it for BPh₄ arising respectively from an enthalpically favorable, exothermic, chaotropic binding process or an enthalpically unfavorable, endothermic, hydrophobic binding process. Importantly, superchaotropic binding can be stronger or weaker than hydrophobic binding depending on temperature.