Interfacial ion specificity modulates hydrophobic interaction.

Ion specificity is crucial in assembly and aggregation of polymers in water driven by hydrophobic interaction. An increasing number of studies have suggested that specific ion adsorption and consequent impact on interfacial water molecules should play an important role in modulating hydrophobic interaction. Here, bubble probe atomic force microscopy (AFM) combined with theoretical modeling analysis was applied to quantify hydrophobic interactions involving three model polymers in solutions containing different ions. For polystyrene, the hydrophobic interaction's decay length D0 was reduced from 0.75 nm to 0.60 nm by introducing weakly hydrated cations (e.g., K+ and NH4+), while varying anion type had little effect. For poly(methyl methacrylate) and polydimethylsiloxane, anion specificity was demonstrated more evident in shortening the hydrophobic interaction range, with D0 decreasing from 0.63 nm to 0.50 nm and from 0.72 nm to 0.58 nm respectively when strongly hydrated F- or Cl- was replaced by weakly hydrated I-. Such results could arise from specific ion adsorption at water/polymer interface which disrupts the water structuring effect. From the nanomechanical perspective, this work has revealed the importance of interfacial ion specificity in modulating hydrophobic interaction, which offers novel implications for tuning assembly behavior of macromolecules in relevant engineering applications such as micelle formation and foam stabilization.