Influence of immobilized cations on the thermodynamic signature of hydrophobic interactions at chemically heterogeneous surfaces

Abstract

Hydrophobic interactions play a central role in bioinspired strategies for molecular self-assembly in water, yet how these interactions are encoded by chemically heterogeneous interfaces is poorly understood. We report an experimental investigation of the influence of immobilized polar groups (amine) and cations (ammonium and guanidinium) on enthalpic and entropic contributions to hydrophobic interactions mediated by methyl-terminated surfaces at temperatures ranging from 298 K to 328 K and pH values between 3.5 to 10.5. We use our measurements to calculate the change in free energy (and enthalpic and entropic components) that accompanies transfer of each surface from aqueous TEA containing 60 vol% methanol into aqueous TEA (i.e., transfer free energy that characterizes hydrophobicity). We find the thermodynamic signature of the pure methyl surface (positive transfer enthalpy and entropy) to be altered qualitatively by incorporation of amine or guanidinium groups into the surface (negative transfer enthalpy and near zero transfer entropy). In contrast, ammonium groups immobilized on a methyl surface do not change the thermodynamic signature of the hydrophobic interaction. Compensation of entropy and enthalpy is clearly evident in our results, but the overall trends in the transfer free energies are dominated by enthalpic effects. This observation and others lead us to hypothesize that the dominant effect of the immobilized charged or polar groups in our experiments is to influence the number or strength of hydrogen bonds formed by interfacial water molecules adjacent to the nonpolar domains. Overall, these results provide insight into entropy–enthalpy compensation at chemically heterogeneous surfaces, and generate hypotheses and a rich experimental dataset for further exploration via simulation.