Influence of Immobilized Cations on the Thermodynamic Signature of Hydrophobic Interactions at Chemically Heterogeneous Surfaces
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. In this paper, weWe report the results of an experimental investigation of the influence of immobilized polar groups (amine) and cations (ammonium and guanidinium)cations on enthalpic and entropic contributions to hydrophobic interactions mediated by mixed-componentmethyl-terminated surfaces monolayers (40% 11-aminoundecanethiol or 40% 11-guanidinoundecanethiol and 60% 1-decanethiol) formed on gold films at temperatures ranging from 298 K to 328 K and pH values between 3.5 to 10.5. We identify the hydrophobic interactions by measuring adhesive forces before and after addition of 60 vol% methanol to aqueous triethanolamine (TEA, 10 mM), and interpret the measurements using Johnson-Kendall-Roberts (JKR) theory to 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 , 〖∆G〗_tr, that characterizes hydrophobicity adhesion). Our measurements reveal that theWe find the thermodynamic signature of a hydrophobic interaction mediated byth a e pure methyl surface (positive transfer enthalpy and negative transfer entropy) is reversed to be altered qualitatively by incorporation of amine or guanidinium groups into the methyl surfacesurface (negative transfer enthalpy and near zero transfer entropy) but not by . In contrast, introduction of ammonium groups immobilized on a methyl surface do not change the thermodynamic signature of the hydrophobic interaction. Although cCompensation of entropic 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 reported in this paper 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.
- This article is part of the themed collection: Bioinspired Materials