Size dependence of hydrophobic hydration at electrified gold/water interfaces

Abstract

Hydrophobic hydration at metal/water interfaces actively contributes to the energetics of electrochemical reactions, e.g. CO2 and N2 reduction, where small hydrophobic molecules are involved. In this work, constant applied potential molecular dynamics is employed to study hydrophobic hydration at a gold/water interface. We propose an adaptation of the Lum–Chandler–Weeks (LCW) theory to describe the free energy of hydrophobic hydration at the interface as a function of solute size and applied voltage. Based on this model we are able to predict the free energy cost of cavity formation at the interface directly from the free energy cost in the bulk plus an interface-dependent correction term. The interfacial water network contributes significantly to the free energy, yielding a preference for outer-sphere adsorption at the gold surface for ideal hydrophobes. We predict an accumulation of small hydrophobic solutes of sizes comparable to CO or N2, while the free energy cost to hydrate larger hydrophobes, above 2.5-Å radius, is shown to be greater at the interface than in the bulk. Interestingly, the transition from the volume dominated to the surface dominated regimes predicted by the LCW theory in the bulk is also found to take place for hydrophobes at the Au/water interface but occurs at smaller cavity radii. By applying the adapted LCW theory to a simple model addition reaction, we illustrate some implications of our findings for electrochemical reactions.

Stripping away ion hydration shells in electrical double-layer formation: Water networks matter

“…This requires new experimental techniques that are able to selectively probe the EDL and simultaneously provide molecular-level information, under operando (i.e., operating or reaction) electrochemical conditions. In the past two decades, a number of theoretical (12)(13)(14)(15)(16)(17) and experimental (18)(19)(20)(21)(22)(23)(24)(25)(26)(27) studies have been performed to explore the microscopic structure of the electrochemical double layer. It is still an experimental challenge to probe interfaces under operando conditions, due to difficulties in regulating all the variables involved in electrochemical reactions (28,29), such as controlling surface structure and mass transport.…”

“…As discussed in more detail in ref. 15 , water molecules within the adlayer preferentially lie flat on the Au surface, forming a two-dimensional hydrogen bond (2D-HB) network composed of HBs oriented parallel to the surface. As a consequence, only a few HBs form between the adlayer and the second water layer, creating a soft liquid–liquid interface ( 33 ).…”

“…As a consequence, only a few HBs form between the adlayer and the second water layer, creating a soft liquid–liquid interface ( 33 ). When an increasing negative potential is applied, interfacial water molecules gradually reorient their H atoms toward the gold surface, hence disrupting the 2D-HB network in the adlayer ( 15 , 21 ). These structural changes affect the EDL formation, since the adsorption of ions at the metal surface was shown to require a free energy cost due to perturbation of the adlayer structure and removal of one (or more) water molecule(s) from it ( 13 ).…”

The role of hydrophobic hydration in the free energy of chemical reactions at the gold/water interface: size and position effects

“…However, hydrophobic hydration free energy depends on the characteristic of the water H-Bond network at the interface. 21,22 Indeed, the hydration of a hydrophobic molecule is determined by the process of perturbing the water H-Bond network a) Electronic mail: alessandra.serva@sorbonne-universite.fr b) Electronic mail: simone.pezzotti@rub.de and creating a cavity in the liquid. 23,24 Recent advances in molecular dynamics (MD) simulations that include multiple layers of explicit water allow to more accurately describe the hydration contributions to interfacial chemical reactions.…”

Local thermodynamics of alcohols from THz-calorimetry: Spectroscopic fingerprints of entropic loss and enthalpic gain

“…Hydrophobic hydration is important to understand and predict fundamental biological processes, such as protein folding and aggregation [1][2][3] , molecular recognition [4][5][6] and liquid-liquid phase separation, [7][8][9] as well as in many other fields, e.g. water-mediated catalysis [10][11][12] and electro-catalysis [13][14][15] . Going from small solutes all the way up to large biomolecules, a subtle balance between hydrophilic and hydrophobic interactions is what dictates hydration free energies.…”