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Dissecting the liquid water organization in contact with hydrophobic and hydrophilic surfaces is essential for understanding the chemical and physical properties of aqueous interfaces. Recently developed descriptors for microscopic hydrophobicity/hydrophilicity based either on molecular dynamics (MD) simulations or on surface-sensitive nonlinear optical techniques, such as sum frequency generation (SFG) spectroscopy, manage to capture and quantify the change in local molecular hydrophobicity at heterogeneous surfaces. However, the connections between the theoretical/structural descriptors and spectroscopic fingerprints have not been established yet. Here, we combine density functional theory-based MD simulations (DFT-MD) and both theoretical and experimental SFG spectroscopy to explore how the interfacial water responds in contact with self-assembled monolayers (SAM) of tunable hydrophilicity. We introduce a microscopic metric to track the transition from hydrophobic to hydrophilic interfaces, which combines a structural descriptor based on the preferential orientation within the water network in the topmost binding interfacial layer (BIL) and spectroscopic fingerprints of H-bonded and dangling OH groups of water pointing towards the surface carried by BIL-resolved SFG spectra. This metric builds a bridge between molecular descriptors of hydrophilicity/hydrophobicity and spectroscopically measured quantities, and provides a recipe to quantitatively or qualitatively interpret experimental SFG signals.

We use second harmonic generation (SHG) spectroscopy, molecular dynamics simulation, and theoretical modeling to study the response of the neat liquid water-air interface to changes in the potential of an external electrode positioned near the liquid, but out of direct contact. We observe a parabolic dependence of second harmonic intensity on applied potential. Based on standard theory, we associate this dependence with the response of the diffuse layer water molecules to changes in interfacial potential profile. Taking the literature value for this response leads to the unexpected conclusion that the electric fields within the diffuse layer are opposite in sign from those originating from the electrodes. This conclusion implies that the traditional continuum-based models of interfacial screening lack the complexity necessary to properly describe the potential profile of the liquid water-vapor interface. Effects such as overscreening in the topmost interfacial layer and extended correlations in the interfacial hydrogen bonding network may play a role in governing the response of the water interface to external fields.

Glycerol is a major cryoprotective agent and is widely used to promote protein stabilization. Through a combined experimental and theoretical study, we show that global thermodynamic mixing properties of glycerol and water are dictated by local solvation motifs. We identify three water populations, i.e., bulk water, bound water H-bonded to hydrophilic groups of glycerol and wrap water hydrating hydrophobic moieties. Each population provides distinct spectroscopic fingerprints in the THz/FIR spectral range, which allow to quantify their respective abundance and their partial contributions to the mixing enthalpy. We uncover a 1:1 connection between the number of bound waters and the mixing enthalpy, as deduced from experiments as well as from simulations. The balance between local hydrophobic wrap and hydrophilic bound contributions at the molecular level dictates macroscopic thermodynamics of mixing. This offers opportunities to rationally design polyol water mixtures to optimize technological applications by tuning mixing enthalpy and entropy based on spectroscopic screening.

Although it is well-known that limited local mutations of enzymes, such as matrix metalloproteinases (MMPs), may change enzyme activity by orders of magnitude as well as its stability, the completely rational design of proteins is still challenging. These local changes alter the electrostatic potential and thus local electrostatic fields, which impacts the dynamics of water molecules close the protein surface. Here we show by a combined computational design, experimental, and molecular dynamics (MD) study that local mutations have not only a local but also a global effect on the solvent: In the specific case of the matrix metalloprotease MMP14, we found that the nature of local mutations, coupled with surface morphology, have the ability to influence large patches of the water hydrogen-bonding network at the protein surface, which is correlated with stability. The solvent contribution can be experimentally probed via terahertz (THz) spectroscopy, thus opening the door to the exciting perspective of rational protein design in which a systematic tuning of hydration water properties allows manipulation of protein stability and enzymatic activity.

Hydrophobicity/hydrophilicity of aqueous interfaces at the molecular level results from a subtle balance in the water-water and water-surface interactions. This is characterized here via density functional theory-molecular dynamics (DFT-MD) coupled with vibrational sum frequency generation (SFG) and THz-IR absorption spectroscopies. We show that water at the interface with a series of weakly interacting materials is organized into a two-dimensional hydrogen-bonded network (2D-HB-network), which is also found above some macroscopically hydrophilic silica and alumina surfaces. These results are rationalized through a descriptor that measures the number of "vertical" and "horizontal" hydrogen bonds formed by interfacial water, quantifying the competition between water-surface and water-water interactions. The 2D-HB-network is directly revealed by THz-IR absorption spectroscopy, while the competition of water-water and water-surface interactions is quantified from SFG markers. The combination of SFG and THz-IR spectroscopies is thus found to be a compelling tool to characterize the finest details of molecular hydrophobicity at aqueous interfaces.

International audience; We show that phase-sensitive vibrational sum frequency generation (SFG) spectra of solid/water and air/water interfaces, neutral and charged, can be successfully predicted using classical molecular dynamics (CMD) simulations in combination with simple nonpolarizable force fields (FFs). This can be achieved when employing velocity–velocity autocorrelation functions weighted by parameterized Raman and atomic polar tensors for the computation of the SFG. This procedure avoids computing polarizability tensors and dipole moments using either costly ab initio molecular dynamics (AIMD) simulations or CMD simulations with more complex and computationally demanding FFs. Such a methodology paves the way to a broad usage and computationally low-cost theoretical SFG spectroscopy, as even flexible nonpolarizable water models and common FFs for inorganic surfaces can provide good predictions of the SFG spectra, in rather good qualitative agreement with AIMD and/or experiments. The strongly reduced computational cost in our approach opens the possibility to study larger systems for long periods of time, for example, allowing a detailed characterization of the electric double-layer formation at interfaces with “environmentally relevant” ionic concentrations (mM), extracting fingerprints by theoretical CMD–SFG spectroscopy.

International audience; Water, being the universal solvent, acts as a competing agent in fundamental processes, such as folding, aggregation or biomolecular recognition. A molecular understanding of hydrophobic hydration is of central importance to understanding the subtle free energy differences, which dictate function. Ab initio and classical molecular dynamics simulations yield two distinct hydration water populations in the hydration shell of solvated tert-butanol noted as “HB-wrap” and “HB-hydration2bulk”. The experimentally observed hydration water spectrum can be dissected into two modes, centered at 164 and 195 cm–1. By comparison to the simulations, these two bands are attributed to the “HB-wrap” and “HB-hydration2bulk” populations, respectively. We derive a quantitative correlation between the population in each of these two local water coordination motifs and the temperature dependence of the solvation entropy. The crossover from entropy to enthalpy dominated solvation at elevated temperatures, as predicted by theory and observed experimentally, can be rationalized in terms of the distinct temperature stability and thermodynamic signatures of “HB-wrap” and “HB-hydration2bulk”.

We use second harmonic generation (SHG) spectroscopy, molecular dynamics simulation, and theoretical modeling to study the response of the neat liquid water-air interface to changes in the potential of an external electrode positioned above the liquid out of contact. We observe a parabolic dependence of second harmonic intensity on applied potential. This dependence is reminiscent of bulk-phase electric field induced second harmonic (EFISH) but more complicated because it combines the second-order response of the topmost water layer and the potential dependent response of the interfacial electrical double-layer. Based on the literature values for these contributions, we derive a physical interpretation of our measurements that reveals new insight into the response of the neat water interface to external electric fields. Specifically, we find that the net dipolar orientation of water molecules within the double-layer is primarily responsive to the internal fields generated by the excess surface concentrations of OH- and H3O+ that arise to screen the external potential. Notably, this interpretation implies that the orientational response of water dipoles at the interface can actually oppose the direction of the external field, a subtle effect that is not captured by traditional models.

Hydration free energies are dictated by a subtle balance of hydrophobic and hydrophilic interactions. which is crucial for many biological processes and technological applications, such as protein folding and molecular recognition. Whereas so far the overall entropy and enthalpy are experimentally determined based on equilibrium measurements using a calorimeter, we present here a pure spectroscopic access to these important observables, which give direct access to the underlying molecular mechanism that determines these driving forces. Using THz calorimetry the contributions due to cavity formation and hydrophilic interactions can be traced back to changes in the intermolecular hydrogen bond stretching region around 150-200 cm−1 and spectroscopic changes due to strong solute-water interactions in the frequency range of the librational modes, i.e. between 540 and 600 cm−1. Thus, we are able to link the thermodynamic model of the Lum-Chandler-Weeks theory, which was a pure ”Gedankenexperiment”, directly to experimental observables. We show that alcohol hydration can be described by a sum of a free energy cost of forming and wrapping a cavity around the solute (which is entropic for small alcohols) and an enthalpic gain due to the hydrogen bonds formed between the alcohol OH group and bound water molecules around it. In the future, our approach will allow to quantify entropic cost and enthalpic gain not only in equilibrium but also in non-equilibrium processes.

The double layer at the solid/electrolyte interface is a key concept in electrochemistry. Here, we present an experimental study combined with simulations, which provides a molecular picture of the double-layer formation in operando processes. By THz spectroscopy we are able to follow the stripping off of the cation/anion hydration shells for a NaCl electrolyte at the Au surface when decreasing/increasing the bias potential. While Na+ is attracted toward the electrode already at the smallest applied negative potentials, stripping-off of the Cl- hydration shell is observed only at higher potential values. These phenomena are directly measured by in operando THz spectroscopy with ultra-bright synchrotron light as a source and rationalized by accompanying molecular-dynamics simulations and electronic-structure calculations.