Hydrophobic but Water-Friendly: Favorable Water–Perfluoromethyl Interactions Promote Hydration Shell Defects

Thiourea as a "Polar Hydrophobic" Hydrogen-Bonding Motif: Application to Highly Durable All-Underwater Adhesion

Here, we report that, in contrast to urea, thiourea functions as a "polar hydrophobic" hydrogen-bonding motif. Although thiourea is more acidic than urea, thiourea exchanges its N-H protons with water at a rate that is 160 times slower than that for urea at 70 °C. This suggests that thiourea is much less hydrated than urea in an aqueous environment. What led us to this interesting principle was the serendipitous finding that self-healable poly(ether thiourea) adhered strongly to wet glass surfaces. This discovery enabled us to develop an exceptionally durable all-underwater adhesive that can maintain large adhesive strength for over a year even in seawater, simply by mechanically mixing three water-insoluble liquid components on target surfaces. Because thiourea is hydrophobic, its hydrogen-bonding networks within the adhesive structure and at the adhesive-target interface are presumed to be dehydrated. For comparison, a reference adhesive using urea as a representative "polar hydrophilic" hydrogen-bonding motif was durable for less than 4 days in water. Highly durable all-underwater adhesives are needed in various fields of marine engineering and biomedical sciences, but their development has been a major challenge because a hydration layer that spontaneously forms in water always inhibits adhesion.

Evaluating aliphatic CF, CF2, and CF3 groups as vibrational Stark effect reporters

Given the extensive use of fluorination in molecular design, it is imperative to understand the solvation properties of fluorinated compounds and the impact of the C-F bond on electrostatic interactions. Vibrational spectroscopy can provide direct insights into these interactions by using the C-F bond stretching [v(C-F)] as an electric field probe through the vibrational Stark effect (VSE). In this work, we explore the VSE of the three basic patterns of aliphatic fluorination, i.e., mono-, di-, and trifluorination in CF, CF2, and CF3 groups, respectively, and compare their response to the well-studied aromatic v(C-F). Magnitudes (i.e., Stark tuning rates) and orientations of the difference dipole vectors of the v(C-F)-containing normal modes were determined using density functional theory and a molecular dynamics (MD)-assisted solvatochromic analysis of model compounds in solvents of varying polarity. We obtain Stark tuning rates of 0.2-0.8 cm-1/(MV/cm), with smallest and largest electric field sensitivities for CFaliphatic and CF3,aliphatic, respectively. While average electric fields of solvation were oriented along the main symmetry axis of the CFn, and thus along its static dipole, the Stark tuning rate vectors were tilted by up to 87° potentially enabling to map electrostatics in multiple dimensions. We discuss the influence of conformational heterogeneity on spectral shifts and point out the importance of multipolar and/or polarizable MD force fields to describe the electrostatics of fluorinated molecules. The implications of this work are of direct relevance for studies of fluorinated molecules as found in pharmaceuticals, fluorinated peptides, and proteins.

Molecular Modeling of the Fluorination Effect on the Penetration of Nanoparticles across Lipid Bilayers

The fluorinated decorations have recently been widely used in many biomedical applications. However, the potential mechanism of the fluorination effect on the cellular delivery of nanoparticles (NPs) still remains elusive. In this work, we systemically explore the penetration of a perfluoro-octanethiol-coated gold NP (PF-Au NP) and, for comparison, an octanethiol-coated gold NP (OT-Au NP) across lipid bilayers. We also investigated the effect of these two types of NPs on the properties of lipid bilayers. Our findings indicate that the lipid type and the surface tension of the lipid bilayer significantly impact the penetration capabilities of the fluorinated gold NP. By examining the distribution of ligands on the surface of the two types of NPs in water and during the penetration process, we unveil their distinct penetration characteristics. Specifically, the PF-Au NP exhibits amphiphobic behavior (both hydrophobic and lipophobic), while the OT-Au NP exhibits solely hydrophobic characteristics. Finally, we observe that the penetration capabilities can be increased by adjusting the degree of fluorination of the ligands on the NP surface. Overall, this study provides useful physical insights into the unique properties of the fluorinated decorations in NP permeation.

Visualizing Water Monomers and Chiral OH-(H2O) Complexes Infiltrated in a Macroscopic Hydrophobic Teflon Matrix

Insights into the interaction of fluoroalkyl groups with water are crucial to understanding the polar hydrophobicity of fluorinated compounds, such as Teflon. While an ordered hydrophobic-like 2D water layer has been demonstrated to be present on the surface of macroscopically hydrophobic fluorinated polymers, little is known about how the water infiltrates into the Teflon and what is the molecular structure of the water infiltrated into the Teflon. Using highly sensitive femtosecond sum frequency generation vibrational spectroscopy (SFG-VS), we observe for the first time that monomeric H2O and chiral OH-(H2O) complexes are present in macroscopically hydrophobic Teflon. The species are inhomogeneously distributed inside the Teflon matrix and at the Teflon surface. No water clusters or single-file water "wires" are observed in the matrix. SFG free induction decay (SFG-FID) experiments demonstrate that the OH oscillators of physically absorbed molecular water at the surface dephase on the time scale of <230 fs, whereas the water monomers and hydrated hydroxide ions infiltrated in the Teflon matrix dephase much more slowly (680-830 fs), indicating that the embedded monomeric H2O and OH-(H2O) complexes are decoupled from the outer environment. Our findings can well interpret ultrafast water permeation through fluorous nanochannels and the charging mechanism of Teflon, which may tailor the desired applications of organofluorines.

Impact of hydrophobicity on local solvation structures and its connection with the global solubilization thermodynamics of amphiphilic molecules

The relationship between the local solvation structures and global thermodynamics, specifically in the case of amphiphilic molecules, is a complex phenomenon and is not yet fully understood. With the prior knowledge that local solvation structures can impose a significant impact on the overall solvation process, we here combine THz spectroscopic analysis with MD simulations to investigate the impact of the altered hydrophobicity and polarity of amphiphilic solute molecules on the local solvation configurations. We use two water soluble alcohols: ethanol (EtOH) and its fluorinated counterpart, 2,2,2-trifluoroethanol (TFE), as model solutes. Our study is aimed to determine the relative abundance of different hydrogen bonded conformers and to establish a correlation between the spectral signatures (as obtained from THz spectroscopic measurements) and microscopic solute-solvent interactions associated with the local solvation structures (as obtained from MD simulations). Finally, we estimate the possible energetic parameters associated with the alcohol solubilization process. We found that while both the alcohols are completely water soluble, they receive a contrasting solvation energy share in terms of entropy and enthalpy. We understand that these findings are not limited to the specific system studied here but can be broadly extrapolated to other amphiphilic aqueous solutions.

Combinational Vibration Modes in H2O/HDO/D2O Mixtures Detected Thanks to the Superior Sensitivity of Femtosecond Stimulated Raman Scattering

Overtones and combinational modes frequently play essential roles in ultrafast vibrational energy relaxation in liquid water. However, these modes are very weak and often overlap with fundamental modes, particularly in isotopologues mixtures. We measured VV and HV Raman spectra of H₂O and D₂O mixtures with femtosecond stimulated Raman scattering (FSRS) and compared the results with calculated spectra. Specifically, we observed the mode at around 1850 cm^-1 and assigned it to H-O-D bend + rocking libration. Second, we found that the H-O-D bend overtone band and the OD stretch + rocking libration combination band contribute to the band located between 2850 and 3050 cm^-1. Furthermore, we assigned the broad band located between 4000 and 4200 cm^-1 to be composed of combinational modes of high-frequency OH stretching modes with predominantly twisting and rocking librations. These results should help in a proper interpretation of Raman spectra of aqueous systems as well as in the identification of vibrational relaxation pathways in isotopically diluted water.

Beyond Aquaporins: Recent Developments in Artificial Water Channels

A molecular scale understanding of the fast and selective water transport in biological water channels, aquaporins (AQPs), has inspired attempts to mimic its performance in synthetic structures. These synthetic structures, referred to as artificial water channels (AWCs), present several advantages over AQPs in applications. After over a decade of efforts, the unique transport properties of AQPs have been reproduced in AWCs. Further, recent developments have shown that the performance of benchmark AQP channels can be exceeded by new AWC designs using novel features not seen in biology. In this Perspective, we provide a brief overview of recent AWC developments, and share our perspective on forward-looking AWC research.

Ultrafast water permeation through nanochannels with a densely fluorous interior surface

Ultrafast water permeation in aquaporins is promoted by their hydrophobic interior surface. Polytetrafluoroethylene has a dense fluorine surface, leading to its strong water repellence. We report a series of fluorous oligoamide nanorings with interior diameters ranging from 0.9 to 1.9 nanometers. These nanorings undergo supramolecular polymerization in phospholipid bilayer membranes to form fluorous nanochannels, the interior walls of which are densely covered with fluorine atoms. The nanochannel with the smallest diameter exhibits a water permeation flux that is two orders of magnitude greater than those of aquaporins and carbon nanotubes. The proposed nanochannel exhibits negligible chloride ion (Cl^-) permeability caused by a powerful electrostatic barrier provided by the electrostatically negative fluorous interior surface. Thus, this nanochannel is expected to show nearly perfect salt reflectance for desalination.

Influence of Methylene Fluorination and Chain Length on the Hydration Shell Structure and Thermodynamics of Linear Diols

The interplay between the local hydration shell structure, the length of hydrophobic solutes, and their identity (perfluorinated or not) remains poorly understood. We address this issue by combining Raman-multivariate curve resolution (Raman-MCR) spectroscopy, simulation, and quantum-mechanical calculations to quantify the thermodynamics and the first principle interactions behind the formation of defects in the hydration shell of alkyl-diol and perfluoroalkyl-diol chains. The hydration shell of the fluorinated diols contains substantially more defects than that of the nonfluorinated diols; these defects are water hydroxy groups that do not donate hydrogen bonds and which either point to the solute (radial-dangling OH) or not (nonradial-dangling OH). The number of radial-dangling OH defects per carbon decreases for longer chains and toward the interior of the fluorinated diols, mainly due to less favorable electrostatics and exchange interactions; nonradial-dangling OH defects per carbon increase with chain length. In contrast, the hydration shell of the nonfluorinated diols only contains radial-dangling defects, which become more abundant toward the center of the chain and for larger chains, predominantly because of more favorable dispersion interactions. These results have implications for how the folding of macromolecules, ligand binding to biomacromolecules, and chemical reactions at water-oil interfaces could be modified through the introduction of fluorinated groups or solvents.

Correlating solvation with conformational pathways of proteins in alcohol-water mixtures: a THz spectroscopic insight

Water, being an active participant in most of the biophysical processes, is important to trace how protein solvation changes as its conformation evolves in the presence of solutes or co-solvents. In this study, we investigate how the secondary structures of two diverse proteins - lysozyme and β-lactoglobulin - change in the aqueous mixtures of two alcohols - ethanol and 2,2,2-trifluoroethanol (TFE) using circular dichroism measurements. We observe that these alcohols change the secondary structures of these proteins and the changes are protein-specific. Subsequently, we measure the collective solvation dynamics of these two proteins both in the absence and in the presence of alcohols by measuring the frequency-dependent absorption coefficient (α(ν)) in the THz (0.1-1.2 THz) frequency domain. The alcohol-water mixtures exhibit a non-ideal behaviour with the highest absorption difference (Δα) obtained at X_alcohol = 0.2. The protein solvation in the presence of the alcohols shows an oscillating behaviour in which Δα_protein changes with X_alcohol. Such an oscillatory behaviour of protein solvation results from a delicate interplay between the protein-water, protein-alcohol and water-alcohol associations. We attempt to correlate the various structural conformations of the proteins with the associated solvation.

Do Docking Sites Persist Upon Fluorination? The Diadamantyl Ether-Aromatics Challenge for Rotational Spectroscopy and Theory

Fluorinated derivatives of biological molecules have proven to be highly efficient at modifying the biological activity of a given protein through changes in the stability and the kind of docking interactions. These interactions can be hindered or facilitated based on the hydrophilic/hydrophobic character of a particular protein region. Diadamantyl ether (C₂₀H₃₀O) possesses both kinds of docking sites, serving as a good template to model these important contacts with aromatic fluorinated counterparts. In this work, an experimental study on the structures of several complexes between diadamantyl ether and benzene as well as a series of fluorinated benzenes is reported to analyze the effect of H→F substitution on the interaction and structure of the resulting molecular clusters using rotational spectroscopy. All experimentally observed complexes are largely dominated by London dispersion interactions with the hydrogen‐terminated surface areas of diadamantyl ether. Already single substitution of one hydrogen atom with fluorine changes the preferred docking site of the complexes. However, the overall contributions of the different intermolecular interactions are similar for the different complexes, contrary to previous studies focusing on the difference in interactions using fluorinated and non‐fluorinated molecules.

Oleophobic composite films based on multi-layer graphitic scaffolding

Free-standing films based on a plasma-exfoliated multi-layer graphitic material composite comprising polyelectrolyte and fluorosurfactant. Such materials show oleophobic/hydrophilic properties and application within oil–water separations.

On the Behavior of Perfluorinated Persistent Organic Pollutants (POPs) at Environmentally Relevant Aqueous Interfaces: An Interplay of Hydrophobicity and Hydrogen Bonding

The behavior of perfluorinated persistent organic pollutants (POPs), especially perfluoroalkyl carboxylic and sulfonic acids, at aqueous interfaces is crucial for their transport and speciation in the environment and subsequent immunotoxicity. Here, we investigate the surface prevalence and interfacial interaction of a prototype perfluorinated-POP, perfluoroheptanoic acid (PFHA), with environmentally relevant amphiphiles of varying hydrophobicity and head groups (C_nH_2n+1-X; n: 8 vs 16; -X: -OH vs -COOH) using interface-selective vibrational sum frequency generation (VSFG) spectroscopy. SFG intensity spectra in the CH- and OH-stretch regions reveal that PFHA prevails at aqueous interfaces that contain amphiphiles of intermediate chain length such as 1-octanol (n = 8) and heptanoic acid (n = 6). PFHA partially expels as well as increases the alkyl chain order of octanol on the water surface. Whereas heptanoic acid, though less hydrophobic than octanol, is retained at the water surface through hydrogen-bonding with the PFHA head group (^(PFHA)COO^-···HOOC^{(heptanoic-acid)}). Long chain amphiphiles (n = 16) such as hexadecanol and palmitic acid expel PFHA from the water surface regardless of the difference in their head groups. Interestingly, the dangling OH (3710 cm^-1) which is diminished at the hydrogenated-amphiphile-water interface is preserved at the perfluorinated-POP-water interface.

Quantifying how step-wise fluorination tunes local solute hydrophobicity, hydration shell thermodynamics and the quantum mechanical contributions of solute–water interactions

Locally tuning solute–water interactions with fluorination.