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Nature 2025, 640, 87.

Water structure and electric fields at the interface of oil droplets

Interfacial water exhibits rich and complex behaviour1, playing an important part in chemistry, biology, geology and engineering. However, there is still much debate on the fundamental properties of water at hydrophobic interfaces, such as orientational ordering, the concentration of hydronium and hydroxide, improper hydrogen bonds and the presence of large electric fields. This controversy arises from the challenges in measuring interfacial systems, even with the most advanced experimental techniques and theoretical approaches available. Here we report on an in-solution, interface-selective Raman spectroscopy method using multivariate curve resolution to probe hexadecane-in-water emulsions, aided by a monomer-field theoretical model for Raman spectroscopy. Our results indicate that oil–water emulsion interfaces can exhibit reduced tetrahedral order and weaker hydrogen bonding, along with a substantial population of free hydroxyl groups that experience about 95 cm−1 redshift in their stretching mode compared with planar oil–water interfaces. Given the known electrostatic zeta potential characteristic of oil droplets, we propose the existence of a strong electric field (about 50–90 MV cm−1) emanating from the oil phase. This field is inferred indirectly but supported by control experiments and theoretical estimates. These observations are either absent or opposite in the molecular hydrophobic interface formed by small solutes or at planar oil–water interfaces. Instead, water structural disorder and enhanced electric fields emerge as unique features of the mesoscale interface in oil–water emulsions, potentially contributing to the accelerated chemical reactivity observed at hydrophobic–water interfaces.

https://www.nature.com/articles/s41586-025-08702-y#citeas

Nat. Commun. 2025,16,7288.

Interfaces govern the structure of angstrom-scale confined water solutions

Nanoconfinement of aqueous electrolytes is ubiquitous in geological, biological, and technological contexts, including sedimentary rocks, water channel proteins, and applications like desalination and water purification membranes. The structure and properties of water in nanoconfinement can differ significantly from bulk water, exhibiting, for instance, modified hydrogen bonds, altered dielectric constant, and distinct phase transitions. Despite the importance of nanoconfined water, experimentally elucidating the nanoconfinement effects on water, such as its orientation and hydrogen bond (H-bond) network, has remained challenging. Here, we study two-dimensionally nanoconfined aqueous electrolyte solutions with tunable confinement from nanoscale to angstrom-scale sandwiched between a graphene sheet and calcium fluoride (CaF2) achieved by capillary condensation. We employ heterodyne-detection sum-frequency generation (HD-SFG) spectroscopy, a surface-specific vibrational spectroscopy capable of directly and selectively probing water orientation and H-bond environment at interfaces and under confinement. The vibrational spectra of the nanoconfined water can be described quantitatively by the sum of the individual interfacial water signals from the CaF2/water and water/graphene interfaces until the confinement reduces to angstrom-scale (<~8 Å). Machine-learning-accelerated ab initio molecular dynamics simulations confirm our experimental observation. These results manifest that interfacial, rather than nanoconfinement effects, dominate the water structure until angstrom-level confinement for the two-dimensionally nanoconfined aqueous electrolytes.

https://www.nature.com/articles/s41467-025-62625-w

J. Am. Chem. Soc.2025, 147, 41, 37328.

Elucidating the Mechanisms of Ion Permeation through Sub-Nanometer Graphene Pores: Uncovering Free Energy Barriers via High-Throughput Molecular Simulations

Aqueous anions play a crucial role in chemical and biological processes. They are traditionally classified as “structure makers” or “structure breakers” based on their impact on the viscosity of electrolyte solutions. Until now, this behavior has been assumed to stem from a single restructuring mechanism of the hydrogen (H) bonding network of water, that could align with macroscopic properties. Correlated Vibrational Spectroscopy (CVS) measurements reveal that this is not the case. Rather, anions modify water–water H-bonds through multiple distinct pathways, with frequency shifts correlating with charge transfer, and intensity changes quantifying variations in the number of interacting/orientationally cross-correlated H-bonds. The different ways through which anions impact water structure can be explained in terms of Hard–Soft-Acid–Base theory. Hard anions only affect water H-bonds through electrostatics. By contrast, soft anions weaken the H-bonds via charge transfer but simultaneously increase their concentration. The two effects for soft anions nearly cancel each other out in terms of structure breaking/making, resulting in macroscopic behavior that is similar to hard anions in spite of dramatically different molecular-level effects.

https://pubs.acs.org/doi/10.1021/acsnano.5c13306

J. Phys. Chem. Lett.2025, 16, 20, 5091.

Z-Bonds in Choline Chloride/Water Deep Eutectic Solvent: X-ray/Neutron Scattering and Density Functional Theory Calculations

Z-bond, a new weak interaction that couples H-bond and electrostatic interactions, plays an important role in ionic liquid and deep eutectic solvent (DES) formation. However, little direct experimental observation of the Z-bonds is available. In the present work, X-ray scattering (XRS) and isotope-substituted neutron scattering (ISNS) multi-data reverse driven all atomic modeling [empirical potential structure refinement (EPSR)] was employed to elucidate the microstructure of choline chloride (ChCl)/3H2O DES. The results show that Z-bonds are the determinative driving force for Ch+ solvation, while H-bonds directly drive Cl– solvation. Density functional theory (DFT) calculations confirm both bond motifs and quantify their strengths. H-bonds facilitate the formation of longer chains and larger rings, whereas Z-bonds predominantly result in the formation of medium-length chains and smaller rings. The size distribution of chains and rings formed by Z-bonds significantly surpasses that of H-bonds. Thus, the Z-bonds result in a lower diffusion coefficient of Ch+ [(0.0336 ± 0.0011) × 10–5 cm2/s] than that of Cl– [(0.0651 ± 0.0013) × 10–5 cm2/s], emphasizing the efficacy of Z-bond structures in the modulation of transport properties.

https://pubs.acs.org/doi/10.1021/acs.jpclett.5c00897

ACS Nano 2025, (published online)

Elucidating the Mechanisms of Ion Permeation through Sub-Nanometer Graphene Pores: Uncovering Free Energy Barriers via High-Throughput Molecular Simulations

Understanding ion transport through subnanometer graphene nanopores is critical for advancing nanoscale filtration technologies and uncovering the molecular mechanisms underlying selective ion permeation. Owing to their atomic thickness and tunable pore sizes, nanoporous graphene membranes serve as a model system for probing ion selectivity and hydration behavior under spatial confinement. This work investigates the transport of Na+, Cl–, K+, and water through graphene nanopores to elucidate their ion-sieving characteristics. Free energy barriers associated with ion and water permeation are quantified, offering insight into the energetic costs of dehydration and translocation through nanopores. Selective ion transport is further examined using the constant potential method (CPM), which more accurately reflects experimental electrochemical conditions, and allows for the selective permeation of K+ over Na+ within nanoporous graphene membranes. The role of externally applied electric fields is also explored to assess their impact on ion hydration and transport dynamics. Together, these results contribute to a deeper mechanistic understanding of ion confinement, hydration, and selective permeation in nanoporous atomically thin membranes.

https://pubs.acs.org/doi/10.1021/acsnano.5c13306