Extraordinary activity of interfacial water on oil droplets

The behavior of water at hydrophobic interfaces has perplexed scientists for over a century, spanning chemistry, biology, materials science, geology, and engineering. Recent discoveries—such as the anomalous chemistry of water microdroplets and contact electro-catalysis—highlight the pivotal role of interfacial water.

A new study published in Nature systematically resolves the disordered molecular structure and ultrahigh electrostatic fields (~40–90 MV/cm) at oil-water mesoscopic interfaces, overturning textbook assumptions about the “inert” nature of hydrophobic surfaces and opening new avenues for catalysis, biomedicine, and green energy.

For decades, sum frequency generation (SFG) spectroscopy has dominated interfacial water studies but suffered from inherent limitations. Researchers have now pioneered a novel approach: integrating high-resolution Raman spectroscopy with multivariate curve resolution (MCR) algorithms.

By isolating solvent background and solute-correlated (SC) spectral signals with unprecedented signal-to-noise ratios, they achieved the first nanoscale-resolution measurements of interfacial layers in oil-water emulsions.

Their key findings include the following:

• Structural disorder: The characteristic OH-stretch shoulder at 3250 cm⁻¹—a hallmark of tetrahedral hydrogen-bonded networks—nearly vanished at oil droplet interfaces, indicating drastic structural disorder. Molecular dynamics simulations revealed ~25% of interfacial water molecules possess unbonded “free” OH groups, contradicting classical predictions of “ice-like ordered layers.”

• Ultrahigh electric fields: By analyzing resonance redshifts (3575 cm⁻¹) of free OH bonds, the team quantified interfacial electrostatic fields (40–90 MV/cm), rivaling the intense fields in enzyme active sites (~100 MV/cm). These fields correlate directly with droplet ζ-potentials: reducing ζ from −60 mV to −20 mV diminished redshifts, implicating charge distribution (e.g., hydroxide adsorption or oil-water charge transfer) as the primary mechanism.

• Catalytic implications: Transition state theory calculations showed such fields reduce activation free energy by ~4.8 kcal/mol, accelerating reaction rates >3,000-fold at room temperature. This provides a mechanistic basis for water microdroplet chemistry (rate enhancements of 10³–10⁶) and explains catalyst-free redox reactions in contact-electrocatalysis.

The discovery of disordered interfaces and colossal electric fields could transform the understanding of biological processes (protein aggregation, membrane interactions) as well as technologies, such as triboelectric nanogenerators, atmospheric aerosol nucleation, water purification and oil-spill remediation.

The paper, titled “Water structure and electric fields at oil droplet interfaces,” was the work of a collaborative team led by Prof. Wei Min of Columbia University and Prof. Teresa Head-Gordon of UC Berkeley. Co-first authors are Dr. Lixue Shi and Dr. Allen LaCour, with critical contributions from Naixin Qian, Joseph Heindel, Xiaoqi Lang, and Ruoqi Zhao.