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Water structure and electric fields at the interface of oil droplets

Nature volume 640, pages 87–93 (2025)Cite this article

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Abstract

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 fields2,3,4,5. 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 resolution6,7 to probe hexadecane-in-water emulsions, aided by a monomer-field theoretical model for Raman spectroscopy8. 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 droplets9, 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 interfaces10,11,12,13.

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Fig. 1: Raman-MCR spectroscopy on oil droplets in water.
Fig. 2: Oil-perturbed, SC water spectra of oil droplets.
Fig. 3: Computational structure of water–oil interface and SC spectra.
Fig. 4: SC water spectrum of oil–Triton nanodroplets.

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Data availability

 Source data are provided with this paper.

Code availability

The source code for Lammps is available at GitHub (https://github.com/lammps/lammps). The source code for OpenMM is available at GitHub (https://github.com/openmm/openmm). Examples of analysis codes and computational Raman calculations are available at GitHub (https://github.com/alacour/water_oil_interface) and Zenodo (https://doi.org/10.5281/zenodo.14618735; ref. 49).

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Acknowledgements

We acknowledge the support of the Multi-University Research Initiative of the Air Force Office of Scientific Research (FA9550-21-1-0170). W.M. acknowledges support from the National Institute of Health (R35 GM149256). T.H.G, R.A.L. and J.P.H. also acknowledge the CPIMS program, Office of Basic Energy Sciences, Chemical Sciences Division of the US Department of Energy under contract DE-AC02-05CH11231 for support of the monomer-field theory applied to this work.

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Author notes

  1. These authors contributed equally: Lixue Shi, R. Allen LaCour

Authors and Affiliations

  1. Department of Chemistry, Columbia University, New York, NY, USA

    Lixue Shi, Naixin Qian, Xiaoqi Lang & Wei Min

  2. Kenneth S. Pitzer Theory Center and Department of Chemistry, University of California, Berkeley, CA, USA

    R. Allen LaCour, Joseph P. Heindel, Ruoqi Zhao & Teresa Head-Gordon

  3. Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

    R. Allen LaCour, Joseph P. Heindel, Ruoqi Zhao & Teresa Head-Gordon

  4. Department of Bioengineering and Chemical and Biomolecular Engineering, University of California, Berkeley, CA, USA

    Teresa Head-Gordon

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  1. Lixue Shi
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Contributions

L.S., N.Q., X.L. and W.M. developed the new experimental approach; R.A.L., J.P.H., R.Z. and T.H.-G. formulated all the theoretical work; L.S., R.A.L., T.H.-G. and W.M. wrote the paper; and all authors contributed to all insights through their discussion.

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Correspondence to Lixue Shi, Teresa Head-Gordon or Wei Min.

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Extended data figures and tables

Extended Data Fig. 1 Raman spectra of pure water and aqueous alcohol solutions.

a, 0.5 M methanol. b, 0.5 M ethanol. c, 0.5 M 2-propanol. d, 0.5 M t-butanol.

Extended Data Fig. 2 Solute-correlated (SC) spectra of aqueous alcohol solutions in literature and from our measurement.

a, Raman-MCR SC spectra (solid curves) obtained from ~0.5 M aqueous methanol solutions at temperatures between 20 °C and 100 °C6. b, Raman-MCR SC spectra (solid curves) obtained from ~0.5 M aqueous solutions of deuterated ethanol at temperatures between 2 °C and 100 °C6. c, Raman spectra of pure water (dashed black curve) and 0.5 M aqueous 1-propanol (solid black curve) at 20 °C, as well as the resulting Raman-MCR SC spectrum (blue curve) of 1-propanol6. The inset shows an expanded view of the dangling OH bands obtained at various temperatures, each normalized by dividing the SC spectrum by the corresponding CH band area. d, Raman-MCR decomposition for 1 M tert-butyl alcohol (TBA) at 20 °C50. e-h, SC spectra measured from our instrument of (e) 0.5 M methanol, (f) 0.5 M ethanol, (g) 0.5 M 2-propanol, (h) 0.5 M t-butanol.

Extended Data Fig. 3 Oil droplets emulsion samples and sub-pixel spectral correction.

a, Oil droplets prepared with different concentrations. b, A sandwiched oil emulsion sample for confocal Raman micro-spectroscopy measurement. c, Size distribution of oil emulsions. DLS intensity-diameter distribution of 2% hexadecane droplets in ultrapure water. d, Setup of neon lamp on home-built Raman system for sub-pixel spectral correction. A neon lamp is placed above the sample stage, and the transmitted light is measured with the detection system of confocal Raman. A 600-nm short-pass filter is added to block emission peaks after 2100 cm−1. e, Top, typical neon emission spectrum, and bottom, neon emission spectrum with a 600-nm short-pass filter. f, Raman spectrum of water. Asterisk indicates the used emission line (~1700 cm−1) for Raman shift correction. g, Fitting the ~1702 cm−1 emission line with a Gaussian profile for spectral correction.

Extended Data Fig. 4 Noise level after sub-pixel spectral drift correction.

Blue line: the SC spectrum of 1% d34-hexadecane oil emulsion. Purple line: SC spectrum from a water-subtract-water negative control. We performed two parallel measurements on the pure water sample, treating one measurement as the solvent spectrum and the other as solution spectrum. After sub-pixel spectral drift correction, the noise level of the negative control is approaching the technical noise level and is way lower than our measured SC spectrum of oil emulsion. This comparison supports the experimental reliability.

Extended Data Fig. 5 Overview of Raman-MCR analysis workflow.

Input: spectral-drift-corrected nanoemlusion Raman spectrum and pure water spectrum measured under the same condition. Output: Raman residual map and SC spectrum. Specifically, after the sub-pixel spectral correction, a variable fraction (x%) of the pure water Raman spectrum is subtracted from the nanoemulsion spectrum to generate the SC spectrum. The intensity within SC spectra is divided into three categories: positive, negative, near-zero (i.e. below a certain threshold), which are used to construct the MCR residual map for each x value. When the minimum non-negative area is observed, this fraction (x=x2) is identified as a critical point.

Extended Data Fig. 6 SC spectrum of regular and deuterated oil nanodroplets.

a, SC spectra of 2% regular hexadecane nanoemulsion in water (blue solid) and scaled Raman spectra of pure water (orange solid). b, SC spectra of 2% d34-hexadecane nanoemulsion in water (blue solid) and scaled Raman spectra of pure water (orange solid). Their gaussian-fitted O-H regions are plotted in Fig. 2c,d. c,Water-perturbed, solute-correlated oil spectrum of d34-hexadecane oil droplets. While Raman-MCR spectroscopy has mostly focused on the water spectrum, the strategy should be general and thus extendable to the oil phase. We therefore measured water-perturbed interfacial oil spectrum with MCR (purple), and compared it to a pure d34-hexadecane (yellow). Although there is a narrowing of multiple peaks at the oil-water interface, the positions of the peaks are largely consistent. The largest difference is that the water-perturbed oil spectrum appears narrower, and the peak ratio of 2105 cm−1 over 2196 cm−1 is increased compared to bare oil, both suggesting the interfacial oil may be more ordered. Indeed, similar behavior is commonly observed in lipids at low temperature and more ordered phase. An increase in molecular order can also be understood by the fact that oil molecules tend to lie parallel to the interface with water, which has been reported elsewhere for hexadecane-water and other oil-water systems.

Extended Data Fig. 7 Structure of water at a water/hexane interface with the AMOEBA model.

a, Liquid density as a function of distance to the WCI for our MD simulation of a water/hexane system simulated with the AMOEBA model. Positive distances correspond to the water side of the surface. b, The average values of the order parameter q4 for waters as a function of distance to the WCI together with the average value for bulk water at two different temperatures. The region of the first peak in the WCI is demarcated with two vertical grey lines. c, The electric field (E) distribution for the AMOEBA model measured within 2 Å of the WCI, dividing the measured distribution into contributions from waters with and without a free OH. We only examine waters within 2 Å of the WCI. See Methods for complete details on the MD simulations and calculation of the order parameters.

Extended Data Fig. 8 Optical imaging of probes in different droplets in oil.

a, Oil droplets prepared with different solvents. Stable oil emulsions only form in water, but not in methanol (MeOH), acetonitrile (MeCN) and DMSO, indicating negligible zeta potential in the latter three systems. b, Chemical structure of positively charged Rhodamine 800 fluorescent probe used in c. c, Top, confocal fluorescence imaging on Rhodamine 800 doped microdroplets in oil. From left to right, the droplet is prepared using a 10 μM Rhodamine 800 solution in water, methanol (MeOH), acetonitrile (MeCN) or DMSO surrounded by hexadecane, respectively. Bottom, fluorescence intensity distributions along the white dashed lines in images. In water microdroplets, the ring-like distribution indicates the probe is enriched at the interface; the signal is strongest at the vertical direction (the same as the laser polarization) and weakest at the horizontal direction, indicating the probe orientation is aligned perpendicular to the interface. Both effects can be readily explained by an interfacial electric field. In contrast, both effects are absent in the other three microdroplets. Scale bars, 5 μm.

Extended Data Fig. 9 Raman spectra produced by theory as a function of distance from the WCI.

a,b, We show Raman spectra for the a, SPC/E and b, AMOEBA models produced by the monomer-field model using only water within 2 Å of the WCI, within 2 – 6 Å of the WCI, and within 6 – 10 Å of the WCI. The interfacial spectra have converged to bulk-like within 2 Å of the WCI.

Extended Data Fig. 10 Correlation between tetrahedral order and stretching frequencies.

The relationship between OH-stretching frequencies and the tetrahedral order parameter q4 in simulation. a,b, We show the frequencies corresponding to the third and fourth excited states (which generally correspond to the stretching modes) for a, the SPC/E and b, the AMOEBA models computed from our local monomer model for a subset of waters in our simulations. We only include waters that have both hydrogens participating in H-bonds. c,d, Average frequencies at different q4 for the SPC/E and AMOEBA models, respectively. The error bars represent the standard deviation.

Extended Data Table 1 A literature overview on the structure of interfacial water by second-order optical spectroscopy
Full size table
Extended Data Table 2 Parameters for Monomer-Field Model
Full size table

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Shi, L., LaCour, R.A., Qian, N. et al. Water structure and electric fields at the interface of oil droplets. Nature 640, 87–93 (2025). https://doi.org/10.1038/s41586-025-08702-y

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