Collective Electronic Polarization Drives Charge Asymmetry at Oil-Water Interfaces

Abstract

Why kinetically stable oil droplets in water spontaneously acquire a negative charge remains one of the most vigorously debated questions in interfacial science. Here, we combine neural-network based deep potential molecular dynamics with a data-driven and information theory approach to probe the real-space electron density at an extended decane-water interface. While decane-water clusters show nearly symmetric forward and backward charge transfer (CT) and thus negligible net CT, the extended interface displays a systematic electronic asymmetry, yielding a net CT from water to the hydrocarbon phase producing an average surface charge density of $\sim0.006~e^{-}\,\mathrm{nm}^{-2}$ on the oil phase. This imbalance is accompanied by much larger intra-phase self-polarization, particularly within the hydrocarbon phase, demonstrating that collective many-body polarization dominates the interfacial electronic response. Structural analysis reveals an asymmetry between forward C--H$\cdots$O and backward O--H$\cdots$C motifs, providing a microscopic origin for a net CT from one phase to the other. Curiously, both the water O--H and decane C--H covalent bonds incur subtle contractions which originate from a response to the charge-separation layers at the interface. These features are fully consistent with the weak improper hydrogen-bonds forming at the oil-water interface that results in blue-shifts of the C-H modes.

Figures (5)

• Figure 1: Electronic $T$-matrix of the water (a), hydrogen fluoride (b), ammonia (c), and carbon dioxide (d) dimer. Molecular geometries are optimized at the M06-2X/def2-TZVPD level whereas the respective electron densities are evaluated at the r2SCAN/def2-TZVPD theory. Diagonal elements (blue) correspond to self-polarization while off-diagonal entries (red) correspond to the inter-molecular charge transfer. Net charge transfer is simply obtained by taking the absolute difference between off-diagonal elements. Color intensity of the cells follows the magnitude of the elements within each $T$-matrix while zero values are not colored.
• Figure 2: (a) Snapshot taken from the Deep Potential Molecular Dynamics simulation of the extended decane-water interface and the extraction procedure of decane-water dimers along with the respective $T$-matrix averaged over 1500 configurations. (b) Distributions of the charge transfer (CT, in $e^{-}$) by direction. Black solid line: water $\rightarrow$ decane CT; red dashed line: decane$\rightarrow$water CT; magenta dotted line: net CT. (c) Distributions of the intra-monomer (self-polarization) CT. Black solid line: water$\rightarrow$water; red dashed line: decane$\rightarrow$decane.
• Figure 3: (a) Conditional mean of the net atomic charge, $\langle q^{\mathrm{net}}\rangle$ (in $e^{-}$), plotted as a function of the cross-phase approach distances. Curves: black solid, water H net charge; red dashed, water O net charge; orange dotted, decane H net charge; blue dash-dot, decane C net charge. The thin cyan dotted horizontal line denotes $q^{\mathrm{net}}=0$. Values are shown as bin-averaged means of $\langle q^{\mathrm{net}}\rangle$ conditioned on the distance of the interaction. Yellow dotted inset displays the forward interaction (decane to water). Black continuous inset shows the backward interaction (water to decane). (b) Distributions of interfacial charge transfer (CT, in $e^{-}$) by direction. Black solid line: water $\rightarrow$ decane CT ; red dashed line: decane$\rightarrow$water CT; magenta dotted line: net CT. (c) Distributions of intra-phase (self-polarization) CT. Black solid line: water$\rightarrow$water; red dashed line: decane$\rightarrow$decane. (d) Snapshot from a Deep Potential Molecular Dynamics trajectory of the decane-water slab, showing the Willard-Chandler instantaneous interface between the two phases (pale pink dots). Carbon, oxygen, and hydrogen atoms are depicted as silver, red, and white spheres, respectively. On the right, the respective average coarse-grained $2\times2$$T$-matrix with the electron density re-sampled @r2SCAN/TZVP level. Entries are in $e^{-}$.
• Figure 4: (a) Two-dimensional probability density $P\!\left(d_{\mathrm{OH\cdots C}},\,\theta_{\mathrm{HOC}}\right)$, where $d_{\mathrm{OH\cdots C}}\equiv d(\mathrm{OH}\cdots\mathrm{C})$ and $\theta_{\mathrm{HOC}}\equiv\angle(\mathrm{H}_{\mathrm{water}}-\mathrm{O}-\mathrm{C})$ of the backward O-H$\cdots$C interaction motif (water $\rightarrow$ decane). (b) Two-dimensional probability density $P\!\left(d_{\mathrm{CH\cdots O}},\,\theta_{\mathrm{HCO}}\right)$, where $d_{\mathrm{CH\cdots O}}\equiv d(\mathrm{CH}\cdots\mathrm{O})$ and $\theta_{\mathrm{HCO}}\equiv\angle(\mathrm{H}_{\mathrm{decane}}-\mathrm{C}-\mathrm{O})$ of the forward C-H$\cdots$O interaction motif (decane $\rightarrow$ water). Only atoms tagged as interfacial via the instantaneous Willard-Chandler surface are sampled, using thresholds $s_{\mathrm{C}}=2.8~\text{\AA}$ (carbons) and $s_{\mathrm{W}}=3.0~\text{\AA}$ (oxygens). Insets show representative configurations used to define the geometric descriptors.
• Figure 5: (a) Conditional mean of the covalent O--H bond length of water molecules, $\langle d_{\mathrm{OH,cov}}\mid d_{\mathrm{WC}}\rangle$, as a function of the distance to the instantaneous WC surface $d_{\mathrm{WC}}$ (black solid line). The cyan dotted line marks the bulk mean $\langle d_{\mathrm{OH,cov}}\rangle_{\mathrm{bulk}}$. (b) Conditional mean of the covalent C--H bond length of decane molecules, $\langle d_{\mathrm{CH,cov}}\mid d_{\mathrm{WC}}\rangle$, as a function of $d_{\mathrm{WC}}$ (black solid line), compared to the bulk mean $\langle d_{\mathrm{CH,cov}}\rangle_{\mathrm{bulk}}$ (cyan dotted line). (c) Probability density of the nearest-neighbor O$\cdots$O distance $d_{\mathrm{OO}}$ in water: interface (black solid) vs bulk (red dashed). (d) Probability density of the nearest-neighbor C$\cdots$C distance $d_{\mathrm{CC}}$ in decane: interface (black solid) vs bulk (red dashed).