Can Charge Transfer Across C-H...O Hydrogen Bonds Stabilize Oil Droplets in Water?

TL;DR

The paper tests whether charge transfer across C–H···O hydrogen bonds can impart a measurable negative charge to oil droplets in water to explain emulsion stability and electrophoretic mobility. Using ALMO-EDA with COVP analysis on water–hexane clusters and AMOEBA MD for extended interfaces, it shows forward and backward CT are bidirectional and largely cancel, yielding net charges on the order of 0.00065 e$^-$ nm$^{-2}$ (≈0.65 me$^-$ nm$^{-2}$) or smaller, far too small to explain observed stability or conductivity. It further argues that vibrational blue shifts seen in VSFS arise from Pauli repulsion rather than CT, and that dynamic polarization cannot produce finite conductivity under an applied field. Consequently, CT cannot account for oil charging or electrophoretic mobility, and the true molecular origin of oil charge remains open, with impurities or residual ions as plausible contributors.

Abstract

Oil-water emulsions resist aggregation due to the presence of negative charges at their surface that leads to mutual repulsion between droplets, but the molecular origin of oil charge is currently under debate. While much evidence has suggested that ionic species must accumulate at the interface, an alternative perspective attributes the negative charge on the oil droplet to charge transfer of electron density from water to oil molecules. While the charge transfer mechanism is consistent with the correct sign of oil charge, it is just as important to provide good estimates of the charge magnitude to explain emulsion stability and electrophoresis experiments. Here we show using energy decomposition analysis that the amount of net flow of charge from water to oil is negligibly small due to nearly equal forward and backward charge transfer through weak oil-water interactions, such that oil droplets would be unstable and coalesce, contrary to experiment. The lack of charge transfer also explains why vibrational sum frequency scattering reports a blue shift in the oil C-H frequency when forming emulsions with water, which arises from Pauli repulsion due to localized confinement at the interface. Finally, unlike ions, neither charge transfer nor dynamic polarization can produce a finite conductivity needed to couple to electric fields that would explain electrophoretic mobility.

Figures (4)

• Figure 1: Charge transfer analysis for optimized water–hexane complexes compared to the water dimer. Optimized structures and dominant COVPs in forward and backward charge transfer processes of A (H$_2$O)(H$_2$O), B (C$_6$H$_{14}$)(H$_2$O), and C(C$_6$H$_{14}$)(H$_2$O$^1$---H$_2$O$^2$) complexes. Forward CT refers to H$_2$O $\rightarrow$ C$_6$H$_{14}$, and backward CT refers to C$_6$H$_{14}$$\rightarrow$ H$_2$O. D and E represent charge transfer and ALMO-EDA energy analysis of optimized complexes in A, B, C, and C$^1$ and C$^2$ that represent the complexes (C$_6$H$_{14}$)(H$_2$O$^1$) and (C$_6$H$_{14}$)(H$_2$O$^2$) after removing one water molecule from complex C. All structural optimizations were performed at the $\omega$B97X-V/def2-TZVP level, and AMLO-EDA calculations were conducted using $\omega$B97X-V/def2-TZVPD. CT unit me$^-$. The original data for D and E are in Supplementary Tables S1 and S2.
• Figure 2: $\textbf{Water/oil interface structures}$. ($\textbf{A, B}$) Sample representation of the clusters considered for the ALMO-EDA calculations highlighting the spatial arrangement of water molecules (red and white spheres) around hexane molecules (cyan). The extended hydrogen-bonded water network at the interface is illustrated in pink. ($\textbf{C}$) Probability density distribution illustrating the spatial and angular relationship between water oxygen and hexane. The x-axis represents the C–O distance, while the y-axis shows the C-H···O angle, describing the relative orientation of water molecules with respect to hexane’s C–H bonds. ($\textbf{D}$) Probability distribution of the closest distance between hydrogen atoms in hexane with water hydrogens and oxygens. This is consistent with the radial distribution functions (Supplementary Figure S1), where the H···H intermolecular distribution is more short-ranged than the O···H distribution in the water–hexane system. ($\textbf{E}$) Probability density distributions of Forward CT, Backward CT, and Net CT. Solid lines for direct water–hexane interactions clusters in (A) and dashed lines correspond to the clusters in (B). The original data is provided in Supplementary Tables S3 and S4. ($\textbf{F}$) Probability density distributions of water oil inter-molecule interaction energy components. The legend includes the percentage contribution of each energy component relative to the total interaction energy. Forward CT refers to H$_2$O $\to$ C$_6$H$_{14}$, and Backward CT refers to C$_6$H$_{14}$$\to$ H$_2$O. Cluster selection details are provided in the Methods section.
• Figure 3: $\textbf{Estimates of surface charge and zeta potentials}$. ($\textbf{A}$) Surface potential as a function of surface charge density ($\sigma_0$) for a 100 nm-radius spherical interface at various ionic strengths (1 mM, 0.01 mM, 0.0001 mM) using the non-linear Poisson Boltzmann equationGouy1910Chapman1913Debye1923. Dashed vertical lines are surface charge densities of $-$0.015 poli2020charge, $-$0.002 vacha2012charge, and $-$0.0007 e$^-$/nm$^2$ from ALMO-EDA. See Supplementary Information.($\textbf{B}$) The surface potential (solid lines) as a function of surface charge density using the Poisson equation.
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