Fig. 2: H-bonding network and charge transfer mechanism are not pH-dependent.

A Vibrational sum frequency scattering: Simultaneous excitation using pulsed fs IR and VIS laser beams results in a simultaneous IR and Raman excitation that gives rise to scattered sum frequency (SF) photons that report on the interfacial molecular vibrational spectrum. B The O-D stretch spectrum of 2 vol% 100 nm radius hexadecane droplets in pure D₂O (black) and in the presence of 1 mM sodium bis(2-ethylhexyl) sulfosuccinate (AOT) surfactant, measured with the SSP polarization combination (SF and VIS beams polarized perpendicular to the scattering plane, IR beam polarized parallel to the scattering plane). See “Methods” for experimental details, supplementary Note 2 and Supplementary Figs. 2 and 3 for details on retrieving the O-D stretch spectra. C, D Vibrational SFS spectra of O-D stretch modes (C) and C-H stretch modes (D) of 2 vol% hexadecane droplets in D₂O at pD 7 (1 mM NaCl, red) and pD 11 (1 mM NaOD, blue) recorded using the SSP polarization combination). The gray curve shows C-H stretch modes of oil droplets that are not interacting with water, achieved by covering them with a dense monolayer of deuterated DPPC lipids. Solid lines in B–D represent the moving average as a guide to the eye, as described in methods. Note that the data in C, D are not normalized. E Schematic illustration of charge distribution at the interface of an oil nanodroplet as a consequence of improper H-bonding/charge transfer effects. The surface charge density in the oil phase in contact with pH neutral water as arising from charge-transfer effects was computed to be ∼−0.015 e/nm², ref. ³⁵.