  Mesoscale water-hydrophobic interfaces are of fundamental importance in
multiple disciplines, but their molecular properties have remained elusive for
decades due to experimental complications and alternate theoretical
explanations. Surface-specific spectroscopies, such as vibrational
sum-frequency techniques, suffer from either sample preparation issues or the
need for complex spectral corrections. Here, we report on a robust "in
solution" interface-selective Raman spectroscopy approach using multivariate
curve resolution to probe hexadecane in water emulsions. Computationally, we
use the recently developed monomer field model for Raman spectroscopy to help
interpret the interfacial spectra. Unlike with vibrational sum frequency
techniques, our interfacial spectra are readily comparable to the spectra of
bulk water, yielding new insights. The combination of experiment and theory
show that the interface leads to reduced tetrahedral order and weaker hydrogen
bonding, giving rise to a substantial water population with dangling OH at the
interface. Additionally, the stretching mode of these free OH experiences a ~80
cm-1 red-shift due to a strong electric field which we attribute to the
negative zeta potential that is general to oil droplets. These findings are
either opposite to, or absent in, the molecular hydrophobic interface formed by
small solutes. Together, water structural disorder and enhanced electrostatics
are an emergent feature at the mesoscale interface of oil-water emulsions, with
an estimated interfacial electric field of ~35-70 MV/cm that is important for
chemical reactivity.
