Measuring dipolar width across liquid–liquid interfaces with ‘molecular rulers’

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Abstract
Molecular dynamics simulations have previously described how the physical properties across immiscible liquid–liquid interfaces should converge from aqueous to organic limits1,2,3,4,5, but these predictions have largely gone untested, owing to difficulties associated with probing buried interfaces. X-ray and neutron scattering experiments have created detailed pictures of molecular structure at these boundaries6,7,8, but such scattering studies cannot probe how surface-altered solvent structures affect interfacial solvating properties. Given that surface-mediated solvent properties control interfacial solute concentrations and reactivities, identifying the characteristic dimensions of interfacial solvation is essential for formulating predictive models of solution phase surface chemistry. Here we use specially synthesized solvatochromic surfactants that act as ‘molecular rulers’9 and resonance-enhanced second-harmonic generation10,11,12,13 to measure the dipolar width of weakly and strongly associating liquid–liquid interfaces. Dipolar width describes the distance required for a dielectric environment to change from one phase to another. Our results show that polarity converges to a nonpolar limit on subnanometre length scales across a water–cyclohexane interface. However, polarity across the strongly associating, water–1-octanol interface is dominated by a nonpolar, alkane-like region. These data call into question the use of continuum descriptions of liquids to characterize interfacial solvation, and demonstrate that interfacial environments can vary in a non-additive manner from bulk solution limits.