Solvation at liquid surfaces I
Measuring the widths of liquid interfaces â€“ Our original work in the area of surface chemistry was motivated by the simple question, â€œDo oil and water really not mix?â€ A more quantitative way of asking this question is â€œOver what lengthscales do the properties of one liquid converge to those of a second, immiscible liquid.â€ Using custom designed surfactants dubbed â€œmolecular rulersâ€ and resonance enhanced, second harmonic generation (SHG) spectroscopy we have measured the distances required for solvent polarity to transition from the aqueous to the organic limit across a wide variety of immiscible liquid/liquid interfaces. As a result of these studies, we discovered that interfacial asymmetry can force solvent species to organize differently compared to their long range structure in bulk solution. Consequently, liquid surfaces may acquire properties that can not be described simply by extrapolating contributions from the two individual phases. For example, water is a very polar solvent and long-chain alcohols have intermediate polarity based on their respective static dielectric constants. However, water/alcohol interfaces are dominated by a nonpolar, alkane-like region. These findings necessarily force one to reconsider proposed mechanisms of solvent extraction, interfacial electrochemistry and colloid stability.