Studies at the solution/air interface by experiment and molecular dynamics simulation

The behaviour of benzo-15-crown-5 and 4-nitrobenzo-15-crown-5 at the aqueous solution/air interface has been studied, as a function of bulk concentration, using surface tension measurements, second harmonic generation (SHG) spectroscopy and molecular dynamics (MD) computer simulations. From the surface tension measurements, the area occupied per molecule at the surface is derived over a bulk concentration range between 0 and 10 mM. This is used to set up a series of MD simulations, which enable investigation of the crown ethers on an atomistic scale. These simulations allow the calculation of parameters that influence the second harmonic response of the system, such as the tilt angle of the molecule to the surface normal and the order parameters ⟨P1⟩, ⟨P2⟩ and ⟨P3⟩. An estimate of the interfacial refractive index, nint of the system may also be obtained. The conformations which may be adopted by the crown ethers at the surface are also examined. SHG analysis also relies upon several assumptions about the system under investigation, and through the simulations, it is possible to examine the validity of these assumptions. The combination of these three techniques greatly increases the understanding of the systems, investigated, and also allows a critical analysis of the limitations and assumptions used.