Alkane adsorption at the water-vapor interface

The adsorption of methane and ethane at the water liquid-vapor interface is investigated using a combination of explicit molecular simulations and scaled-particle theory. Calculated adsorption coefficients are found to be approximately half the experimental values for both hydrocarbons. Simulation enthalpies of adsorption are in excellent agreement with the experimental values, while the heats predicted by scaled-particle theory are just below the lower limit of the experimental error. The simulation underprediction of the experimental adsorption coefficients corresponds to a similar tok/2 increase in the entropic adsorption penalty. Potential sources for the discrepancy between simulation and experiment are explored using the scaled-particle model. Based on this analysis, it is unlikely that the differences arise from either long-wavelength capillary fluctuations or bulk water density differences between simple point charge and real water. Contributions from surface field polarization of these two adsorbates appear to be negligible.