Molecular dynamics simulations have been employed to determine the contact angles of alkylthiol passivated gold nanocrystals adsorbed at the air-water interface. Simulations were performed using butane-, dodecane-, and octadecanethiol passivated nanoparticles. We demonstrate how the length of the surfactant chain can profoundly influence the wetting behavior of these nanoparticles. All particles were found to be stable at the air-water interface, possessing large, well-defined contact angles. We find that the shape of the dodecane- and octadecanethiol particles is strongly perturbed by the interface. We also present an analysis of the orientational ordering of water molecules at the dodecane-water interface and around butane- and dodecanethiol passivated nanoparticles. The orientational ordering translates into an electrostatic field around the nanoparticles, the magnitude of which corresponds with that of the water liquid-vapor interface.

We investigate the water-oil interface using molecular dynamics simulations of realistic models of alkanes and water. The intrinsic density profiles are computed using a methodology that removes the smoothing effect of the capillary waves. We show that at 300 K the intrinsic width of the gap separating the oil and water phases spans little more than one water molecule diameter, and undergoes very weak short-ranged fluctuations, indicating that the water-oil interface is a rigid molecular structure at ambient temperature. Only near the drying transition (above 500 K for dodecane), the gap features uncoupled fluctuations of the oil and water surfaces, as expected in a typical drying structure. We find that the intrinsic structure of water next to the oil phase is remarkably similar to the bare water-vapor interface.PACS numbers: 68.03. Hj, 68.03.Kn, 68.05.Cf, 82.70.Uv Understanding the properties of water next to hydrophobic surfaces is essential to develop a microscopic description of biological macromolecules and materials in solution. The oil-water interface is a particularly useful model in this instance, since the structural changes undergone by water at this hydrophobic surface are expected to be similar to those found at biological and materials surfaces. Stillinger suggested that the interfacial structure of water at a "flat repelling" hydrophobic surfaces should be similar to that of the water-air interface [1]. An experimental proof of this idea is far from trivial. Modern spectroscopic and diffraction techniques provide a powerful approach to investigate buried interfaces, such as those appearing in hydrophobic materials. As a matter of fact, sum frequency spectroscopy (SFS) experiments of the alkane-water interface have suggested that water hydrogen bond interactions at the alkane surface are weaker than at the corresponding liquid-vapor interface [2]. Moreover X-ray and neutron diffraction techniques have been used to investigate the oil-water interface [3,4]. One of these works [3] has suggested that the intrinsic structure of the alkane-water interface could add a significant contribution to the average density profile.The global structure of water at a hydrophobic surface has been discussed in terms of the so called depletion layer. Water depletion is one element of recent theories of hydrophobic forces [5,6]. Moreover, capillary drying between hydrophobic materials has been put forward as an explanation of the hydrophobic force in extended hydrophobic surfaces [7]. This idea has raised a hot controversy (see for instance [5, 8-11]).Recent experiments [12][13][14] provide support for the for- * Electronic address: f.bresme@imperial.ac.uk,e.chacon@icmm. csic.es,pedro.tarazona@uam.es,kafui.tay@lcp.u-psud.fr mation of a narrow depletion layer at hydrophobic surfaces. The width has been reported to be of the order of 1-6Å [12], whereas the density of the depletion layer appears to be about 40-70 % [12] of the bulk water density. The two most recent X-ray reflectivity experiments [12] show, within t...

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