  Adsorption of small amphiphilic molecules occurs in various biological and
technological processes, sometimes desired, the other times unwanted (e.g.,
contamination). Surface-active molecules preferentially bind to interfaces and
affect their wetting properties. We study the adsorption of short-chained
alcohols (simple surfactants) to the water-vapor interface and solid surfaces
of various polarities using molecular dynamics simulations. The analysis
enables us to establish a theoretical expression for the adsorption
coefficient, which exponentially scales with the molecular surface area and the
surface wetting coefficient, and which is in good agreement with the simulation
results. The competition of the adsorptions to both interfaces of a sessile
droplet alters its contact angle in a nontrivial way. The influence of
surfactants is strongest on very hydrophilic and very hydrophobic surfaces,
whereas droplets on surfaces of moderate hydrophilicity are much less affected.
