The self-assembled monolayers of alkane thiolates on Ag (111) surfaces and nanoparticles are studied using molecular dynamics. Reactive force fields allow simulations of very large systems such as nanoparticles of 10 nm. Stable (sqrt(7) X sqrt(7))R19.1{\deg} assemblies are obtained as experimentally observed for these systems. Only nanoparticles smaller than 4 nm show a spontaneous restructuration of the metallic core. The preferred adsorption site is found to be in an on-top position, in good agreement with recent X-ray absorption near edge structure experiments. Moreover, similar distances between the sulfur headgroups are found on the facets and edges.