We use molecular dynamics simulations in order to understand the microscopic origin of the asymmetric growth mechanism in gold nanorods. We provide the first atomistic model of the different surfaces on the gold nanoparticles in the growing electrolyte solution and we describe the interaction of the metal with the surfactants, namely the cetyltrimethylammonium bromide (CTAB), and the ions. An innovative aspect is the inclusion of the role of the surfactants, which are explicitly modelled. We find that on all the investigated surfaces, namely (111), (110) and (100), the CTAB form a layer of distorted cylindrical micelles where channels among micelles would provide direct ions access to the surface. In particular, we show how AuCl2- ions, which are found in the growth solution, can freely diffuse from the bulk solution to the gold surface. We also find that the (111) surface exhibits the higher CTAB packing density and the higher electrostatic potential. Both elements would favor the grow of the gold nanoparticle along the (111) direction. These findings are in agreement with the growth mechanisms proposed by the experimental groups of Murphy and Mulvaney.