Significance By means of an advanced molecular model, we identify the optimum conditions under which the Capacitive mixing technique can output maximum energies. In accord with experimental observations, we show that, when the salinity difference is fixed, a hydrophilic electrode surface can promote the energy extraction, the electrolyte solution containing monovalent ions with smaller ionic size will give larger energy output, and the temperature gradient between the salt water and fresh water should make significant contribution. (c) 2017 American Institute of Chemical Engineers AIChE J, 63: 1785-1791, 2017