The proposed experiments will directly determine the influence of surface potential on the packing of surfactant molecules at a silica/solution interface. Competitive effects of hydrophobic interactions, solvation and molecular and ionic packing are all known to influence the structure and the amount of material in adsorbed layers. Neutron reflection measurements with an applied electric potential permit the determination of the role of potential and results will be compared with previous work where simulations have been made and studies where pH is used to change adsorption but also influences simultaneously the activity of molecules in solution.