Surfactants modulate interfacial processes. In electrochemical CO2 reduction, cationic surfactants promote carbon product formation and suppress hydrogen evolution. The interfacial field produced by the surfactants affects the energetics of electrochemical intermediates, mandating their detailed understanding. We have used two complementary tools-vibrational Stark shift spectroscopy which probes interfacial fields at molecular length scales and electrochemical impedance spectroscopy (EIS) which probes the entire double layer-to study the electric fields at metal-surfactant interfaces. Using a nitrile as a probe, we found that at open-circuit potentials, cationic surfactants produce larger effective interfacial fields (similar to-1.25 V/nm) when compared to anionic surfactants (similar to 0.4V/nm). At a high bulk surfactant concentration, the surface field reaches a terminal value, suggesting the formation of a full layer, which is also supported by EIS. We propose an electrostatic model that explains these observations. Our results help in designing tailored surfactants for influencing electrochemical reactions via the interfacial field effect.