We report information regarding the structure at the interface between hexadecane and an aqueous solution of trimethyl tetradecyl ammonium bromide (C(14)TAB) with appropriate deuterium labeling. We also report the role of the headgroup size and the charge on structures at the oil-water interface by comparing the data for C(14)TAB to that for a deuterated trimethyl tetradecyl ammonium sulfate (C(14)TAS) solution at the oil-water interface. As the charge on the counterion increases, this results in a reduction in the limiting area per molecule and an increase in the adsorbed amount at the oil-water interface.