Graphene oxide (GO) is a promising material for separations. Nanoscale GO thin films at the air/water interface are excellent experimental models to understand molecular-scale interactions of ions and water with GO. However, thin film formation strongly depends on how the GO was processed. This paper reports a simple, reliable, and quick method of preparing ultra-thin GO films, irrespective of their origin. This method allows the quantitative investigation of differences in film structure, ion adsorption, and interfacial water behavior with multiple surface sensitive probes. The data show that functional groups and oxidative debris vary significantly between different commercially available GO samples. These differences strongly affect ion adsorption and interfacial water behavior near GO, which are vital properties in separation applications. The results demonstrate the importance of the GO process conditions and provide experimental methods to quantify molecular-scale differences between different GO films.