Colloidal self-assembly and aggregation are controlled by molecular scale interaction forces across (sub)nanometric films of an ambient fluid, often thin water layers of variable salt content and pH. In this lecture, I will give an overview over insights that we have gained using in situ Atomic Force Microscopy (AFM) measurements on the structure of the electric double layer and the resulting nano-scale interaction forces. I will discuss aspects of ion adsorption, hydration, and electrostatic forces for a variety of systems ranging from clay minerals to semi-conducting nanoparticles. In particular, I will illustrate the potential of operando-AFM for the characterization of photocatalytically active materials.