This thesis probes interfacial dynamics of complex molecular thin films. This work is focused on sticking and hydrogen bonding of small molecules on and in astrophysical ices, differential condensation of isotopologues, oxidation of an important industrial alkene, and thermal destruction of a chemical warfare agent. While these systems vary greatly, they all focus on elucidating how film structure and morphology impact adsorption and reactivity. When examining different astrophysical ices (crystalline, non-porous amorphous solid water, and porous amorphous-solid water), we find that ice morphology vastly changed the interaction with small hydrocarbons. Not only do the pores allow more multiple collisions yielding a higher sticking probability for methane, the undercoordinated water molecules in the porous films form more hydrogen bonds with acetone. When switching away from ices to methane isotopologues, we determine that film composition vastly impacts adsorption behavior. By making a small mass adjustment from methane to heavy methane, we find that there is preferential condensation for heavy methane (CD4). This finding, confirmed from both experimental as well as novel theoretical gas-surface chemical trajectory simulations indicates a better energy transfer for the heavier isotopologue. We next focus on a larger hydrocarbon (propene) to facilitate increased chemical complexity upon exposure to oxygen. We conclude that oxygen is only able to diffuse through and react with the ordered film indicating the important role that film structure and morphology play in limiting reactivity even for reactions with low barriers. Lastly, we determined the mechanism for the destruction of the nerve agent simulant, diisopropyl methylphosphonate (DIMP), under atmospheric and oxygen depleted conditions which can directly inform chemical warfare mitigation strategies.