Oil recovery yields from sandstone reservoirs strongly depend on the wetting properties of the rock. Carboxylic acids present in crude oil may decrease the water wettability by adsorbing onto the mineral surface via cation interactions. A highly simplified version of this scenario has been mimicked in the lab to study these mechanisms in more detail. In previous studies on oil/brine/mineral systems the formation of fatty acid monolayers on mica was observed, yielding water contact angles in ambient oil of up to 60°. Here we demonstrate that the presence of 2 mM bicarbonate (typical for brines) has a strong influence at temperatures above 40 °C (as in reservoirs), yielding water contact angles in ambient oil up to 160°. Similar behavior was found for a variety of carboxylic acids. On increasing the (even) carbon number of simple fatty acids from 8 to 20, the contact angle becomes larger until it saturates at 16 carbon atoms. Similar hydrophobic layers are formed by pulling a sheet of mica through an oil/water interface at comparable velocities. By studying the nanometer-scale topography and chemistry of these dip-coated samples, we can infer that the adsorbed layer is composed of alternating carboxylic acid bilayers that are held together by a very thin intermediate layer containing calcium and (bi)carbonate ions. Exposure to low-salinity water makes the multilayers disappear and the mineral surface become water-wet again, demonstrating that the presence of these structures can lead to a strong salinity-dependent wettability alteration.