It is shown that metastable fluid films on solid surfaces are destabilized when the applied capillary pressure exceeds a critical disjoining pressure. This leads to a wettability reversal of the solid substrate that can be detected using AFM measurements. Force vs distance curves reveal large hysteresis when the intervening liquid film is ruptured, causing wettability alteration. No hysteresis is observed for a stable film. A series of such experiments are conducted to investigate the effect of pH, salinity, and surface characteristics on wetting behavior of solid surfaces. Quantitative measurements of disjoining pressure isotherms have been made with octadecane and crude oil to better understand wettability alteration in oil reservoirs, These measurements also provide some insight into the short-range forces of interaction between a hydrocarbon (hydrophobic) phase and mineral (hydrophilic) surface separated by aqueous electrolytes.