Abstract: Confined nano-scale liquids can display very different behaviors compared to those on the macro scale. These characteristics can have important implications in many areas,such as the transport of biological fluids, liquid pumps with no moving mechanical parts, drug mixing/separation, the selection of colloidal particles, and ion-specific transport through ion channels.The study of the characteristics of nano-fluids requires a thorough understanding of one of the most basic phenomena at fluid—solid interfaces——the electrical double layer (EDL). This article points out some inadequacies in the Poisson—Boltzmann equation that are central to the traditional studies of EDL, and proposes a new framework based on the constraint of global charge neutrality, in conjunction with the introduction of a surface potential trap model.