An interfacial region, near the boundary between two different phases of a substance, in which physical properties vary markedly (in contrast with those in the bulk phases). For electrically conducting phases, charge distribution occurs in this interfacial region, which may be approximated as two parallel sheets of charge of opposite sign, hence the term double layer. This name is retained even if the interfacial region is more complex. Double layers arise from an excess of charge, which may be electrons, ions, or oriented dipoles, in the interfacial region.