We reveal the fundamental microscopic structure of the Helmholtz layer at a sodium solution-platinum interface using our original scheme based on ab initio molecular dynamics to decompose electrostatic responses in the electrochemical environment. The total potential variation is shown to mainly occur within "the contact layer", a thin surface region with a low dielectric constant formed by the Pauli repulsion. This has not been previously considered by conventional electric double-layer models. An estimation of its capacitance is also achieved without the use of empirical parameters, where the value obtained mostly agrees well with previous experiments. (C) 2012 Elsevier B. V. All rights reserved.