Double layer structure in a supercritical fluid

This work reports the first double layer capacitance measurements in a polar supercritical fluid. The capacitance of a tetrabutylammonium tetrafluoroborate (TBA BF4) electrolyte in supercritical difluoromethane (HFC 32) at a platinum electrode is shown to be strongly dependent on fluid density. At high pressures the capacitance-potential plots exhibit a double layer response similar to that in the liquid state, but as the pressure is lowered toward the critical value the diffuse layer collapses and a pure Helmholtz response is observed.