Figure 26-20a shows the electric double layer (Box 17-3) at the wall of the capillary. The double layer consists of fixed negative charge on the wall and excess cations near the wall. A tightly adsorbed, immobile layer of cations partially neutralizes the negative charge. The remaining negative charge is neutralized by mobile cations in the diffuse part of the double layer in solution near the wall. The thickness of the diffuse part of the double layer ranges from when the ionic strength is 1 mM to 0.3 nm when the ionic strength is 1 M.