Fig. 2

Conceptual picture showing a single silica nanoparticle with a mostly dissociated surface surrounded by an alkali water solution. The sodium ions inside the slipping plane are completely locked in place and make up the stern layer. a Represents a particle dissolved in low concentrated NaCl solution and therefore have several unshielded SiO⁻ surface groups. This particle retains most of its electrostatic repulsion forces which is shown by a large Debye sphere and is therefore either stable or will aggregate at a significantly slower rate than the particle shown in b. b Represents a particle that is dissolved in a highly concentrated NaCl aqueous solution and is completely covered by sodium ions which leads to a greatly reduced Debye sphere. The b particle can be considered to be unstable i.e. it would aggregate with other particles