When placed into solution, salts begin to dissolve and form ions. This is not always in equal proportion, due to the preference of an ion to be dissolved in a given solution. The ability of an ion to preferentially dissolve (as a result of unequal activities) over its counterion is classified as the potential determining ion. The properties of this ion are strongly related to the surface potential present on a corresponding solid. This unequal property between corresponding ions results in a net surface charge. In some cases this arises because one of the ions freely leaves a corresponding solid and the other does not or it is bound to the solid by some other means. Adsorption of an ion to the solid may result in the solid acting as an electrode. (e.g., H⁺ and OH⁻ on the surfaces of clays). In a

colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others exten ...

al dispersed system,

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

dissolution arises, where the dispersed particles exist in equilibrium with their saturated counterpart, for example: :NaCl_(s) Na⁺_(aq) + Cl⁻_(aq) The behavior of this system is characterised by the components

activity coefficient In thermodynamics, an activity coefficient is a factor used to account for deviation of a mixture of chemical substances from ideal behaviour. In an ideal mixture, the microscopic interactions between each pair of chemical species are the same (o ...

s and

solubility product Solubility equilibrium is a type of dynamic equilibrium that exists when a chemical compound in the solid state is in chemical equilibrium with a solution of that compound. The solid may dissolve unchanged, with dissociation, or with chemical reac ...

: :''a''_Na⁺ · ''a''_Cl⁻ = ''K''_sp In clay-aqueous systems the potential of the surface is determined by the activity of ions which react with the mineral surface. Frequently this is the

hydrogen ion A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particl ...

H⁺ in which case the important activity is determined by pH. The simultaneous adsorption of protons and hydroxyls as well as other potential determining cations and anions, leads to the concept of

point of zero charge The point of zero charge (pzc) is generally described as the pH at which the net charge of total particle surface (i.e. absorbent's surface) is equal to zero, which concept has been introduced in the studies dealt with colloidal flocculation to ex ...

or PZC, where the total charge from the cations and anions at the surface is equal to zero. The charge must be zero, and this does not necessarily mean the number of

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s versus

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s in the solution are equal. For clay minerals the potential determining ions are H⁺ and OH⁻ and complex ions formed by bonding with H⁺ and OH⁻.

References

Further reading