A clay treated either chemically, thermally, or physically (milling) to improve its properties or increase its reactivity, e.g. by increasing the specific surface area and porosity, creating broken bonds and lattice distortions.
Cf., acid activated clay, alkaline activated clay, thermally activated clay, ageing

The minimum energy two molecules, atoms, or ions must have to initiate (by colliding) a reaction is called the activation energy, E_a, and has units of kilojoules per mole.

A pharmaceutical term denoting the active ingredient(s) in a drug. For a drug containing clay, the active principle may be loaded in clay lumen (e.g., halloysite, zeolite) or adsorbed to clay surfaces, including interlayer surfaces (e.g., as found in smectite). The clay portion, as an inactive carrier or coating, is referred to as the excipient or vehicle. The excipient is often used to improve the physical appearance, taste, smell, or smoothness of a pill or to promote disintegration (time release) of capsules or pills.
Syn., active ingredient, bulk active
Cf., aesthetic medicine, antibacterial clay, excipient, geotherapy, healing clay, lumen, medicinal clay, pelotherapy

In thermodynamics, activity refers to an ‘effective’ concentration, which takes nonideal behavior into account. Thus, ion-to-ion interactions, such as charge shielding, may affect the activity by attractive or repulsive intermolecular forces. The activity of a dissolved species is calculated as a_i = γ_im_i where γ_i is the activity coefficient and m_i is the molality. Activity is the term used to describe concentrations in solutions, whereas fugacity describes the activity of a gas.

In geotechnical engineering, the activity number (A) of a soil is the ratio of the plasticity index to the mass percentage of the clay fraction.
See also Atterberg limits, consistency number, liquid limit, plastic limit, plasticity index, shrinkage limit.

Skempton (1953) defines clay activity, or the colloidal activity of clay, as the ratio of the plasticity index to the percentage of the clay-size fraction. According to Skempton, three types of activity can be determined: inactive (activity <0.75), normal (activity 0.75 to 1.25) and active (activity > 1.25).
See low-activity clay, high-activity clay

An obsolete term for muscovite.

An atom attached to a substrate.
Syn., adsorbed atom

Adhesion refers to the attractive forces between two or more chemically dissimilar molecules or surfaces of different materials (e.g., glass vs. water, clay vs. sand). Thus, wet-clay and dry-clay particles tend to adhere to different surfaces.
Cf., cohesion

A micelle structure adsorbed to a surface.
See micelle.