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The terms stable and stability are used in rather special and often different senses in colloid science: the relationship between these usages and the formal thermodynamic usage is outlined below.

Thermodynamically stable or metastable means that the system is in a state of equilibrium corresponding to a local minimum of the appropriate thermodynamic potential for the specified constraints on the system (e.g. Gibbs energy at constant $ T$ and $ p$). Stability cannot be defined in an absolute sense, but if several states are in principle accessible to the system under given conditions, that with the lowest potential is called the stable state, while the other states are described as metastable. Unstable states are not at a local minimum. Transitions between metastable and stable states occur at rates which depend on the magnitude of the appropriate activation energy barriers which separate them. Most colloidal systems are metastable or unstable with respect to the separate bulk phases, with the (possible) exception of lyophilic sols, gels and xerogels of macromolecules.

Colloidally stable means that the particles do not aggregate at a significant rate: the precise connotation depends on the type of aggregation under consideration. For example, a concentrated paint is called stable by some people because oil and pigment do not separate out at a measurable rate, and unstable by others because the pigment particles aggregate into a continuous network.

An aggregate is, in general, a group of particles (which may be atoms or molecules) held together in any way: a colloidal particle itself (e.g. a micelle, see below) may be regarded as an aggregate. More specifically, aggregate is used to describe the structure formed by the cohesion of colloidal particles.

Aggregation is the process or the result of the formation of aggregates.

When a sol is colloidally unstable (i.e. the rate of aggregation is not negligible) the formation of aggregates is called coagulation or flocculation. These terms are often used interchangeably, but some authors prefer to introduce a distinction between coagulation, implying the formation of compact aggregates, leading to the macroscopic separation of a coagulum; and flocculation, implying the formation of a loose or open network which may or may not separate macroscopically. In many contexts the loose structure formed in this way is called a floc. While this distinction has certain advantages, in view of the more general (but not universal) acceptance of the equivalence of the words coagulation and flocculation, any author who wishes to make a distinction between them should state so clearly in his publication.

The reversal of coagulation or flocculation, i.e. the dispersion of aggregates to form a colloidally stable suspension or emulsion, is called deflocculation (sometimes peptization).

The rate of aggregation is in general determined by the frequency of collisions and the probability of cohesion during collision. If the collisions are caused by Brownian motion, the process is called perikinetic aggregation; if by hydrodynamic motions (e.g. convection or sedimentation) one may speak of orthokinetic aggregation.

In hydrophobic sols, coagulation can be brought about by changing the electrolyte concentration to the critical coagulation concentration (c.c.c.) (preferably expressed in = ). As the value of the critical coagulation concentration depends to some extent on the experimental circumstances (method of mixing, time between mixing and determining the state of coagulation, criterion for measuring the degree of coagulation, etc.) these should be clearly stated.

The generalization that the critical coagulation concentration for a typical lyophobic sol is extremely sensitive to the valence of the counterions (high valence gives a low critical coagulation concentration) is called the Schulze-Hardy rule.

If the critical coagulation concentration of a mixture of two electrolytes $ A$ and $ B$ corresponds to concentrations of the two components of $ c_A$ and $ c_B$ whereas the c.c.c.'s of $ A$ and $ B$ taken separately are $ c^0_A$ and $ c^0_B$ then the effects of the electrolytes are said to be additive if $ (c_A/c^0_A) + (c_B/c^0_B) = 1$; they are synergistic if $ (c_A/c^0_A) + (c_B/c^0_B) < 1$; and antagonistic if $ (c_A/c^0_A) + (c_B/c^0_B) > 1$. It is often found in the latter case that the individual values of $ (c_A/c^0_A)$ and/or $ (c_B/c^0_B)$ exceed unity.

Addition of small amounts of a hydrophilic colloid to a hydrophobic sol may make the latter more sensitive to flocculation by electrolyte. This phenomenon is called sensitization. Higher concentrations of the same hydrophilic colloid usually protect the hydrophobic sol from flocculation. This phenomenon is called protective action. Colloidally stable mixtures of a lyophobic and lyophilic colloid are called protected lyophobic colloids; although they may be thermodynamically unstable with respect to macroscopic phase separation, they have many properties in common with lyophilic colloids.

Sedimentation is the settling of suspended particles under the action of gravity or a centrifugal field. If the concentration of particles is high and interparticle forces are strong enough, the process of sedimentation may be better described as compaction of the particle structure with pressing out of the liquid. This particular kind of settling is also called subsidence.

Sediment is the highly concentrated suspension which may be formed by the sedimentation of a dilute suspension.

Coalescence is the disappearance of the boundary between two particles (usually droplets or bubbles) in contact, or between one of these and a bulk phase followed by changes of shape leading to a reduction of the total surface area. The flocculation of an emulsion, viz. the formation of aggregates, may be followed by coalescence. If coalescence is extensive it leads to the formation of a macrophase and the emulsion is said to break.

The breaking of a foam involves the coalescence of gas bubbles.

Coalescence of solid particles is called sintering.

Creaming is the macroscopic separation of a dilute emulsion into a highly concentrated emulsion, in which interglobular contact is important, and a continuous phase under the action of gravity or a centrifugal field. This separation usually occurs upward, but the term may still be applied if the relative densities of the dispersed and continuous phases are such that the concentrated emulsion settles downward. Some authors, however, also use creaming as the opposite of sedimentation even when the particles are not emulsion droplets.

Cream is the highly concentrated emulsion formed by creaming of a dilute emulsion. The droplets in the cream may be colloidally stable or flocculated, but they should not have coalesced.

As a rule all colloidal systems, initially of uniform concentration, establish, when subjected to the action of gravity or a centrifugal field, a concentration gradient as a result of sedimentation or creaming (see §1.10); but if the system is colloidally stable the particles in the sediment or cream do not aggregate and can be redispersed by the application of forces of the same magnitude as those which caused sedimentation or creaming.

The loss of the stability of a lyophilic sol (equivalent to a decrease in the solubility of the lyophilic colloid) quite often results in a separation of the system into two liquid phases. The separation into two liquid phases in colloidal systems is called coacervation. It occurs also, though rarely, in hydrophobic sols. The phase more concentrated in colloid component is the coacervate, and the other phase is the equilibrium solution.

If coacervation is caused by the interaction of two oppositely charged colloids, it is called complex coacervation.

Coacervation usually begins with the separation of the second phase in the form of small droplets which may coalesce to a continuous phase. Sometimes with extremely anisotropy particles the droplets have the shape of spindles or cylinders (tactoids). If the colloidal system is highly concentrated, droplets of the dilute phase are formed in the concentrated one (negative tactoids). The phenomenon of tactoid formation is not restricted to lyophilic systems.

In some systems, sedimenting particles form layers separated by approximately equal distances of the order of the wavelength of light. This gives rise to strong colours when observed in reflected light and the system is said to form irridescent layers or schiller layers.

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