Colloids metastable

In colloid science the term colloid stability means that a specified process that causes the colloid to become a macrophase, such as aggregation, does not proceed at a significant rate. Colloid stability is different from thermodynamic stability (see Ref. [978]). The term colloid stability must be used with reference to a specific and clearly defined process, for example, a colloidally metastable emulsion may signify a system in which the droplets do not participate in aggregation, coalescence, or creaming at a significant rate. See also Kinetic Stability, Thermodynamic Stability. 

At equilibrium, in order to achieve equality of chemical potentials, not only tire colloid but also tire polymer concentrations in tire different phases are different. We focus here on a theory tliat allows for tliis polymer partitioning [99]. Predictions for two polymer/colloid size ratios are shown in figure C2.6.10. A liquid phase is predicted to occur only when tire range of attractions is not too small compared to tire particle size, 5/a > 0.3. Under tliese conditions a phase behaviour is obtained tliat is similar to tliat of simple liquids, such as argon. Because of tire polymer partitioning, however, tliere is a tliree-phase triangle (ratlier tlian a triple point). For smaller polymer (narrower attractions), tire gas-liquid transition becomes metastable witli respect to tire fluid-crystal transition. These predictions were confinned experimentally [100]. The phase boundaries were predicted semi-quantitatively. 

Fig. 11. Effects of pH in the colloidal siUca-water system (1), where A represents the point of zero charge regions B, C, and D correspond to metastable gels, rapid aggregation, and particle growth, respectively. Positive and negative correspond to the charges on the surface of the siUca particle.

Bifunctional spacer molecules of different sizes have been used to construct nanoparticle networks formed via self-assembly of arrays of metal colloid particles prepared via reductive stabilization [88,309,310]. A combination of physical methods such as TEM, XAS, ASAXS, metastable impact electron spectroscopy (MIES), and ultraviolet photoelectron spectroscopy (UPS) has revealed that the particles are interlinked through rigid spacer molecules with proton-active functional groups to bind at the active aluminium-carbon sites in the metal-organic protecting shells [88]. 

Mechanical agitation of the cream - a process called whipping - creates a metastable foam (i.e. it contains much air). Further whipping causes this foam to collapse some water separates out, and the major product is yellow butter. Incidentally, butter is a different form of colloid from milk, since its dispersed medium is water droplets and its dispersal phase is oil (milk is an oil-in-water colloid). Forming butter from milk is a simple example of emulsion inversion. 

For catalytic purposes, it is generally important to optimize the exposed surface area of active sites, and as a consequence one must control the degree of aggregation of the colloidal particles. These suspensions are metastable thermodynamic systems... 

Eberhart, J. G., Kremsner, W., and Blander, M. (1975). Metastability of superheated liquids AD753S61, bubble nucleation in hydrocarbons and their mixtures. J. Colloid Interface Sci. 50(2), 369. 

Surfactants are commonly used to kinetically stabilize colloidal systems. An alternative way to achieve long-term metastability consists of adsorbing... 

FIGURE 7.1 Stability criteria of any colloidal system metastable-unstable-stable states. 

A colloidal suspension may be unstable and exhibit separation of particles within a very short time, or it may be stable for a very long time, such as for over a year. In between, a metastable state can be found. This is an oversimplified example, but it shows that any colloidal system should be analyzed following these three criteria. 

It should be realized, at the outset, that colloidal solutions (unlike true solutions) will almost always be in a metastable state. That is, an electrostatic repulsion prevents the particles from combining into their most thermodynamically stable state, of aggregation into the macroscopic form, from which the colloidal dispersion was (artificially) created in the first place. On drying, colloidal particles will often remain separated by these repulsive forces, as illustrated by Figure 1.1, which shows a scanning electron microscope picture of mono-disperse silica colloids. 

Zeolite crystallization represents one of the most complex structural chemical problems in crystallization phenomena. Formation under conditions of high metastability leads to a dependence of the specific zeolite phase crystallizing on a large number of variables in addition to the classical ones of reactant composition, temperature, and pressure found under equilibrium phase conditions. These variables (e.g., pH, nature of reactant materials, agitation during reaction, time of reaction, etc.) have been enumerated by previous reviewers (1,2, 22). Crystallization of admixtures of several zeolite phases is common. Reactions involved in zeolite crystallization include polymerization-depolymerization, solution-precipitation, nucleation-crystallization, and complex phenomena encountered in aqueous colloidal dispersions. The large number of known and hypo-... 

Although most colloidal systems are metastable or unstable with respect to the separate bulk phases, they can have an appreciable kinetic stability. That is, the state of dispersion can exist for an appreciable length of time. Colloidal species can come together in very different ways therefore, kinetic stability can have different meanings. A colloidal dispersion can be kinetically stable with respect to coalescence but unstable with respect to... 

In colloid science, the terms thermodynamically stable and metastable mean that a system is in a state of equilibrium corresponding to a local minimum of free energy (Ref. [978]). If several states of energy are accessible, the lowest is referred to as the stable state and the others are referred to as metastable states unstable states are not at a local minimum. Most colloidal systems are metastable or unstable with respect to the separate bulk phases. See also Colloid Stability, Kinetic Stability. 

---