Colloidal stability definition

The following mechanism emerges form the observation reported so far on the colloidal stability of multichain PNIPAM particles at elevated temperature. Let us note first that such PNIPAM particles are examples of mesoglob-ules, according to the definition of Timoshenko and Kuznetsov [215] ... 

The question to be discussed is whether saturation of the electric field (asserted by Proposition 2.1) implies saturation of the interparticle force of interaction. Consider for definiteness repulsion between two symmetrically charged particles in a symmetric electrolyte solution. In the onedimensional case (for parallel plates) the answer is known—the force of repulsion per unit area of the plates saturates. (This follows from a direct integration of the Poisson-Boltzmann equation carried out in numerous works, primarily in the colloid stability context, e.g., [9]. Recall that again in vacuum, dielectrics, or an ionic system with a linear screening, the appropriate force grows without bound with the charging of the particles.)... 

This is a somewhat academic question because the answer depends on the definition of this. Two possibilities for this have been given in fig. 2.6. Nevertheless, for interpretational purposes it is sometimes useful to have some feeling for the thickness t. The issue may be compared with that of defining the thickness of an electric double layer. Strictly speaking such layers are infinitely thick but for many practical purposes, like colloid stability and electrokinetics, it has proved convenient to introduce the Debye length, v" as a representative measure of t. Making this choice for double layers involves three elements ... 

Colloid Stability In colloid science, an indication that a specified process, such as aggregation, does not proceed at a significant rate, which is different from the definition of thermodynamic stability (4). 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. 

The most significant distinction between the two classes of semi-batch process is, therefore, that an emulsion addition process necessarily involves the addition of smfactant throughout the particle growth period during which monomer is added. In the strict definition of a monomer addition process, the surfactant is introduced completely at the outset of the polymerization however, it is more usual for surfactant to be added during the reaction (to ensure colloidal stability as the particles grow), but normally as a separate input stream. 

Such designations of stable and unstable colloids are very relative and must be made in the context of the apphcation in question. It may be, for example, that a colloid that maintains its characteristics for two days would be considered stable in one application, while another would require that a minimum of two years pass without change. Obviously, then, one must be careful when discussing colloidal stability and instability. From this point on in the discussion, unless otherwise indicated, the kinetic (rather than energetic) definition of stability will be employed in its most general sense, it being assumed that all (or almost all) colloids are in reality metastable systems. Also, it must be kept in mind that stability in the present context is used in terms of... 

In Section 1.1, we defined colloidal systems as systems in which particles dispersed in a medium are snbjected to both thermal motion and motion due to external forces (e.g., gravity). This definition leads directly to the notion of stability of a colloidal dispersion. A colloidal dispersion is considered to be stable if no rapid phase separation occnrs through sedimentation (if the density of the particles is higher than that of the medium) or creaming (if the density of the particles is lower than that of the medinm). Thus, colloidal stability refers to the ability of a dispersion to resist aggregation into larger entities that then would segregate from the medium. 

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian Hquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin Hquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a large number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the lUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

The methods of disintegration rely entirely upon increasing the dispersity of a solids which process can, at least theoretically, be stopped at any instant resulting in the formation of a suspension of definite dispersity but one that is not necessarily stable. The processes of suspension formation by methods of condensation on the other hand are more complicated, owing to the fact that unless the resulting colloidal suspension possesses at least some degree of stability the process of condensation once set in operation will not cease but proceed until the transformation to the macrocrystalline structure is complete. 

Emulsions are colloidal dispersions of liquid droplets in another liquid phase, sometimes stabilized by surface active agents. Emulsions thus consist of a discontinuous phase, dispersed in a continuous phase. The most common types of emulsions are water-in-oil (W/O) in which oil is the continuous phase, and oil-in-water (OAV) in which water forms the continuous phase. However, this traditional definition of an emulsion is too narrow to include most food emulsions. For example, in foods the dispersed phase may be partially solidified, as in dairy products or the continuous phase may contain crystalline material, as in ice cream. It may also be a gel, as in several desserts. In addition to this, air bubbles may have been incorporated to produce the desired texture. 

Although most colloidal dispersions are not thermodynamically stable, a consequence of the small size and large surface area in colloids, and of the presence of an interfacial film on droplets, bubbles or particles, is that dispersions of these species, having reasonable kinetic stability, can be made. That is, suspended droplets or particles may not aggregate quickly nor settle or float out rapidly and droplets in an emulsion or bubbles in a foam may not coalesce quickly. Many food and personal care product emulsions and suspensions, for example, are formulated to remain stable for months to years. It is crucial that stability be understood in terms of a clearly defined process, and one must consider the degree of change and the time-scale in the definition of stability. 

In DLVO theory, the secondary minimum can only be created by the van der Waals force, which is essentially independent of the salt concentration across the concentration range 0.001 M < c < 0.1 M. This force has to be balanced with a force that decays exponentially as a function of k, which means that it decays by a factor exp(-10) across this range. The unhappy consequence of this prediction is that the position of the secondary minimum, and therefore the interlayer d value, varies very rapidly as a function of k, in contradiction to the experimental results. A further unhappy consequence of this balance is that it always produces a primary minimum much deeper than the secondary minimum. The full, standard DLVO thermodynamic potential energy curve, which also includes a very-short-range Bom repulsion, is shown in Figure 1.13 [23], It is therefore a definite prediction of DLVO theory that charge-stabilized colloids can only be kinetically, as opposed to thermodynamically, stable. The theory does not mean anything at all if we cannot identify the crystalline... 

Ligand-stabilized species can be described as colloids, according to the definition given above. Colloids used in polymer matrices or other stabilizing liquid media are not considered here. Continuing the principle of ligating metal particles of cluster size, only such metal particles are taken into account which exist as individuals outside the liquids in which they are produced. 

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