Disperse Systems

DISPERSE SYSTEMS (from Latin dispergere to scatter), systems consisting of two (or several) phases, in which one of them is scattered or pulverized in the other in the form of extremely small (microscopic or ultramicroscopic) particles. - Basic concepts. Each part of the system that is homogeneous throughout is called its "phase"; a system consisting of several phases separated from each other by interfaces is called heterogeneous. The interface is not always continuous. A mixture of oil and water, for example, represents a two-phase system, regardless of whether the oil is located on the water in a continuous layer or, breaking up into separate droplets, forms an emulsion of oil in water. In this latter case, one of the phases consists of a multitude of separate parts, identical in composition but spatially isolated. Such a phase, at the suggestion of W. Ostwald, is called the dispersed or disperse (sometimes also "internal") phase, whereas the other phase, in which the particles of the first are suspended and dispersed, is called the dispersion medium (also the "external phase"). In contrast to the dispersed phase, the dispersion medium is also called the continuous or solid phase, since continuous communication exists between all its parts. Thus, a disperse system consists of a dispersed phase and a dispersion medium. In two-phase systems, heterogeneity is not always equally sharply expressed. Frequently it catches the eye immediately. In disperse systems, for example in emulsions and especially in colloidal solutions, a microscope or even an ultramicroscope must be used to detect it. In contrast to grossly heterogeneous systems, disperse systems are microheterogeneous. Classification. For the classification of disperse systems (or dispersoids, as they were named by Weimarn), the size of the disperse particles is of primary importance. A whole series of properties of disperse systems depends on their dimensions. Between 1 μ and 0.1 μ (more precisely, about 0.2 μ) lies the boundary of microscopic visibility and the resolving power of the microscope. Disperse systems having a smaller particle size appear homogeneous upon microscopic examination. A whole series of other properties also undergoes significant changes on both sides of the specified boundary of dispersion (see table). Particle size 0.1 mμ 1 mμ 10 mμ 100 mμ 1 μ 10 μ 100 μ 1 mm Ultramicroscopic region Microscopic region Quartz particles in water do not settle appreciably settle Particles pass through paper filters

do not pass Brownian motion very rapid slow

The resolving power of a microscope is determined not by the greater or lesser perfection of its construction, but by the length of the light wave itself. Submicroscopic particles can still be detected with an ultramicroscope, although the latter does not give an idea of the shape and true size of the particles. The limit of application of the ultramicroscope lies approximately a thousand times below the microscopic boundary. Thus, particles of the dispersed phase can be conveniently divided into three groups depending on whether a) they are visible in the microscope, b) they can be detected by the ultramicroscope, or c) they lie beyond the latter. To distinguish these three groups, Zsigmondy introduced the names: microns, submicrons, and amicrons. In addition to the size of the particles, the state of aggregation of the phases making up the system is of great importance for characterizing heterogeneous systems. Each of the two phases of a dispersed system can be in a solid, liquid, or gaseous state. Since gases mix freely with each other and cannot form two separate gaseous phases, eight possible combinations remain, examples of which are presented in the following table. Dispersing medium Dispersed phase solid liquid gaseous solid Ruby glass Liquid inclusions in minerals Pumice liquid Suspension Emulsion Foam gaseous Smoke Fog For classification, it is necessary to use both of the features considered here—the size of the particles and the state of aggregation of both phases, which gives a very large number of possible subdivisions. The greatest interest (and are most thoroughly studied) are systems consisting of a liquid and a solid or of two liquid phases. If the dispersing medium is a liquid and the dispersed phase consists of microscopic solid particles, we have a suspension; if the dispersed phase consists of microscopic droplets of liquid—an emulsion. When the particles are reduced to ultramicroscopic sizes (approximately from 200 μ to 1 mμ), suspensions and emulsions turn into colloidal solutions, and depending on the state of aggregation of ultramicrons (and a number of related properties, see sols), one can in this case also speak of suspension or emulsion colloids, or, for short, of suspensionsoids and emulsoids. However, the concept of state of aggregation applies not to molecules, but to bodies built from their aggregates. Therefore, the smaller the particle, the fewer individual molecules it contains, the more difficult it is to distinguish its state of aggregation, and the less meaningful this subdivision becomes. With a further increase in the dispersity of matter (to amicrons), the dispersed particles break down into individual molecules or ions (the size of a crystalloid molecule is approximately 0.1-1 mμ). The criteria of state of aggregation and the very concept of phase (as a part of the system bounded by a physical interface) are no longer applicable to them. One then obtains a molecularly- or ionally-dispersed system, or the so-called 'true solution' (as opposed to a colloidal one). The complete sequence and continuity of transitions from suspensions and emulsions to colloidal solutions (suspensionsoids and emulsoids), and from them to true solutions, clearly shows the conventional and relative nature of the main division of systems into homogeneous and heterogeneous. Due to their structure of molecules, atoms, and electrons, matter itself is heterogeneous, and a system is called homogeneous simply in those cases when its heterogeneity cannot be detected by our research methods. Research methods. Thus, the study of the heterogeneity and dispersity of a system entirely depends on the methods available for distinguishing or actually separating both phases of the dispersed system. Optical methods serve the first purpose. In the case of suspensions and emulsions, heterogeneity is detected directly by microscopic observation. For colloidal solutions, optical heterogeneity is detected by the Faraday-Tyndall phenomenon: if a strong beam of light is directed sideways onto the solution, its path in the solution will be revealed as a luminous cone—the so-called 'Tyndall cone'; molecularly- and ionally-dispersed solutions remain dark, 'optically empty' under these conditions. The possibility of observing colloidal particles in the ultramicroscope is based on similar light scattering by them. In many cases, the color of colloidal solutions also depends on the size of the particles. Colorless substances, at certain sizes of their particles, can give one or another color by light scattering. According to the theory developed by Rayleigh, this explains the color of the so-called 'turbid media' (e.g., a hydrosol of mastic has a yellow or brown color in transmitted light, and a bluish color in reflected light). The blue color of the sky is explained, as is known, by the scattering of light by the smallest particles in the upper layers of the atmosphere. When the dispersity of a colloid and the size of its particles change, its color also changes in many cases. However, the possibility of detecting the heterogeneity of a system by optical methods depends not only on the size of the dispersed particles, but also on their optical properties. The more the refractive index of a particle differs from the refractive index of the surrounding medium, the more easily it can be detected. Therefore, for example, in hydrosols of heavy metals (gold, silver, etc.), which have very dense particles, the latter are easily detected under the ultramicroscope, whereas swollen protein particles (and therefore differing little from the surrounding water) may—at considerably larger sizes—remain undetected. A chemically heterogeneous system can be optically homogeneous. The techniques that allow separating the phases of dispersed systems from each other can serve no less essential a characteristic for dispersed systems. A substance in a true solution can be completely separated from the solvent only by means of more or less specific chemical reactions. For colloidal solutions and especially for coarser dispersed systems, purely mechanical methods can be used for such separation. Coarsely heterogeneous suspensions can be separated from their dispersing medium by filtration. The same method is applicable to colloidal solutions, provided a denser filter with correspondingly smaller pores is used. Depending on the diameter of their pores, such 'ultrafilters' retain all or some colloidal particles. By using a series of ultrafilters of different porosity and investigating which ones pass the colloidal solution and which retain its particles, the size of the latter can be determined. The passage of liquid occurs under more or less considerable pressure and requires the use of special apparatus. Another method of mechanical separation of the two phases of dispersed systems is centrifuging. In some cases, with a significant increase in the number of revolutions, this method can be applied not only to suspensions and emulsions, but also to colloidal solutions. Specific surface area. As one of the phases of a heterogeneous system is broken down into smaller and smaller particles, as the degree of dispersity increases, the boundary surface separating the two phases continuously increases. This increase in the boundary surface can be easily expressed quantitatively. As is known, the surface of a body is proportional to the square, and the volume (or mass) to the cube of its linear dimension. If a spherical oil droplet suspended in an aqueous solution is broken into smaller droplets having a diameter ten times smaller, the surface of each new droplet will be a hundred times smaller than the original. At the same time, the mass of the droplet will decrease a thousand times, i.e., instead of one droplet, we will now get a thousand droplets, whose total surface will be ten times larger than the original. The total surface of the dispersed phase, referred to a unit of its volume, is called the specific surface area. Thus, with an increase in the dispersity of a dispersed system, its specific surface area increases. The following table shows the relationship between the diameter of particles and their specific surface area. Diameter Specific surface area 1 CM 1 MM 1 μ 1 mμ 6 cm² 60 cm² 6 m² 6,000 m² In a living organism, where there are many heterogeneous systems built from combinations of liquid and solid phases, boundary surfaces reach enormous development. The surface of contact between blood and cells, for example, increases by distributing the blood through increasingly smaller vessels and capillaries. In other cases, the size of the surface increases due to the complication of its shape. Thus, the absorbing surface of the intestines increases due to the development of intestinal villi. However, the formation of dispersed systems is also a particularly powerful means of increasing boundary surfaces in the organism. An example of a coarse dispersed system can be blood, which is a suspension of erythrocytes in the dispersing medium—plasma. If the surface of an erythrocyte is taken as an average of 120 μ², then with 5 liters of blood and the normal number of erythrocytes (5 million in 1 mm³), their total surface will be about 3,000 m².

Finally, the cellular structure of all living organisms itself can serve as the most vivid proof of the importance of finely divided state and large specific surface area for life. Only in a state of rest can protoplasm collect into more or less significant continuous masses, as can be seen, for example, in the large eggs of some animals. The development of the organism begins with the division of the egg into smaller cells: first of all, it is not the mass that grows, but the surface of the embryo. The protoplasm of a multicellular organism forms a huge 'surface washed by blood and tissue fluids. The nuclei are even more finely divided, forming their own interface surface with the protoplasm. Finally, in addition to these easily noticeable microscopic structures, the cell contains smaller colloidal formations, gradually decreasing to ultramicroscopic sizes. In heterogeneous systems, the speed of chemical processes in most cases depends on the size of the boundary surface, along which the exchange of substances between the two contacting phases occurs. This same surface is the site of action of significant surface forces. Changes in the degree of dispersion and the associated changes in specific surface area are therefore a powerful means of influencing the intensity of biological processes. Boundary forces. The free energy of a system in many cases depends on the volume or mass of the substance (or its concentration). It is proportional, for example, to the osmotic pressure exerted by the dissolved substance, its chemical energy, etc. Along with this, there is a force in surface tension associated with the size of the free surface. The product of this force by the surface of the system determines its surface energy. With the enormous values that specific surface area has in D. s., the surface energy must reach a very significant magnitude. It should be borne in mind that the forces acting in D. s. are connected not with an ideal geometric, but with a physical surface having a certain, although extremely small, thickness. As the size of particles approaches molecular dimensions, surface forces rapidly decrease. Therefore, although in terms of the size of their particles, suspensions, emulsions, and colloidal solutions occupy an intermediate position between more coarsely heterogeneous systems and true solutions, surface energy reaches its maximum development in them. The total energy of the system consists of volumetric and surface energy. In D. s., the latter plays a predominant role. Many researchers try to reduce almost all processes occurring in such systems to the action of surface capillary forces and treat the chemistry of dispersed and in particular colloidal systems as 'capillary chemistry.' The action of surface tension is primarily associated with adsorption (see), which in D. s. can be very significant. All good adsorbents are finely divided substances with a highly developed surface. Such is, for example, the carbon powder most often used for adsorption. The enormous development of boundary surfaces in the organism creates in it very favorable conditions for the phenomena of adsorption. In this respect, the adsorption of dissolved substances by erythrocytes has been most studied. In particular, in recent years attention has been drawn to their ability to retain amino acids on their surface that enter the blood during digestion. By capturing amino acids coming from the intestinal tract, erythrocytes can transport them and deliver them to the tissues (Zbarsky); similarly, they can adsorb various other substances. Furthermore, the accumulation of dissolved substances in increased concentration on boundary surfaces can accelerate the chemical reactions occurring between them. Bodies with a highly developed boundary surface, such as spongy platinum or colloidal solutions of heavy metals, are therefore used as catalysts. In this respect, living cells also exhibit the properties of D. s. As shown by Warburg's research, many enzymatic processes in the cell proceed only as long as its structure is preserved, which determines the significant size of the boundary surface. They can continue even in a killed cell. However, crushing the cellular structures immediately sharply affects the course of oxidative processes.