Colloidal System A heterogeneous mixture in which solute particles are larger than molecules or ions but cannot be seen by the naked eye is called colloidal solution. (Also called colloidal dispersion, colloidal suspension.) An intimate mixture of two substances, one of which, called the dispersed phase (or colloid), is uniformly distributed in a finely divided state through the second substance, called the dispersion medium (or dispersing medium). The dispersion medium may be a gas, a liquid, or a solid and the dispersed phase may also be any of these, with the exception of one gas in another. A system of liquid or solid particles colloidally dispersed in a gas is called an aerosol. A system of solid substance or water-insoluble liquid colloidally dispersed in liquid water is called a hydrosol. There is no sharp line of demarcation between true solutions and colloidal systems or between mere suspensions and colloidal systems. When the particles of the dispersed phase are smaller than about 10-3 ?m in diameter, the system begins to assume the properties of a true solution; when the particles dispersed are much greater than 1 ?m, separation of the dispersed phase from the dispersing medium becomes so rapid that the system is best regarded as a suspension. According to the latter criterion, natural clouds in the atmosphere should not be termed aerosols; however, since many cloud forms apparently exhibit characteristics of true colloidal suspensions, this strict physicochemical definition is often disregarded for purposes of convenient and helpful analogy. Condensation nuclei and many artificial smokes may be regarded as aerosols. The dispersion medium may be a gas, a liquid, or a solid. · Smoke is composed of a solid dispersed in gas. · Milk is a liquid dispersed in liquid. · Pumice stone is a gas dispersed in solids. There are two forms of colloidal systems. 1. Sol: A system composed of non-viscous colloidal solution is called sol. For example milk. 2. Gel: A system composed of viscous colloidal solution is called gel. For example butter. The colloidal system can be classified into two general classes on the basis of their affinity for liquids: · Lyophilic System: The system in which dispersed phase and liquid dispersion medium attract each other is called lyophilic system. · Lyophobic System: The system in which the dispersed phase and liquid dispersion phase repel each other is called lyophobic system. Types of Colloidal Dispersions Dispersed phase and dispersion medium can be solid, liquid or gas. Depending upon the state of dispersed phase and dispersion medium, eight different types of colloidal dispersions can exist. Eight Different Types of Colloidal Dispersions are: 1. Foam 2. Solid foam 3. Liquid Aerosol 4. Emulsions 5. Gels 6. Solid Aerosol 7. Sol (Colloidal suspension) 8. Solid sol (Solid suspension) Dispersed Phase Dispersion Medium Type of Colloidal Dispersions Gas Liquid Foam Gas-Solid Solid foam Gas Gas Does not exist Liquid Gas Liquid Aerosol Liquid Liquid Emulsions Liquid Solid Gel Solid Gas-Solid Aerosol Solid Liquid Sol or Colloidal Suspension Solid Solid Solid sol(solid suspension) It is important to note that when one gas is mixed with another gas, a homogeneous mixture is formed i.e. gases are completely miscible into each other. Colloidal dispersions are heterogeneous in nature and gas dispersed in another gaseous medium does not form colloidal system. When the dispersion medium is gas, the solution is called Aerosol and when the dispersion medium is liquid, the colloidal dispersion is known as Sol. Sols can further be classified into different types depending upon the liquid used. · If the liquid used is water, the solution is Hydrosol or Aquasol. · If liquid used is Benzene, the solution is Benzosol. · If liquid used is Alcohol, the solution is Alcohol. · If any organic compound is used, the solution is Organosol. Example of Colloidal Dispersions. Different Types of Colloidal Dispersion and their examples are summarized in table below. Type of Colloidal Dispersions Examples Foam Soap, beer, lemonade Solid foam Pumice stone Does not exist Liquid Aerosol Fog, dust Emulsions Milk, rubber Gel Butter, Cheese Solid Aerosol Dust Sol or Colloidal Suspension Paste, ink Solid sol(solid suspension) Pearls, gemstones Properties of Colloidal System The colloidal system shows following properties. 1. Adsorption: The tendency of molecules and ions to adhere to the surface of certain solids or liquids is called adsorption. Colloidal particles show a high tendency of adsorption. Thus, a colloidal system provides a large surface area of adsorption of molecules and ions. 2. Brownian Movements: Robert Brown in1927 observed that colloidal particles show random dancing movements. These movements were named Brownian movements. 3. Tyndall Effect: The colloidal particles scatter light. Ths is called Tyndall Effect. The path of light appears as a cone. It is known as Tyndall cone. This property helps to detect the presence of colloidal particles. 4. Precipitation: The additions of an electrolyte remove the electrical double layer present around the colloidal particles. As a result, the dispersed particles of a colloidal suspension will aggregate and precipitate. 5. Electrical Properties: All colloidal particles carry a same electric charge. This charge may be positive or negative. There is a adsorption of free ions in the dispersion medium. It produces an electrical double layer around the colloidal particles. The electric charges on the colloidal particles stabilize the colloidal system. 6. Filtration: The colloidal particles cannot pass through a parchment membrane. This property of colloidal dispersions is used to separate them from true solution by a process called dialysis. 7. Phase Reversal: The sol and gel form of a colloidal system can be interchanged due to change in certain conditions. Certain lyophilic sols form the gel under certain conditions. For example, aqueous agar sols are cooled. It forms a jelly-like gel. The conversion of a sol to a gel is called gelation. If a gel of gelation or agar is heated, it will convert back to a sol. This process is known as the solution. The property of colloidal dispersions is called phase reversal. 8. Surface Charge The most important characteristic of colloidal systems is the surface charge of the particles. Keep in mind that a “particle” is a group of bonded atoms or molecules. Charged particles repel each other, overcoming the tendency to aggregate (the attraction force) and remaining dispersed. Particle size plays a major role in the capacity to bear a charge, and the colloidal size range is set by this capacity. In manufactured systems, the charge can be greatly increased over what might occur naturally. Within the range, the smaller the particle, the greater the surface and the greater the charge that can be applied in manufacture. Only heterogeneous, highly dispersed colloidal systems, containing the smallest possible particles, have a well-developed surface area. Given a constant voltage applied to the system, particle charge is not automatically increased as the substance is made finer, but total charge in the system will increase. Already coarse particles will tend to fall out even if they have received an electrical charge like the smaller particles because gravity will have a greater influence than the electrical forces which maintain the dispersion. Metallic particles have a great affinity for each other at the atomic level. They are magnetically attracted to each other and want to bond. But the magnetism of metals does not create an added difficulty of attraction against maintaining a colloidal system because of the superior capacity of metals to hold a charge. Given a constant particle size, the higher the concentration in a solution, the more likely the attraction force will overcome the repelling charge, creating larger masses. At some point, the mass will precipitate out due to gravitation. At lesser concentrations, the attraction force is insufficient for precipitative particle bonding, and groups are light enough that gravitation will not pull them out of solution. This is an ideal colloidal system. Biological Significance Of Colloidal Systems 1. Protoplasm a colloidal system: Protoplasm a living, and viscous substance. Viscous mean semi-fluid, jelly-like substance. It is surrounded by the cell wall. It is present in prokaryotic and eukaryotic cells. Protoplasm is colloidal in nature. The small molecules and ions are true solute particles. But the larger particles remain suspended in water and form a colloidal suspension or colloidal solution. 2. Cyclosis and ameboid movements: The cyclosis occurs due to phase reversal of colloidal property. The cyclosis usually occurs in sol phase. Amoeboid movements in amoeba occur due to colloidal properties. 3. Fruits: Fruits store a large number of proteins and starch. They also exhibit colloidal properties. These properties help in the storage of food in fruits. 4. Blood: The plasm protein forms a colloidal system in blood. This system maintains the pH and osmotic concentration of blood. 5. Milk: Milk is perfect colloidal system. It contains all essential nutrients for young. Manufacturing Colloids and Systems At least five methods were used to manufacture colloids before 1938, including: (1) Grind, (2) Wave, (3) Liquid, (4) Chemical, (5) Electrical. For medical or health purposes, the FDA now allows both the grind and electrical manufacturing techniques to be used. Of these two methods, however, the electro-colloidal process is generally considered to be far superior. (The chemical method, described below, is restricted to industrial or commercial applications.) With the grinding method, the inorganic or organic particles are usually no finer than four one-hundred-thousandths of an inch, or about one micron, which is outside the upper end of the ideal size range by a factor of 10. Such particles may or may not be electrically charged. Even if a charge is present, the size of the particles may be great enough that the repelling forces are unable to overcome the pull of gravity. Thus, particles will tend to settle to the bottom of the solution, and much of the effectiveness of the colloidal system will be lost. While some sols owe their stability to particle size, charge and high dispersion, others employ a mechanical stabilizer added to the medium. Such stabilizers include gelatin, glycoproteins, and starch, among other things, which increase solution viscosity and cause the particles to settle much more slowly. The downside to this is that stabilizers tend to block the effects of the colloids, and the particles will still eventually settle if the solution is allowed to stand long enough. If the inorganic or organic particles are within the size range of 1 to 100 nm and are uniformly charged, no stabilizer is required to maintain suspension indefinitely in deionized water, as long as no disruptive influence intrudes. Thus, the integrity and power of a colloidal system is a factor of the interplay among size, charge, concentration, and interaction between particle and medium. It should be mentioned that shape is also a factor. In recent years, the chemical process has been widely employed to replace the inferior grind method, because it provides a convenient shortcut to the more difficult electro-colloidal process. But it also has drawbacks, one of which is the difficulty in getting the chemicals (acids) back out of the colloidal solution. Consequently, traces of the chemicals are frequently left in solution, which can cause unwanted effects, especially in nutritional/medical applications. After studying the health benefits of various forms of colloidal silver, Dr. Leonard Keene Hirschberg, A.M.M.D. (Johns Hopkins) concluded, “There are two principal ways of producing metallic colloids, viz., chemical and physical (electrical). The two methods yield widely different results, and from a therapeutic point of view I need only deal with the electric colloid metals since only these present the necessary homogeneity, minuteness of granules, purity, and stability.” A simple illustration will suggest the immense power potential of a colloidal system. The total surface of a one-inch cube of iron is six square inches. By colloidal chemistry, the cube can be divided into particles having a total surface area in the range of 800,000,000 square inches, all expressing electrical energy. The total surface area of the particles in a quarter teaspoon is greater than that of a football field.