A colloidal solution generally represents a solution system in which the particles comprising that system have a particle size intermediate that of a true solution and a coarse dispersion, roughly ranging between 1nm to 500 nm (or 1nm to 0.5µm). A colloidal solution may be considered as a two-phase (heterogeneous) system under some circumstances, while it may be considered as a one-phase (homogeneous) system under other circumstances.
Types of Colloidal Solutions
There are three types of colloidal solutions which are discussed as follows:
1. Lyophilic Colloids
The colloidal systems in which the colloidal particles interact to an appreciable extent with the dispersion medium are referred to as the lyophilic colloids. The term lyophilic mean solvent loving. Owing to their affinity for the dispersion medium, such materials form colloidal sols.
The lyophilic colloidal sols are usually obtained by simply dissolving the required material (whose sol is to be prepared) into the solvent that is being used. The most common examples of the formation of sols are dissolving acacia in water, dissolving gelatin in water or dissolving celluloid in amyl acetate. The various properties of this class of colloids are due to the attraction between the dispersed phase and the dispersion medium, which leads to salvation; the attachment of solvent molecules to the molecules of the dispersed phase. If water is taken as the dispersion medium, the colloids prepared are known as hydrophilic colloids. Most lyophilic colloids are organic molecules, for example, gelatin, acacia, insulin, albumin, rubber, and polystyrene. Of these, insulin, albumin, gelatin and acacia produce lyophilic or hydrophilic sols. Rubber and polystyrene form lyophilic colloids in nonaqueous, organic solvents. These materials accordingly are referred to as lipophilic colloids. These examples illustrate the important point that the term lyophilic has meaning only when applied to the material dispersed in a specific dispersion medium. A material that forms a lyophilic colloidal system in one liquid (e.g., water) may not do so in another liquid (e.g., benzene).
2. Lyophobic Colloids
Lyophobic colloids are composed of substances which have very little attraction, if existing, for the dispersion medium. These are the lyophobic (solvent-hating) colloids and, predictably, their properties differ from those of the lyophilic colloids. This is primarily due to the absence of a solvent sheath around the particle. These types of colloids are generally constituted when inorganic particles are dispersed in water. Examples of such materials are gold, silver, sulfur, arsenous sulfide, and silver iodide. Unlike lyophilic colloids, lyophobic colloids require special methods of preparation. These include two types of methods. First, dispersion methods, in which size reduction of coarse particles is done, and second, condensation method, which requires the aggregation of small-sized particles to form bigger particles which lie within colloidal size range.
3. Association colloids
Association or amphiphilic colloids are the third type of colloidal systems. In these types of colloids, certain molecules or ions, termed amphiphiles or surface-active agents, are characterized by having two distinct regions of opposing solution affinities within the same molecule or ion. They have one polar region which is attracted towards a polar solvent and within the same molecule; they have a no-polar region which is attracted towards the non-polar solvent. These amphiphiles can arrange themselves according to the type of solution (polar or non-polar) they are put into. When they are put into a polar solution, they expose their polar regions towards the solvent while covering their non-polar regions towards the inner core, and vice cersa. When present in a liquid medium at low concentrations, the amphiphiles exist separately and are of such a size as to be subcolloidal. As the concentration of amphiphiles increases, they start to aggregate faster. These aggregates may comprise of 50 or more amphiphiles and are called micelles.
Properties of Colloidal Solutions
The colloidal solution exhibit a wide range of properties which are classified into three broad types discussed below:
1. Optical Properties of Colloidal Solutions
• The Faraday-Tyndall Effect: When a strong beam of light is passed through a colloidal sol, a visible cone, resulting from the scattering of light by the colloidal particles, is formed. This is the Faraday–Tyndall effect.
• Elicitation in Electron Microscope: The electron microscope, capable of yielding pictures of the actual particles, even those approaching molecular dimensions, is now widely used to observe the size, shape, and structure of colloidal particles. The success of the electron microscope is due to its high resolving power, which can be defined in terms of ‘d’, the smallest distance by which two objects are separated and yet remain distinguishable. The smaller the wavelength of the radiation used, the smaller is ‘d’ and the greater is the resolving power. The source of radiation for the optical microscope is visible light which can resolve only two particles at a time of about 20 nm (200 Å). The radiation source of the electron microscope is a beam of high energy electrons having wavelengths in the region of 0.01 nm (0.1 Å).
• Light Scattering:Light scattering property of the Colloidal solution particles is based on the Faraday-Tyndall Effect, discussed above. A perfect example of this is the blue colour of the sky which is visible to our eyes due to the scattering the light of blue wavelength by the colloidal particles present in the atmosphere. This property of colloidal particles is used to determine their molecular weight.
2. Kinetic Properties of Colloidal Solutions
• Brownian Motion: Brownian motion describes the random movement of colloidal particles. The erratic motion, which may be observed with particles as large as about 5 µm, was explained as resulting from the bombardment of the particles by the molecules of the dispersion medium. The motion of the molecules cannot be observed because they are too small to see. The velocity of the particles increases with decreasing particle size. Increasing the viscosity of the medium, which may be accomplished by the addition of glycerin, decreases and finally stops the Brownian movement.
• Diffusion:Colloidal particles diffuse spontaneously from a region of higher concentration to one of lower concentration until the concentration of the system is uniform throughout. Diffusion is a direct result of the Brownian movement. Diffusion of the colloidal particles are governed by a law known as Fick’s first law of diffusion which states that the amount of substance diffusing at a particular time across a plane of area is directly proportional to the change of concentration across both sides.
• Osmotic Pressure: The osmotic pressure of the colloidal particles is described by the Van’t Hoff Equation, π = cRT, where π is the osmotic pressure, c is the concentration of the solute in the system, R is the universal gas constant and T is the temperature. According to this equation, the osmotic pressure of the colloidal particles is directly proportional to all these components.
• Sedimentation: The colloidal particles do not have any tendency to sediment because the particles are constantly in Brownian motion, as already discussed. This Brownian motion in the colloidal particles is enough to combat the gravitational force applied on them. Hence, a stronger force must be applied to bring about the sedimentation of colloidal particles in a quantitative and measurable manner. This is accomplished by use of the ultracentrifuge which can produce a force one million times that of gravity.
• Viscosity: Viscosity is an expression of resistance to the flow of a system under an applied stress. If a liquid is more viscous, a greater amount of force is required to initiate its flow and regulate it at a particular rate. The viscosity of the colloidal solution is given by an equation developed by Einstein, η = ηo(1 + 2.5ϕ) where y, ηo is the viscosity of the dispersion medium, η is the viscosity of the dispersion and φ is the volume fraction.
3. Electrical properties of Colloidal Solutions
• Electrokinetic Phenomena: The movement of a charged surface with respect to an adjacent liquid phase is the basic principle underlying four electro-kinetic phenomena: electrophoresis, electro-osmosis, sedimentation potential, and streaming potential. Electrophoresis is a phenomenon of the movement of charged particles in a liquid medium upon application of a potential difference. Electro-osmosis is a phenomenon in which the application of a potential causes a charged particle to move relative to the liquid, which is stationary. Sedimentation potential is the production of a potential difference when charged particles undergo sedimentation. The streaming potential differs from electro-osmosis in that forcing a liquid to flow through a plug or bed of particles creates the potential.
• Donnan Membrane Equilibrium: If sodium chloride is placed in a solution on one side of a semi-permeable membrane and a negatively charged colloid together with its counter ions R-Na+ is placed on the other side, the sodium and chloride ions can pass freely across the barrier but not the colloidal anionic particles.