Can colloids be separated
Colloids, colloidal systems, Molecules or aggregates made up of about 103 until 109 Putting atoms together. It can be particles (particle diameter 10-7 until 10-9 m) or thin films (thickness < 100="" nm)="" handeln.="" k.="" nehmen="" eine="" zwischenstellung="" zwischen="" den="" molekulardispersen="" und="" grobdispersen="" verteilungen="" ein="" (disperses="" system).="" zum="" unterschied="" von="" den="" molekulardispersen="" verteilungen="" läßt="" sich="" der="" zustand="" eines="" k.="" nicht="" allein="" durch="" die="" zustandsgrößen="" druck,="" temperatur="" und="" konzentration="" beschreiben,="" sondern="" die="" eigenschaften="" hängen="" noch="" ab="" von="" größe,="" gestalt="" und="" struktur="" der="" teilchen.="" aufgrund="" des="" großen="" verhältnisses="" von="" oberfläche="" zum="" volumen="" sind="" kolloide="" thermodynamisch="" instabil.="" diese="" instabilität="" läßt="" sich="" mit="" hilfe="" der="" beziehung="">G = explain γ dσ. dG means the change in free enthalpy, γ the surface tension, dσ the change in the surface. As the surface area decreases, dσ becomes negative and thus dG. Surfactant solutions with concentrations above that are an exception critical micelle concentration (engl. critical micell concentration, abbr. CMC). In a micelle, the surfactant molecules accumulate in such a way that the hydrophobic tails point inwards and the hydrophilic heads point outwards into the surrounding solvent. The enthalpy of formation is positive in aqueous solutions. However, since the solvent molecules can move more freely after micelle formation, the entropy of the solvent increases. This compensates for the decrease in entropy during micelle formation and the free enthalpy of the overall system becomes negative. K. are most appropriately divided into dispersion colloids, association colloids and molecular colloids.
I) Dispersion colloids. All C. which can in principle be produced by dispersion are referred to as dispersion colloids. Depending on the physical state of the disperse phase and dispersion medium, a distinction can be made between the following dispersions: distributions of a) solid particles in gases (Aerosol), b) liquid droplets in gases (fog), c) gaseous particles in liquids or solids (Foams, solid foams), d) liquid particles in a second liquid phase (emulsion), e) solid particles in liquids (Brine, Solid dispersion), f) liquid particles in solids, g) dispersions of solid particles in solids (colored glasses, Ceramics). High conc. structured solid dispersions are called gels.
Manufacturing. 1) The Dispersing and grinding processes compact matter is broken up by the action of external forces. Thereby fracture processes can take place; H. the compact primary particles are broken (solids) or torn (liquids and gases). However, it is also possible to separate particle aggregates, in which case one speaks of disaggregation or dispersion. Solids can be crushed in colloid mills in which coarse particles are exposed to strong mechanical stress in narrow shear gaps (≈ 0.05 mm gap width). It can be dispersed by the action of ultrasound. Liquids can be dispersed by dividing centrifugal force. The disadvantage of dispersion processes is the low energy yield (a large part of the energy is converted into heat) and the broad particle size distribution. 2) The Condensation process are of much greater importance for the production of K. z. B. for image and data recording materials, catalyst supports, polymer dispersions. On the basis of molecular distributions, in principle any particle size, homodisperse systems and also particles of different shapes can be produced. Colloid particles are obtained from both highly conc. (saturated or supersaturated) solutions through homogeneous nucleation or from very dilute solutions through heterogeneous nucleation. It is important that the particle growth process is interrupted in the desired size range. The molecularly disperse substances that arise during production are separated off by dialysis or ultrafiltration (filtration through membrane filters). 3) Under Peptization one understands the spontaneous dispersion of aggregates of colloidal particles, e.g. B. by adsorption of potential-determining ions. This gives the particles an electrical charge in the same direction, which causes repulsion between the particles.
resistance. Since atoms or molecules at the phase boundary and near the phase boundary have a greater free energy than inside the phase (interfacial tension), all dispersion colloids tend to reduce the free energy by reducing the surface. A stabilization, i.e. H. An inhibition or prevention of coagulation can be achieved: a) if colloidal particles carry a sufficiently strong electrical charge in the same direction, which is created by dissociation of surface groups or by preferential ion adsorption. The same number of oppositely charged ions are then located around the particles, which compensate for the surface charge. The electrostatic repulsion of the particles charged in the same direction prevents them from approaching one another. Practically all dispersion colloids carry an electrical charge. Colloid metals are usually negatively charged, whereas colloidal oxides are negative or positive, depending on the pH value; b) when the colloidal particles have a strong solvate layer which hinders the approach of the particles. Strongly solvated K. are often called lyophilic K. designated; c) when surface-active compounds such as surfactants or macromolecules are added. The surfactants are then often called Dispersants, Emulsifiers or. Foam stabilizers called the macromolecules as protective colloids. Due to their spatial expansion, they also prevent the dispersed particles from migrating together. Just as important as the stabilization of K. (drugs, latices, emulsion paints) is their destruction (sewage and gas purification, drainage of petroleum). For example, C., which are stabilized by electrical repulsive forces, can be coagulated by adding electrolytes. The effect of the electrolyte is based either on the fact that the oppositely charged type of ion (counter-ions) is adsorbed by the colloid particle and the particle is thereby discharged (Neutralization coagulation) or that the range of the electrostatic repulsive forces decreases due to compression of the diffuse electrical double layer, which leads to a lowering of the energy barrier to be overcome by the approaching particles (DLVO theory). The effectiveness of an electrolyte is greater, the higher the valency of the counterion. The threshold values for the coagulation concentrations of mono-, bivalent and trivalent counterions are like 10000: 50: 1 (Schulze-Hardy rule).
Determination of the particle size. Dispersion colloids only have a uniform particle size in exceptional cases, as a rule they are polydisperse. The properties are influenced by the degree of dispersion and the polydispersity. The mean particle size can be determined by osmotic measurements from the diffusion rate, by scattered light or turbidity measurements and electron microscopy. The latter method and sedimentation analyzes in a gravitational or centrifugal field also make it possible to determine particle size distributions.
properties. 1) Electrical properties: Colloid particles move in an electrical field. The potential that occurs at the particle / electrolyte solution shear surface is called the zeta potential or electrokinetic potential. The potential at the shear plane is smaller than the surface potential, which is calculated from the number of charge carriers per unit area on the surface; it roughly corresponds to the star potential (electrochemical double layer). In capillary systems, when an electric field is applied, a migration of the liquid phase, the electroosmosis, is observed. The electrokinetic potential arises again at the shear plane. If a liquid is not allowed to flow under the influence of an electric field, but rather under pressure along a solid surface, a streaming potential is created. When particles sediment under the influence of gravity, a sedimentation potential is established accordingly. 2) Optical properties: Colloid solutions show strong light scattering (Tyndall phenomenon). Rod-shaped and sheet-shaped anisometric particles (e.g. vanadium pentoxide brine) orient themselves during flow processes. One observes flow birefringence, i. That is, a brightening occurs between two crossed Nicol prisms. 3) Rheological properties: The viscosity of colloidal solutions depends on the particle concentration and temperature, on the particle shape, on the particle size in the case of anisometric particles and on the interaction forces between the disperse particles. With strong interaction forces, very high viscosities occur, e.g. B. in gels. In these cases, the viscosity does not obey Newton's law of friction, but depends on the shear stress (structural viscosity, thixotropy, dilatancy).
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