There are two principal ways of preparation of colloids:
1. Dispersion of large particles to the colloidal dimensions. There are three methods of colloidal preparation using this method : mechanical, peptization, and Briedig arc methods.
a. Mechanical method
In the mechanical method, large particles are broken down into colloidal size particles by grinding. The colloidal size particles obtained are then dispersed in the dispersing medium by stirring. Paint pigments, for example, are produced by dispersing large particles by grinding in special mills.
b. Peptization method
A few solid substances such as gelatin, glue, and starch, when brought into contact with water, disperse spontaneously and form colloidal system. Gelatin , glue, and starch particles are already in colloidal size, the water simply disperses them. Gelatin, glue, and starch particles in water are said to undergo peptization.
c. Bredig arc method
Bredig arc method is used to prepare metallic sol such as colloid of gold in water. Gold metal that will be dispersed in water is used as electrode and is immersed in water as shown in figure above.
Gold atoms are released from gold electrodes when potential difference is given between the electrodes. The gold atom disperse and condense to produce colloidal particles in water.
2. Condensation of molecules dissolved in a true solution into larger colloidal particles.For examples, clouds form when water condense and produce very small droplets of colloidal size. Many con densation methods employ chemical reactions such as redox, double decomposition adn hydrolisis reactions.
a. By oxidation : A colloidal solution of sulphur can be obtained by bubbling oxygen (or any other oxidising agent like HNO3, Br2 etc.) through a solution of hydrogen sulphide in water.
2H2S + O2 (or any other agent) → 2H2O + 2S
b. By reduction : A number of metals such as silver, gold and platinum, have been obtained in colloidal state by treating the aqueous solution of their salts, with a suitable reducing agent such as formaldehyde, phenyl hydrazine, hydrogen peroxide, stannous chloride etc
2AuCl3 + 3SnCl2 → 3SnCl + 2Au
Gold sol
2AuCl3 + 3HCHO + 3H2O → 2Au + 3HCOOH + 6HCl
Gold sol
The gold sol, thus prepared, has a purple colour and is called purple of cassius.
c. By hydrolysis : Many salt solutions are rapidly hydrolysed by boiling dilute solutions of their salts. For example, ferric hydroxide and aluminium hydroxide sols are obtained by boiling solutions of the corresponding chlorides.
FeCl3 + 3H2O → Fe(OH)3 + 3HCl
Colloidal sol
Similarly silicic acid sol is obtained by the hydrolysis of sodium silicate.
d. By double decomposition : A sol of arsenic sulphide is obtained by passing hydrogen sulphide through a cold solution of arsenious oxide in water.
As2O3 + 3H2S → As2S3 + 3H2O
Tampilkan postingan dengan label colloid. Tampilkan semua postingan
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Rabu, April 13, 2011
EMULSIFIER *THE IMPORTANT THING IN DAILY LIFE*
Emulsion are mixture of liquids which ae immiscible. You have found out that when two liquids are immiscible they do not mix but form two different layers. Oil and water are like this but if you shake the mixture it becomes cloudy.
The apparent mixing that you see is due to the fact that ine of the liquids has been broken into tiny droplets which float suspended in the other liquid. If the mixture of oil and water now left to stand the two layers will re-form. To make emulsion, such as mayonnaise, an emulsifier is used to stop the droplets joining back together again to form a seperate layer. The emulsifier used when making mayonnaise is egg yolk. In may countries of the world, if you examine the ingridients on the side of many packets found in kitchen cupboards you will find that emulsifier have ‘E-numbers’ in the range E322 to E494. For example, ammonium phospatide E422 is used as the emulsifier in cocoa and chocolate. Other food addictives such as colourings and preservatives, are also given E-numbers but in different range of the emulsifier.
Cleaning agents are emulsifier
Soaps are sodium or potassium fatty acids salts, produced from the hydrolysis of fats in a chemical reaction called saponification. Each soap molecule has a long hydrocarbon chain, sometimes called its 'tail', with a carboxylate 'head'. In water, the sodium or potassium ions float free, leaving a negatively-charged head.
Soap is an excellent cleanser because of its ability to act as an emulsifying agent. An emulsifier is capable of dispersing one liquid into another immiscible liquid. This means that while oil (which attracts dirt) doesn't naturally mix with water, soap can suspend oil/dirt in such a way that it can be removed.
The organic part of a natural soap is a negatively-charged, polar molecule. Its hydrophilic (water-loving) carboxylate group (-CO2) interacts with water molecules via ion-dipole interactions and hydrogen bonding. The hydrophobic (water-fearing) part of a soap molecule, its long, nonpolar hydrocarbon chain, does not interact with water molecules. The hydrocarbon chains are attracted to each other by dispersion forces and cluster together, forming structures called micelles. In these micelles, the carboxylate groups form a negatively-charged spherical surface, with the hydrocarbon chains inside the sphere. Because they are negatively charged, soap micelles repel each other and remain dispersed in water.
Grease and oil are nonpolar and insoluble in water. When soap and soiling oils are mixed, the nonpolar hydrocarbon portion of the micelles break up the nonpolar oil molecules. A different type of micelle then forms, with nonpolar soiling molecules in the center. Thus, grease and oil and the 'dirt' attached to them are caught inside the micelle and can be rinsed away.
Although soaps are excellent cleansers, they do have disadvantages. As salts of weak acids, they are converted by mineral acids into free fatty acids:
CH3(CH2)16CO2-Na+ + HCl → CH3(CH2)16CO2H + Na+ + Cl-
These fatty acids are less soluble than the sodium or potassium salts and form a precipitate or soap scum. Because of this, soaps are ineffective in acidic water. Also, soaps form insoluble salts in hard water, such as water containing magnesium, calcium, or iron.
2 CH3(CH2)16CO2-Na+ + Mg2+ → [CH3(CH2)16CO2-]2Mg2+ + 2 Na+
The insoluble salts form bathtub rings, leave films that reduce hair luster, and gray/roughen textiles after repeated washings. Synthetic detergents, however, may be soluble in both acidic and alkaline solutions and don't form insoluble precipitates in hard water. But that is a different story.
G. Supplemental concepts
Water purification is the process of removing undesirable chemicals, materials, and biological contaminants from contaminated water. The goal is to produce water fit for a specific purpose. Most water is purified for human consumption (drinking water) but water purification may also be designed for a variety of other purposes, including meeting the requirements of medical, pharmacology, chemical and industrial applications. In general the methods used include physical processes such as filtration and sedimentation, biological processes such sallow sand filters or activated sludge, chemical processes such as flocculation and chlorination and the use of electromagnetic radiation such as ultraviolet light.
The apparent mixing that you see is due to the fact that ine of the liquids has been broken into tiny droplets which float suspended in the other liquid. If the mixture of oil and water now left to stand the two layers will re-form. To make emulsion, such as mayonnaise, an emulsifier is used to stop the droplets joining back together again to form a seperate layer. The emulsifier used when making mayonnaise is egg yolk. In may countries of the world, if you examine the ingridients on the side of many packets found in kitchen cupboards you will find that emulsifier have ‘E-numbers’ in the range E322 to E494. For example, ammonium phospatide E422 is used as the emulsifier in cocoa and chocolate. Other food addictives such as colourings and preservatives, are also given E-numbers but in different range of the emulsifier.
Cleaning agents are emulsifier
Soaps are sodium or potassium fatty acids salts, produced from the hydrolysis of fats in a chemical reaction called saponification. Each soap molecule has a long hydrocarbon chain, sometimes called its 'tail', with a carboxylate 'head'. In water, the sodium or potassium ions float free, leaving a negatively-charged head.
Soap is an excellent cleanser because of its ability to act as an emulsifying agent. An emulsifier is capable of dispersing one liquid into another immiscible liquid. This means that while oil (which attracts dirt) doesn't naturally mix with water, soap can suspend oil/dirt in such a way that it can be removed.
The organic part of a natural soap is a negatively-charged, polar molecule. Its hydrophilic (water-loving) carboxylate group (-CO2) interacts with water molecules via ion-dipole interactions and hydrogen bonding. The hydrophobic (water-fearing) part of a soap molecule, its long, nonpolar hydrocarbon chain, does not interact with water molecules. The hydrocarbon chains are attracted to each other by dispersion forces and cluster together, forming structures called micelles. In these micelles, the carboxylate groups form a negatively-charged spherical surface, with the hydrocarbon chains inside the sphere. Because they are negatively charged, soap micelles repel each other and remain dispersed in water.
Grease and oil are nonpolar and insoluble in water. When soap and soiling oils are mixed, the nonpolar hydrocarbon portion of the micelles break up the nonpolar oil molecules. A different type of micelle then forms, with nonpolar soiling molecules in the center. Thus, grease and oil and the 'dirt' attached to them are caught inside the micelle and can be rinsed away.
Although soaps are excellent cleansers, they do have disadvantages. As salts of weak acids, they are converted by mineral acids into free fatty acids:
CH3(CH2)16CO2-Na+ + HCl → CH3(CH2)16CO2H + Na+ + Cl-
These fatty acids are less soluble than the sodium or potassium salts and form a precipitate or soap scum. Because of this, soaps are ineffective in acidic water. Also, soaps form insoluble salts in hard water, such as water containing magnesium, calcium, or iron.
2 CH3(CH2)16CO2-Na+ + Mg2+ → [CH3(CH2)16CO2-]2Mg2+ + 2 Na+
The insoluble salts form bathtub rings, leave films that reduce hair luster, and gray/roughen textiles after repeated washings. Synthetic detergents, however, may be soluble in both acidic and alkaline solutions and don't form insoluble precipitates in hard water. But that is a different story.
G. Supplemental concepts
Water purification is the process of removing undesirable chemicals, materials, and biological contaminants from contaminated water. The goal is to produce water fit for a specific purpose. Most water is purified for human consumption (drinking water) but water purification may also be designed for a variety of other purposes, including meeting the requirements of medical, pharmacology, chemical and industrial applications. In general the methods used include physical processes such as filtration and sedimentation, biological processes such sallow sand filters or activated sludge, chemical processes such as flocculation and chlorination and the use of electromagnetic radiation such as ultraviolet light.
A. The Differences among Solution, Colloidal dispersion, and Suspension
Colloids represent a state intermediate between a solution and a suspension. Some of the noticeable differences among solution, colloidal dispersion, and suspension are given in table 1.
The differences of solution, colloidal dispersion, and suspension.
The differences of solution, colloidal dispersion, and suspension.
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