Chemistry · Ch 10 — Surface Chemistry
Applications of Colloids
Applications of Colloids
Colloids and surface chemistry touch nearly every corner of daily life. The human body itself is built substantially around colloidal systems — blood, the protoplasm of plant and animal cells, and the fats present in the intestines all exist in the form of colloidal emulsions — and familiar synthetic polymers such as polystyrene, silicones and PVC are themselves colloidal in nature.
Food. Everyday foodstuffs such as milk, cream and butter are present in colloidal form.
Medicines. Antibiotics such as penicillin and streptomycin are formulated in colloidal form for suitable injections. Colloidal gold and colloidal calcium are used as general tonics; milk of magnesia (a colloidal suspension) is used for stomach troubles; and silver sol protected by gelatine, known commercially as Argyrol, is used as an eye lotion.
Industry — nine distinct applications. (i) Water purification: drinking water is purified by coagulating suspended colloidal impurities using alum, which supplies ions. (ii) Washing: soap's cleansing action works by forming an emulsion of soap molecules with dirt and grease, allowing them to be rinsed away. (iii) Tanning of leather: skin and hides contain protein particles carrying a positive charge, which are coagulated by adding tannin (or chromium salts, in chrome tanning) to produce hardened, soft, polishable leather. (iv) Rubber industry: latex is itself an emulsion of natural rubber carrying negatively charged particles; heating rubber together with sulphur (vulcanisation) produces the vulcanised rubber used for tyres and tubes. (v) Sewage disposal: sewage contains dirt, mud and waste dispersed colloidally in water, and passing an electric current through it deposits the waste material (which can then be used as manure) — a process worked through in more detail alongside the accompanying sewage-treatment figure. (vi) Cottrell's precipitator: carbon dust and other particulate pollution in air is removed using Cottrell's precipitator, which applies a high potential difference of about 50,000 V to neutralise the charge on the carbon particles, causing them to solidify and drop out, leaving the air free of carbon particles. (vii) The blue colour of the sky arises from the Tyndall scattering of sunlight by colloidal-sized dust and aerosol particles suspended in the atmosphere. (viii) Formation of deltas: as river water meets sea water at a river mouth, the electrolytes present in the sea water coagulate the colloidal sol …
What this figure shows. Raw sewage first passes through primary treatment: screens and a comminutor remove large debris, a grit chamber settles out sand and grit (disposed of separately), and a primary clarifier lets suspended solids settle out as primary sludge, producing a primary effluent. That effluent then enters secondary treatment: an aeration tank, fed by an air compressor, promotes the growth of microorganisms that break down dissolved organic matter into an activated sludge, part of which is recycled back into the aeration tank by a return sludge pump while the rest goes for disposal; a secondary clarifier settles this treated water further, and a disinfectant is added before the secondary effluent is discharged to surface water or sent on to tertiary treatment. The whole layout illustrates how colloidal and coagulation chemistry — settling suspended and co …