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Colloids Surface Chem

The document discusses the formation of associated colloids, which are substances that aggregate in a solvent to form colloidal particles. It details methods for preparing lyophilic and lyophobic sols, including mechanical dispersion and Bredig's Arc Method for creating hydrosols of metals. The document also mentions the use of ultrasonic vibrations and the preparation of non-metal sols through arc striking between electrodes.

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27 views99 pages

Colloids Surface Chem

The document discusses the formation of associated colloids, which are substances that aggregate in a solvent to form colloidal particles. It details methods for preparing lyophilic and lyophobic sols, including mechanical dispersion and Bredig's Arc Method for creating hydrosols of metals. The document also mentions the use of ultrasonic vibrations and the preparation of non-metal sols through arc striking between electrodes.

Uploaded by

jane
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© © All Rights Reserved
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The molecules of substances as soaps and artificial detergents are smaller than the colloidal

particles. However in concentrated solutions these molecules form aggregates of colloidal size.
Su~tances whose molecules aggregate spontaneomly in a given solvent to form particles of colloidal
dimensions are called Associated or Association Colloids.

Na+ C17 H:3sCOO-


Na+
" •
- - Stearate ion
).
Na+ - - Na+ --r---\-
1 I
•
- Insoluble Polar
Na+ tail head
Na+

Micelle

Figure 22.26
A soap micelle.
PREPARATION OF SOLS
Lyophil ic sols may be prepared by s imply warming the solid with the liquid dispersion medium
e.g., starch with water. On the other hand, lyophob ic sols have to be prepare d by special method s.
These method s fall into two categor ies :
(a) Dispersion Method s in which larger macro-s ized particles are broken down to colloidal s ize.
(b) Aggreg atio n Method s in which colloida l size particles are built up by aggrega ting single
ions or molecules.
DISPERSION METHODS
ln these method s, materia l in bulk is dispers ed in another medium .
(1) Mecha nical dis persion using Colloid mill
The solid along with the liquid dispersi on medium is fed into a Colloid mill. The mill consists of
two steel plates nearly touchin g each other and rotating in opposit e directio ns with high speed. The
solid particle s are ground down to colloida l s ize and are then dispers ed in the liquid to give the sol.
' Colloid al graphite • (a lubrican t) and printing inks are made by this method .

Steel plate

-=) Sol
Micro-sized
particles +
dispersion
medium

Electric motor
Figure 22.4
A Colloid mJII.
812 22 PHYSICAL CHEMISTRY

Recently, mercury sol has been prepared by disintegrating a layer of mercury into sol particles in
water by means of ultrasonic vibrations.
(2) Brcdig's Arc Method
It is used for preparing hydrosols of metals e.g., si lver, gold and platinum. An arc is struck
between the two metal electrodes held close together beneath de-ionized water. The water is kept
cold by immersing the container in ice/ water bath and a trace of alkali (KOH) is added. The intense
heat of the spark across the electrodes vaporises some of the metal and the vapour condenses under
water. Thus the atoms of the metal present in the vapour aggregate to form colloidal particles in
water. Since the metal has been ultimately converted into sol particles (via metal vapour), this
method has been treated as of dispersion.

High voltage ---...


source

Metal
~ - - electrodes

Deionized
water + trace
of alkali
- Ice-bath

Figure 22.5
Bredig's Arc method.

Non-metal sols can be made by suspending coarse particles of the substance in the dispersion
medium and striking an arc between iron electrodes.
( ~ \ R u P ..nti,~tinn

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