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The PROPERTIES OF GELS

Structure. The structure of various xerogels and jellies has been thoroughly studied in the last decades by several methods, such as the X-ray method, observations in the ultramicroscope, studies with the electron microscope, and so on. From all these studies the important conclusion may be drawn that jellies which contain large amounts of liquid have a network structure in which the liquid is bound to the fibrous particles and is also mechanically immobilised between them. The less asymmetric are the colloidal particles, the higher must be their concentration to be able to form a jelly.

The shape of the colloidal particles however, is not the only factor, which determines the ability to form stable jellies. For instance, a 4% solution of nitrocellulose in a mixture of ether and alcohol does not set, although the molecules of the nitrocellulose are very long and thin. Another important factor, which determines the gelation, is the possibility of entanglement of the fibres or rods. The rods and fibres must be linked if a network is desired. Hence the structural elements of a jel need not have long fibrous macromolecules. The solid framework of a jelly may be composed also of plate or needleshaped crystals as micelles. In such instances the concentration of the solid is always relatively high, whereas with linear macromolecules the concentration may be lower. The framework of a jelly must be coherent. If the particles do not stick together there is no gelation. For example, a concentrated graphite dispersion does not conduct electricity in the fluid state, although it becomes a conductor after it sets, because countless contact points are formed throughout the network. All gels can be classified into three structural groups: 1) gels with unstable frameworks; 2) jellies with metastable frameworks, and 3) systems with stable networks. A very labile framework is encountered in ferric hydroxide, alluminium hydroxide, bentonite, graphite and many other gels in which the structural elements are not very asymmetric. The crystallites or particles are joined in such cases by very weak cohesive forces (van der Waals attraction). Such gels are often thixotropic, i.e. the framework is so weak that it is destroyed by shaking. To this group belong also the entanglement gels of linear macromolecules, such as unvulcanized or rubber polystyrene. The corresponding solids, e.g. polystyrene, when placed in liquid in which they can dissolve, will swell first and then slowly go into solution. In the instances of weak frameworks of linear macromolecules the gels swell without limit; if the instance structural elements are not joined by sufficiently strong forces, the solvent may in time destroy the framework by gradually disconnecting the building units. Metastable frameworks are encountered in most of the protein jellies, for in those of gelatin. Finally, there are gels with stable frameworks, for example the gels of relatively concentrated silicic acid. Such gels can be obtained upon addition of acid to a solution of sodium silicate. The resulting gel is rigid and stable, and it cannot be reversibly transformed into a liquid system.

Exercises

I. Answer the following questions:

1. By what methods has the structure of various xerogels and jellies been studied?

2. What conclusion may be drawn from all these studies?

3. What factors determine the ability to form stable jellies?

4. What must be linked if a network is desired?

5. What may the solid framework of a jelly be composed of?

6. In what cases may the concentration be high and low?

7. Is there gelation when the particles do not stick together?

8. Into what groups can all gels be classified?

9. What are metastable frameworks encountered in?

10. What examples of gels with stable frameworks can you give?

11. How can gels with stable frameworks be obtained?

II. Give antonyms to the following words:

Immobilised

Asymmetric

Unable

Short

Thick

Unimportant

undesirable

liquid

low

fluid

stable

destroy

unvulcanized

disconnect

rigid

strong.

III. Translate the following derivatives into Vietnamese:

structure, structural

vary, various

conclude, conclusion, conclusive

liquid, liquidfy

fibre, fibrous

ability, able, unable

stable, unstable, stability

solution, solvent, soluble, dissolve

possible, impossible

line, linear

coherence, coherent, cohesive

gelation, gel

conduct, conductive, conduction

ferrous, ferric

IV. Translate the following sentences paying attention to the words in bold type:

1. The less asymmetric are the colloidal particles, the higher must be their concentration to be able to form a jelly.

2. The larger the viscosity, the larger will be the inefficient expenditure of energy.

3. The smaller the number of valence electrons, the more readily the atom yields them.

4. If the acid is nonvolatile, only the ammonia is driven off while the acid remains in the vessel.

5. The shape of the colloidal particles, is not the only factor, which determines the ability to form stable jellies.

6. Hence the structural elements of a jel need not have long fibrous macromolecules.

V. Translate into Vietnamese paying attention to the Participles:

1. Some adsorbents may contain sufficient acid or alkali to after the pH of the water being treated.

2. Having been compressed the air is to be cooled.

3. Having been composed of liquid particles emulsions disperse in other liquids.

4. Being used in different fields of chemistry, industry and medicine colloid chemistry becomes increasingly important.

5. When placed in a liquid, the corresponding solids will swell first and then slowly go into solution

VI. Translate into English

1. H¹t keo cã tÝnh ®èi xøng cµng thÊp th× nång ®é cña chóng ph¶i cµng cao.

2. H×nh d¹ng cña h¹t keo kh«ng ph¶i lµ yÕu tè ®éc nhÊt quyÕt ®Þnh kh¶ n¨ng t¹o thµnh keo bÒn.

3. NÕu c¸c h¹t keo kh«ng dÝnh vµo nhau th× sù t¹o gel kh«ng s¶y ra.

4. Mét sè gel thu ®­îc khi thªm axit vµo dung dÞch natri silicat. Gel t¹o thµnh nh­ vËy th× r¾n, bÒn v÷ng vµ kh«ng thÓ chuyÓn thµnh tr¹ng th¸i láng.

VII. Write an outline of the text and retell