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SOLID SOLS
CHAPTER
SOLID
19
SOLS
Restriction of the field of solid sols. These sols, according to the definition of colloidal substances, should be represented by a very large group of systems in which solid (gas or liquid) is dispersed in another solid. Two cases can be distinguished: (1) the dispersion medium has a low absorption of light (it is transparent) ; and (2) the absorption of light is very strong, and, therefore, the medium is opaque. In the first case it is possible to study the optical properties of the sol, whereas this is impossible in the second case in which only the definition links these systems with the colloids. If one wishes to explore the hardening of quenched aluminium alloys, in which the alloyed metals, immediately after quenching, are in a highly dispersed ( 1) s t a t e , one cannot use the methods of colloid chemistry, but rather one should apply the methods of metallography and physical metallurgy. These methods and techniques are entirely different from those used in colloid chemistry. Therefore the second group of solids belongs to the wide fields of metallurgy, mineralogy and petrography, and only the first group of solids, having a transparent dispersion medium, will be mentioned in any detail. The transparent dispersion medium may be either amorphous or crystalline in nature. Amorphous substances as dispersion medium. To this group belong the coloured glasses. Gold ruby glass was first prepared by K U N K E L in 1 6 7 9 , and was mentioned by L I B A V I U S . This glass was of importance in colloid chemistry, because in 1 9 0 3 Z S I G M O N D Y and S I E D E N T O P F succeeded in disclosing the nature of the colloidal state by studying this substance in the ultramicroscope. A red-coloured substance is obtained when small amounts of gold ( 0 - 1 g or more per 1 kg of lead or other glass) in the form of a gold preparation, or gold foil, is dissolved in molten glass. The mass, if quickly cooled, is at first colourless and optically * empty ' if observed in the ultramicroscope, evidently because of complete solution of the gold. But if the mass is cooled slowly, or if the cold mass is tempered afterwards at a temperature below the softening temperature of the glass, it assumes a beautiful, clear, red colour, which resembles that of a gold hydrosol. It is then possible to demonstrate ultramicroscopically the presence of myriads of colloidal particles (1
14
) e.g. Quenching very rapidly a supersaturated Al-Si alloy, 10 silicon particles/ cm in the alloy were found: see H . S. R O S E N B A U M and D. T U R N B U L L ; Acta Met. 6, 653 (1958). 469 3
470
S O L I D
SOLS
in this mass. These look like those in a gold hydrosol except that they are motionless because of the great viscosity of the dispersion medium. The number of these particles can easily be counted. Such a solid sol has optical properties very similar to those of hydrosols. For instance, the same changes in colour are observed if the particle size is increased. This takes place if the gold ruby glass is tempered for a considerable time at an elevated temperature : at first the red colour becomes blue (by reflected light the glass appears brown), and finally, tiny, brilliant, solid gold particles can be detected in the glass. The effect of the heat treatment is to accelerate the diffusion of gold atoms by diminishing the viscosity of the glass. The gold atoms then have the opportunity of collecting on separate crystallisation centres, with the subsequent formation of submicroscopic and then of larger gold crystals if the heat treatment is continued. Ruby glass and other glasses can also be prepared by dissolving copper, selenium, silver and other colouring substances instead of the more expensive gold. The behaviour of these glasses is very similar to that of gold ruby glass. The glass containing copper is also colourless if quenched. With heat treatment a ruby glass with ultramicroscopic particles is first obtained; this glass then becomes red and opaque, containing microscopic particles with a metallic lustre; and finally a product containing macroscopic crystal dendrites is reached. Silver produces a yellow, brown or red-brown stained glass, resembling the silver hydrosols in colour. In 1937, R. H. D A L T O N discovered that a colourless copper ruby glass, obtained by rapid cooling of the heated glass, develops its colour more readily if exposed to ultraviolet light prior to the heat treatment (annealing at about 580° for 15 minutes to one hour). Such a ' p h o t o sensitive glass' is used for the production of pictures in glass. A plate of the above-mentioned glass is covered by a negative, and then exposed from the negative side to short-wave light (violet or ultraviolet). X-rays are also effective. At the intensely irradiated spots on the glass, corresponding to the transparent parts of the negative, a greater concentration of metallic nuclei is developed. There is evidence that gold, copper or silver are dissolved in the colourless glass in the form of ( 2) On subions, which are reduced to atoms as a result of irradiation. sequent heating the metallic nuclei act as crystallisation centres even for non-metallic crystals, and the respective spots on the glass become the darker the stronger the irradiation. The colour depends on the nature of the dispersed substance. Instead of glass various plastic organic materials such as Bakelite, 2
( ) S. D .
STOOKEY;
Industr. Engng. Chem. 41, 856 (1949); see also S. D . S T O O K E Y , A L E X A N D E R ' S Colloid Chemistry, Vol. VII, p. 697 (New
' Photosensitive glass ' in J. York 1950).
471
GLASSES
vinyl resins can be used as suspension media for photo-sensitive substances. Films containing the latter (Calimar films) are photosensitive throughout. If exposed to ultraviolet light through a standard photographic negative for about one minute, the light activates metal oxides a n d dye formers present in the film (creation of nucleation centres). The exposed film is then ' developed ' in a hot-air oven at 100° C in about 6 min. The image becomes visible because the lightactivated compounds form dyes in the film (growth of the nuclei) at the temperature mentioned, while simultaneously the non-irradiated dyeforms are passivated. A very similar process occurs in ordinary photographic films and plates during light exposure and subsequent ( 3 4 5) wet development in the dark-room. ' ' The colour of borax a n d sodium phosphate beads, which were used for the detection of elements in qualitative analysis, may originate in many cases from colloidally dispersed substances in the glassy masses. Transparent crystalline substances as dispersion media. T o this class of solid sols belong the pyrosols, described by L O R E N Z and E I T E L . Pyrosols are prepared by mixing metals with low melting points under ( 6) their molten salts. F o r instance, under molten lead chloride lead expels grey clouds which disappear completely a t higher temperatures. It has been shown that a t this stage the metal forms a true solution ( 7) with the molten salt. U p o n cooling, the grey fog again appears. It is very probable that this phenomenon is the same as that already described in the case of gold ruby glass : the high viscosity of the solid salt a t low temperatures prevents the coagulation, or aggregation, of the separate, highly dispersed metal particles; with slow cooling the particle size is larger. Coloured masses are also obtained during the electrolysis of many molten salts. Metallic dispersions are produced upon heating some active metals (titanium, zirconium, hafnium) in molten salts, for instance sodium ( 8 9) chloride. ' The dispersion occurs due t o oxygen absorption (under the melt) by the surface of these metals and the subsequent breakdown ( 1 0) of the oxygen-containing l a y e r s . U p o n heating white phosphorus with mercury, or better still with a 3
<> J . W. M I T C H E L L ; Z . Elektroch. 60, 557 (1956). 4 <δ) H . S T A U D E ; ibid. 6 0 , 563 (1956). ( ) Ε. K L E I N ; ibid. 60, 998 (1956). Μ Ε. H E Y M A N N ; Austral. Chem. Inst., Journal and Proceedings 4 , 3 8 (1937). See also7 G . C L E A R Y and D . C U B I C C I O T T I ; / . Amer. Chem. Soc. 74, 557, 1198 (1952). <> J. F A R Q U H A R S O N and E. H E Y M A N N ; Trans. Faraday Soc. 3 1 , 1004 (1935). E. H E Y M A N N and E. F R I E D L Ä N D E R ; Z . Physik. Chemie 148A, 177 (1930). (8 > C . B . G I L L , M . E. S T R A U M A N I S and A. W. S C H L E C H T E N ; / . Electrochem.
102, 4 2 (1955). (E
) M . E. S T R A U M A N I S , S. T. S H I H and A. W. S C H L E C H T E N ; J. Electrochem.
104,1 0 17(1957). ( ) M . E. S T R A U M A N I S and C H .
CHIOU;
Z . Elektrochem.
62, 201 (1958).
Soc.
Soc.
472
SOLID SOLS
mercury salt, a colourless product is obtained which, after quenching, 4 turns black (but is not identical with black phosphorus '). G E R N E Z was able to show, by means of an ultramicroscope, that the black substance consists of white phosphorus in which mercury is dispersed in the form of minute spheies. Mercuric iodide also dissolves, at elevated temperatures, in white phosphorus, as well as in naphthalene, salol, thymol, benzophenone and other substances; upon cooling it segregates out in a finely dispersed form. The preparations are at first yellow, but later turn red, corresponding to the low temperature red form of H g l 2 (yellow above 126°). It is also very likely that many organic dyes dissolve at elevated temperatures in the organic substances mentioned above, precipitating on cooling in a colloidal form. The colours of certain gem-stones such as ruby, amethyst and sapphire, may be due to small amounts of colloidally dispersed solutes. Natural, as well as synthetic rubies contain some chromic oxide or iron oxide dispersed in the crystalline aluminium oxide. On the other hand, the blue colour of ultramarine is due to a solid solution of sulphur, selenium, or tellurium in the main substance of ultramarine (F.
J.
JAEGER).
Salts can be coloured by irradiating them with the rays of radioactive substances, X-rays or ultraviolet light. In crystals of potassium, sodium and certain other chlorides there are regions, possibly around lattice defects, where the chlorides are decomposed under the influence of the radiation, and where the free metal accumulates in the interior of the crystal. As a consequence, the crystals become coloured, usually yellow or brown, but in some instances blue. Coloured natural rock salt is mostly blue. S I E D E N T O P F was able to show ultramicroscopically that in all these crystals, except the slightly coloured ones, particles are present which are exactly like those observed in ruby glass. In support of the free metal dispersion theory, the preparation of coloured sodium and potassium chlorides by heating the respective dry chlorides in sodium or potassium vapours may be cited. Evidently the vapours penetrate the crystals through the microscopic channels and condense inside in the form of very small metallic crystals. At higher temperatures, 500° and above, the colours of such salts disappear. The property of penetration of some substances, e.g. metals a n d their vapours, into the interior of crystals through the microcracks, and condensation in or near the crystal defects (dislocations), is used to make 12 the latter visible and is called ' decoration ' (of dislocations). > The cracks and vacancies in salt crystals may be empty, or may be filled with water or a gas other than air; then they represent colloidal dispersions of liquid or gas in a solid. By heating such crystals (e.g. (11
> J. T. B A R T L E T T and J. W . M I T C H E L L ; Phil. Mag. 3 , 3 3 4 (1958). 12 < > W . G . J O H N S T O N ; / . Metals 11, 60 (1959).
COLOURED
CRYSTALS
473
lead nitrate, sodium chloride) they frequently explode : a phenomenon known as decrepitation. As a further example of a colloidal dispersion of a gas in a transparent substance, the blue feathers of some birds can be mentioned, the horny parts of these feathers being the dispersion medium for the finest of gas bubbles. Floating soap also contains small globules of air which make the soap lighter and cause it to float on water. Some other porous products such as baked bread, sponges, pumice stone etc. can be included in this class of colloidal systems.
SOLID
19
SOLS
Restriction of the field of solid sols. These sols, according to the definition of colloidal substances, should be represented by a very large group of systems in which solid (gas or liquid) is dispersed in another solid. Two cases can be distinguished: (1) the dispersion medium has a low absorption of light (it is transparent) ; and (2) the absorption of light is very strong, and, therefore, the medium is opaque. In the first case it is possible to study the optical properties of the sol, whereas this is impossible in the second case in which only the definition links these systems with the colloids. If one wishes to explore the hardening of quenched aluminium alloys, in which the alloyed metals, immediately after quenching, are in a highly dispersed ( 1) s t a t e , one cannot use the methods of colloid chemistry, but rather one should apply the methods of metallography and physical metallurgy. These methods and techniques are entirely different from those used in colloid chemistry. Therefore the second group of solids belongs to the wide fields of metallurgy, mineralogy and petrography, and only the first group of solids, having a transparent dispersion medium, will be mentioned in any detail. The transparent dispersion medium may be either amorphous or crystalline in nature. Amorphous substances as dispersion medium. To this group belong the coloured glasses. Gold ruby glass was first prepared by K U N K E L in 1 6 7 9 , and was mentioned by L I B A V I U S . This glass was of importance in colloid chemistry, because in 1 9 0 3 Z S I G M O N D Y and S I E D E N T O P F succeeded in disclosing the nature of the colloidal state by studying this substance in the ultramicroscope. A red-coloured substance is obtained when small amounts of gold ( 0 - 1 g or more per 1 kg of lead or other glass) in the form of a gold preparation, or gold foil, is dissolved in molten glass. The mass, if quickly cooled, is at first colourless and optically * empty ' if observed in the ultramicroscope, evidently because of complete solution of the gold. But if the mass is cooled slowly, or if the cold mass is tempered afterwards at a temperature below the softening temperature of the glass, it assumes a beautiful, clear, red colour, which resembles that of a gold hydrosol. It is then possible to demonstrate ultramicroscopically the presence of myriads of colloidal particles (1
14
) e.g. Quenching very rapidly a supersaturated Al-Si alloy, 10 silicon particles/ cm in the alloy were found: see H . S. R O S E N B A U M and D. T U R N B U L L ; Acta Met. 6, 653 (1958). 469 3
470
S O L I D
SOLS
in this mass. These look like those in a gold hydrosol except that they are motionless because of the great viscosity of the dispersion medium. The number of these particles can easily be counted. Such a solid sol has optical properties very similar to those of hydrosols. For instance, the same changes in colour are observed if the particle size is increased. This takes place if the gold ruby glass is tempered for a considerable time at an elevated temperature : at first the red colour becomes blue (by reflected light the glass appears brown), and finally, tiny, brilliant, solid gold particles can be detected in the glass. The effect of the heat treatment is to accelerate the diffusion of gold atoms by diminishing the viscosity of the glass. The gold atoms then have the opportunity of collecting on separate crystallisation centres, with the subsequent formation of submicroscopic and then of larger gold crystals if the heat treatment is continued. Ruby glass and other glasses can also be prepared by dissolving copper, selenium, silver and other colouring substances instead of the more expensive gold. The behaviour of these glasses is very similar to that of gold ruby glass. The glass containing copper is also colourless if quenched. With heat treatment a ruby glass with ultramicroscopic particles is first obtained; this glass then becomes red and opaque, containing microscopic particles with a metallic lustre; and finally a product containing macroscopic crystal dendrites is reached. Silver produces a yellow, brown or red-brown stained glass, resembling the silver hydrosols in colour. In 1937, R. H. D A L T O N discovered that a colourless copper ruby glass, obtained by rapid cooling of the heated glass, develops its colour more readily if exposed to ultraviolet light prior to the heat treatment (annealing at about 580° for 15 minutes to one hour). Such a ' p h o t o sensitive glass' is used for the production of pictures in glass. A plate of the above-mentioned glass is covered by a negative, and then exposed from the negative side to short-wave light (violet or ultraviolet). X-rays are also effective. At the intensely irradiated spots on the glass, corresponding to the transparent parts of the negative, a greater concentration of metallic nuclei is developed. There is evidence that gold, copper or silver are dissolved in the colourless glass in the form of ( 2) On subions, which are reduced to atoms as a result of irradiation. sequent heating the metallic nuclei act as crystallisation centres even for non-metallic crystals, and the respective spots on the glass become the darker the stronger the irradiation. The colour depends on the nature of the dispersed substance. Instead of glass various plastic organic materials such as Bakelite, 2
( ) S. D .
STOOKEY;
Industr. Engng. Chem. 41, 856 (1949); see also S. D . S T O O K E Y , A L E X A N D E R ' S Colloid Chemistry, Vol. VII, p. 697 (New
' Photosensitive glass ' in J. York 1950).
471
GLASSES
vinyl resins can be used as suspension media for photo-sensitive substances. Films containing the latter (Calimar films) are photosensitive throughout. If exposed to ultraviolet light through a standard photographic negative for about one minute, the light activates metal oxides a n d dye formers present in the film (creation of nucleation centres). The exposed film is then ' developed ' in a hot-air oven at 100° C in about 6 min. The image becomes visible because the lightactivated compounds form dyes in the film (growth of the nuclei) at the temperature mentioned, while simultaneously the non-irradiated dyeforms are passivated. A very similar process occurs in ordinary photographic films and plates during light exposure and subsequent ( 3 4 5) wet development in the dark-room. ' ' The colour of borax a n d sodium phosphate beads, which were used for the detection of elements in qualitative analysis, may originate in many cases from colloidally dispersed substances in the glassy masses. Transparent crystalline substances as dispersion media. T o this class of solid sols belong the pyrosols, described by L O R E N Z and E I T E L . Pyrosols are prepared by mixing metals with low melting points under ( 6) their molten salts. F o r instance, under molten lead chloride lead expels grey clouds which disappear completely a t higher temperatures. It has been shown that a t this stage the metal forms a true solution ( 7) with the molten salt. U p o n cooling, the grey fog again appears. It is very probable that this phenomenon is the same as that already described in the case of gold ruby glass : the high viscosity of the solid salt a t low temperatures prevents the coagulation, or aggregation, of the separate, highly dispersed metal particles; with slow cooling the particle size is larger. Coloured masses are also obtained during the electrolysis of many molten salts. Metallic dispersions are produced upon heating some active metals (titanium, zirconium, hafnium) in molten salts, for instance sodium ( 8 9) chloride. ' The dispersion occurs due t o oxygen absorption (under the melt) by the surface of these metals and the subsequent breakdown ( 1 0) of the oxygen-containing l a y e r s . U p o n heating white phosphorus with mercury, or better still with a 3
<> J . W. M I T C H E L L ; Z . Elektroch. 60, 557 (1956). 4 <δ) H . S T A U D E ; ibid. 6 0 , 563 (1956). ( ) Ε. K L E I N ; ibid. 60, 998 (1956). Μ Ε. H E Y M A N N ; Austral. Chem. Inst., Journal and Proceedings 4 , 3 8 (1937). See also7 G . C L E A R Y and D . C U B I C C I O T T I ; / . Amer. Chem. Soc. 74, 557, 1198 (1952). <> J. F A R Q U H A R S O N and E. H E Y M A N N ; Trans. Faraday Soc. 3 1 , 1004 (1935). E. H E Y M A N N and E. F R I E D L Ä N D E R ; Z . Physik. Chemie 148A, 177 (1930). (8 > C . B . G I L L , M . E. S T R A U M A N I S and A. W. S C H L E C H T E N ; / . Electrochem.
102, 4 2 (1955). (E
) M . E. S T R A U M A N I S , S. T. S H I H and A. W. S C H L E C H T E N ; J. Electrochem.
104,1 0 17(1957). ( ) M . E. S T R A U M A N I S and C H .
CHIOU;
Z . Elektrochem.
62, 201 (1958).
Soc.
Soc.
472
SOLID SOLS
mercury salt, a colourless product is obtained which, after quenching, 4 turns black (but is not identical with black phosphorus '). G E R N E Z was able to show, by means of an ultramicroscope, that the black substance consists of white phosphorus in which mercury is dispersed in the form of minute spheies. Mercuric iodide also dissolves, at elevated temperatures, in white phosphorus, as well as in naphthalene, salol, thymol, benzophenone and other substances; upon cooling it segregates out in a finely dispersed form. The preparations are at first yellow, but later turn red, corresponding to the low temperature red form of H g l 2 (yellow above 126°). It is also very likely that many organic dyes dissolve at elevated temperatures in the organic substances mentioned above, precipitating on cooling in a colloidal form. The colours of certain gem-stones such as ruby, amethyst and sapphire, may be due to small amounts of colloidally dispersed solutes. Natural, as well as synthetic rubies contain some chromic oxide or iron oxide dispersed in the crystalline aluminium oxide. On the other hand, the blue colour of ultramarine is due to a solid solution of sulphur, selenium, or tellurium in the main substance of ultramarine (F.
J.
JAEGER).
Salts can be coloured by irradiating them with the rays of radioactive substances, X-rays or ultraviolet light. In crystals of potassium, sodium and certain other chlorides there are regions, possibly around lattice defects, where the chlorides are decomposed under the influence of the radiation, and where the free metal accumulates in the interior of the crystal. As a consequence, the crystals become coloured, usually yellow or brown, but in some instances blue. Coloured natural rock salt is mostly blue. S I E D E N T O P F was able to show ultramicroscopically that in all these crystals, except the slightly coloured ones, particles are present which are exactly like those observed in ruby glass. In support of the free metal dispersion theory, the preparation of coloured sodium and potassium chlorides by heating the respective dry chlorides in sodium or potassium vapours may be cited. Evidently the vapours penetrate the crystals through the microscopic channels and condense inside in the form of very small metallic crystals. At higher temperatures, 500° and above, the colours of such salts disappear. The property of penetration of some substances, e.g. metals a n d their vapours, into the interior of crystals through the microcracks, and condensation in or near the crystal defects (dislocations), is used to make 12 the latter visible and is called ' decoration ' (of dislocations). > The cracks and vacancies in salt crystals may be empty, or may be filled with water or a gas other than air; then they represent colloidal dispersions of liquid or gas in a solid. By heating such crystals (e.g. (11
> J. T. B A R T L E T T and J. W . M I T C H E L L ; Phil. Mag. 3 , 3 3 4 (1958). 12 < > W . G . J O H N S T O N ; / . Metals 11, 60 (1959).
COLOURED
CRYSTALS
473
lead nitrate, sodium chloride) they frequently explode : a phenomenon known as decrepitation. As a further example of a colloidal dispersion of a gas in a transparent substance, the blue feathers of some birds can be mentioned, the horny parts of these feathers being the dispersion medium for the finest of gas bubbles. Floating soap also contains small globules of air which make the soap lighter and cause it to float on water. Some other porous products such as baked bread, sponges, pumice stone etc. can be included in this class of colloidal systems.