- Email: [email protected]
SOME SIMPLE, BASIC METHODS IN EXPERIMENTS WITH COLLOIDS
CHAPTER 3
SOME SIMPLE, BASIC M E T H O D S I N E X P E R I M E N T S WITH COLLOIDS I N this chapter we shall discuss the simplest experimental procedures by means of which some of the most important properties of colloids can be investigated. The first question we may ask is : By what means may we find out whether a particular substance is a colloid or not? Further usual questions are : Is the substance a fibrous or a globular colloid? Is it a molecular or micellar colloid? Is it lyophobic or lyophilic? Most of these and similar questions can be answered by means of simple experimental procedures. These a r e : filtration and ultrafiltration, dialysis, diffusion in a gel, optical investigations, viscosity measurements, and some coagulation experiments. First, however, it is necessary to know how to prepare the colloids. The preparation of certain colloids The general conditions under which colloids are formed, and several special methods for their preparation will be treated more particularly in a later chapter. Here we shall discuss only some very simple cases. It is noteworthy that many of the molecular colloids are produced naturally in biochemical reactions. Our task, then, is merely to separate and purify these substances. Less simple is the preparation of most of the inorganic colloids which usually are lyophobic, because the substances (sulphur, gold, arsenious sulphide, ferric hydroxide, etc.) are insoluble in water. Some of these substances dissolve in acids, but in such solutions they are completely changed chemically, with the result that not colloids but micromolecular or ' true ' solutions are formed. Colloidal solutions of these substances can, however, be prepared either by condensation or dispersion methods. The simplest dispersion method is mechanical grinding; another dispersion method is called peptisation (partial or incomplete dissolution) of coarse precipitates. In a condensation process the colloidal particles are built up from smaller units starting with a micromolecular solution. Thus a sulphur sol can, (after V O N WEIMARN), be prepared as follows : a small amount of sulphur is first dissolved in absolute alcohol. This micromolecular solution is then poured into water. Since the solubility of sulphur in the resulting water-alcohol mixture is less than in alcohol, the excess sulphur precipitates in the form of tiny colloidal particles. The chemical methods of condensation by which colloidal particles of sparingly soluble substances are produced in chemical reactions have much wider appli19
20
SIMPLE,
BASIC
EXPERIMENTAL
METHODS
cations. Examples are provided by the following p r e p a r a t i o n s : arsenious sulphide from arsenious acid a n d hydrogen sulphide; gold or silver sols by reduction of the corresponding salts of gold or silver, silver chloride from very dilute solutions of silver nitrate a n d sodium chloride. In analytical procedures especially, colloids appear quite frequently and unintentionally, a n d since they can not be separated by the usual filtration procedure they are strongly disliked by analysts. W e shall now present three examples of the preparation of lyophobic micellar colloids of inorganic substances ; the experiments are so simple that they can be carried out in any laboratory.* Preparation of arsenious sulphide sol (after SCHULZE). 1-2 grams of pure, glassy arsenious oxide is dissolved in 900 ml. of boiling distilled water. The more finely ground the material the more easily will it dissolve. After prolonged boiling the solution should be cooled to room temperature and, if necessary, filtered. The resulting saturated solution of arsenious acid is now diluted with three to four volumes of distilled water. Into this solution hydrogen sulphide gas is introduced. The latter must be carefully purified by slow bubbling through several flasks of distilled water, and must be free of HCl before introduction into the arsenious acid. After five minutes' bubbling the solution will be saturated with hydrogen sulphide, and the arsenious sulphide has been formed. The excess of hydrogen sulphide is expelled with a current of hydrogen (20 to 3 0 minutes' bubbling of pure hydrogen through the colloid). Usually a small amount of coarse sulphide is also formed ; this may be removed by filtration. The preparation of this attractive yellow sol always succeeds if the substances used are sufficiently pure. Preparation of ferric hydroxide sol (after KRECKE). Into 750 ml. of boiling distilled water are poured 12 ml. of a 3 2 % ferric chloride solution. The hydrolysis of ferric chloride occurs instantly, and a beautiful, deep red sol of ferric hydroxide forms. The colloid is quite stable, and may be purified by dialysis. Preparation of silver sol by reduction of silver carbonate with tannic acid. To 500 ml. of distilled water are added 2 0 ml. of a 0-1 iV silver nitrate solution and 5 - 1 0 ml. of a 1% tannic acid solution. The mixture is then heated to 70-80° C, and 10 ml. of a 1% sodium carbonate solution is added in portions with stirring. The silver carbonate formed is instantly reduced by the tannic acid to metallic silver. This remains in the solution as a colloid which has the colour of tea and is quite clear. All three of these sols are lyophobic inorganic colloids. As examples of lyophilic sols egg albumin or glycogen solutions are recommended. 1-2 grams of the finely ground solid organic substance is poured into 100 ml. distilled w a t e r ; the mixture is stirred a n d then allowed to stand for two hours. After that time the solutions are filtered. Gelatin is a typical lyophilic linear colloid. Two grams of gelatin are placed in distilled water a n d kept there for several hours. Unlike albumin a n d glycogen, gelatin does not dissolve in cold water although it does swell. T h e swollen material, however, may be dis* The following experiments were established as very useful for students at the University of Latvia in Riga, 1922-1944, where the Laboratory for Colloid Chemistry was organised and supervised by Professor A . JANEK.
FILTRATION
AND
21
ULTRAFILTRATION
solved by heating with water to 80-90° C. If the two grams of gelatin are dissolved in 400 ml. water, a clear sol is formed on cooling. A 1-2% gelatin solution will set to a jelly on cooling. Filtration and ultrafiltration
( 1)
By simple filtration experiments it can be shown that all the abovementioned colloids pass through the usual filter papers. The particles are not retained even by the hardest (finest) sorts of filter paper. Filter paper is composed of cellulose fibres which are interwoven in an irregular network which contains pores of different sizes and shapes through which the fine colloidal particles pass. Several methods are available for determining the average pore size of filters. For instance, microscopically measured particles can be filtered, or the rate of filtration of liquids under a known constant pressure can be determined. By means of such methods it has been found that the average diameter of the capillaries in filter papers is about 0-003-0-004 mm. The fine or hard papers have a pore diameter of 0-0009-0-0016 mm. Since the particles of arsenious sulphide sol, as well as those of ferric hydroxide, silver, gelatin, albumin and other colloids pass through these filters, it is clear that the particles must be smaller than the pores. The same applies to the usual glass and porcelain filters, the pores of which are even coarser than those of paper filters. The coarse, sintered glass filters have a pore diameter of about 0-035-0-04 mm, and the fine about 0-002-0-003 mm. Filtration
Ultrafiltration
FIG. 2.
Filtration and ultrafiltration.
The following facts should be considered in the evaluation of particle sizes by means of filtration : If the particles run through a filter, they are certainly smaller than the pores. If they are retained, however, it is impossible to conclude with certainty that they are larger than the capillaries, because they may sometimes be adsorbed on the surfaces of the filter and so clog the capillaries. In such cases only the first portions of the filtrate will give reliable information: if they differ L
( ) A. B. CUMMINS and F . Β. Ηυττο, Jr., in Weissberger's
Chemistry,
Technique
vol. Ill, Part I (Interscience, New York 1956) p. 608 ff.
of
Organic
22
SIMPLE,
BASIC EXPERIMENTAL
METHODS
from the original solution (i.e. if they are clearer or less strongly coloured), the particles may be assumed to be coarser than the pores of the filter. Colloidal particles are more or less completely retained by ultrafilters, i.e. extremely fine filters whose membranes are solid gels. The structure of the membrane is the same as that of the paper, but the pore size is much smaller. There are various ultrafilters now available, with different average pore size varying from 100 Â to 2000 Â, i.e. from 10 m/x to 200 m^. 9 One of the best materials for an ultrafilter is ' Cellophane . From a sheet of thin ' Cellophane' a disc is cut and folded into the shape of a dish. By means of a rubber ring the border is then fastened to a funnel as shown in Fig. 3. Into this ' C e l l o p h a n e ' bag a sol is now poured. Filtration is very slow, but after a certain time the liquid penetrates
4
FIG. 3. Formation of a ' Cellophane ' bag. (1) Cellophane' sheet, (2) the edges of the sheet bent upward; (3) the bag hanging on a funnel tube.
the membrane, and a drop of liquid is formed on the bottom outside the bag. Investigation of these drops shows whether the particles penetrate the membrane or not. Another good material for making ultrafilters is collodion. The usual collodion is a 4 % solution of nitrocellulose in a mixture of alcohol and ether. Sintered glass filters or unglazed porcelain crucibles are the best supporters for the collodion membranes. These are formed upon partial evaporation of the alcohol-ether mixture. Practically, collodion is simply poured into a sintered glass filter or filter crucible to a thickness of several mm, and kept until it solidifies. Another method is to impregnate filter paper with collodion. The pore size of these collodion ultra-filters depends mainly on the degree of drying of the collodion layer. The longer it dries the finer the capillaries get. After a very long time the nitrocellulose film may even become so hard that water cannot permeate such a membrane. That is why collodion ultrafilters, after a certain time of drying, are placed in water to prevent further drying and consequent decrease in pore size. BECHHOLD (1907) was one of the pioneers in ultra-filtration; he even succeeded in separating certain viruses by means of this method. ZSIGMONDY, BACHMANN and ELFORD continued to develop the technique. Several kinds of commercial ultrafilters with varying pore sizes, and suitable for filtration under pressure, have now been developed. Any sintered glass filter or Büchner funnel with a collodion
VARIOUS
ULTRAFILTERS
23
membrane can be used for ultra-filtration under pressure. According to ZSIGMONDY, the periphery in the bottom of the Büchner funnel must be first wetted with rubber solution, after which the disc-shaped ultrafilter is inserted ; collodion does not stick well to porcelain. Discs of filter paper impregnated with collodion and dried to a certain extent can be used as membranes. The ultrafiltration can be promoted by stirring the solution, by warming it with a jacketed filter funnel, or by introducing a heating device into the colloid. It is interesting that in certain cases high pressure only produces blocking of the pores ; this happens when the particles are strongly adsorbed by the walls of the capillaries or when the particles are solid. If they 4 are liquid or soft as it is in the case of emulsions or lecithin sols, particles even larger than the pore size can thus be forced through the filter. Some particles which usually are retained by an ultrafilter can be made to pass by the addition of certain stabilising agents which counteract the adsorption. For instance, fine carbon, which is retained by hard filter paper, slips through after the addition of certain protein solutions. This shows clearly that filtration and ultrafiltration are not quite the same as separation by a sieve.
Most convenient for simple ultrafiltration experiments are nonwaterproofed ' Cellophane ' membranes and coloured colloids, like silver sol or arsenious sulphide colloid. The coloured particles are completely retained on these membranes. By means of moderately dried collodion membranes, however, it is possible to ascertain that the particles of silver are smaller than those of arsenious sulphide, since the former pass through such filters while the latter are retained. The pore size of BECHHOLD'S ultrafilters may be determined by the
rate of penetration of water or by forcing air through the wetted membrane.
ZSIGMONDY determined the pore size of his ultrafilters by the
filtration of carefully prepared gold sols whose particle size had been determined by means of an ultramicroscope. Such standardised ultrafilters have been used for the estimation of the particle sizes of other colloids. Substances like dextrins, with a molecular weight about 5 0 0 0 - 8 0 0 0 , penetrate ultrafilters but slowly. They are called semicolloids. ELFORD investigated the virus proteins
( l )a
by means of ultrafiltration.
He used collodion membranes of fairly large pore size, and found that only in the case of coarse ultrafilters was the diameter (/?) of particles which just passed through equal to the diameter of the capillaries (d). For an ultrafilter with large pores (d = 0 O 0 1 mm) the ratio p\d=\. The finer the pores the smaller is this ratio. TABLE 8
(LA
Ratio p\d
Pore size (diameter)
0.33-0.50 0.50-0.75 0.75-1.00
10 m μ- 100 m/χ 100 m/χ- 500 m/x 500 m/x-1000 m/x
> W . F . ELFORD ; / . Pathol. Bacteriol. 34, 505 (1931) ; Proc. Roy. Soc. (London), Β 112, 384 (1933). Κ . SOLLNER; / . Phys. Colloid Chem. 49, 47, 171, 265 (1945). J. FERRY; Chem. Revs. 18, 373 (1936).
24
SIMPLE,
BASIC EXPERIMENTAL
METHODS
Thus, using ultrafilters with very narrow capillaries the particles appear smaller than they actually are because they adsorb on and block the capillaries. Diffusion and dialysis It is remarkably difficult to perform accurate diffusion experiments in solutions. This was early noted by GRAHAM, who proposed to observe diffusion not in liquids but instead in soft jellies. By means of the following simple experiments it is easy to distinguish coloured colloids from coloured micromolecular solutions. A two per cent gelatin sol, prepared by dissolving gelatin in hot water, is distributed between several test tubes ; each is only half full. The tubes then are left for one hour at room temperature without stirring. During this time the gelatin sol sets to a jelly. On to these several jellies are poured the coloured colloids, and for comparison, into other tubes are poured some of the coloured salt solutions, e.g. copper sulphate, cobaltous chloride or nickel nitrate. After one or two days the various tubes are compared, and it is seen that while the salts have penetrated deeply into the jelly the colloids have not diffused at all. If the substances are not coloured, a chemical investigation of the upper layer of the jelly is necessary. This diffusion in jellies is related to ultrafiltration and to dialysis : here the sol is separated from the pure solvent by a semipermeable membrane through which the penetration of a substance is observed. The membrane is a gel with such tiny pores that only micromolecular substances can pass it. If a substance dialyses, i.e. penetrates through the membrane into the pure solvent, its particles must be composed of less than 1000 atoms. Thus, by means of dialysis, too, we can estimate particle sizes. Actually, however, the main goal of dialysis is to free a sol from accompanying electrolytes and other micromolecular impurities. The devices so used are called dialysators. The main part of a dialysator is the membrane. Generally the same types of membranes are used as in ultrafiltration experiments. For instance we can make a bag of non-waterproofed ' cellophane ', fasten it to a tube, pour a sol into the bag and finally place the bag in a container with distilled water. There are now on the market ' cellophane ' tubes which can be cut to different lengths, and these are particularly convenient. For the same purpose defatted animal membranes (e.g. bladders) are sometimes used. Parchment, which was formerly used, is now largely abandoned, since the rate of dialysis through it is very low. Very efficient dialysing membranes, according to Wo. OSTWALD, can be prepared by impregnating extraction socks with collodion. The porous extraction casing is first wetted with warm water and then filled with collodion. From the first casing the collodion is poured into another, and so on. The viscous layer adhering to the walls is now made more uniform by rotating and swinging the sock until the film
DIALYSIS
EXPERIMENTS
25
solidifies. Finally the casing is filled with water in order to prevent further drying of the membrane. If there are no extraction socks on hand, good dialysing bags of collodion can be prepared in the following way. Collodion is poured into a large, wide test tube (diameter 3-6 cm), which must be completely clean and dry; the tube is then emptied, and the remaining layer of collodion, which sticks to the walls, may be distributed evenly by rotating and swinging as in the preceding experiment. After about 5 minutes' rotation, enough of the ether and alcohol have evaporated for the collodion layer to have turned into a soft film which does not stick to the finger. The longer the drying out lasts the smaller will be the pores. The solidification may be stopped when required by pouring water into the tube. We now have a collodion bag adhering to the glass tube. It is removed from the glass by first freeing the upper border with the aid of forceps; a glass rod then is pushed between the film and the wall of the tube, and into the space thus formed water is poured. If the membrane has not been hardened too much by excessive drying, the bag will come out of the tube quite easily. Before using it as a dialysator it must be thoroughly washed in distilled water (submerged for at least one day).
Collodion membranes prepared in this way have a moderately large pore size. They dialyse faster than ' cellophane ' membranes. Chemi4 cally, a cellophane ' membrane is composed of fibrous regenerated cellulose molecules, while collodion is composed of similarly shaped molecules of nitrocellulose. Ferric hydroxide sol is quite a useful substance for dialysis experiments. It is introduced into a collodion bag or cellophane tube, and placed into a container with distilled water (Fig. 4). After several
FIG. 5. Electrodialyser. Ε—electrodes, M— membranes, C—the colloid in the middle compartment. FIG. 4. ' Cellophane ' bag in water ; M—the membrane, S—sol, A—water.
hours of dialysing, hydrochloric acid can readily be detected in the water. The H+ and C I ions, formed through hydrolysis of ferric chloride, penetrate the membrane but the colloidal particles do not. The semicolloids, which possess very tiny particles (diameter 5-10 Â ; molecular weight, if they are molecular colloids, about 4000-10,000), penetrate the coarse membranes slowly, and it is possible from this rate of dialysis to estimate the particle size of these substances (p. 190).
26
SIMPLE,
BASIC
EXPERIMENTAL
METHODS
For more efficient dialysis the water outside the bag must be constantly changed. W a r m water may also be used. Several types of apparatus for this purpose have been proposed, while simple devices for dialysing against flowing distilled water can be easily arranged in any laboratory. Stirring both the sol and water considerably increases the rate of dialysis. Dialysis is promoted also by means of a direct electric current which pulls the micromolecular ions out of the sol. This electrodialysis is carried out in a three compartment apparatus (Fig. 5) ; the middle compartment is separated from each outer compartment by a semipermeable membrane. The sol is poured into the middle compartment ; through the other two cells flows distilled water. The two electrodes, usually platinum gauzes or perforated platinum ( 2) The discs, are inserted into the outer cells close to the m e m b r a n e s . applied electrical potential then pulls all micromolecular ions through the membranes into the water. By means of such electrodialysing it is possible in a short time to liberate colloids from associated micromolecular electrolytes, although the electric current does not affect the dialysis of non-conducting impurities (alcohol, sugar). The current may also cause electrolytic decomposition a n d changes in acidity of ( 2 a) the s o l . Flocculation or coagulation of colloids Coagulation or flocculation is the increase of particle size in a sol, whereby the sol usually becomes turbid, or may even precipitate. There are several ways of causing coagulation. They include the action of electrolytes, radiation, heat, and various sols, although there are minor differences in their various effects. Some sols are very stable towards electrolytes, while others can be flocculated very easily. Let us compare the stability of our silver, ferric hydroxide, arsenious sulphide, albumin and gelatin colloids toward different reagents. F o r that purpose we shall mix 5 ml. each of the sol in test tubes with 5 ml. lTVNaCl solution. In a short time ( i - i minute) the silver sol, the arsenious sulphide a n d the ferric hydroxide colloids have become turbid, whereas there is no change in the sols of albumin and gelatin. After longer periods the first three sols will be more or less completely precipitated, but no change will be observed in the sols of albumin a n d gelatin. The former three are easily precipitated by NaCl, while the two latter sols are resistant to this reagent. Just the opposite is observed in flocculation with alcohol. Again, we add to 5 ml. of each sol 10 ml. alcohol and mix thoroughly. After ( 2)
H . HOLMES ; Laboratory
3rd2 aed. 1934.
Manual of Colloid Chemistry
(Wiley, New York 1928),
( ) R. E . STAUFFER, in WEISSBERGER'S Technique of Organic
Part I (Interscience, New York 1956) p. 65 if.
Chemistry,
vol. Ill,
F L O C C U L A T I O N
E X P E R I M E N T S
27
standing for five minutes the tubes are compared. N o w there is no change in the first three, but the sols of albumin and gelatin have become quite turbid. The colloids like Ag-sol and ferric hydroxide, which are easily flocculated by electrolytes, are lyophobic or hydrophobic. The hydrophilic sols of albumin or gelatin, however, are stable toward electrolytes, but are flocculated by alcohol. The coagulation problem will be thoroughly treated in several later chapters. However, having the three hydrophobic sols on hand we are able to perform quite easily some very instructive experiments. If instead of sodium chloride we use amounts of 0-1 Ν calcium chloride equal to the volume of each sol to flocculate the colloids, we find that silver and arsenious sulphide will coagulate easily, but that ferric hydroxide is unaffected. The latter, however, will be easily flocculated by salts containing such polyvalent anions as sulphate or phosphate. The two former sols, however, are quite stable toward these electrolytes. Silver and arsenious sulphide sols are very sensitive toward polyvalent cations, while ferric hydroxide sol is sensitive toward polyvalent anions. The selectivity depends on the sign of the electrical charge of the particles. The charge, as we know, is one of the stabilising factors. The silver and arsenious sulphide particles are negatively charged, and thus are most easily discharged by polyvalent cations. Ferric hydroxide particles carry a positive charge and are discharged and coagulated by anions, especially if polyvalent. Viscosity The viscosity of a liquid or solution is its resistance to flow. To stir glycerin more energy is needed than to stir water. Glycerin is hence said to be more viscous than water, and similarly water is more viscous than ether. Collodion, solutions of rubber and jellies are all very viscous. Still higher resistance to structural disruption is offered by the solids. It is obvious that liquids whose molecules tend to associate in larger aggregates are more viscous. Such are the liquids with polar molecules, e.g. glycerin, glycol, water, formamide, sulphuric acid, as well as liquids composed of large molecules, such as the long chain molecules of oleic acid. All these liquids also have a high boiling point. The molecules of water, according to DEBYE, are dipoles, i.e. the distribution of the positive and negative charges in the molecule of water is asymmetric. The molecules are V-shaped as illustrated below :
...+ - O ...+ - O ....
28
SIMPLE,
BASIC
EXPERIMENTAL
METHODS
The positive end of the water molecule attracts the negative end of another water molecule, a n d loose aggregates a n d chains are formed. This phenomenon of mutual interaction a n d association in water, organic acids a n d similar substances can be interpreted also in terms of the so-called hydrogen bonds. The latter are weak chemical bonds between hydrogen atoms or protons a n d a strongly electronegative atom like oxygen or nitrogen. According t o this concept liquid water has the following structure : ...Hx Ο ...Η, Ο ...Η Η
Ο . . . Η ^
Η /
Since the other Η also may form bonds with the oxygen atoms of adjacent molecules, the actual structure is even more complicated than that shown. Such hydrogen bonds are also formed in glycerin a n d in concentrated sulphuric acid, which is why these are so viscous a n d have relatively high boiling points. T h e attractive forces in non-polar liquids, however, are weak, a n d hence the viscosities a n d boiling points are low (e.g. carbon disulphide, hexane a n d benzene). The viscosity of a viscous, polar solvent will be still further increased if a polar solute is dissolved in i t ; this increase will be very high if long, fibrous, polar molecules are introduced (see p . 153). However, liquids of low viscosity (e.g. hexane, octane, ether) also show an enhanced viscosity after dissolving a fibrous colloid. T h e very viscous solutions of rubber in low viscous hydrocarbons are the best examples of this. The viscosity of a liquid or solution can be measured precisely either by moving a solid body through the liquid or by allowing the liquid itself t o flow through a capillary. T h e second method is the simpler, a n d is the most commonly used device in colloid chemistry. The viscometer of Wi. OSTWALD was invented
in Riga (Latvia) some seventy years ago, a n d FIG. 6. Capillary viscometer after WILHELM OSTWALD. M x—upper mark, M 2—lower mark, Κ—the capillary.
VISCOSITY
29
MEASUREMENTS
is now used throughout the world. The viscometer is a U-shaped tube which includes a capillary (Fig. 6). The viscosity is determined by measuring the flow time of a definite volume of a liquid through this capillary. For instance, the flowing time of 5Ό ml. of a sol is compared with the flowing time of 5-0 ml. water. The relative viscosity is the ratio of the flowing time of the solution to the flowing time of the solvent. If 5 ml. of a gelatin sol flows through in tx seconds, and 5 ml. of water in / seconds, the relative viscosity, ^ r ei , of the sol is given by 1
^rel = - ·
Since temperature has a pronounced influence on viscosity, all measurements are carried out in a thermostat at constant temperature. In exact calculations the density of the solutions must be considered. Another common term is specific viscosity ; this is the rise in viscosity of the solvent produced by the dissolved substance. The specific viscosity ^ s p = ^ r e i - l , or
We can now try to measure the viscosities of the colloids we have prepared. For this we need a suitable viscometer, a thermostat, and a stopwatch. The viscometer must be quite clean and dry. It is fastened vertically in the thermostat. 5Ό ml. of distilled water is now introduced into the wider tube, and the water pulled (by applied suction) over the mark of the upper bulb ; by releasing the suction the water is allowed to flow back through the capillary. The knob of the stopwatch is pressed at the moment the level passes the upper mark ; this is the beginning of the flow time. When the level passes the second (lower) mark, the watch is stopped Several readings of the flow time should be obtained, and the average may then be calculated in the usual way. Thus, the value for t is obtained. Measuring in the same way the flow time of the colloids, we obtain tx. From these figures the relative and specific viscosities can be calculated. In Table 9 the results are presented. TABLE 9.
The viscosity of some colloids at 25° C.
Ferric hydroxide sol, 0-5% tx in sec. *7rel *?sp
80-8 1010 0010
Albumin 0-5% 1-0%
t = 8 0 0 sec.
Glycogen 0-5% 1-0%
Gelatin 0-5% 1-0%
83-3 86-4 110-2 82-4 84-6 1-377 1 040 1 080 1 030 1 057 0080 0-377 0030 0057 0040
156-6 1-957 0-957
Ferric hydroxide evidently has the lowest viscosity, and approximately the same figures can be obtained for sols of silver and arsenious sulphide. The hydrophobic sols thus have a very low viscosity. The viscosities of the three hydrophilic colloids differ appreciably : albumin and glycogen have a much lower viscosity than gelatin. The reason
30
SIMPLE,
BASIC
EXPERIMENTAL
METHODS
lies in the differences in particle shape: albumin and glycogen are spherocolloids, but gelatin has fibrous particles. Differences in the viscosities of our spherocolloids, too, are quite easy to explain. The particles of all our inorganic colloids are evidently very compact ; even the ferric hydroxide, despite the polar O H groups, cannot be much hydrated. The albumin molecules are not so compact, and are somewhat more hydrated. The globular molecules of glycogen are still more loosely built, and the solutions have a higher viscosity than the solutions of albumin. It is noteworthy that among the linear colloids gelatin shows only a moderate viscosity. A much higher viscosity is found in 0-5% solutions of rubber, which even in such concentrations form jellies. The specific viscosities of 0 - 1 % solutions of different nitrocellulose fractions are 0-6-26. The above facts indicate that the chief reason for the high viscosities of colloids is the fibrous shape of the particles ; solvation or hydration has a much smaller effect. Some optical properties of sols It is noteworthy that many substances appear quite highly coloured if their particles are of colloidal dimensions. Thus, silver ions are colourless, precipitated silver is grey, but silver colloids have intense red-brown or greenish-brown colours. The strong colour of colloidal silver is changed and reduced quickly upon coagulation with a few drops of sodium chloride. Similar results are observed with gold: diluted solutions of gold chloride or chlorauric acid H [ A u C l 4 ] are slightly yellow, while in the reduction of this substance (e.g. with alcohol) a deep red or violet gold sol is formed. In flocculation with some electrolytes the colour is first changed to blue, then turbidity sets in and a greyish brown precipitate is finally formed. Very characteristic and interesting are the ' colours ' of colourless colloids. Every chemist knows that in the precipitation of very dilute solutions of halides (for instance, KCl, NaBr, KI) with silver ions a very fine, milky turbidity appears; looking through the solution it appears orange but on looking on it from the side it seems to be bluish. This phenomenon is called opalescence, and is explained as follows : the short waves of light (producing the sensations of blue and violet) are strongly scattered by the particles, whereas the long waves (yellow, orange, red) pass undisturbed through the sol. Opalescence depends mainly on particle size. Our albumin or gelatin sols contain very small particles, too small to produce a distinct opalescence. But if the degree of dispersion is decreased (the particle size increased) by gradual addition of alcohol or acetone, the opalescence will suddenly appear. Many colloidal solutions, for example our silver and ferric hydroxide
THE
FARADAY-TYNDALL CONE
31
sols, are completely clear. (The colour of the particles here overshadows the opalescence.) But if a sharp, intense beam of light passes through such clear sols, the path appears turbid. The best source of
FIG. 7. Tyndall cone in colloids. L—the light source, S—the sol.
light for such experiments is the projection illuminator which produces a sharp, conical beam. Observed from the side the path of the light through the sol has the shape of a cone. That is the Faraday-Tyndall cone. The reason for this phenomenon is the same as in opalescence : the light is scattered by the tiny colloidal particles. The phenomenon can be observed in a dark room in which a sharp ray of sunlight enters through a slit; the path is visible, because dust particles reflect a n d scatter the light, enabling us t o observe them. If the Faraday-Tyndall cone is observed under a microscope on a dark background, it is ' resolved ' into separate, bright particles. That is the principle of the ultra-microscope. T h e bright, coloured, rapidly moving discs we observe in the ultra-microscope are caused by the colloidal particles, all acting as separate scattering centres. The lyophilic linear colloids show a weak Tyndall cone which is usually n o t resolved in the ultra-microscope into particles. This indicates that the fibrous particles, mostly those of organic molecular colloids, scatter the light comparatively little. The scattering depends on the difference in refractive indices between the solvent and particles, as well as on the structure of the particles, their size, and their solvation. (See Chapter 6.) Concentration and density of sols The concentration of most of the colloids is small. Metal sols usually contain 0 Τ - 0 · 5 % metal, the sols of hydroxides and sulphides 1 - 5 % of the solid. T h e concentration of the solutions of fibrous molecular colloids, t o o , is usually only about 0 - 2 - 1 % . More concentrated sols of fibrous substances set easily. It is possible, however, to prepare quite concentrated sols of hydrophilic spherocolloids, for instance 1 0 - 1 5 % sols of albumin or casein. Concentrated sols of ferric hydroxide or arsenious sulphide are also known. F o r instance, BOUTARIC a n d VUILLAUME ( 1 9 2 4 ) prepared a n arsenious sul-
phide sol which contained 3 0 0 grams A s 2 S 3 per litre. However, since the sulphide particles are very large compared with micromolecular units, the molar concentration, even of this concentrated sol, is very low. FREUNDLICH, indeed, computed the molar concentrations by assuming that the spherical particles have a diameter of 1 0 0 m/x. Such
32
SIMPLE,
BASIC
EXPERIMENTAL
METHODS 8
particles have a ' molecular weight ' of 8-6 . 1 0 , while the molar con5 centration of the 3 0 % arsenious sulphide sol is only 3 - 5 . 1 0 ~ mol. per litre. The density of sols, according to CHOLODNY ( 1 9 0 3 ) increases linearly with the concentration. If p s denotes the density of a sol, p the density of the dispersed substance, and p 0 the density of the solvent, and if c is the number of grams of the dispersed substance in one ml., then the relation between these is expressed by the following equation :
By means of this equation it is possible to calculate the density of the particles. In a number of cases (Ag, Se, Z n O sols) the density of the particles is the same as that of the substance in bulk.
SOME SIMPLE, BASIC M E T H O D S I N E X P E R I M E N T S WITH COLLOIDS I N this chapter we shall discuss the simplest experimental procedures by means of which some of the most important properties of colloids can be investigated. The first question we may ask is : By what means may we find out whether a particular substance is a colloid or not? Further usual questions are : Is the substance a fibrous or a globular colloid? Is it a molecular or micellar colloid? Is it lyophobic or lyophilic? Most of these and similar questions can be answered by means of simple experimental procedures. These a r e : filtration and ultrafiltration, dialysis, diffusion in a gel, optical investigations, viscosity measurements, and some coagulation experiments. First, however, it is necessary to know how to prepare the colloids. The preparation of certain colloids The general conditions under which colloids are formed, and several special methods for their preparation will be treated more particularly in a later chapter. Here we shall discuss only some very simple cases. It is noteworthy that many of the molecular colloids are produced naturally in biochemical reactions. Our task, then, is merely to separate and purify these substances. Less simple is the preparation of most of the inorganic colloids which usually are lyophobic, because the substances (sulphur, gold, arsenious sulphide, ferric hydroxide, etc.) are insoluble in water. Some of these substances dissolve in acids, but in such solutions they are completely changed chemically, with the result that not colloids but micromolecular or ' true ' solutions are formed. Colloidal solutions of these substances can, however, be prepared either by condensation or dispersion methods. The simplest dispersion method is mechanical grinding; another dispersion method is called peptisation (partial or incomplete dissolution) of coarse precipitates. In a condensation process the colloidal particles are built up from smaller units starting with a micromolecular solution. Thus a sulphur sol can, (after V O N WEIMARN), be prepared as follows : a small amount of sulphur is first dissolved in absolute alcohol. This micromolecular solution is then poured into water. Since the solubility of sulphur in the resulting water-alcohol mixture is less than in alcohol, the excess sulphur precipitates in the form of tiny colloidal particles. The chemical methods of condensation by which colloidal particles of sparingly soluble substances are produced in chemical reactions have much wider appli19
20
SIMPLE,
BASIC
EXPERIMENTAL
METHODS
cations. Examples are provided by the following p r e p a r a t i o n s : arsenious sulphide from arsenious acid a n d hydrogen sulphide; gold or silver sols by reduction of the corresponding salts of gold or silver, silver chloride from very dilute solutions of silver nitrate a n d sodium chloride. In analytical procedures especially, colloids appear quite frequently and unintentionally, a n d since they can not be separated by the usual filtration procedure they are strongly disliked by analysts. W e shall now present three examples of the preparation of lyophobic micellar colloids of inorganic substances ; the experiments are so simple that they can be carried out in any laboratory.* Preparation of arsenious sulphide sol (after SCHULZE). 1-2 grams of pure, glassy arsenious oxide is dissolved in 900 ml. of boiling distilled water. The more finely ground the material the more easily will it dissolve. After prolonged boiling the solution should be cooled to room temperature and, if necessary, filtered. The resulting saturated solution of arsenious acid is now diluted with three to four volumes of distilled water. Into this solution hydrogen sulphide gas is introduced. The latter must be carefully purified by slow bubbling through several flasks of distilled water, and must be free of HCl before introduction into the arsenious acid. After five minutes' bubbling the solution will be saturated with hydrogen sulphide, and the arsenious sulphide has been formed. The excess of hydrogen sulphide is expelled with a current of hydrogen (20 to 3 0 minutes' bubbling of pure hydrogen through the colloid). Usually a small amount of coarse sulphide is also formed ; this may be removed by filtration. The preparation of this attractive yellow sol always succeeds if the substances used are sufficiently pure. Preparation of ferric hydroxide sol (after KRECKE). Into 750 ml. of boiling distilled water are poured 12 ml. of a 3 2 % ferric chloride solution. The hydrolysis of ferric chloride occurs instantly, and a beautiful, deep red sol of ferric hydroxide forms. The colloid is quite stable, and may be purified by dialysis. Preparation of silver sol by reduction of silver carbonate with tannic acid. To 500 ml. of distilled water are added 2 0 ml. of a 0-1 iV silver nitrate solution and 5 - 1 0 ml. of a 1% tannic acid solution. The mixture is then heated to 70-80° C, and 10 ml. of a 1% sodium carbonate solution is added in portions with stirring. The silver carbonate formed is instantly reduced by the tannic acid to metallic silver. This remains in the solution as a colloid which has the colour of tea and is quite clear. All three of these sols are lyophobic inorganic colloids. As examples of lyophilic sols egg albumin or glycogen solutions are recommended. 1-2 grams of the finely ground solid organic substance is poured into 100 ml. distilled w a t e r ; the mixture is stirred a n d then allowed to stand for two hours. After that time the solutions are filtered. Gelatin is a typical lyophilic linear colloid. Two grams of gelatin are placed in distilled water a n d kept there for several hours. Unlike albumin a n d glycogen, gelatin does not dissolve in cold water although it does swell. T h e swollen material, however, may be dis* The following experiments were established as very useful for students at the University of Latvia in Riga, 1922-1944, where the Laboratory for Colloid Chemistry was organised and supervised by Professor A . JANEK.
FILTRATION
AND
21
ULTRAFILTRATION
solved by heating with water to 80-90° C. If the two grams of gelatin are dissolved in 400 ml. water, a clear sol is formed on cooling. A 1-2% gelatin solution will set to a jelly on cooling. Filtration and ultrafiltration
( 1)
By simple filtration experiments it can be shown that all the abovementioned colloids pass through the usual filter papers. The particles are not retained even by the hardest (finest) sorts of filter paper. Filter paper is composed of cellulose fibres which are interwoven in an irregular network which contains pores of different sizes and shapes through which the fine colloidal particles pass. Several methods are available for determining the average pore size of filters. For instance, microscopically measured particles can be filtered, or the rate of filtration of liquids under a known constant pressure can be determined. By means of such methods it has been found that the average diameter of the capillaries in filter papers is about 0-003-0-004 mm. The fine or hard papers have a pore diameter of 0-0009-0-0016 mm. Since the particles of arsenious sulphide sol, as well as those of ferric hydroxide, silver, gelatin, albumin and other colloids pass through these filters, it is clear that the particles must be smaller than the pores. The same applies to the usual glass and porcelain filters, the pores of which are even coarser than those of paper filters. The coarse, sintered glass filters have a pore diameter of about 0-035-0-04 mm, and the fine about 0-002-0-003 mm. Filtration
Ultrafiltration
FIG. 2.
Filtration and ultrafiltration.
The following facts should be considered in the evaluation of particle sizes by means of filtration : If the particles run through a filter, they are certainly smaller than the pores. If they are retained, however, it is impossible to conclude with certainty that they are larger than the capillaries, because they may sometimes be adsorbed on the surfaces of the filter and so clog the capillaries. In such cases only the first portions of the filtrate will give reliable information: if they differ L
( ) A. B. CUMMINS and F . Β. Ηυττο, Jr., in Weissberger's
Chemistry,
Technique
vol. Ill, Part I (Interscience, New York 1956) p. 608 ff.
of
Organic
22
SIMPLE,
BASIC EXPERIMENTAL
METHODS
from the original solution (i.e. if they are clearer or less strongly coloured), the particles may be assumed to be coarser than the pores of the filter. Colloidal particles are more or less completely retained by ultrafilters, i.e. extremely fine filters whose membranes are solid gels. The structure of the membrane is the same as that of the paper, but the pore size is much smaller. There are various ultrafilters now available, with different average pore size varying from 100 Â to 2000 Â, i.e. from 10 m/x to 200 m^. 9 One of the best materials for an ultrafilter is ' Cellophane . From a sheet of thin ' Cellophane' a disc is cut and folded into the shape of a dish. By means of a rubber ring the border is then fastened to a funnel as shown in Fig. 3. Into this ' C e l l o p h a n e ' bag a sol is now poured. Filtration is very slow, but after a certain time the liquid penetrates
4
FIG. 3. Formation of a ' Cellophane ' bag. (1) Cellophane' sheet, (2) the edges of the sheet bent upward; (3) the bag hanging on a funnel tube.
the membrane, and a drop of liquid is formed on the bottom outside the bag. Investigation of these drops shows whether the particles penetrate the membrane or not. Another good material for making ultrafilters is collodion. The usual collodion is a 4 % solution of nitrocellulose in a mixture of alcohol and ether. Sintered glass filters or unglazed porcelain crucibles are the best supporters for the collodion membranes. These are formed upon partial evaporation of the alcohol-ether mixture. Practically, collodion is simply poured into a sintered glass filter or filter crucible to a thickness of several mm, and kept until it solidifies. Another method is to impregnate filter paper with collodion. The pore size of these collodion ultra-filters depends mainly on the degree of drying of the collodion layer. The longer it dries the finer the capillaries get. After a very long time the nitrocellulose film may even become so hard that water cannot permeate such a membrane. That is why collodion ultrafilters, after a certain time of drying, are placed in water to prevent further drying and consequent decrease in pore size. BECHHOLD (1907) was one of the pioneers in ultra-filtration; he even succeeded in separating certain viruses by means of this method. ZSIGMONDY, BACHMANN and ELFORD continued to develop the technique. Several kinds of commercial ultrafilters with varying pore sizes, and suitable for filtration under pressure, have now been developed. Any sintered glass filter or Büchner funnel with a collodion
VARIOUS
ULTRAFILTERS
23
membrane can be used for ultra-filtration under pressure. According to ZSIGMONDY, the periphery in the bottom of the Büchner funnel must be first wetted with rubber solution, after which the disc-shaped ultrafilter is inserted ; collodion does not stick well to porcelain. Discs of filter paper impregnated with collodion and dried to a certain extent can be used as membranes. The ultrafiltration can be promoted by stirring the solution, by warming it with a jacketed filter funnel, or by introducing a heating device into the colloid. It is interesting that in certain cases high pressure only produces blocking of the pores ; this happens when the particles are strongly adsorbed by the walls of the capillaries or when the particles are solid. If they 4 are liquid or soft as it is in the case of emulsions or lecithin sols, particles even larger than the pore size can thus be forced through the filter. Some particles which usually are retained by an ultrafilter can be made to pass by the addition of certain stabilising agents which counteract the adsorption. For instance, fine carbon, which is retained by hard filter paper, slips through after the addition of certain protein solutions. This shows clearly that filtration and ultrafiltration are not quite the same as separation by a sieve.
Most convenient for simple ultrafiltration experiments are nonwaterproofed ' Cellophane ' membranes and coloured colloids, like silver sol or arsenious sulphide colloid. The coloured particles are completely retained on these membranes. By means of moderately dried collodion membranes, however, it is possible to ascertain that the particles of silver are smaller than those of arsenious sulphide, since the former pass through such filters while the latter are retained. The pore size of BECHHOLD'S ultrafilters may be determined by the
rate of penetration of water or by forcing air through the wetted membrane.
ZSIGMONDY determined the pore size of his ultrafilters by the
filtration of carefully prepared gold sols whose particle size had been determined by means of an ultramicroscope. Such standardised ultrafilters have been used for the estimation of the particle sizes of other colloids. Substances like dextrins, with a molecular weight about 5 0 0 0 - 8 0 0 0 , penetrate ultrafilters but slowly. They are called semicolloids. ELFORD investigated the virus proteins
( l )a
by means of ultrafiltration.
He used collodion membranes of fairly large pore size, and found that only in the case of coarse ultrafilters was the diameter (/?) of particles which just passed through equal to the diameter of the capillaries (d). For an ultrafilter with large pores (d = 0 O 0 1 mm) the ratio p\d=\. The finer the pores the smaller is this ratio. TABLE 8
(LA
Ratio p\d
Pore size (diameter)
0.33-0.50 0.50-0.75 0.75-1.00
10 m μ- 100 m/χ 100 m/χ- 500 m/x 500 m/x-1000 m/x
> W . F . ELFORD ; / . Pathol. Bacteriol. 34, 505 (1931) ; Proc. Roy. Soc. (London), Β 112, 384 (1933). Κ . SOLLNER; / . Phys. Colloid Chem. 49, 47, 171, 265 (1945). J. FERRY; Chem. Revs. 18, 373 (1936).
24
SIMPLE,
BASIC EXPERIMENTAL
METHODS
Thus, using ultrafilters with very narrow capillaries the particles appear smaller than they actually are because they adsorb on and block the capillaries. Diffusion and dialysis It is remarkably difficult to perform accurate diffusion experiments in solutions. This was early noted by GRAHAM, who proposed to observe diffusion not in liquids but instead in soft jellies. By means of the following simple experiments it is easy to distinguish coloured colloids from coloured micromolecular solutions. A two per cent gelatin sol, prepared by dissolving gelatin in hot water, is distributed between several test tubes ; each is only half full. The tubes then are left for one hour at room temperature without stirring. During this time the gelatin sol sets to a jelly. On to these several jellies are poured the coloured colloids, and for comparison, into other tubes are poured some of the coloured salt solutions, e.g. copper sulphate, cobaltous chloride or nickel nitrate. After one or two days the various tubes are compared, and it is seen that while the salts have penetrated deeply into the jelly the colloids have not diffused at all. If the substances are not coloured, a chemical investigation of the upper layer of the jelly is necessary. This diffusion in jellies is related to ultrafiltration and to dialysis : here the sol is separated from the pure solvent by a semipermeable membrane through which the penetration of a substance is observed. The membrane is a gel with such tiny pores that only micromolecular substances can pass it. If a substance dialyses, i.e. penetrates through the membrane into the pure solvent, its particles must be composed of less than 1000 atoms. Thus, by means of dialysis, too, we can estimate particle sizes. Actually, however, the main goal of dialysis is to free a sol from accompanying electrolytes and other micromolecular impurities. The devices so used are called dialysators. The main part of a dialysator is the membrane. Generally the same types of membranes are used as in ultrafiltration experiments. For instance we can make a bag of non-waterproofed ' cellophane ', fasten it to a tube, pour a sol into the bag and finally place the bag in a container with distilled water. There are now on the market ' cellophane ' tubes which can be cut to different lengths, and these are particularly convenient. For the same purpose defatted animal membranes (e.g. bladders) are sometimes used. Parchment, which was formerly used, is now largely abandoned, since the rate of dialysis through it is very low. Very efficient dialysing membranes, according to Wo. OSTWALD, can be prepared by impregnating extraction socks with collodion. The porous extraction casing is first wetted with warm water and then filled with collodion. From the first casing the collodion is poured into another, and so on. The viscous layer adhering to the walls is now made more uniform by rotating and swinging the sock until the film
DIALYSIS
EXPERIMENTS
25
solidifies. Finally the casing is filled with water in order to prevent further drying of the membrane. If there are no extraction socks on hand, good dialysing bags of collodion can be prepared in the following way. Collodion is poured into a large, wide test tube (diameter 3-6 cm), which must be completely clean and dry; the tube is then emptied, and the remaining layer of collodion, which sticks to the walls, may be distributed evenly by rotating and swinging as in the preceding experiment. After about 5 minutes' rotation, enough of the ether and alcohol have evaporated for the collodion layer to have turned into a soft film which does not stick to the finger. The longer the drying out lasts the smaller will be the pores. The solidification may be stopped when required by pouring water into the tube. We now have a collodion bag adhering to the glass tube. It is removed from the glass by first freeing the upper border with the aid of forceps; a glass rod then is pushed between the film and the wall of the tube, and into the space thus formed water is poured. If the membrane has not been hardened too much by excessive drying, the bag will come out of the tube quite easily. Before using it as a dialysator it must be thoroughly washed in distilled water (submerged for at least one day).
Collodion membranes prepared in this way have a moderately large pore size. They dialyse faster than ' cellophane ' membranes. Chemi4 cally, a cellophane ' membrane is composed of fibrous regenerated cellulose molecules, while collodion is composed of similarly shaped molecules of nitrocellulose. Ferric hydroxide sol is quite a useful substance for dialysis experiments. It is introduced into a collodion bag or cellophane tube, and placed into a container with distilled water (Fig. 4). After several
FIG. 5. Electrodialyser. Ε—electrodes, M— membranes, C—the colloid in the middle compartment. FIG. 4. ' Cellophane ' bag in water ; M—the membrane, S—sol, A—water.
hours of dialysing, hydrochloric acid can readily be detected in the water. The H+ and C I ions, formed through hydrolysis of ferric chloride, penetrate the membrane but the colloidal particles do not. The semicolloids, which possess very tiny particles (diameter 5-10 Â ; molecular weight, if they are molecular colloids, about 4000-10,000), penetrate the coarse membranes slowly, and it is possible from this rate of dialysis to estimate the particle size of these substances (p. 190).
26
SIMPLE,
BASIC
EXPERIMENTAL
METHODS
For more efficient dialysis the water outside the bag must be constantly changed. W a r m water may also be used. Several types of apparatus for this purpose have been proposed, while simple devices for dialysing against flowing distilled water can be easily arranged in any laboratory. Stirring both the sol and water considerably increases the rate of dialysis. Dialysis is promoted also by means of a direct electric current which pulls the micromolecular ions out of the sol. This electrodialysis is carried out in a three compartment apparatus (Fig. 5) ; the middle compartment is separated from each outer compartment by a semipermeable membrane. The sol is poured into the middle compartment ; through the other two cells flows distilled water. The two electrodes, usually platinum gauzes or perforated platinum ( 2) The discs, are inserted into the outer cells close to the m e m b r a n e s . applied electrical potential then pulls all micromolecular ions through the membranes into the water. By means of such electrodialysing it is possible in a short time to liberate colloids from associated micromolecular electrolytes, although the electric current does not affect the dialysis of non-conducting impurities (alcohol, sugar). The current may also cause electrolytic decomposition a n d changes in acidity of ( 2 a) the s o l . Flocculation or coagulation of colloids Coagulation or flocculation is the increase of particle size in a sol, whereby the sol usually becomes turbid, or may even precipitate. There are several ways of causing coagulation. They include the action of electrolytes, radiation, heat, and various sols, although there are minor differences in their various effects. Some sols are very stable towards electrolytes, while others can be flocculated very easily. Let us compare the stability of our silver, ferric hydroxide, arsenious sulphide, albumin and gelatin colloids toward different reagents. F o r that purpose we shall mix 5 ml. each of the sol in test tubes with 5 ml. lTVNaCl solution. In a short time ( i - i minute) the silver sol, the arsenious sulphide a n d the ferric hydroxide colloids have become turbid, whereas there is no change in the sols of albumin and gelatin. After longer periods the first three sols will be more or less completely precipitated, but no change will be observed in the sols of albumin a n d gelatin. The former three are easily precipitated by NaCl, while the two latter sols are resistant to this reagent. Just the opposite is observed in flocculation with alcohol. Again, we add to 5 ml. of each sol 10 ml. alcohol and mix thoroughly. After ( 2)
H . HOLMES ; Laboratory
3rd2 aed. 1934.
Manual of Colloid Chemistry
(Wiley, New York 1928),
( ) R. E . STAUFFER, in WEISSBERGER'S Technique of Organic
Part I (Interscience, New York 1956) p. 65 if.
Chemistry,
vol. Ill,
F L O C C U L A T I O N
E X P E R I M E N T S
27
standing for five minutes the tubes are compared. N o w there is no change in the first three, but the sols of albumin and gelatin have become quite turbid. The colloids like Ag-sol and ferric hydroxide, which are easily flocculated by electrolytes, are lyophobic or hydrophobic. The hydrophilic sols of albumin or gelatin, however, are stable toward electrolytes, but are flocculated by alcohol. The coagulation problem will be thoroughly treated in several later chapters. However, having the three hydrophobic sols on hand we are able to perform quite easily some very instructive experiments. If instead of sodium chloride we use amounts of 0-1 Ν calcium chloride equal to the volume of each sol to flocculate the colloids, we find that silver and arsenious sulphide will coagulate easily, but that ferric hydroxide is unaffected. The latter, however, will be easily flocculated by salts containing such polyvalent anions as sulphate or phosphate. The two former sols, however, are quite stable toward these electrolytes. Silver and arsenious sulphide sols are very sensitive toward polyvalent cations, while ferric hydroxide sol is sensitive toward polyvalent anions. The selectivity depends on the sign of the electrical charge of the particles. The charge, as we know, is one of the stabilising factors. The silver and arsenious sulphide particles are negatively charged, and thus are most easily discharged by polyvalent cations. Ferric hydroxide particles carry a positive charge and are discharged and coagulated by anions, especially if polyvalent. Viscosity The viscosity of a liquid or solution is its resistance to flow. To stir glycerin more energy is needed than to stir water. Glycerin is hence said to be more viscous than water, and similarly water is more viscous than ether. Collodion, solutions of rubber and jellies are all very viscous. Still higher resistance to structural disruption is offered by the solids. It is obvious that liquids whose molecules tend to associate in larger aggregates are more viscous. Such are the liquids with polar molecules, e.g. glycerin, glycol, water, formamide, sulphuric acid, as well as liquids composed of large molecules, such as the long chain molecules of oleic acid. All these liquids also have a high boiling point. The molecules of water, according to DEBYE, are dipoles, i.e. the distribution of the positive and negative charges in the molecule of water is asymmetric. The molecules are V-shaped as illustrated below :
...+ - O ...+ - O ....
28
SIMPLE,
BASIC
EXPERIMENTAL
METHODS
The positive end of the water molecule attracts the negative end of another water molecule, a n d loose aggregates a n d chains are formed. This phenomenon of mutual interaction a n d association in water, organic acids a n d similar substances can be interpreted also in terms of the so-called hydrogen bonds. The latter are weak chemical bonds between hydrogen atoms or protons a n d a strongly electronegative atom like oxygen or nitrogen. According t o this concept liquid water has the following structure : ...Hx Ο ...Η, Ο ...Η Η
Ο . . . Η ^
Η /
Since the other Η also may form bonds with the oxygen atoms of adjacent molecules, the actual structure is even more complicated than that shown. Such hydrogen bonds are also formed in glycerin a n d in concentrated sulphuric acid, which is why these are so viscous a n d have relatively high boiling points. T h e attractive forces in non-polar liquids, however, are weak, a n d hence the viscosities a n d boiling points are low (e.g. carbon disulphide, hexane a n d benzene). The viscosity of a viscous, polar solvent will be still further increased if a polar solute is dissolved in i t ; this increase will be very high if long, fibrous, polar molecules are introduced (see p . 153). However, liquids of low viscosity (e.g. hexane, octane, ether) also show an enhanced viscosity after dissolving a fibrous colloid. T h e very viscous solutions of rubber in low viscous hydrocarbons are the best examples of this. The viscosity of a liquid or solution can be measured precisely either by moving a solid body through the liquid or by allowing the liquid itself t o flow through a capillary. T h e second method is the simpler, a n d is the most commonly used device in colloid chemistry. The viscometer of Wi. OSTWALD was invented
in Riga (Latvia) some seventy years ago, a n d FIG. 6. Capillary viscometer after WILHELM OSTWALD. M x—upper mark, M 2—lower mark, Κ—the capillary.
VISCOSITY
29
MEASUREMENTS
is now used throughout the world. The viscometer is a U-shaped tube which includes a capillary (Fig. 6). The viscosity is determined by measuring the flow time of a definite volume of a liquid through this capillary. For instance, the flowing time of 5Ό ml. of a sol is compared with the flowing time of 5-0 ml. water. The relative viscosity is the ratio of the flowing time of the solution to the flowing time of the solvent. If 5 ml. of a gelatin sol flows through in tx seconds, and 5 ml. of water in / seconds, the relative viscosity, ^ r ei , of the sol is given by 1
^rel = - ·
Since temperature has a pronounced influence on viscosity, all measurements are carried out in a thermostat at constant temperature. In exact calculations the density of the solutions must be considered. Another common term is specific viscosity ; this is the rise in viscosity of the solvent produced by the dissolved substance. The specific viscosity ^ s p = ^ r e i - l , or
We can now try to measure the viscosities of the colloids we have prepared. For this we need a suitable viscometer, a thermostat, and a stopwatch. The viscometer must be quite clean and dry. It is fastened vertically in the thermostat. 5Ό ml. of distilled water is now introduced into the wider tube, and the water pulled (by applied suction) over the mark of the upper bulb ; by releasing the suction the water is allowed to flow back through the capillary. The knob of the stopwatch is pressed at the moment the level passes the upper mark ; this is the beginning of the flow time. When the level passes the second (lower) mark, the watch is stopped Several readings of the flow time should be obtained, and the average may then be calculated in the usual way. Thus, the value for t is obtained. Measuring in the same way the flow time of the colloids, we obtain tx. From these figures the relative and specific viscosities can be calculated. In Table 9 the results are presented. TABLE 9.
The viscosity of some colloids at 25° C.
Ferric hydroxide sol, 0-5% tx in sec. *7rel *?sp
80-8 1010 0010
Albumin 0-5% 1-0%
t = 8 0 0 sec.
Glycogen 0-5% 1-0%
Gelatin 0-5% 1-0%
83-3 86-4 110-2 82-4 84-6 1-377 1 040 1 080 1 030 1 057 0080 0-377 0030 0057 0040
156-6 1-957 0-957
Ferric hydroxide evidently has the lowest viscosity, and approximately the same figures can be obtained for sols of silver and arsenious sulphide. The hydrophobic sols thus have a very low viscosity. The viscosities of the three hydrophilic colloids differ appreciably : albumin and glycogen have a much lower viscosity than gelatin. The reason
30
SIMPLE,
BASIC
EXPERIMENTAL
METHODS
lies in the differences in particle shape: albumin and glycogen are spherocolloids, but gelatin has fibrous particles. Differences in the viscosities of our spherocolloids, too, are quite easy to explain. The particles of all our inorganic colloids are evidently very compact ; even the ferric hydroxide, despite the polar O H groups, cannot be much hydrated. The albumin molecules are not so compact, and are somewhat more hydrated. The globular molecules of glycogen are still more loosely built, and the solutions have a higher viscosity than the solutions of albumin. It is noteworthy that among the linear colloids gelatin shows only a moderate viscosity. A much higher viscosity is found in 0-5% solutions of rubber, which even in such concentrations form jellies. The specific viscosities of 0 - 1 % solutions of different nitrocellulose fractions are 0-6-26. The above facts indicate that the chief reason for the high viscosities of colloids is the fibrous shape of the particles ; solvation or hydration has a much smaller effect. Some optical properties of sols It is noteworthy that many substances appear quite highly coloured if their particles are of colloidal dimensions. Thus, silver ions are colourless, precipitated silver is grey, but silver colloids have intense red-brown or greenish-brown colours. The strong colour of colloidal silver is changed and reduced quickly upon coagulation with a few drops of sodium chloride. Similar results are observed with gold: diluted solutions of gold chloride or chlorauric acid H [ A u C l 4 ] are slightly yellow, while in the reduction of this substance (e.g. with alcohol) a deep red or violet gold sol is formed. In flocculation with some electrolytes the colour is first changed to blue, then turbidity sets in and a greyish brown precipitate is finally formed. Very characteristic and interesting are the ' colours ' of colourless colloids. Every chemist knows that in the precipitation of very dilute solutions of halides (for instance, KCl, NaBr, KI) with silver ions a very fine, milky turbidity appears; looking through the solution it appears orange but on looking on it from the side it seems to be bluish. This phenomenon is called opalescence, and is explained as follows : the short waves of light (producing the sensations of blue and violet) are strongly scattered by the particles, whereas the long waves (yellow, orange, red) pass undisturbed through the sol. Opalescence depends mainly on particle size. Our albumin or gelatin sols contain very small particles, too small to produce a distinct opalescence. But if the degree of dispersion is decreased (the particle size increased) by gradual addition of alcohol or acetone, the opalescence will suddenly appear. Many colloidal solutions, for example our silver and ferric hydroxide
THE
FARADAY-TYNDALL CONE
31
sols, are completely clear. (The colour of the particles here overshadows the opalescence.) But if a sharp, intense beam of light passes through such clear sols, the path appears turbid. The best source of
FIG. 7. Tyndall cone in colloids. L—the light source, S—the sol.
light for such experiments is the projection illuminator which produces a sharp, conical beam. Observed from the side the path of the light through the sol has the shape of a cone. That is the Faraday-Tyndall cone. The reason for this phenomenon is the same as in opalescence : the light is scattered by the tiny colloidal particles. The phenomenon can be observed in a dark room in which a sharp ray of sunlight enters through a slit; the path is visible, because dust particles reflect a n d scatter the light, enabling us t o observe them. If the Faraday-Tyndall cone is observed under a microscope on a dark background, it is ' resolved ' into separate, bright particles. That is the principle of the ultra-microscope. T h e bright, coloured, rapidly moving discs we observe in the ultra-microscope are caused by the colloidal particles, all acting as separate scattering centres. The lyophilic linear colloids show a weak Tyndall cone which is usually n o t resolved in the ultra-microscope into particles. This indicates that the fibrous particles, mostly those of organic molecular colloids, scatter the light comparatively little. The scattering depends on the difference in refractive indices between the solvent and particles, as well as on the structure of the particles, their size, and their solvation. (See Chapter 6.) Concentration and density of sols The concentration of most of the colloids is small. Metal sols usually contain 0 Τ - 0 · 5 % metal, the sols of hydroxides and sulphides 1 - 5 % of the solid. T h e concentration of the solutions of fibrous molecular colloids, t o o , is usually only about 0 - 2 - 1 % . More concentrated sols of fibrous substances set easily. It is possible, however, to prepare quite concentrated sols of hydrophilic spherocolloids, for instance 1 0 - 1 5 % sols of albumin or casein. Concentrated sols of ferric hydroxide or arsenious sulphide are also known. F o r instance, BOUTARIC a n d VUILLAUME ( 1 9 2 4 ) prepared a n arsenious sul-
phide sol which contained 3 0 0 grams A s 2 S 3 per litre. However, since the sulphide particles are very large compared with micromolecular units, the molar concentration, even of this concentrated sol, is very low. FREUNDLICH, indeed, computed the molar concentrations by assuming that the spherical particles have a diameter of 1 0 0 m/x. Such
32
SIMPLE,
BASIC
EXPERIMENTAL
METHODS 8
particles have a ' molecular weight ' of 8-6 . 1 0 , while the molar con5 centration of the 3 0 % arsenious sulphide sol is only 3 - 5 . 1 0 ~ mol. per litre. The density of sols, according to CHOLODNY ( 1 9 0 3 ) increases linearly with the concentration. If p s denotes the density of a sol, p the density of the dispersed substance, and p 0 the density of the solvent, and if c is the number of grams of the dispersed substance in one ml., then the relation between these is expressed by the following equation :
By means of this equation it is possible to calculate the density of the particles. In a number of cases (Ag, Se, Z n O sols) the density of the particles is the same as that of the substance in bulk.