T H E HYDRATION O F COLLOIDAL SULPHUR.

BY T. R. BOLAM AND A. K. M. TRIVEDI.

Received 20th January, 1942.

The behaviour of sulphur sols of the OdCn type is usually considered to indicate that the colloidal particles are in some manner “ hydrated.” 1s 2* 39 l2 In the present investigation we have attempted to obtain direct quantitative information of such hydration by measure- ments of the distribution of certain reference substances in suitable systems containing dispersed or coagulated sulphur. The principle employed has been applied to the determination of the hydration of the colloidal units in soap solutions, but does not appear to have been used previously for any inorganic colloid.

and protein

ExRerimental. Preparation of Sols.-The colloidal sulphur was prepared by Raffo’s

method, i. e., by the interaction of sodium thiosulphate and concentrated sulphuric acid. In all cases the initial procedure of Bolam and Muir f

was followed, but coagulation with NaCl was repeated until acid was completely eliminated. To obtain a high concentration of sulphur, each sol was prepared by peptising several batches of acid-free sulphur in one portion of water. In many cases the sols were used without further treatment, and since such sols are stabilised by sodium polythionate, and contain sodium chloride as the only other electrolyte, they may be called “ sodium sols.” A number of experiments were performed with “ lithium sols,” prepared from sodium sols by repeated coagulation with LiC1.

Analysis of Sols.-Chloride was determined by Mohr’s method. A known volume of sol was gently boiled for half-an-hour with 0.5 N HNO,, whereby the sulphur was completely coagulated, and the polythionate, both bound (ie., adsorbed on the sulphur particles) and free (i.e., present in the intermicellar liquid), completely decomposed. The acid was neu- tralised with solid CaC03,7 the solution filtered, the residue washed with warm water, and the cold filtrate and washings titrated with 0-1 N AgNO,.

To determine the sulphur content, the coagulum obtained from a known volume of sol, by treatment with HNO,, was collected in a Gooch crucible, washed with water, and then repeatedly melted at 1 2 0 O until constant weight was obtained.

Bound polythionate was determined as -4g,S by Bassett and Durrant’s method,3 as slightly modified by Bolam and Muir.s Free polythionate was estimated as BaSO, in the following way. After removal of the bound polythionate by coagulation with KC1, the liquid was treated with AgNO,

1 Hatschek, Kolloid-Z., 1912, I I , 280 ; T h e Viscosity of Liquids, 1928, 199. Freundlich, N e w Concel!x!ions of Colloid Chemistry, 1926, 88 ; Kapillarchemie, 1933 11, 386. Dorfman, Kolloid-Z., 1928, 46, 186, 198 ; 1930, 52, 66. Dorfman and &erbaCewa, ibid., 1930, 52, 289.

a Freundlich and Scholz, Kolloid-Beih., 1922, 16. 234, 267.

4 McBain and Bowden, ibid., 1923, 123, 2417. 5 Greenberg and Gunther, J . Biol. Chem., 1929-30, S5, 491 ; Greenberg and

7 Mellor and Thompson, A Treatise on Quantitative Inorganic Analysis , 1938, 65.

Bassett and Durrant, J.C.S., 1931, 2919.

Greenberg, ibid., 1931-32, 94, 373. Bolam and Muir, J.C.S., 1933, 1022 ; 1934, 754.

140

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T. R. BOLAM AND A. K. M. TRIVEDI 141

and HNO,, the Ag2S filtered off, and the sulphate in the filtrate precipi- tated as barium sulphate.

As a check on the analytical procedure, in a number of instances the following determinations were made with the same sample of sol : (a) total nTa as Na,SO,, ( b ) chloride, (c) total polythionate as BaSO,. It was found that the total polythionate as given by the difference (a)-@) was always close to the value obtained under (c). Moreover, parallel estimations of polythionate as BaSO, and as Ag,S gave good agreement provided 0.2 N HNO, was used for the decomposition. Since Bassett and Durrant state that the concentration of acid may be varied between the limits 0.13 N and 0.2 N, it might appear that the optimum concentration should be 0.16 N. However, our experience showed that the quantity of Ag,S obtained under these conditions is about 50 too high.

Distribution of Raffinose in Membrane Equilibria.-About 50 ml. of a sodium or lithium sol, containing a suitable amount of raffinose (C,,H,,016. 5H,O, supplied by Hopkin and Williams), was placed in a collodion bag and kept suspended in water for one week, by which time equilibrium was undoubtedly established. Care was taken to avoid evaporation, and to protect the sol from any action of light. The apparatus was placed in an air-thermostat maintained a t 25 f 0.05' by electrical regulation. The bags were prepared by coating the internal surface of a test-tube with an alcohol-ether solution of collodion. Although a 10 yo

TABLE DISTRIBUTION OF RAFFINOSE IN MEMBRANE EQUILIBRIA. - Expt.

I 2

3 4 5

-

sol.

Lithium 9 ,

Sodium ,, 3 ,

S1.

81-54 67.82

82.40

80.80 122'12

3-10 14.76

I 4.86 3'73

16.07

19.89 2 j .89

31.01

42-08 28-43

aa .

20.41 26-30

31.91 30'74 43.58

D.

1-19 1.23 Mean 1.07 0.89 0.96 Mean

V.

1.60 1-55 1-58 1.78 2-14 1-98

1'97

solution was employed and the liquid allowed to drain very slowly, it was not found possible to obtain a membrane which retained the whole of the colloidal sulphur. Moreover, repeated changes of the outside water decreased the amount of colloid within the bag to an undesirable extent, without giving an external solution free from sulphur. However, the con- centration of sulphur inside the bag was always much greater than outside. The osmotic pressure developed was very small, and no attempt was made to measure it. Some formation of coarse sulphur occurred, but this had all sedimented by the time the liquids were ready for sampling.

Equilibrium having been established, equal volumes of the two sols were completely coagulated by the addition of equal amounts of solid barium chloride, and the rotations ( a=) of the perfectly clear supernatant liquids determined. A Schmidt and Hansch polarimeter was used, and the liquids were examined in a tube, 2 decimetres in length, and provided with fused ends. The measurements were made at room temperature, which was very constant. The experimental data are given in Table I, where S, and S, are the concentrations of colloidal sulphur inside and outside the bag respectively, and a1 and a2 are the observed rotations for the corresponding supernatant liquids. As elsewhere in the paper, sulphur concentrations are expressed as grams per litre of sol.

Coagulation Experiments with Sucrose and Raffinose as Reference Substances.-The experiments with sucrose were carried out with

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142 THE HYDRATION O F COLLOIDAL SULPHUR

sodium sols, and the chlorides of Li, Na, K and Ba were used as coagulants. Exactly 10 g. of sucrose (ordinary) was placed in each of two 50 ml. graduated flasks, 30 ml. of sol measured into one of the flasks, and 30 ml. of water into the other. After the sugar had been dissolved by heating to 40°, the mixtures were brought t o room temperature, the volume made up to 50 ml. with water, and complete mixing ensured by thorough shaking. The mixtures were then cooled to oo, in order to coagulate the sulphur. A sample of the supernatant liquid from the coagulated sol and a sample of the “com- parison solution ” were brought to room temperature, and their rotations measured. Although the supernatant liquid still contained colloidal sulphur, i t was quite clear, and a satisfactory polarimetric reading was obtained. The concentrations of sulphur in the original sol and in the supernatant liquid were determined. In the course of preliminary experi- ments with sucrose the interesting observation was made that the addition of a sufficiently large amount of the sugar produced coagulation of the sol. In the final experiments the concentration of sucrose was limited to 20 ?& to avoid any complication from this source.

The procedure for LiCl was the same as for NaCl, except that LiCl was added before the volume was made to 50 ml. In the case of KCI and BaCI, the concentration of added salt was made high enough to produce complete coagulation at room temperature, and the supernatant liquid, after the removal of suspended particles of coagulum by filtration, was quite clear and free from sulphur. To keep conditions as uniform as possible, the comparison solution was also filtered.

Co- agulation was brought about by adding LiCl or NaCl and cooling to o’. The procedure was the same as that described for sucrose with LiCl as coagulant. In every case 5 g. of raffinose was present in 50 ml. of diluted sol or comparison solution.

So is the concentration of sulphur in the uncoagulated sol (= concentration in original sol x 30/50), and S is the concentration in the supernatant liquid. Under co and t are given the respective rotations for the comparison solution and the super- natant liquid. In some cases the previously described polarimeter tube was used, and in others a similar tube, I decimetre in length, was employed,

Coagulation Experiments with Chloride and Free Polythionate as Reference Substances .-Sodium sols were used for these experiments and NaCl, HNO,, KNO, and Ba(NO,), were employed as coagulants. In the case of NaC1, a portion of sol, containing a suitable concentration of the salt, was placed in a tightly stoppered Jena bottle and kept at oo for four hours. Most of the somewhat turbid supernatant liquid was decanted into another Jena bottle and allowed to reach room temperature. Portions of the liquid, which was now quite clear, were analysed for sulphur, chloride, and free polythionate. Similar analyses were made on a portion of the original sol kept at room temperature. In one or two experiments the supernatant liquid and a portion of the original sol were brought to 2 5 O in the air-thermostat, but no significant difference in the results was observed.

In the case of HNO,, 25 ml. of sol and 35 ml. of 1.0 N acid were mixed in a tightly stoppered Jena bottle, and heated to 80°, until the whole of the sulphur was coagulated (and the polythionate decomposed). 35 ml. of water was added to another 25 ml. of the sol, and heated under the same conditions. Both liquids were then brought to 2 5 O , the supernatant liquid from the coagulated sol analysed for chloride, and the uncoagulated sol analysed for sulphur and chloride. In the case of KNO, and Ba(NO,), 25 ml. of sol was completely coagulated by the addition of 25 ml. of salt solution of suitable concentration. The coagulated sulphur was separated from the supernatant liquid by filtration, the first portions of the filtrate discarded, and the concentration of chloride in the remainder determined,

The procedure for NaCl was as follows.

Lithium sols were employed in the experiments with raffinose.

The data are shown in Tables I1 and 111.

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T. R. BOLAM AND A. K. M. TRIVEDI I43

1'0371 1.0386

after decomposition of the free polythionate. For comparison, 25 ml. of sol was diluted with 25 ml. water, and the sulphur and chloride determined in the usual way.

So and S are the respective concentrations of sulphur in uncoagulated sol (original or diluted, as the case may be) and supernatant liquid. Under co is given either the AgNO,

The data are shown in Tables I1 and 111.

0.0398 0.0415 Mean

TABLE II.-DISTRIBUTION DATA FOR PEPTISABLE SULPHUR COAGULA.

Expt.

I 2 3 4

5 6

7 8 9

I0

I1 I2

I 3 I 4

I5 16 I 7 18 19 20 21 22

21 22

Salt.

LiCl

NaCl

Concn. (N).

4'8 3'9

3'2

0'2

1'0

0'2

0.2

Reference Substance.

Sucrose

Raffinose

Sucrose

R a ffi n o s e

Chloride

Free poly thiona te

SO.

85-07 86-20 86-20 81.86

73-01 72-65

60.87 02.72 02-78 02.34

73.01 72'65 66-62 67.02

96.37 03-30 99'57 96'35 I 3-80 70.78 23'40 19.60

23-40 19.60

S.

22.76

22.76 10.46

23-56

5-17 5'25

37'64'

13.56 6.60

21-20

0'00 0-00 0.00 0'00

12.79 8-14 5.18

13-52 19.12 2-40

40.58 30.36

40.58 30.36

cg. 1 c.

O rotation

I 2.68 12.79 12-79 12-79

2 I .08 21.08

I 3.28 13.28 13.31 13-28

20.98 20.98 20.98 20.98

13-48

13-67

13-51 13-59

Mean 22-16 22-14 Mean

13-51 14-16 14.24 14-17 Mean 22.07

22-13

Mean

22.10

22.12

ml. AgNO,

20.20

23'45

19-75 17.15 26.35 19-38 22.05 Mean

20.10

g. BaSO,

D .

1.08 1 -21 1'10 1-13 1-13

1-39 1-41 I -40 1.41 1.56 1'39 1.31 1-42 1.48 1'43 - 1-28 1-30

1'37

1-22 1.18 1.23 1-18 1-37 1-25

1.25 1-24

1'21

1.26 1-31 1-29

V .

1-68

1-36

1-34

1-39

1'53

1'47

titre for, or the weight of BaSO, from, a given volume (10-35 ml.) of un- coagulated sol, and under c, the corresponding figure for an equal volume of supernatant liquid.

In the case of each coagulant it was ascertained by actual experiment how far the coagulated sulphur could be repeptised by removing the supernatant liquid and treating the coagulum with water. The coagula

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I44 THE HYDRATION O F COLLOIDAL SULPHUR

13'32 13'33 13-35 13'35

26.69 26.69

produced by the Na and Li salts were readily peptised at room temperature, but the HNO, and the potassium and barium salts gave coagula which showed no tendency to disperse under these conditions. Some peptisation occurred with potassium coagula on heating to So0, provided the treatment was carried out soon after coagulation.

13-96 13'97 14-03 13-95

27.38 28-07

TABLE III.-DISTRIBUTION DATA

23.60 10.80

17-73 14'97

15-83 21-13 39'75

Expt.

I 3

3 4

5 6

7 S

9 I 0

I1 I 2 I3

24'15 11-18

18.23 15-47

16-13 22-28 41-15

Salt.

KCL .

BaC1, .

KNO, .

Ba(N03)2

HNO, .

Concn. (N).

0'20

0'10

0.25

0.10

0.58

Reference Substance.

Sucrose

Sucrose

NaCl .

NaCl .

NaCl .

'OR NON-PEPTISABLE SULPHUR COAGULA.

SO.

86.59 83.16 96-92 81-86

47'87 94'09

54-15 59'24

47-60 60.64

36.28

64.26 00'22

S.

0'00 0'00 0'00 0'00

0'00 0'00

0'00 0'00

0'00 0'00

0'00 0'00 0'00

cg. 1 c.

O rotation

D .

1.89 1.82

I 'go

1-90 1.91

2'00

(2.38) 1'74

1-74 1.88

1'95 1'94 1-89

- V.

1-01 I '04 0.95 1-00

I '00 0.99

(0.80) I '09

I '09 1'01

0.97 0.98 1'01

Discussion. It will be convenient to consider first the data in Tables I1 and 111.

O d h * has shown that coagulation of a sulphur sol is not accompanied by any appreciable change in the total volume of the system. Hence, provided adsorption of chloride, or sugar, by the coagulated sulphur is negligible, the volume occupied by the coagulum (= sulphur + water of hydration) is given by : o = 1000 ( I - c,,/c), and the " apparent density " of the coagulated sulphur by : 13 = (So - (1000 - ~)S/IOOO)V.

It is generally assumed 2 s 3~ * that the sulphur in Raffo sols is in the amorphous condition, and this view is supported by the data obtained with potassium and barium salts and nitric acid. In most cases the value of D lies within a few per cent. of rego, which is identical with the average of the values given by the International Critical Tables for the density of amorphous sulphur. Thus it appears that the sulphur in potassium and barium coagula is not hydrated. When, however, coagulation is brought about by sodium or lithium salts, the value of D is always less than 1-90. The difference in behaviour is perhaps more strikingly shown by comparison of the values obtained for the " specific hydrodynamic volume " lo of the sulphur, i.e., the volume of coagulum

O d h , Nova Acta Ufisala. Series IV, 1913, 3, No. 4. International Critical Tables, 1928, I I I, 21.

l o Kraemer, Treatise of Physical Chemistry, by H. S . Taylor, 1931, 11, 1615- 1616.

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T. R. BOLAM AND A. K. M. TRIVEDI I45

containing unit volume of sulphur, or V = I .~o/D. In the case of lithium and sodium coagula Y is always much greater than unity, showing quite definitely that the sulphur is hydrated.

it was found that although lithium and sodium coagula are readily peptised, coagula- tion by potassium and barium rapidly becomes irreversible. Weiser and Cunningham 12 have examined, by means of the ultramicroscope, the behaviour of lithium and sodium coagula when treated with potassium or barium salt. The clumps of aggregated particles were seen to shrink, and in some cases convection currents were observed, as though water was being released from the coagulum. Weiser and Cunningham con- clude tha t in coagulation by sodium or lithium salts, the particles retain an envelope of water, and hence do not coalesce. When the coagulum is treated with potassium or barium salt, the water film is eliminated, the particles actually unite, and peptisation becomes impossible. It is evident that this view is fully supported by our results, since the coagulum was peptisable only in those cases where the coagulated sulphur was shown to be hydrated.

The following relations were employed in the calculation of the values of D for dispersed sulphur given in Table I. Let vl be the volume occu- pied by the micelles (= sulphur + water of hydration) in 1000 ml. of sol ( I ) . Now the volume of supernatant liquid from 1000 ml. of sol ( I ) = 1000 - (S,/I.~O), since the coagulated sulphur is not hydrated (barium coagulum). Hence, if c, is the concentration of raffinose in the supernatant liquid, and c,

In agreement with the observations of others,llP 8 9 l 2 ~

Then the volume of intermicellar liquid = 1000 - vl.

( I 0 0 0 - v,)c that in the intermicellar liquid, we have : c - Simi-

larly, since the intermicelar concentration of raffinose must be the same - 1000 - (S1/1.go)'

(1000 - V2)C on both sides of the membrane, we have for sol (2) : c - - 1000 - (S2/1-90)'

Also c1/c2 = allot2. Finally, i t may be assumed, as a first approximation, tha t the degree of hydration of the sulphur is the same in ( I ) and (2), so that SJv1 = S2/v2 = D.

The figures indicate a very considerable degree of hydration, and i t is of interest to find that our values for Y are comparable with tha t (1.62) deduced by Hatschek from the viscosity data of O d h 8 In the case of the sodium sols, the total volume of the micelle is about twice that of the sulphur it contains. of water to 1.9 g. of sulphur, or 0.03 mol. of water to I g. of sulphur. &ce the results of numerous determinations showed that in a sodium sol the bound polythionate amounted to about one millieqdivalent per g. of sulphur, it follows that there were 60 mols. of water to I mol. of bound sodium polythionate (probably Na2S606).3 If i t be assumed with Bassett and Durrant that the hydration is actually that of the bound polythionate ions and their compensating sodium ions, and, further, that (as a first approximation) the two ions are equally hydrated, there will be 20 mol. of water per g. ion. In view of the available data for ionic hydration,13 this result is not unreasonable.

Comparison of the values of V for dispersed sulphur with those for coagulated sulphur, with raffinose as reference substance, indicates that

l1 Sobrero and Selmi, T h e Foundations of Colloid Chemistry, by E. Hatschek,

la Weiser and Cunningham, Colloid Symposium Monograph, 1928, VI, 319. l3 Glasstone, The Electrochemistry of Solutions, 1937, Chap. 111.

Thus there is approximately I

1929, 59.

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THE HYDRATION O F COLLOIDAL SULPHUR

coagulation by sodium or lithium salts is accompanied by partial de- hydration of the micelles. With potassiclm salts the dehydration is complete. Since the compensating cations in the coagulum are always of the same nature as those of the coagulating electrolyte,37 169 67 14 our results suggest that the coagulating powers of salts of the alkali metals give the lyotropic sequence K > Na > Li,* because the coagulating power depends upon the ease with which the cation is dehydrated.

The results in Table I appear anomalous, in that the values of V for lithium sols are less than those for sodium sols. I t is possible that the difference is due to a difference in the degree of dispersion of the sulphur. Our determinations showed that the smaller the particles, the greater is the amount of polythionate bound per g. of sulphur. I t may therefore be expected that if the average size of the particles was greater in the lithium than in the sodium sols, the degree of hydration would be less.

It is evident that if the sulphur adsorbs appreciable amounts of chloride or sugar, the true values of Y will be greater than those given in the tables under these reference substances. There is reason to believe, however, that the adsorption of chloride is of very minor im- portance, since the values of V obtained in Experiments 21 and 2 2 (Table II), with chloride as reference substance, are slightly higher than those deduced from determinations of the free polythionate, which is undoubtedly located in the intermicellar liquid. I t also seems likely that the adsorption of sugar is inconsiderable, since the values of V obtained with sucrose in the presence of 0.2 N sodium chloride (Table 11, Expts. 7-10) are only a few per cent. less than those given by free poly- thionate under similar conditions, and a t least part of the difference is probably due to dehydration by the sugar.

The absence of adsorption of chloride in the sols examined by us is in agreement with the results obtained by Bassett and Durrant and by Bolam and Muir under similar conditions. On the other hand, Rinde l5 and Bolam and Bowden l6 found evidence of adsorption in the case of well dialysed sols. At present i t is difficult to explain this dif- ference in behaviour. I t should be pointed out that OdCn’s 8 experi- ments on the thermo-coagulation of undialysed sodium sols do not demonstrate the existence of adsorption of chloride by the sulphur, as is often assumed.179 l5 OdCn actually established that when an approxi- mately monodisperse sodium sol is coagulated in stages by gradual lowering of the temperature, the following relation holds :

y = (A(1oo - S/D)/IOO) + gs, where y = total amount of sodium and S = amount of sulphur in 100 ml. of supernatant liquid at any given stage of the coagulation, A = amount of sodium in the same volume of liquid after complete coagulation, D = apparent density of the coagulated sulphur, and jl = constant. Unaware of the presence of polythionate in his sols, O d h concluded that /3 was the amount of sodium adsorbed, as

1 4 Weiser and Gray, J . Physical Chern., 1935, 39, 1163 ; Bolam and Currie,

Rinde, Phil . Mag., Ser. 7, 1926, I , 32. l6 Bolam and Bowden, J.C.S., 1932, 2684. 1 7 Gustaver, Kolloid-Beih., 1922, 15, 213 ; Svedberg, Colloid Symposium

Monograph, 1923, I , 18 ; Hatschek, loc. c i t . l ; Freundlich, Kupillarchemie, 1933, 11, 392.

J.C.S., 1939, 296.

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T. R. BOLAM AND A. K. M. TRIVEDI I 4 7

sodium chZuride, by unit mass of sulphur. His values for /3 are, however, very similar to those obtained by various workers for the sodium corresponding to the bound polythionate in these sols. Moreover, O d h ’ s data reveal that, for a given sol, the value of /? does not change significantly when A is varied by the addition of sodium chloride. In view of the foregoing, it is clear that by far the greater part of /I, if not the whole, represents bound polythionate.

Summary. I. The micelles in Od6n sulphur sols are hydrated, probably owing to

the presence of bound polythionate ions and the compensating cations. 2. Reversible coagulation (lithium and sodium salts) is accompanied

by partial dehydration, and irreversible coagulation (potassium and barium salts) by complete, dehydration of the micelles.

3. The adsorption of chloride ions by the micelles in undialysed Od6n sols is inappreciable.

We thank the Moray Fund Committee for a grant for the purchase of apparatus.

King’s Buildings , University of Edinburgh.

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