T H E OPTICAL INACTIVITY O F T H E ACTIVE SUGARS I N T H E ADSORBED STATE:-A CONTRIBUTION TO T H E CHEMICAL THEORY OF ADSORPTION. I BY SHANTI SWARUP BHATNAGAR AND DASHARATH LAL SHRIVASTAVA
There are at present three well-known theories regarding the mechanism of protective action of colloids: ‘
( I ) That the protecting agent concentrates a t the interface of the colloid particles and the dispersion medium according to Gibbs-Thomson law, and forms a mechanical envelope round them, thus protecting them from coalescence. Familiar examples of this kind of action are emulsions prepared from oil and water with the aid of soaps as emulsifying agents. ( 2 ) . That the protecting agent on adsorption by the colloidal particles increases the density of the charge on them and thus confers greater stability. Protection by similarly charged colloids fall in this category.
(3). That on account of the chemical affinity the protecting agent and the colloid particles join hands as it were and acquire greater stability. Zsigmondy’ has given a very beautiful example of this in the case of gelatin adsorbed by gold foil. The fact that sugars have protective action is well-known. They are generally used as peptising agents2 and it is therefore interesting to know as t o which of these theories can explain the mechanism of the protective action of the sugars satisfactorily. ‘A priori’ it is clear that the application of the second theory is not probable as sugars in aqueous solutions do not exist as charged particles, and thus the choice would fall between the first and the third theories. If the protection is a purely physical phenomenon then the optical rotation of the solution mixed with sugar would be as follows :Let the original rotation of the sugar solution be @,and further let ‘a’ C.C. of this be diluted with h a ’ C.C. of a collodial solution where n is a positive integer. Then on Beer’s law,
the new rotation
@’=--P
X
(1)
n+I where x denotes the rotation corresponding to the decrease in the effective length ‘of the polarimetric tube, due to the presence of suspended particles; z is a factor which is determined experimentally as described later on. 1 2
“The Chemistry of Colloids, 1 1 2 (1917). Bancroft: Second, Colloid Chem. Rep. Brit. Assn. 1919, 2.
OPTICAL INACTIVITY OF ADSORBED SUG.4RS
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If however, chemical forces as postulated by Zsigmondy and Lanpmuir are operating, the optical rotation of the system may undergo anomalous change. It was with a view to obtain information regarding this point that the present investigation was undertaken. Incidentally it has thrown much interesting light on the mechanism of adsorption as would be apparent from the discussion of the results. Experimental The colloidal solutions used were those of arsenic and antimony hydrosulphides. The former sol was prepared by dissolving Kahlbaum’s pure arsenious oxide in distilled water and then by passing hydrogen sulphide gas for sometime. The excess of the gas was removed by bubbling in a rapid current of hydrogen gas through the solution. Similarly antimony hydrosulphide sol was prepared by dissolving Merck’s potassium antimony tartrate in distilled water and by passing sulphuretted hydrogen and getting rid of the excess as described above. The former sol was marked A/I and the latter B/I. The quantity of the equivalent amounts of As203 and Sbz 0 3 in A / I and B/I respectively were determined volumetrically by the Kessler’s methodl Four samples marked A/I, A/2, A/3, A/4 were prepared from A/I by diluting it with water; the ratio of sol to water being respectively I :o, I : I , I : Z , 1:3. Similarly different samples B / I , B/2, B/3, B/4 were prepared from B/I sol. All these were preserved in well-stoppered bottles. Optically active sugars used were Merck’s sucrose, glucose, and galactose. Solutions of these were prepared by dissolving them in distilled water. The optical rotations were measured by the Tripartite field, International Sugar Scale Polarimeter manufactured by Adam Hilger Ltd. The tubes used were the water-jacketed 50 mm. and 2 2 0 mm. ones. The monochromatic light used was the arc lamp with a potassium bichromate filter, the sodium light being found too feeble for these experiments. First of all, the specific optical rotations of the three sugars for the particular light used were determined. After this a solution of sugar to be experimented upon was prepared and its rotation measured. Then a sample of the colloidal solution was taken and three dilutions of the sugar solution were made in which sugar and colloidal solutions were in the ratio of I:I, I : Z , and I :3. I n order that the volumes of the two taken out for the dilution may be the same, the same pipette was used for measuring the volume of the liquids, special care being taken in washing and rinsing the pipette when taking in any liquid. Such dilutions as above were made in the case of all the samples. 1
Pogg. Ann. 118, 17 (1863).
SHANTI SWARUP BHATNAGAR AND DASHARATH LAL SHRIVASTAVA
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But before doing regular experiments, attempts were made to determine the factor z in equation ( I ) . Two series of experiments were performed. ( I ) I n the water-jacketed polarimeter tube there is an opening through the water jacket, in the middle, so that any liquid to be examined can be poured into the tube without opening it a t any one of the ends. Advantage was taken of this to put in small crystals of aluminium chloride into the tube containing the mixture of the solution and sugar. The addition of AICls resulted in the settling down of the particles. The reading in the polarimeter was again taken, and some of the results are given below in Table I.
The mixture of the sol and sugar solution was coagulated outside and the rotation due to the supernatant liquid was examined. The results of these also are given below in Table I, column 3. (2)
TABLE I All the values of rotation given in this table are positive. I
2
3
Rotation of the mixture of the sol and sugar solution.
Rotation after the coagulation of the solution in the tube.
Rotation of the supernatant liquid obtained by coagulating the sol particles outside the tube.
0.41 0.58 I .66 I .83
0.84 0.41 0.58 I .66 I .83
0.84 0.41 0.58
2.51
2.51
2.51
0.84
.66 I .83 I
From the above table it is clear that the effect of the presence of the particles on rotation are, if any, undetectable i.e. the factor z is negligible. Therefore the equation becomes
P p’= n+I
After this the rotations of the various mixtures of sols and sugars prepared as described above, were examined and the results are entered in Tables IIV~TT.
OPTICAL INACTIVITY OF ADSORBED SUGARS
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