PTL-DIES I S ELECTROCAPILLARITY 1
Part 11. Selective Adsorption in Solutions Containing Two Active Substanres BY CHARLES OCKRETT AND J
The surface tensions of solutions of two or more active substances have not been widely investigated. Morgan and McKirahanl determined a few cases of mixtures of salts of organic acids and found that if the surface tension lowerings produced by the two salts separately were not far removed from water, that of the mixture is nearly equal to their sum. If, however, one of the solutes causes a much greater lowering than the other, the value for the mixture is nearer to the one with the greater lowering, an observation which is supported by the measurements of Strebnitzki.* Szyszkowski3 also found that in a mixture of isovaleric acid and its barium salt, the lowering produced by the former being much greater than that of the latter, the surface tension lowering is to be attributed to the undissociated part of the acid. In experiments with two mixtures of fatty acids he found that the surface tension lowerings were an additive property of those of the components. According to the thermodynamical principles of Gibbs, the surface energy at an interface tends to a minimum. If a substance is present in a solution which lowers its interfacial tension, it will be present at the interface in greater amount than in the bulk of the solution. In a solution containing two active substances we might expect that one which causes the greatest surface tension lowering to be adsorbed to the greatest extent. It is easy to see that under certain circumstances one component may be exclusively adsorbed. Consider a solution of a substance A, which causes a greater surface tension lowering (in equivalent concentrations) than a substance B. If we add B to this solution it is possible that the conditions may be such that the surface energy is not lowered by its adsorption, since this might involve the displacement of some of the more active substance A. Under these conditions we should get the exclusive adsorption of A. The use of the mercury-solution interface instead of the air-solution interface has the advantages that the potential difference between the mercury and the solution may be varied by polarisation and the effect of change of the electrical conditions on the behaviour of the mixture may be observed. 11-e have determined the electrocapillary curves of solutions of a number of mixtures of active substances in ?\I.z sodium sulphate. Their behaviour is summarised below. J. .4m. Chem. Soc., 35, r i j o (1913) J. Russ.Phys. Chern. SOC.,44, 145. 3 Z . physik. Chem., 64, 3 S j (1908).
*
2298
CHARLES OCKREST A S D J. A . V. BUTLER
A. Mixtures of Salts M / z o Sodium cinnamate, 11#'zo sodium trichloracetate, M / z sodium sulphate. M,'zo Sodium cinnamate, hI/zo potassium benzene sulphonate, M / z (2) sodium sulphate (3) ?lI/'zo Sodium cinnamate, 1 1 / 2 0 sodium terephthalate, M / n sodium sulphate. I n these cases the curve of the mixture is identical with that of N / z o sodium cinnamate alone, within the experimental error (Fig. I ) . The surface tension lowering produced by sodium cinnamate is in each case greater than y that produced by the other component and it appears to be exclusively adsorbed. (4) 11,:zo Sodium cinnamate, hI/zo 3 sodium phthalate, M / z sodium sulk 25 3 phate. a ( 5 ) 11,'zo Sodium salicylate, M / 2 0 'I ; n, c.9 '9 _e=',
v
-1
7 0 7 .3
.o
-2
899 4
i96 8 754 9 io7 3
i55.4
.6
-I
20
Mixture (6) Mixture ( 7 ) Potass benzene bodium terephthalsulphonate JI 20 a t e 1I 20 and a n d Sodium trihodiurn trichlorchloracetate 11 20 acetate .\I/zo
11
20
8jY 8 900 i 929 3 941 0 954 5 960 4 962 o 960 2 951 5 945 3 930 0 910 o 88j 2 8j; 8 826 I 790 2
--
1 3 0
I
io4 0 653 3 600 2 541 I
4i6 A
j
11/20
887.5 85j.1 820.3 783. I 742 . o
697.8 649.4
657.4
Lactose
Tetra-methyl ammonium hydroxide
602.I
596'5 538.1 474. I
B
A
Mixture (9) Phenol Xi 20 and Lactose 11 '20
Caffein
829 I 848 8
864.4 898.
Si5
2
911.0
894 910 920 926 927
8
9'6.3 918.3 9'8.3 9'5.7 911.8
3 o I
3 928 Y 927 0 920 o
906 6 88j o 858 5 826 3 i90 7 750 3 704 3 653 3 600 9
hl/1oo
906,o 898'7 889.8 881 , j
866.I 8j2.4
829,I i92 7 i j 2 . 3
707.7
656.7
541
6
601 . i 541 . 8
477
I
476.4
x
-1
STUDIES IN ELECTROZAPILLARITY
2305
TABLA I (Continued) Electrocapillary Curves of Solutions of Organic Substances Mixture ( I O ) Caffein h l / ~ o o and Lactose R l i z o
v
-
+O.I
-0.1 -0.2
-0.3 -0.4 -0.5
-0.6 -0.7
-0.8 -0.9 .o
--I
-1.1 -1.2
-1.3 -1.4 -1.5
-1.6 -1.7 --I
.8 .9
-2
.o
-I
-
752 .o
752.2
475.2
707 .o 656,I 600.8 540,8 476.4
704.8 675'1 597.5 536.8 470 ' 2
A
A
C
887 . 2 879,; 865.6 852.8 826.3 792.2 751.7 706.6 655. I 600.6 540.2
654 . o 600.6 541 ' 1 476. 5
Mixture (13) Mixture (14) Aniline X3/zo Phenol M / z o and and Phenol hl,/20 Sodium benzoate
Mixture (13) Mixture (14) .4niline M/zo Phenol M i 2 0 and Phenol and Sodium benzoate M/20 M izo
v
M 120
i 0 . I 0.0 -0.1
-0.2
-0.3 -0.4 --0. j
-0.6
7 -0.8 -0.9 -0
810.4
800,I
--I
846.0 866.8 881.6 889.2 894.6 896.j 896.3 893.6 889.8 883 7
837.9 864 . o 883 ' 5 898,j 908. 7 914.9 918.6 920.5 917.8 913.8
-1.1
'
M/20
814.5 858.4 889.0 902.8 909.4 913.2 912.4 911.2 906.3 902.3 894.2 887 . o 875.9 859 . o 831 . o 794.2
'
jo4. I
Aniline
8j2 .3 876.9 887.1 901.2 892.8 892.8 890.9 886.9 882.8 875.8 86j.3 856.4 843.3 827.2 793.2
A V
Mixture (12) Caffein hliroo and p-Toluidine (satd.)
827.2 846.6 874.2 891.1 g o o .6 905.4 906 7 906.3 902.9 897 . o
896.9 908.7 915.3 917.1 918.1 91 j . 6 911.4 906,7 899.1 890.0 881 . o 868.3 852.4 823 .o j87.6 750.6
0.0
Mixture (11) Caffein M/roo and Phenol M/zo
.o
-1.2
-1.3
-1.4 -1.5 -1.6 -1.7 -I
.8
-1.9 -2.0
876 . o 864.3 8j0.4 828.0 792.8 751.0 704.0 653.6 597 . o 536,9 470-3
C
908 5 892.8 866.6 833.3 795.9 753.4 707.8 6j5.8 600.2 539.6 473 7 '
A
2306
CHARLES OCKREXT AND J. A. V. BUTLER
summary The electrocapillary curves of a number of solutions containing two active substances have been determined. 2. I t is shown that in mixtures of substances of the same type (ions or non-ionised molecules), if one compound causes a greater surface tension lowering than the other over the whole range of potential differences, it alone is adsorbed. 3 . When the components cause nearly the same surface tension lowerings, both substances are adsorbed from the mixture. 4. When the adsorption curves cut each other, several cases have been found at which one component is exclusively adsorbed at potential differences below the point of intersection and the other at p.ds. above the point of intersection. 5 . Except in the case of a solution containing active positive and negative ions, the surface tension lowerings produced by the mixtures are less than the sum of those produced by the components separately. I n conclusion we wish to express our appreciation of a grant from Imperial Chemical Industries, Ltd. which defrayed the expense of part of the apparatus employed in this investigation. I.
Chemistry Depnrtnienfs, The Cni(;ersifu Edinburgh . February 6, 1930.
