- scheme in the upper Aare. 

5, 
JUNE 16, 1923] 

NATURE 
827 
The Electric Charge of Colloids.! 
By Prof. H. R. Kruyt, University of Utrecht. 
GINCE Hardy’s publication in 1900, the electric 
charge of the particles has been the central 
problem of colloid chemistry. I propose to develop 
this point of view for both suspensoids and emulsoids, 
and indeed in the same manner for both types. 
In 1907, Freundlich propounded his theory, ac- 
cording to which the origin of the electric double 
layer was to be sought in a preferential adsorption 
of one of the ions of the liquid. This theory was 
applicable to colloid particles with regard to the 
external phase, and to capillary electric phenomena 
with regard to the moving liquid. It gave a satis- 
factory explanation of many facts concerning the 
coagulation of suspensoids and of the investigations 
of Perrin and Elissafoff on electro-omosis, of Kruyt 
on streaming potentials, and of Powis on cataphoresis. 
Several problems of colloid chemistry could be 
elucidated by these investigations, e.g. the irregular 
series of flocculation, peptisation, etc. Nevertheless 
this theory, according to which the double layer 
is built up only by ions coming from the external 
liquid, so that the material of the solid wall does 
not take any part in the process, could not explain 
all the facts. Especially the investigations on the 
alkalinesol of SnO,, carried out by pupils of Zsigmondy, 
make it obvious that in this case the inner side of 
the double layer is built up by stannate and not 
by hydroxyl ions. 
_ Thespecial conditions of the atoms at the periphery 
_ of a crystalline particle can account for the formation 
of a double layer, as Fajans has pointed out. For 
example, when a negative sol of AgBr is made from 
solutions of AgNO, and KBr, with a slight excess 
of the latter, the Ag-atoms in the crystal lattice are 
each surrounded by six Br-atoms, whereas an Ag- 
atom at the crystal boundary is connected to five 
only; thus it will attract a Br-ion from the sur- 
rounding liquid towards the vacant place. This ion, 
however, is accompanied by a K-ion, which will 
place itself near the attracted Br-ion. Thus the 
double layer is formed by the special selective 
attraction of the solid phase. This train of thought, 
when slightly modified, holds too for a disperse 
amorphous phase. According to Langmuir and 
Harkins, the molecules at a phase interface are 
oriented with their electrically polar parts towards 
the water; therefore the conditions are similar to 
those at the surface of a crystal lattice. 
Has the electric charge in the case of lyophilic 
colloids, like the proteins, the same capillary electric 
character as in that of the suspensoids? If not 
(and most physiologists consider it so), colloid 
chemistry is on the wrong track. The behaviour 
of proteins is often explained as if they gave real 
solutions, electrolytically dissociated as amphoteric 
electrolytes, following Ostwald’s law of dilution. 
Kruyt and De Jong have made investigations on 
1 Synopsis of a lecture delivered at the Universities of London, Edin- 
burgh, and Aberdeen in May 1923. 
Plant Ecology. 
fed “Die Vegetationsverhaltnisse der Grimselgegend 
im Gebiet der zukunftigen Stauseen’’ (Bern, 
Wyss Erben, fr. 8), Dr. Eduard Frey records the 
character of the vegetation of an area which will 
soon be submerged in connexion with a water-power 
The area is of special 
interest in affording a unique opportunity for study- 
ing the colonisation of naked siliceous rock and 
NO. 2798, VOL. 111] 
_ (according to the lyotropic strength). 
the sol of agar, the behaviour of which cannot 
possibly be interpreted in that way, the agar being 
a carbohydrate though giving a typical lyophilic 
colloid. They pointed out that there is a consider- 
able decrease of viscosity when small amounts of 
electrolytes are added, the effect being a function 
only of the valency of the cation, just as is the case 
with suspensoids and capillary electric phenomena. 
This effect is the electro-viscous effect, already 
predicted years ago by Hardy, and thoroughly dis- 
cussed in the late von Smoluchowski’s last paper. 
As the electric charge of the agar particles has, 
without any doubt, just the same character as that 
| of, say, a gold sol, why should a gelatine sol have 
a charge of quite another origin? Investigations in 
collaboration with different pupils (unpublished until 
now for the most part) have convinced me that 
with gelatin, glycogen, casein, starch, gum arabic, 
and even with rubber in benzene, the capillary 
electric phenomena play the principal réle and can 
account for the behaviour, which is often interpreted 
as if we were not dealing with colloids, but with 
electrolytes in realsolution. The influence, especially, 
of neutral salts can now be understood much better. 
The only difference between suspensoids and 
_ emulsoids lies in the fact that the latter are hydrated 
to a large extent, viscosity showing this fact clearly. 
Water bound by hydration acts as a stabilising 
factor, just as the electric charge does. The latter 
can be removed by electrolytes, as mentioned before, 
and the hydration by adding alcohol or acetone. 
When hydration only is removed, there remains a 
suspensoid with all the typical properties of such. 
Salting out a protein is a combination of the removal 
of charge (according to the valency) and of hydration 
Special experi- 
ments with agar have made this obvious. 
Dr. Bungenberg de Jong has pointed out that the 
action of tanning agents, like tannin, is a mere 
dehydration, causing just the same effects as alcohol, 
though by a very different mechanism. 
As a general conclusion, I wish to emphasise the 
view that the electric charge of all colloids has the same 
origin, namely, a capillary electric one. The electric 
charge of suspensoids is their only stabilising factor, 
the emulsoids having a second in their hydration. 
With both, the way in which the double layer is 
built up is not always independent of the material 
from which the particle is made: with a gold sol, 
as well as with an agar sol, the double layer behaves 
in perfect accord with the adsorption theory of 
Freundlich. With the sol of stannic oxide, as well 
as with a protein sol, the ions of the molecules 
situated in the periphery of the particles play an 
important réle in the constitution of the double 
layer. 
The advantage of the train of thought developed 
here lies in the principle of unity according to which 
| colloid-chemistry can be treated. 
/ 
broken soil exposed by the prolonged retreat of the 
glaciers. Dr. Frey describes in detail the physical 
characters of the district and the statics of the 
different plant associations, and also traces the 
succession of plant life from the original colonisation 
of unoccupied rock and debris by lichens and mosges 
to the ultimate condition in which vascular plants 
are mainly concerned. He remarks on the crowding 
