The Original Nature Article

Reprinted with permission from Macmillan Publishers Ltd: Nature, Vol. 127, No. 3211, pg. 741, copyright May 16, 1931.

Coherent Expanded Aerogels and Jellies.

THE continuity of the liquid permeating jellies is
demonstrated by diffusion, syneresis, and ultra-
filtration, and the fact that the liquid may be replaced
by other liquids of very diverse character indicates
clearly that the gel structure may be independent of
the liquid in which it is bathed. Hitherto the attempt.
to remove the liquid by evaporation has resulted in
shrinkage so great that the effect upon the structure
may be profound.

Mr. Charles Learned and I, with the kindly assistance
and advice of Prof. J. W. McBain, undertook to test
the hypothesis that the liquid in a jelly can be
replaced by a gas with little or no shrinkage. Our
efforts have met with complete success.

The procedure that we have adopted is as follows :
The jelly is first formed in a suitable liquid in dilute
form. The liquid is then replaced by another which
does not dissolve the structure and has a reasonably
low critical temperature. Alcohol has proved quite
satisfactory for most of the inorganic gels, ether has
advantages in the case of easily reduced substances,
and propane was used for all of the organic jellies.
In making the replacement, it is necessary that each
liquid used be completely miscible with both that
which precedes and that which follows it. For
example, water may be replaced by alcohol and then
by ether. Mere evaporation would inevitably cause
shrinkage. However, the jelly is placed in a closed
autoclave with an excess of liquid and the temperature
is raised above the critical temperature of the liquid,
while the pressure is maintained at all times at or
above the vapour pressure, so that no evaporation of
liquid can occur and consequently no contraction of
the gel can be brought about by capillary forces at
its surface.

When the critical temperature is passed, the liquid
has been converted directly into a permanent gas
without discontinuity. The jelly has had no way of
‘ knowing ‘ that the liquid within its meshes has
become a gas. All that remains is to allow the gas
to escape, and there is left behind a coherent aerogel
of unchanged volume.

Silica aerogel with a density so low as 0.1 is very
easy to prepare, and we have prepared some with a
density of only 0.02. The silica aerogels are highly
opalescent, although quite transparent ; they display
a glassy fracture and small pieces emit a metallic ring
when dropped.

So far, we have prepared silica, alumina, nickel
tartarate, stannic oxide, tungstic oxide, gelatine,
agar, nitrocellulose, cellulose, and egg albumin aerogels
and see no reason why this list may not be extended
indefinitely. Apart from the scientific significance
of these observations, the new physical properties
developed in the materials are of unusual interest.


College of the Pacific, Stockton, and
Stanford University, California,
April 8.

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