Studies on the Effect of Chemisorbed Impurities 
on Heterogeneous Nucleation 
Seymour J. BrrsTern 
Geophysics Research Directorate, Air Force Cambridge Research Center, 
Bedford, Massachusetts 
Abstract—Studies have been made on the effects of the chemisorption of ethyl 
amine in lead iodide insofar as they affect the nucleation of supercooled water. It 
has been found that concentrations on the order of one part per two million of ethyl 
amine in the carrier gas will decrease the temperature of nucleation by seven degrees. 
Higher concentrations cause an even greater temperature change. The implications of 
these results in terms of atmospheric phenomena are discussed. 
Introduction—Previous work on the inhibition 
of ice nucleation by ethyl amine has shown that 
in a dynamic system it is possible to inhibit lead 
iodide nucleation of supercooled water by chemi- 
sorbing ethyl amine on the surface of the lead 
iodide [Birstein, 1957]. These results showed that 
it is possible to lower the temperature of water 
nucleation by 17° by passing the lead iodide nu- 
clei over a saturator filled with ethyl amine held 
to a vapor pressure of approximately 0.2 mm 
at a standard flow rate and nuclei generator 
setting. Higher vapor pressures of the amine 
gave greater degrees of inhibition of the nu- 
cleation process (Fig. 1). 
Because this work was done in a dynamic 
system the question arose concerning whether 
equilibrium conditions were reached between the 
nuclei and the nuclei inhibitor. The following 
series of experiments were run to study the re- 
action under equilibrium conditions to determine 
whether it is possible to inhibit nucleation of wa- 
ter droplets with traces of impurity approaching 
that of trace contaminants in the atmosphere. 
Experimental—The apparatus used in this 
work consisted of our standard nuclei generator 
(Fig. 2) described in previous publications 
|Birstein and Anderson, 1955], connected to an 
especially designed cryostat (Fig. 3). The eryo- 
stat was essentially a block of lead in which was 
imbedded a copper coil. Liquid nitrogen was 
circulated through the coil to bring it to a de- 
sired temperature. The block was insulated with 
styrofoam. A Tag controller-indicator regulated 
the flow of liquid nitrogen through the coil. The 
‘Tag, in turn, was controlled by a thermocouple 
set within the cryostat. Two glass tubes were 
located in the cryostat. One tube was filled with 
247 
ethyl amine and held to a given temperature 
which could be interpreted in terms of the vapor 
pressure of the amine at that temperature. The 
second tube was the control and, therefore, was 
empty. These tubes were connected to flasks lo- 
cated outside of the eryostat and the flasks, im 
turn, were connected to the nuclei generator 
through a three way stopcock. All ground glass 
joints and stopcocks were ungreased to prevent 
contamination by the extremely reactive ethyl 
amine. 
In making a run, the Tag was set for the de- 
sired cryostat temperature and the liquid nitro- 
gen flow was started. When the ethyl amine was 
at the set temperature the nuclei generator was 
started and nuclei samples prepared in a nitro- 
gen atmosphere were collected in both flasks. 
The flasks were then opened to the thermo- 
statted tubes in the cryostat and the system was 
allowed to come to equilibrium. Preliminary ex- 
periments to determine the equilibrium time 
showed that fifteen minutes was sufficient. A 
nuclei sample was then removed from the control 
flask and injected into the cold chamber to de- 
termine whether nuclei were present in the sys- 
tem. This was next repeated with the nuclei 
sample in the flask exposed to the ethyl amine 
vapor. The cold box temperature was lowered 
and nuclei injection was repeated until the 
temperature was reached at which ice crystals 
were first formed with the lead iodide sample 
treated with the nuclei ‘poison.’ These experi- 
ments were repeated until the nucleation tem- 
perature for the lead iodide had been obtained 
after exposure to ethyl amine vapor at pressures 
corresponding to the amine vapor pressure be- 
tween —75° and —171°. 
