GEL-GROWTH OF SILVER ACETATE CRYSTALS
291
acid. The less concentrated solutions gelled in the probably heterogeneously; growth of existing nuclei is
shorter times. For each different gel, 40cm3 of AgNO3 preferential to the formation of additional ones.
solution of 1, 2, 3, 4 and 5 molarity was added to
The crystals were identified to be silver acetate in
provide twenty-five different crystallizing conditions. two experiments: Wright and Blitzer, using chemical
Crystallization was continued at 25 ± 2 ° C for ap- analysis12), showed the presence of acetate ion in the
proximately 200 hr without disturbance of the AgNO3 crystals. Proof of identity was completed by X-ray dif-
solution. The chemical reaction in the gel was:
fraction powder pattern of pulverized crystals: A
AgNO3+ CH3COOH CH3COOAg , ~ + HNO3.
—
‘
positive correlation between the d-spacings of the pow-
The crystals were white, translucent and acicular. dered sample and published spacings’3) for silver ace-
Many grew to a length of 5 cm, a width of 3 mm and tate was observed. Additional X-ray analysis by R. S.
thickness of 1—2 mm. These crystals were easily gather- McEwen revealed the gel-growth crystals to have a
ed by either placing the gel in a vacuum and drying to monoclinic structure.
a powder or by vertically sectioning the gel and remov-
ing the crystals with tweezers.
In rotating crystal photographs when the long axis
of the crystal was aligned with the rotation axis a layer
Some general observations on growth evolved from line pattern was obtained.The repeat distance along the
a comparison of the results from the twenty-five dif- growth vector is 5.6 A, a value twice a published d-
ferent conditions of preparation. On adding the AgNO3 spacing of 2.76 A 1 3 ) . Identification of the growth di-
solution, prolific nucleation and rapid growth occurred rection was not pursued further.
in the gel at the interface with the solution. However,
The acicular crystals could be split readily with a
only a few of the initially nucleated crystals continued pointed instrument into micron or smaller filamentary
to grow after 24 hr to form an acicular morphology, fragments. The resulting small fiberscould be deformed
There was a variation in growth rate among the various elasticallytoanexceedingly smallradius ofcurvature, to
gels but no general trends could be attributed to acid- suggest a highdegree of perfection. A 10 pm diameter
ity. The highest rate occurred during the first five hours, fiber was bent to a radius of 0.04 mm. The extreme
an expected result, since the diffusion distance for fiber strain corresponding to this curvature is 0.125. If
AgNO3 was a minimum at the start of growth. Aside weassume a modulus of i0 ~psi, computation shows the
from elapsed time, the concentration of AgNO3 in the strength in bending to be 1.2 x 106 psi. Superstrength
range studied was the most important factor in growth acicular crystals are of interest in high strength fiber
rate. The initial rate was greater and the final crystals composites. Crystals of the structure of silver acetate
longer for the higher concentrations of silver ions. For can have interesting non-linear optical characteristics.
example, after 150 hr crystals from 1 M AgNO3 were
2.5—5 cm in length; from 5 M solution, 6.5—8 cm in Acknowledgments
length, an approximate two-fold increase.
The authors are pleased to thank J. P. Wright and
In another experiment growth occurred unexpectedly L. D. Blitzer for the acetate ion determination, and
in the AgNO3 solution atop the gel. In this instance R. S. McEwen for identifying the crystal system.
the gel was prepared by mixing two liters of 0.8 M
acetic acid with two liters of sodium silicate solution of References
1.06 specific gravity in a 7 in. x 9 in. x 13 in. glass jar.
After gelation, a liter of 0.1 M silver nitrate was flowed
over the gel. In seventeen hours a few silver acetate
crystals, 3 cm long, had grown in the silver nitrate solu-
tion near to and above the interface.
1) R. E. Liesegang, Z. Physik. Chem. 88 (1914) 1.
2) S. C. Bradford, in: Colloid Chemistry, Vol. I, Ed. J. Aiexan-
der, (Chemical Catalogue Co., New York, 1926) p. 790.
3) H. N. Holmes, m: Colloid Chemistry, Vol. I, Ed. J. Alexan-
der, (Chemical Ca$alogue Co., New York, 1926) p. 796.
4) H. K. Henisch, J. Dennis and J. I. Hanoka, J. Phys. Chem.
It is proposed that with so dilute a AgNO3 solution
Solids 26 (1965) 493.
.
.
5) H. K. Henisch, J. I. Hanoka andJ. Dennis, J. Electrochem.
Soc. 112 (1965) 627.
diffusion is principally by the acetic acid upward from
the gel. Moreover, due to the dilution in the solution
and in the gel, diffusion is slow and supersaturation
small. Under these conditions only a few nuclei form,
6) J. J. O’Connor, M. A. DiPietro, A. F. Armington and B.
Rubin, Nature 212 (1966) 68.
7) J. J. 0 Connor and A. F. Armington, J. Crystal Growth I
(i967) 327.
Boulin, D.
