This invention relates to the production of silica gels and in particular to a process for the preparation of silica gels which can be used as a crystal growth medium that simulates the convectionless environment of space to produce structurally perfect crystals. Modern utilizations of substances in electronics, such as radio transmitters and high frequency microphones, often require single crystals with controlled purity and structural perfection. The near convectionless environment of silica gel suppresses nucleation, thereby reducing the competitive nature of crystal growth. This competition limits the size and perfection of the crystal; and it is obviously desirable to suppress nucleation until, ideally, only one crystal grows in a predetermined location. A silica gel is not a completely convectionless environment like outer space, but is the closest known environment to that of outer space that can be created on Earth

Barber, Patrick G.

Simpson, Norman R.

English

NASA Technical Reports Server

United States Patent [I91 p i ]  Patent Number: 4,696,808 
Barber et al. [45] Date of Patent: Sep. 29, 1987 
PROCEDURE TO PREPARE TRANSPARENT 
SILICA GELS 
Inventors: Patrick G. Barber, Keysville, Va.; 
Norman R. Simpson, Rockville, Md. 
The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
Assignee: 
Appl. No.: 933,961 
Filed: Nov. 24,1986 
Int. Cl.4 .............................................. COlB 33/12 
U.S. C1. ..................................... 423/338; 423/339 
Field of Search ................ 423/338, 339; 501/1 12; 
502/233; 106/74 
References Cited 
U.S. PATENT DOCUMENTS 
2,092,163 9/1937 
2,593,652 4/1952 
2,933,371 4/1960 
3,077,389 2/1963 
3,391,950 8/1968 
3,674,709 ?'/I912 
4,528,112 7/1985 
Johnson ................................ 23/132 
Blanchard ............................. 23/1 10 
Alexander et ai. ................... 23/110 
Schulze et al. ....................... 23/312 
Schnorch et al. .................... 23/110 
Barrer et al. ........................ 252/454 
Rieck et al. ......................... 423/329 
OTHER PUBLICATIONS 
Chemical Abstracts vol. 105, No. 24, 213052s, 
Wakabayashi et al., Ion Exchange Equilibria (between 
Glasses and Molten Salts) and Thermodynamic Proper- 
ties in Mixed Alkali Glasses. 
Chemical Abstracts, vol. 97, No. 16, 132173w, Monseev 
et al., Ion Exchange in Silicate Glasses. 
Primary Examiner-John Doll 
Assistant Examiner-Lori S. Freeman 
Attorney, Agent, or Firm-George F. Helfrich; John R. 
Manning 
[571 ABSTRAm 
A process for the preparation of transparent silica gels 
which can be used as crystal growth media is disclosed. 
A quantity of cation exchange resin beads is placed in a 
solution of a soluble potassium salt and stirred for sev- 
eral hours to allow the potassium ions to replace the 
cations on the resin beads. The supernatant liquid is then 
decanted through a filter, and the treated resin beads are 
rinsed with distilled water to remove excess salts. The 
treated resin beads are then transferred into a sodium 
silicate solution. The resulting mixture is stirred to 
allow the potassium ions on the treated resin beads to 
exchange with the sodium ions in the sodium silicate 
solution. The treated silicate solution is then poured 
through a coarse filter. A volume of boiled, cooled 
distilled water is added to a volume of the treated sili- 
cate solution, after which a volume of acetic acid solu- 
tion is added to the mixture of distilled water and 
treated silicate solution. This mixture is then allowed to 
gel at room temperature, whereby a transparent silica 
gel is produced. The transparent silica gel is used as a 
crystal growth medium to simulate the convectionless 
environment of space and produce structurally perfect 
crystals. 
6 Claims, No Drawings 
https://ntrs.nasa.gov/search.jsp?R=19870019927 2020-03-23T15:28:42+00:00Z
4,696,808 
1 
It is another object of the present invention to pro- 
PROCEDURETOPREPARETRANSPARENT duce silica gels which will not contaminate crystals that 
SILICA GELS are grown in the gels. 
Another object of the present invention is to produce 
ORIGIN OF THE INVENTION 5 silica gels which are transparent and allow the observa- 
tion of crystals grown therein during the growth per- The invention described herein was made in the per- iod. formance of work under NASA Grant No. NAG-1-498 
with Longwood College. In accordance with 35 U.S.C. SUMMARY OF THE INVENTION 
According to the present invention, the foregoing 202, the grantee elected not to retain title. 
TECHNICAL FIELD OF THE INVENTION and additional objects are achieved by employing the 
following process: 
This invention relates to the production of silica gels A quantity of cation exchange resin beads is placed in 
and in particular to a process for the Preparation of a solution of a soluble potassium salt, and the resulting 
transparent silica gels which can be used as a crystal 15 mixture is stirred to allow the potassium ions to replace 
ronment of space to Produce structurally Perfect then decanted through a filter, and the treated resin 
tals. beads are rinsed with distilled water leaving the cation 
exchange beads fully charged with potassium ions. A OF THE INVENTION 20 sodium silicate solution is then prepared, into which the 
Modem utilizations of substances in electronics, such treated resin beads are transferred. The resulting mix- 
as radio transmitters and high frequency microphones, ture is stirred to allow the potassium ions on the treated 
often require single crystals with controlled purity and resin beads to exchange with the sodium ions in the 
structural perfection. Previous techniques include sodium silicate solution. Then the treated solution is 
growing the crystal from a melt, from a solution, or 25 poured through a coarse filter. A volume of boiled, 
from a silica gel. However, the methods which require cooled distilled water is added to a volume of the 
high temperatures, the melt and solution techniques, are treated silicate solution, after which a volume of acetic 
likely to produce less perfect crystals, due to the higher acid solution is added to the mixture of distilled water 
frequency of dislocations produced by thermal vibra- and treated silicate solution. This mixture is then a]- 
tion, the increased solubility of impurities, and the 30 lowed to gel at room temperature, whereby a transpar- 
greater number of point imperfections caused by therm ent silica gel is produced. The transparent silica gel can 
mal stress during cooling. Accordingly, the use of silica be used as a crystal growth n-~edium to simulate the 
gels as a crystal growth medium has developed into a convectionless environment of space and produce 
common method for growing single crystals with struc- 
tural perfection and controlled purity. The near con- 35 DETAILED DESCRIPTION OF THE 
vectionless environment of silica gel suppresses nucle- INVENTION 
ation, thereby reducing the competitive nature of crys- 
tal growth. This limits the size and perfec- It has been found that the near convectionless envi- 
40 for growing single crystals with structural perfection 
and controlled purity. According to the method of this press nucleation until, ideally, only one crystal grows in a predetermined location. However, a silica gel is not a invention, the silica gels are produced by employing an completely convectionless environment like outer ion exchange method to clarify and improve the trans- space: but it is the closest known environment to that of parency of the gel to observation of the crystals outer space that can be created on earth. 
Alkali metal silicate solutions for the preparation of According to one of the present inven- 
silica gels are known in the art. In general, a silica gel is tion, a quantity of approximately one hundred grams of 
solution, preferably sodium silicate, and a dilute acid solution of a soluble potassium such as potassium 
and the mixture to gel at a temperature be- 50 nitrate or potassium chloride. Preferably, the concen- 
tween 20" c. and 450 The use Of this method pro- tration of the solution should be greater than one molar. 
duces a cloudy, translucent gel. However, crystals The mixture is stirred for several hours to allow the 
which are grown in gels can be more easily observed if potassium ions to replace other cations On the cation 
the gel is transparent. exchange resin beads. The resin beads are then sepa- 
Gels can also be formed by dissolving an aluminum 55 rated by decanting the supernatant liquid and rinsed 
salt in a quantity of acid before mixing With waterglass several times with distilled water to remove excess salts 
or sodium metasilicate. The use of this method produces and salt solution and allowed to drain on a filter paper. 
a transparent gel within a few minutes. These gels have A sodium silicate solution is then prepared by dis- 
complicated ionic adsorption properties, and when solving 244 grams of sodium silicate in 500 millileters of 
crystals are grown in them, there is a tendency for con- 60 distilled or demineralized water. For best results, the 
tamination of the crystals with aluminum. For this rea- solution should have a density of approximately 1.06 
son, they are ordinarily not used as a crystal growth g/cm3. 
medium, despite their otherwise- attractive transpar- The pretreated resin beads are then transferred to the 
ency. sodium silicate solution and stirred for several- hours. 
Accordingly, it is an object of the present invention 65 This allows the sodium ions in the sodium silicate solu- 
to produce silica gels which can be used as a crystal tion to exchange with the potassium ions on the resin 
growth medium to simulate the convectionless environ- beads. The supernatant liquid is then decanted through 
ment of space. a filter. The success of the above procedure is not de- 
10 
growth medium that SimUlateS the COnVeCtiOnkSS enVi- the cations on the resin beads, The supernatant liquid is 
perfect crysta1s. 
tion of the crystal: and it is obviously desirable to sup- ronment Of gels provides a crysta1 growth medium 
45 during growth. 
prepared by mixing Of an cation exchange resin beads is placed in a concentrated 
4,696,808 
3 
pendent upon the amount of resin beads or potassium 
solution concentration used as long as the amounts are 
at least as great as the values stated above. 
The transparent silica gel is then prepared by first 
mixing approximately sixteen milliliters of the decanted 
supernatant with between twenty-five and twenty-nine 
milliliters of boiled, cooled distilled water. Improved 
reproducibility is obtained if the solutions are made 
with boiled and cooled distilled water, and the boiling 
also removes dissolved gas such as carbon dioxide. 
Then, a quantity of between twenty-five and twenty-six 
milliliters of between one molar and four molar acetic 
acid solution is stirred into the above mixture, which is 
allowed to gel at room temperature. The resulting gels 
have greatly improved gel transparency over gels pre- 
pared by prior methods. 
In using the above method to prepare the silica gel, 
the use of the smaller volumes, i.e., twenty-five milli- 
liters of distilled water and twentv-five milliliters of 
20 acetic acid solution, results in firm gels. Conversely, the 
use of the larger volumes, Le., twenty-nine milliliters of 
distilled water and twenty-six milliliters of acetic acid 
solution, results in much softer gels. The firmness of the 
gel will also vary depending on the atmosphere in the 
laboratory. Furthermore, the use of the more concen- 
trated acetic acid solution causes the silica gels to set 
faster than do the weaker concentrations. It is desirable 
for the gels to be firm enough to maintain their integ- 
rity, while at the same time, soft enough to allow crystal 
growth to occur unperturbed. 
In order that the invention may be better understood, 
reference should be made to the following illustrative 
examples: 
EXAMPLE I 
In one example of the present method using the de- 
scribed embodiment, a stock solution of silica gel was 
prepared by dissolving 48.8 grams of sodium silicate in 
l00ml of boiled, de-ionized water. The gel was treated 
with cation exchange beads to exchange sodium for 
potassium ions. A quantity of sixteen milliliters of the 
clarified stock solution was poured into twenty-six mil- 
liliters of boiled, distilled water. After thoroughly mix- 
ing this solution, twenty-six milliliters of 4.0 molar 
acetic acid solution was added with stirring. This clear, 
colorless solution was then immediately poured into a 
glass tube which has a 24/40 standard taper, ground 
glass joint at each end. The glass tube had been pre-pre- 
pared by placing a clean cork in one end and by holding 
the glass tube in an upright position with a metal clamp. 
Within two hours the gels had solidified into a clear, 
colorless plug at the lower end of the glass tube. The 
cork was removed, and the ground glass joints were 
lubricated with silicon grease. The glass tube was then 
held horizontally and a 250 ml round bottom glass flask 
was placed on each end. The round bottom flasks have 
a 5 mm glass tube protruding upward through which 
the crystallizing solutions were poured. Into the bulb on 
the left end, which is the end away from the gel plug, 
200 ml of a solution that was 0.01 molar in lead acetate 
and 0.001 molar in ethylenediaminetetraacetic acid 
(EDTA) were added. This bulb is filled first so that the 
gel plug remains firmly wedged against the taper of the 
ground glass joint. Then into the round bulb on the 
right side, which is adjacent to the gel plug, 200 ml of a 
solution of 0.01 molar thioacetamide were added. 
Within twenty-four hours dark Liesagang rings of lead 
' 
25 
30 
35 
40 
45 
50 
55 
60 
65 
4 
sulfide crystals formed in the gel. The rings of crystals 
were tilted at a small angle to the plane face of the gel. 
EXAMPLE I1 
In a second example of the present method, the same 
silica stock solution and gel preparation procedures 
described in Example I above were used to prepare 
another clear, colorless plug of silica gel at one end of 
the glass tube which has two standard taper 24/40 
joints. Into the bulb on the left of this tube were placed 
200 ml of a solution of 0.001 molar iodine and 0.02 
molar potassium iodide. Into the bulb on the right, 
which is adjacent to the gel plug, was placed 200 ml of 
a solution of 0.01 molar mercury (11) acetate. Within 
twelve hours small crystals of reddish mercury (11) 
iodide formed in the gel. 
EXAMPLE I11 
In a third example of the present method, the same 
silica stock solution and gel preparation procedures 
described in Example I above were used to prepare 
another clear, colorless plug of silica gel at one end of 
the glass tube which has two standard taper 24/40 
joints. Before assembling the double-bulb apparatus, a 
very thin glass fiber that had been freshly pulled from a 
Pasteur pipette was used to poke three very thin diame- 
ter holes half-way into the gel from the end adjacent to 
the ground glass joint. The purpose of this procedure 
was to prepare sites favorable for the formation of crys- 
tal nuclei and thus encourage the formation of larger 
crystals in the gel. Into the round bottom glass flask on 
the left, which is the side furtherest away from the gel 
plug, 200 ml of a solution of 0.01 molar lead acetate and 
0.01 molar ethylenediaminetetracetic acid (EDTA) 
were added. Into the round bottom glass flask on the 
right, which is the side adjacent to the gel plug and the 
side from which the holes were poked, 200 ml of a 
solution of 0.01 molar thioacetamide were added. 
Larger black crystals formed in the gel at the thioaceta- 
mide side of the plug, i.e., the right side. The usual 
heavy black rings of small crystals and nuclei also 
formed in the gel. 
EXAMPLE IV 
In a fourth example of the present method, the proce- 
dure of Example 111 was repeated using 200 ml of a 
solution that had lead acetate and EDTA in the left 
round bottom flask. In the right round bottom flask, 
which was adjacent to the clear, colorless gel plug that 
had been poked with a glass fiber, 200 ml of thioaceta- 
mide were added. A series of nine experiments were 
performed with only the concentrations of the solutions 
changed. The concentrations of the solutions used are 
summarized in the following table: 
Run Number Concentration/Molar 
1 0.01 
2 0.02 
3 0.03 
4 0.04 
5 0.05 
6 0.06 
7 0.07 
8 0.08 
9 0.09 
The purpose of this series of experimental runs was to 
determine the effect of changing concentrations on the 
4,696,808 
5 6 
crystals formed in the gel. In solutions # 1 and #2, a few 
white crystals were formed and the gel acquires a milk- 
ish color. In solution #3, some crystal nucleation OC- 
curred with the formation of black and grey bands of 
time to allow the potassium ions to replace the 
cations on the resin beads; 
decanting the supernatant liquid from the mixture of 
the previous step through a filter and rinsing the 
very small crystals in the gel. In solution #4, black 5 
crystals formed in horizontal bands on the lower surface 
treated resin beads with distilled water; 
preparing a sodium silicate solution; 
of the gel Plug in the glass tube. The color lightens to 
grey toward the middle of the gel and remains clear in 
the top half. In solution #5, long dendrites of opaque 
white crystals formed in the gel along with the usual 10 
transferring said treated resin beads to said sodium 
silicate solution and stirring the mixture for a per- 
iod of time to allow the sodium ions in the solution 
to exchange with the potassium ions on the beads; 
black bands of small crystals. In solutions $6 and # 7 7  
the gel Plugs broke free Of the glass taper and 
pouring the treated silicate solution through a filter; 
adding a volume ofboiled, cooled distilled water to a 
volume of said treated silicate solution and then 
adding a volume of acetic acid solution to the mix- 
ture of distilled water and treated silicate solution; 
allowing the mixture of the previous step to remain at 
room temperature; 
the solutions to mix, so no crystals formed in the gels. In 
solution #8, massive quantities of small black crystals 
formed in the gel while white, dendritic crystals devel- 15 
oped in the solutions just adjacent to the gels. In soh- 
tion #9, some of the white dendritic crystals formed 
small crystals. 
reference to particular embodiments thereof, it will be 
tions, and substitutions will be apparent to those skilled 
in the art without departing from the and scope of 
the claims. 
What is claimed as new and desired to be secured by 
Letters Patent of the United States is: 
1. A process for the preparation of a transparent silica 
gel which is useful as a crystal growth medium which 
comprises the sequential steps of: 
obtaining a quantity of cation exchange resin beads; 
mixing said cation exchange resin beads with a con- 
stimng the mixture of cation exchange resin beads 
adjacent to the gel, which was by whereby a transparent gel is produced. 
2. The process of claim 1 wherein the soluble potas- While the present invention has been described with 20 
is potassium nitrate’ 
is potassium 
understood, of course, that certain changes, modifica- 3. The process of wherein the potas- 
sium 
4. The process of claim 1 wherein the concentration 
25 of the soluble potassium salt solution is greater than one 
5. The process of claim 1 wherein the sodium silicate 
solution has a density of approximately 1.06 g/cm3. 
6. The Process of claim 1 wherein the final step em- 
30 ploys between twenty-five and twenty-nine milliliters of 
distilled water, approximately sixteen milliliters Of 
treated silicate solution, and between twenty-five and 
twenty-six milliliters of between one molar and four 
molar acetic acid solution. 
centrated solution of a soluble potassium salt; 
and soluble potassium salt solution for a period of 35 * * * * *  
40 
4s 
so 
5s 
65 


Procedure to prepare transparent silica gels

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