THE ACTION O F ULTRA-VIOLET LIGHT ON GELS" BY EDWARD 0. HOLMES, JR. AND WALTER A. PATRICK
Introduction The present investigation was undertaken with the idea of explaining certain phenomena that have been observed in connection with the action of light upon gels. It is known that exposure to light is sufficient to bring about marked changes in the properties of many gels ; as examples : the hardening of gelatine and the embrittling of celluloid under the influence of light may be cited. It occurred to us that possibly the explanation of these phenomena was, in all cases, due to the loss of solvent and not to a specific chemical reaction that was brought about by the light. In other words, it was our belief that the action of the light simply effected a change that produced a loss of associated solvent, and it was due to the loss of the latter that the change in the properties was brought about. Historical The mechanism of this loss of solvent as produced by the action of light was suggested by experiments on the adsorption of sulphur dioxide by silica ge1.l It was found that the presence of a small amount of air reduced the amount of sulphur dioxide adsorbed by the gel. This point is best made clear by reference to the adsorption isotherms which are here reproduced. I n Figure l2 Curves (A) and (B) show the adsorption of sulphur dioxide in the presence of small amounts of air while those in Figure 2 indicate the adsorption in the absence of air. It is to be noted that the curves in Figure 2 are reversible; that is, the removal of sulphur dioxide follows the same path as the adsorption. In the case of the removal of -
* Contribution from the Chemical Laboratory of Johns Hopkins University
2
MacGavack and Patrick: Jour. Am. Chem. SOC.,May, 1920. Ibid.
26
Edward 0. Holmes, J r . and Walter A. Patrick 181
16(
I4C
I 20 100
80
60 40 10
0 Pig. 1
Fig. 2
Action of Ultra-Violet Light on. Gels
27
sulphur dioxide after adsorption in the presence of air (Curve B in Figure 1) the path coincides with that of the reversible curves. From these curves it is easily seen that the adsorbent under the same partial pressure of sulphur dioxide will retain more of the latter in the absence of air. This effect has been explained by the assumption that the presence of permanent gases prevents complete wetting of the pores by the adsorbed vapors. In order to test this theory it was thought that gels containing liquids that could be decomposed by ultra-violet light with the formation of permanent gases should show a loss of liquid when exposed to the light. With the foregoing in mind the purpose of the following research and the method of attack may be outlined more clearly. We first attempted to prove that ultra-violet light caused a loss of solvent from those gels that contained liquids decomposable by this light; that furthermore, this loss of solvent was not simply the results of the decomposition of the solvent itself but that the production of the permanent gases in the pores of the gels caused the solvent to escape. Upon proof of the above we then proposed to extend the study to other organic gels and see if the same explanation was sufficient to account €or the observed effects.
Description of Apparatus The principle of the apparatus is simply that of a tensimeter: namely, a means of reading the difference in pressure between two bulbs containing portions of the same sample of gel, one of which is being exposed to the ultra-violet light, the other being shielded from it. In order to keep the temperature of the two bulbs the same, both bulbs were immersed in a water bath which was constantly stirred by air. The ultra-violet light was generated by an iron arc, passed thru a quartz window in the side of the water bath and into the quartz reaction flask where it hit the gel. The compensation bulb was of glass and was protected from the light by a metal screen. Fig. 3 shows an iron arc
Edward 0. Holmes, J r . and Walter A . Patrick
28
I& ARC
*
Y
Fig. 3
giving our ultra-violet light, which passes thru the quartz window (Q) in the bath. The light traverses the quartz bulb (R) and strikes the gel contained therein. A metal plate protects the bulb (G) from any scattered rays. This bulb (G) is the same size as the bulb (R) and contains the same amount of the same sample of gel as is placed in bulb (R)functioning merely as a compensating bulb. The two bulbs are connected with a manometer having a long, gently-sloping straight arm which makes the manometer very sensitive. Behind this arm is a scale, the movement of the mercury along one division of which represents mm. The two arms of the manometer are connected by a tube which contains a mercury sealed stopcock ((2,). The whole system is connected to a barometer column, a mercury pump and an oil pump thru stopcock ((2,). The cock (C,) a t the bottom of the manometer is placed there to facilitate replacing the mercury and cleaning the arms in case the mercury be attacked by the gases liberated by the action of the light on the gel. Description of Runs The gel was originally made up for use in gas masks and contained about nine percent of water. This was partially
Action of Ultra-Violet Light o n Gels
29
replaced by immersing the gel four times in fresh portions of the desired liquid during which the water is replaced by the liquid used to about fifty percent. This gel is now dried in an air bath until it has no appreciable vapor pressure. Each bulb was filled with the same quantity of dry gel. After attaching the bulbs to the apparatus thru a mercurysealed joint, the whole was evacuated, the stopcocks being opened. When the apparatus was completely evacuated these cocks were closed and the system allowed to stand to be sure that no vapors escaped from the gel. If any increase in pressure in the system was observed, the whole was re-exhausted and allowed to stand once again. It was necessary to have the vapor pressure of the gel in the two bulbs the same and below one millimeter to avoid any “air effect’’which is described later. When the manometer had not moved after long standing, which showed that the gel in each bulb was in equilibrium with that in the other and that the apparatus did not leak, the arc was started and the readings of the mercury column in the long arm of the manometer taken a t certain intervals.
Preliminary Runs on Liquids and Air It became necessary to know what effect any air left in the bulbs would have upon the readings; so that a run was made in which the bulbs contained nothing but forty millimeters air pressure. The arc was started and the curve rose immediately, giving the air curve No. 1 in Fig. 4. When the arc waszstopped, the pressure fell rapidly to zero. This “air
s6 50
d
$50 ,$
$.IO t
2
7
4
5
Fig. 4 Curves for Liquid Acetic Acid and Air
6
30
Edward 0. Holmes, J r . and Walter A.Patrick
effect” was regarded as purely thermal and showed the necessity of excluding air from all runs. A great many organic liquids can be decomposed by ultra-violet light; but two were chosen to impregnate the gel which were easily decomposed and readily attainable : namely, acetone and acetic acid. In order to obtain some data as to the rate of decomposition of these liquids, a run was made by filling both bulbs, with glacial acetic acid and exposing in the usual manner. Curve No. 2 in Fig. 4 was obtained. It should be noticed that even after the light is turned off the curve continues to rise for some time. This is due to the lag caused by the time it takes the gaseous decomposition products formed in the liquid to migrate to the vapor space above and hence to the manometer. On account of this lag, it was decided to find curves for the rate of decomposition of the vapors of the liquids to be used.
Rate of Decomposition of the Vapor
This was found by filling the bulbs about one-quarter full of the liquids to be used and then boiling under reduced pressure until only a small amount remained in the bottom, so as to expel all the air. The arc was started and readings. taken. In Fig. 5, Curve No. 1 shows the results obtained with acetic acid. The curve is a straight line as would be supposed, flattening out when the light is shut off.
Fig. 5 Curves for Acetone and Acetic Acid Vapor
Action of Ultra-Violet Light on Gels
31
Curve No. 2 shows the results with acetone. This curve has a greater slope than that of the acetic acid curve and drops when the arc is shut off. The difference in slope is due to the fact that a t the temperature used, acetone has a vapor pressure sixteen times that of acetic acid. However, the slope .of the acetone curve’ is only four and three-tenths times t h a t of the acetic acid curve, indicating that acetic acid is more readily decomposed than acetone. On the other hand, this is contrary to the literature on the subject and is accounted for by the fact that on photo-chemical decomposition, acetic acid gives carbon monoxide, carbon dioxide and hydrogen,-three molecules ; whereas acetone gives only carbon monoxide and ethane, or a smaller number of molecules.
Runs of Impregnated Gels This chapter is divided into two parts : the first titled “Total Illumination” and the second, “Partial Illumination.’’ I n the first part, the whole of the quartz reaction flask was exposed to the light, which hits the gel aswell as the vapors above the gel. However, it became necessary in order to prove certain points to make similar runs exposing sometimes the solid gel, sometimes the vapor above, or even as before, both; so that this second part was headed “Partial Illumination.” In the following runs, we endeavored to keep a constant source of light, but this could only be done approximately, as we used a powerful source of ultra-violet light,-an iron arc taking eight amperes a t two hundred and twenty volts. For a single run, it was much easier to keep the arc constant than t o have it the same in two runs ; hence the slopes of the curves are not strictly comparable to one another as the amount of decomposition depends on the amount of light absorbed, and this, other things being the same, on the intensity. The temperature of the bath gradually rose during the run due to the heat absorbed from the arc. However, as both bulbs were kept a t the same temperature, they compensated each other for any change in vapor pressure of the gel.
32
Edward 0. Holmes, J r . and Walter A . Patrick (A) Total Illumination
Water Gel In order to show that if any increase in vapor pressure resulted on exposure of an impregnated gel it was not due to the water in the gel, asample containing water (9.73%) was exposed in the apparatus. Curve No. 1 in Fig. 6 shows the
Fig. 6
flat line obtained, proving that there was no water vapor given off by the gel. This result was anticipated inasmuch as the light was of wave length not short enough to decompose water, and therefore no gas could be generated which would prevent the absorbed water from wetting the surfacc of the pore. Acetic Acid Gel A gel containing twenty-one percent volatile matter, about half of which was acetic acid, was run and quite different results obtained, as is shown in Curve No. 2, Fig. 6. The Curve consists of two parts (A) and (B). Part (A) was run one day and (B) the following, allowing the apparatus to stand overnight. For the first two hours of exposure, the pressure increases slowly, the rate becoming constant during
Action of Ultra-Violet Light on Gels
33
the next hour. The arc was shut off and the curve gradually flattened out in the manner of an exponential curve. At this point, the apparatus was allowed to stand undisturbed overnight. That part of the curveghowing a gradual increase was the result of two factors: firstly, the arc was gradually increasing in intensity; and secondly, there was the natural lag of the apparatus-namely the time necessary for the molecules of liquid liberated to diffuse to the manometer. The latter of these explains the rise in the curve after the light was shut off. The next morning the run was continued, forming part (B) of the curve. However, this time the arc was brought to full intensity before the gel was exposed, which accounts for the much quicker rise in pressure. The gel filled about two-thirds of the reaction bulb and the vapors the remaining third. Hence the ultra-violet light played upon both the gel and the vapors above it.
Acetone Gel In place of the acetic gel, a gel containing about seventeen percent of volatile matter, half of which was acetone and the other water was introduced into the apparatus. This run was carried out in a similar manner to the previous one,-namely in two parts, (A) and (B). The pressure rose very slowly as can be seen from Curve No. 3 in Fig. 6, the curve becoming flat when the arc was shut off. The next morning the curve was (B). continued and gave another straight line,-part These three curves show that when a gel impregnated with some organic substance which can be decomposed by ultra-violet light is exposed to the same, some sort of vapor is set free which causes the manometer to show a reading. However, as yet, there is no evidence as to whether this vapor consists of the solvent itself or the gases formed by the photochemical decomposition of the solvent.
(B) Partial Illumination In the following runs, a lead screen (seen on the diagram) was added to the apparatus, and if the bulb was half-
34
Edward 0 . Holmes, J r . and Walter A.Patrick
filled with gel, by moving the screen, the gel itself, the vapors above, or both could be illuminated as desired. Acetic Acid Gel The gel used in this case was from the same large sample as that used in the former run under this title, only re-dried at a higher temperature and for a much longer time, thereby reducing the content of total volatiles t o nine and one-half percent. However, recent work has shown that a larger fraction of this residual solvent was acetic acid than that in the gel used in the preceding run with acetic acid. The bulb was only half-filled with the gel. The run consists of three parts (A), (B), and (C) which followed one another consecutively without the apparatus being disturbed, save the lead window. The results of this run are plotted in the form of a discontinuous curve in Fig. 7. When the system had come to equilibrium as usual, the space above the gel was exposed to the light for one and onehalf hours with no result; then the screen was removed and
Fig. 7
Aciion of Ultra-Violet Light ow Gels
35
the gel also exposed. The rate of decomposition grew to a constant value, as was the case in the preceding run. When the arc was shut off, the curve flattened out immediately. The following day the arc was started again, the gel and the space above it being exposed until the rate of increase of pressure reached a constant value. At this point, the lead screen was placed in such a position that the light fell on the vapor only. The pressure continued to rise for some time; the rate falling off and finally becoming zero. Then the arc was turned off and the pressure remained constant overnight. With the lead window still allowing the vapors only to be hit by the light, the arc was lighted and a small rate of increase observed. However, when the screen was lowered allowing the gel itself to be hit by the light, the pressure rose rapidly and the curve regained its usual slope as was obtained in the first two parts of this run when the light hit the gel itself. The important feature of this curve (the upper half of part B) is that after the gel and the vapor above it have been exposed for some time and the increase in pressure has become constant, when the lead screen is so placed that only the vapor is hit by the light, the pressure still continues to rise a t a gradually decreasing rate. This has only one meaning which is that there must be some substance present in the vapors above the gel that can be decomposed by the light. Obviously this substance cannot be the gaseous decomposition products from the pores of the gel and therefore must be acetic acid itself which has been liberated intact. As this acetic acid vapor is gradually broken up by the light the curve flattens out, there remaining in the gaseous phase only the decomposition products consisting chiefly of carbon monoxide, carbon dioxide and hydrogen.
Acetone Gel In order to check this point, a similar run was made with the acetone gel used in the former run, and the results of this run are.shown in Fig. 8.
36
Edward 0. Holmes, J r . and Walter A.Patrick
(Ordinates are
Fig. 8 mm and not mm.)
'/OO
The curve consists of three parts (A), (B), and (C). The light was allowed to hit the gel only until a constant rate of decomposition was attained. When the arc was turned off, the curve rose for a short time and then flattened. This completed part (A) of the acetone curve. The fact that the curve flattens proves that the gases liberated by the action of the light are not readsorbed by the gel. Now the light was allowed to strike the vapor above the gel only, which produced the lower part of section (B). Then the position of the screen was so shifted that the light struck the gel only, and the same steep slope was obtained as under similar conditions in part (A). The light was now turned off and the curve flattened as usual, completing part (B) of the curve. The next morning the run was resumed with the light on the vapor above the gel and another straight line was obtained which constitutes part (C) of the curve.
Action of Ultra-Violet Light on Gels
37
We have now proven that our results are perfectly reproducible, and that the organic liquid with which the gel is impregnated is set free intact when ultra-violet light plays upon the gel, and also that neither it nor the gaseous decomposition products are readsorbed by the gel.
Nitric Acid Gel It was now thought advisable to impregnate silica gel with an inorganic liquid which would be decomposed by the light. For this purpose some original gel was exposed to the fumes of concentrated nitric acid in a vacuum desiccator for several days. The gel was dried until it had no appreciable vapor pressure and was found on analysis to contain five percent nitric acid. The curve obtained, No. 2 in Fig. 8, consists of a straight line parallel to the abscissa,-in other words, no pressure a t all was generated.
Fig. 9 (Ordinates are ~/c,o mm and not mm.)
A t first, this result may seem strange as nitric acid is easily decomposed by light. However, when it is considered that silica gel adsorbs oxides of nitrogen very strongly, which are the photo-chemical decomposition products of the nitric
38
Edward 0. Holmes, Jr. and Walter A. Patrick
acid, it is not remarkable a t all that nitric acid gels show no pressure under the influence of the ultra-violet light. As fast as the nitric acid is decomposed the products are adsorbed and thus there is no gas which will prevent wetting of the surface of the pores by the nitric acid solution.
Celluloid As the embrittling and the discoloring of celluloid by light furnished the original idea of the problem, it was now an opportune time to try the effect of ultra-violet light on celluloid itself. Inasmuch as the gel structure as well as the solvent are both easily decomposed by the light, celluloid could not be used for a study of the mechanism of the process but a gel having a stable structure was necessary-silica gel. A run on celluloid was made by cutting the celluloid up into small pieces and dropping them'in the necks of the bulbs, until each bulb contained the same amount and was approximately full. The celluloid in the quartz bulb was exposed to the light and Curve No. 3 in Fig. 10 obtained. The rate of decomposition increased to a constant value. The products are undoubtedly gases from the decomposition of the nitrocellulose and its solvent, and also the undecomposed solvent itself, When the arc is shut off, the curve drops rapidly, finally coming to a constant reading. Fig. 10 This drop can be the result of two things; the thermal effect, or more likely the re-adsorption of the oxides of nitrogen, The curve is made up of the results of three similar runs made at different times, the apparatus being undisturbed in between. The second and third sections show a peculiar twist which cannot be accounted for in light of the present knowledge on the subject.
Action of Ultra-Violet Light on Gels
39
Mechanism of the Liberation of the Solvent from the Gel Let us suppose a number of tiny pores or cells to be in a completely evacuated space. For simplicity, consider one cell as pictured in figure No. 10. Now introduce into this space a gas such as sulphur dioxide which can be adsorbed. Immediately, some will enter the pores or cells and be conphedensed on the surface forming a liquid film &)-the nomena of adsorption. The surface (S) of this film will tend to become smaller due to the force of surface tension. This tension is very great and puts the liquid under a negative pressure which reduces its vapor pressure t o , a very small value. As the pressure in the surrounding space is above that of the liquid in the pores, more gas will condense in the liquid surface and tend to make this smaller, which decreases the negative pressure and in turn causes an increase in the vapor . pressure of the liquid film. Obviously, the process will continue until the vapor pressure of the film is equal to that of the surrounding space. This same state can be reached from the other direction,that is by filling the pores with a liquid and evacuating the space around, which causes the liquid to evaporate from the pores and thereby reduce its vapor pressure. In this manner the gels used in the previously described runs were brought to a condition of equilibrium with the apparatus. Hence by either of these two processes, we have a large number of pores partly filled with the adsorbed substance under great negative pressure and therefore having a very low vapor pressure. The negative pressure on the film is caused by the surface tension acting along the sides of the pores and around their circumference. If by any merdns, this force of surface tension is reduced the vapor pressure of the liquid film becomes greater than that of the surrounding space and some of the liquid evaporates in order that equilibrium shall be restored. Now ultra-violet light does reduce this force by decomposing the liquid of the
Edward 0. Holwzes, Jr. and Walter A.Patrick
40
film with the formation of gas bubbles, which prevent the film from completely wetting the surface of the pore, resulting in a decrease in negative pressure and subsequently an increase in the vapor pressure of the adsorbed substance. Inasmuch as the vapor pressure of the film is now above that of the surrounding space some of the liquid evaporates. This same mechanism will explain the liberation of gases by celluloid under the influence of ultra-violet light only here ’ we have the complication that the light decomposes the structure of the gel itself (cellulose nitrate) as well as the solvent in the pores. Therefore the reason for celluloid turning brown and becoming brittle under the action of the ultraviolet light of the sun is quite evident. Thus in the case of sulphur dioxide where there is residual air present, as already cited, the air prevents the adsorbed sulphur dioxide from completely wetting the surface (for small pressures) and therefore the adsorbed film has a higher vapor pressure than if it completely wet the surface. This is a case of what might be called “mutual adsorption.’’ Recent work of Patrick1 and Ryerson on the adsorption of bromine by silica gel in air shows that when the concentration reaches a certain point, the air no longer exerts an influence and is squeezed out or removed in some such way. However, on emptying the pore, the air is swept out alone with the adsorbed substances, and so when equilibrium is obtained a t any point on this curve, the surface can be completely wet,-the air having been removed from the pore. Hence the explanation of the irreversible curves obtained for the adsorption of sulphur dioxide when air is present, which was revealed by our work on the action of ultra-violet light on impregnated silica gel.
Summary (1) Silica gel impregnated with certain organic liquids will, on exposure to ultra-violet light, give off gaseous prod1
To be published
Action of Ultra-VioletLight on Gels
41
ucts, provided the liquid can be decomposed photo-chemically into gases which are not adsorbed by the gel. (2) The gaseous products consist of a mixture of the vapor of the organic liquid itself along with those gases resulting from the photo-chemical decomposition of the liquid. (3) Celluloid acts in a similar way under the influence of ultra-violet light. (4) The mechanism of the liberation of the adsorbed liquid is explained. (5) These results show in a very striking way the manner in which gas bubbles in the pores affect adsorption, which accounts for the irreversibility of the sulphur dioxide isotherms obtained by MacGavack and Patrick. (6) The embrittling of celluloid is also due to this effect.
