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RALPH E. MONTONKA .4XD L. T. J I L K
T H E ACTION O F LIGHT O S CELLULOSE. VI
A METHODFOR
THE
MEASUREMEKT OF THE OSMOTICPRESSURE OF COLLOIDAL SOLUTIONS~
RALPH E. MONTOXXA A N D L. T. JILK* Divzsaon of Chemical Engineering, Universaly of Mznnesota, Mznneapolis, 3lznnPsvfu
Received May 5, 1941
The desirability of determining the effect of ultraviolet irradiation on the molecular weight of cellulose nitrate in acetone solution by measuring the osmotic pressures of the solutions after various intervals of exposure led to the development of an improved apparatus for this work. The measurement of osmotic pressures by the static method requires an excessive amount of time; furthermore, it is impossible to use this method on solutions which are liable to change. Still another disadvantage is that, a t no definite instant, is it certain that the hydrostatic pressure is exactly equal to the osmotic pressure. A method for measuring the osmotic pressures of aqueous sugar solutions in a much shorter period of time was developed by Berkeley and Hartley (1, 2). This method is based on the principle of balancing the osmotic pressure by an artificial pressure. Sorenson (7) used this method to measure the lower osmotic pressures of albumin solutions, but the speed of his measurements was reduced considerably by the tendency of the membrane to undergo changes in shape. Van Campen (8) describes an apparatus for use in the measurement of the osmotic pressures of colloidal solutions by the dynamic method. He used collodion membranes, prepared by the method of Bjerrum (3). Buchner and Samwell (4) applied this method to the determination of the molecular weights of cellulose acetate and cellulose nitrate. Although collodion membranes were not suitable as such, these investigators found that, by denitrating them with alcoholic ammonium sulfide according to the instructions of Hess ( 5 ) , they obtained membranes which could be used with organic solvents and which proved to be strictly semipermeable. T H E OSMOTIC CELL
The design of the osmotic cell for the pressure-balancing method is of fundamental importance in obtaining good results. The first cell constructed according to Van Campen’s design (8) proved to have several 1 This paper was abstracted from a thesis submitted by L. T. Jilk t o the Faculty of the Graduate School of the University of Minnesota in partial fulfillment of the requirements for the degree of Doctor of Philosophy, August, 1937. 2 Present address: E. I . duPont de Nemours and Company, Belle, Kest Virginia.
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serious disadvantages. The rubber gaskets used to seal the cell were slowly acted upon by the acetone. The spaces between the cell proper and the membrane were filled with two copper gauzes of the correct thickness. This made the assembling of the cell a very painstaking task and yet did not prevent movement of the membrane. After thorough trial of this cell, it was decided to design and construct a cell which would not have these disadvantages. The osmotic cell finally developed was very simple and proved to be quite satisfactory. It consisted of two similar pieces of half-hardened brass, 14 cm. square and 1.2 cm. thick, each of which was provided with fifteen circular grooves spaced so that the ridges between them mere 1 mm.
P FIG.1 . The osmotic cell. A, the interior view of one section; B, the assembled cell
wide. These grooves were 2 mm. deep. The outermost groove was 3 mm. wide, and from it there issued a pair of soldered copper tubes the inside diameter of which was 2 mm. These tubes were located diagonally across the cell from each other. Surrounding this outer groove was a ridge 13 mm. wide, which was machined so that it was a t exactly the same height as the ridges between the grooves and so that the outer ridge on one piece made perfect contact with the corresponding ridge on the other piece. I n closing the cell, a membrane, 12.5 em. in diameter, was laid on one half of the cell, and the other half was then laid on top, two guide posts insuring that it be laid in the correct position. The entire cell was then made acetonetight by tightening eight cap screws. I n this design the membrane itself serves as a gasket. The membrane cannot move, since it is held in position
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RALPH E. MONTONNA AND L. T. JILK
by all contact grooves. The volume of the cell, including the volume of the lead-in tubes, was approximately, 15 cc. Figure 1 is a line drawing of the cell. PREPARATION OF MEMBR.4XES
The preparation of suitable membranes proved to be the most difficult part of all of the laboratory operations. Cellophane was the first material tried, but the results were unsatisfactory. Various types of sheet rubber were next tried, but without success. At this point, it was decided to prepare the membranes in the laboratory by the method used by Buchner and Samwell (4). This consists in preparing a film of collodion and then denitrating the film with alcoholic ammonium sulfide. Specimens of collodion of the U.S.P. grades manufactured by the General Chemical Company and by the Mallinckrodt Chemical Works were first tried, as they were the only grades immediately available. The results obtained with these membranes were unsatisfactory. The membranes were either totally permeable or would become so after developing a small osmotic pressure. Buchner and Samwell had used Kahlbaum’s “Kollodion zur Herstellung von Membranen,” and it was therefore decided to try this material. The results given by this collodion were far superior to any previously obtained, but it was thought that still better results should be possible. Merck’s C.P. collodion was then investigated, and the first few membranes prepared from it gave very good results. Consequently, all membranes used were made from this grade of collodion. The following is a description of the apparatus and technique employed in’ the production of the semipermeable membranes. The bottom of a large widemouthed bottle was cut off and placed mouth downward in a large tripod. A desiccator plate was then placed in the bottle. On this plate was placed a large flat dish, which was filled to a depth of about 8 in. with pure, clean mercury. A round iron ring, 14 cm. in diameter, constructed from a piece of $-in. band iron, was then floated on the surface of the mercury. A piece of wall board into which numerous small holes had been drilled was placed over the top of the bottle. A small fan, driven by an electric motor a t between 200 and 400 R.P.M., was placed about 3 in. above this board so that a small current of air would flow continuously through the bottle and thus materially shorten the time necessary for the evaporation of the solvent. In preparing a membrane, 45 cc. of Merck’s C.P. collodion was run onto the surface of the mercury, inside the iron ring, from a pipet held so that the tip was not over 3 in. above the surface of the mercury. The perforated cover was then put in place and the fan started. After 2 hr. the fan was stopped and the membrane removed from the mercury by lifting the iron ring. The iron ring, to which the membrane was now firmly fastened, was then submerged in distilled water
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until the membrane broke loose from the ring. The last traces of the solvent were removed by the water. The ridge on the periphery of the membrane was next cut off and the membrane denitrated. The denitrating solution consisted of 900 cc. of 5 N ammonium hydroxide solution saturated with hydrogen sulfide and 100 cc. of ethyl alcohol. The membrane, after being submerged in a solution of this composition for 2 hr., was carefully washed with water, carbon disulfide, and acetone in the order named, and after drying in air for several minutes was ready for use. If the membrane was not to be used for some time it, was stored under distilled water, a desiccator plate being laid on it to prevent it from mushrooming. Membranes prepared by this method generally were found to be perfectly semipermeable; however, occasionally a membrane would prove to be totally permeable. Exactly the same technique was always followed and no explanation for the formation of these non-semipermeable membranes can be given. A new membrane was used for each measurement of the osmotic pressure of a solution. The membranes could be used more than once if it were possible to clean out the osmotic cell without taking it apart; however, in the cell herein described it was impossible to do this satisfactorily. OSXOTIC PRESSURE MEASUREMENTS
The entire apparatus used in the measurement of osmotic pressures is shown on the accompanying diagram (figure 2). The action of the pressure apparatus is easily seen by referring to this diagram. The dimensions of the parts were so chosen that the distance between the overflows was in all ('ases approximately 10 cm. In order to determine what pressure is being applied, it was only necessary to observe which of the taps 1 to 6, inclusive, was open. A slow stream of acetone guaranteed that the levels in each of the sections always remained a t the overflow. The pressure apparatus was attached to the osmotic cell and to a graduated capillary tube. A trap was placed in the system in order to allow the meniscus to be brought back to the zero position on the scale reading. When the apparatus was first used it was piaced in a 25-liter all-glass aquarium provided with a mercury regulator and a vacuum-tube relay so that the temperature could be held constant a t any desired point. I t was found, however, that the very small fluctuations in the temperature of the bath had a serious effecton the readings obtained. As a result of this the apparatus was set up in the air, protected from drafts, where no trouble of this nature was encountered. In measuring the osmotic pressure of an acetone solution of cellulose nitrate the following procedure was followed: Taps 9 and 10 were opened and 15 cc. of the solution was introduced into one side of the cell, the air
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RALPH E. MONTONNA AND L. T. JILK
escaping through tap 10. Simultaneously, the other side of the cell was filled with pure acetone by opening taps 6,7, and 11, care being taken that no air remained in the system. The rest of the apparatus was filled with pure acetone by opening tap 12. Any air bubbles which might remain can be removed by opening tap 13 for a few seconds. All taps with the exception of Nos. 8 and 12 were then closed. The meniscus in the graduated capillary tube was then brought back to the zero position by opening tap 14 for a few seconds. The next step was to determine the least pressure necessary to cause the meniscus to move away from the cell. This was done by opening one of the taps 1 to 6, inclusive. The velocity of the liquid in the capillary was then determined by means of a stop watch for
FIG.2. Apparatus used in the measurement of osmotic pressures
this pressure and a few higher pressures. These values were then plotted on a graph with the velocities as abscissae and the pressures as ordinates. The points obtained lay on a straight line, showing that the amount of liquid forced through the membrane in a given unit of time was directly proportional to the applied pressure. This applied pressure is not the pressure caused by the height of the liquid column but this pressure minus the osmotic pressure. Thus, when the applied pressure is exactly equal to the osmotic pressure, no liquid will flow through the membrane in either direction. The point representing this condition is the point of intersection of the line drawn through the points with the ordinate. I t is only necessary to extrapolate a straight line until it cuts the ordinate in order to obtain the osmotic pressure. When a solution is being tested, the
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VI
osmotic pressure of which is so high that perhaps the meniscus will move away from the cell only when taps 5 and 6 arc opened, some negative rates may be recorded and plotted. This was found t o be quite satisfactory in most cases, although care must be taken in using this method, since TABLE 1 Osmotic pressures of a solution of cellulose nitrate in acetone SOLUTION
TEMPERtTCHE
PRESSURE
RATE OF I O T I O X OF UENISCLS
c m of acetone
c m . per second
46 56 67 77
0,0048 0.0067 0,0096 0.0116
36 46 67 77
0.016s
-
"C.
Solution 1,* after 2.08 days' exposure to ultraviolet light
Solution 2 : solution 1 after 10.2 days' exposure to ultraviolet light
* This solution
25
O.CQ68 0.0108 0.0200
contained 10.28 g. of cellulose nitrate per liter.
Y
i3
v)
10
w
VELOCITY OF MENISCUS CM. PER SEC. FIG.3 . Plot showing graphical method of calculating osmotic pressures
very slight deformations of the membrane are likely to take place when the direction of fluid fiow is reversed. The data given in table 1 were obtained on two different solutions of cellulose nitrate chosen at random from a large number of samples. The original solution was made up to contain 10.28 g. of cellulose nitrate in
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RALPH E. MOKTONNA AND L. T. SILK
1 liter of solution. The high concentration was necessary because the solution was being used in another investigation. The exposure was made in 250-cc. quartz flasks a t a distance of about 12 in. from a 200-volt A.C. Cooper Hewitt Uviarc mercury lamp. Solutions of lower concentration gave similar results. Plots of the data are shown in figures 3 and 4. From figures 3 and 4 it is seen that the osmotic pressures of these two solutions are 24.5 cm. and 54 cm. of acetone, respectively. In calculating the molecular weights from the measured osmotic pressures, the equation PV = nRT has been applied, since it supposedly is applicable to solutions
6 30
' '
I
I
' ' '
I
FIG.4. Plot showing graphical method of calculating osmotic pressures
in which the volume of the solute is small in coinparieon to the volume of the solvent. Substituting the data given for these two solutions in this equation we obtain the values shown below: From figure 3: Molecular weight =
1033 X 0.082 X 298 X 10.28 = 13,600 24.5 X 0.78 ~
From figure 4: Molecular weight =
1033 X 0.082 X 295 X 10.28 = 6,100 54.0 X 0.78
The molecular weight of the nitrocellulose in the unexposed solution was 19,600; the two different values obtained above show the degradation produced by the light with continued exposure. This method of calculating molecular weights has been used in all of
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our work. It cannot be said definitely that the molecular weights thus calculated are absolutely correct. It has been noticed that the ratio of osmotic pressure to concentration increases with concentration (6). This effect is probably more exaggerated when materials of high molecular weight are used. Since the materials used in this research were samples having a low degree of polymerization, it is believed that no serious error has been made in the calculations. As regards the accuracy of the measurements of the osmotic pressures, it is reasonably certain that the percentage error was never greater than 5 per cent and usually much lower than that. The basis for this statement is that check determinations always agreed with each other within the range given. It, must be remembered that colloids such as the various cellulose derivatives consist of a large number of different sized molecules and that the molecular weights determined experimentally represent an average value only. I n a subsequent paper the results obtained by the use of this method in following the changes in molecular weight of cellulose nitrate gpon exposure in acetone solution to ultraviolet irradiation will be presented and discussed. REFERENCES BERKELEY AND HARTLEY: Phil. Trans. London Azo6,481 (19C6); AZO9,319 (1908). BERKELEY AND HARTLEY: Proc. Roy. SOC. (London) M M , 271 (1909). BJEHRUM AND MANEQOLD: Kolloid-X.4!4, 98 (1927). BCCHNER AND SAMWELL: Trans. Faraday SOC.29,32 (1933). HESS:Die Chemie der ZeZEulose und ihrer Begleiter, p. 380. Akademische Verlagsgesellschaft, Leipeig (1928). (6) KRAEMER AND LANSING: J. Phys. Chem. 89,153 (1935). (7) SORENIJON: 2.physiol. Chem. 108, 2 (1919). Rec. trav. chim. 60,915 (1931). (8) VANCAMPEN: (1) (2) (3) (4) (5)
