PERMSELECTIVE COLLODION MEMBRANES
53
(3) LEGGETT,C. W., JR.: Thesis for degree of Engineer in Engineering Chemistry, Stanford University, 1941. (4) LEGGETT, C. W., JR.,VOLD,R . D., AND MCBAIN,J. W.: J. Phys. Chem. 46,429 (1942). (5) MCBAIN,J. W., VOLD,R. D., AND FRICK, M.: J. Phys. Chem. 44, 1013 (1940). (6) PALIT,S. R.: J. Indian Chem. SOC.19, 271 (1942). J. M., HELDMAN, M. J., LYON,L. L., AND VOLD,R. D.: Oil & Soap 21, (7) PHILIPSON, 315 (1944). (8) VOLD,M . J., MACOMBER, M., AND VOLD,R . D.: J. Am. Chem. SOC.63, 168 (1941). (9) VOLD,R. D.: J. Am. Chem. SOC.63, 2915 (1941). (10) VOLD,R. D., LEGGETT, C. W., JR.,AND MCBAIN,J. W.: J. Phys. Chem. 44,1058 (1940). (11) VOLD,R. D., AND VOLD,M. J.: J. Am. Chem. SOC.61, 808 (1939).
IMPROVED METHODS OF PREPARATION O F “PERMSELECTIVE” COLLODION MEMBRANES COMBINING EXTREME IONIC SELECTIVITY WITH HIGH PERMEABILITY HARRY P. GREGOR‘
AND
KARL SOLLNER
Department of Physiology, University of Minnesota, Minneapolis, Minnesota Received J u n e 21, 1046
I The preparation and properties of “permselective”2 collodion membranes which combine extreme ionic selectivity with high permeability have been described recently (3, 9), and a preliminary account was given of their usefulness in various types of physicochemical studies (6, 8, 9, 10, 15). The permselective collodion membranes were prepared (3) as follows: A solution of collodion in ether-alcohol mas poured over test tubes which were rotated slowly in a horizontal position. The film thus formed was allowed to dry for several minutes; another layer was applied in the same manner, and yet another a few minutes after the second. After several more minutes the tubes mere immersed in distilled mater. The membranes, still on the tubes, were then oxidized in 1 M sodium hydroxide3 for measured lengths of time and then soaked in water to remove the base. Still on the tubes, the membranes were dried in air. New address: The Permutit Company, Birmingham, New Jersey. Interchangeably with the term “permselective” the word “megapermselective” was used previously (3, 9); since this latter term is philologically not entirely desirable, the briefer and linguistically more convenient term “permselective” will be used exclusively here. The alkali causes a complicated decomposition of the collodion with the formation of nitrites and probably other nitrous compounds. The nitrous compounds act upon the collodion, causing thorough oxidation (12). This oxidation greatly increases the surface concentration of fixed acidic (anionic) groups on the pore walls of the membrane. This surface concentration determines the electrochemical characteristics of the membrane (9, 11, 12, 14). Thus, thoroughly oxidized membranes are electrochemically very active. 1
54
HARRY P. GREGOR AND KARL SOLLNER
Membranes produced in this manner consistently gave characteristic concentration potentials of $54 t o $55 mv. Their absolute permeability was from two to four orders of magnitude greater than that of previously available membranep. Their ohmic resistance in 0.1 M potassium chloride solution correspondingly was only 25 to 5000 ohms per square centimeter, as compared with 100,000 to 500,000 ohms per square centimeter in the case of the more conventional dried collodion membranes (3, 9). Carr and Sollner, however, have stated that their method of preparation of permselective collodion membranes was neither adequately reproducible nor satisfactorily controllable. Without a more rigorous control of the conditions under which the membranes are prepared, too much was left t o the skill and experience of the experimenter. In addition, some of the membrane properties could be varied at will only within rather narrow limits. An improvement of this situation was necessary for the use of the permselective collodion membranes in a variety of systematic physicochemical studies-now in progress-which require membranes adjusted in their properties to the different specific experimental situations. We therefore have undertaken the solution of the following two interrelated problems: (1) To increase the reproducibility of the preparation of permselective collodion membranes; (2) t o develop preparation methods which allow the production at will of various types of permselective membranes having desired properties. The obvious solution of the first of these problems consists of a more rigorous definition and more careful control of the conditions under which the membranes are prepared. A successful solution of the second of the above problems-namely, to prepare membranes with a variety of characteristics a t will-was based on the following considerations : The electrochemical properties of the membranes depend upon the geometry of their pore systems, i.e., the absolute and relative abundance of pathways of different effective diameter across the membranes, and the distribution of fixed charged groups, the frequency of their occurrence, and their location (10). One cannot control the specific location of the fixed charged groups in the pore system, but one knows that more thorough oxidation of the membranes in the porous state results in permselective membranes of greater porosity (3). Increasing oxidation of the membranes in the porous state, therefore, could be expected not only to change their qualitative intrinsic character, e.g., their relative permeability for the different alkali cations, but also to increase their absolute permeability. The pore system of permselective membranes may be expected also t o be influenced by the relative humidity a t which these membranes are finally dried while still on the mandrel. If the membranes once prepared are stored in water and not allowed t o dry out, differences due t o different drying humidities should be permanent. Another way t o prepare membranes of graded porosity consists in slightly swelling permselective collodion membranes with aqueous solutions of ethyl
PERMSELECTIVE COLLODION MEMBRANES
55
alcohol of different concentrations and stabilizing them by immersion in water. Such membranes show increased permeabilities, according to the alcohol concentration used (1, 4).
I1 REAGENTS AND SOLUTIONS USED
,
All of the membranes described in the present paper were prepared from “Baker Collodion Cotton, U. S. P., P y r ~ x y l i n . ” ~ It is essential to use ether and alcohol of highest purity. “AbsoluteEther Merck (Reagent)” and “Rossville Gold Shield Absolute Ethyl Alcohol,” a commercial product of high purity, were used. Several other commercial grades of “absolute” alcohol contained impurities which made the membranes soft and caused them to smell in water to undefined degrees higher than that which occurs in their absence (2, 13). Such alcohols do not allow the preparation of satisfactory membranesa5 A freshly prepared 4 per cent solution of collodion in a mixture of equal volumes of ether and alcohol was used. When poured over the rotating tube i t flowed evenly, forrping a smooth film. Solutions of appreciably higher alcohol content yield membranes that are too porous, while higher ether content makes it more difficult to obtain reproducible results. Solutions of lower collodion content required too many successive coatings for the formation of membranes of proper thickness; more concentrated solutions produced uneven membranes. All the water used was distilled from alkaline permanganate solution in an all-glass still. Distilled mater produced for general laboratory use in repeated instances was unsuitable. Certain impurities contained sometimes in traces in such wat>er(like those in many alcohol preparations) made the membranes swell abnormally. CASTING EQUIPMER’T AND PROCEDURE
The membrane-casting machine consisted essentially of six parallel horizontal axles driven by a small variable-speed motor, the entire machine resting upon three set screws. One of the two bearings of each axle could be adjusted with a set screw so that all six axles could be lined up perfectly horizontally. Fixed to the one end of each axle was a brass cylinder provided with leaf springs which firmly held the tubes upon which the membranes mere cast. Before the casting of membranes, the tubes were adjusted t o an exactly horizontal position. It is necessary to control not only the time of drying of the freshly cast membranes but also the temperature at which this occurs. With given drying times Several other brands of commercial collodion were found t o be equally useful; appropriate changes in collodion concentration and solvent composition to give proper viscosity are necessary in some instances, as well as minor adjustmehts in the casting procedure. On evaporation, the ether and alcohol used left only negligible residues; inferior reagents leave an appreciable quantity of gummy material. The benzene contained in many commercial preparations of “absolute alcohol” also may have a detrimental influence on the membrane properties.
0
56
HARRY P. GREGOR AND KARL SOLLNER
in cold rooms, too porous membranes result, while in warm rooms dense membranes are produced. The temperature of the casting room-a constant-temperature room-was held a t 20°C. f 0.5". The air was circulated slowly, with no direct draft upon the rotating membranes. Any constant room temperature of less than 22°C. was satisfactory (with appropriate changes in the casting time) ; a t higher temperatures bubble formation in the membranes became troublesome. The relative humidity of the air in the constant-temperature room in which the original highly porous membranes were cast (which could be varied from 20 to 80 per cent) did not affect the properties of the final permselective membranes. Relative humidities greater than 80 per cent may possibly alter the membrane properties, particularly if moisture condenses upon the membranes while the organic solvents evaporate. The mandrels on which the membranes were cast were Pyrex test tubes (25 x 100 mm.) having a 2-mm. hole in the bottom. These holes were sealed smoothly by allowing a femdrops of caramelized sugar to harden in the hole while the tubes stood in an oven. The sugar dissolves when the tubes are later placed into water; this allows removal of the membranes without damage from the casting tubes. The optimum speed of rotation of the casting tubes wak 18-20 R.P.M. At greater speed the drops of collodion which form on the under side of the tubes did not fall off but formed ridges on the membranes; if the rotation was slower, uneven spreading of the solution resulted. (Tubes of different diameters required different speeds of rotation.) The collodion solution was poured slowly and evenly from a narrow-mouth bottle over the six tubes in succession while they were rotated. (If any spots are missed they cannot be patched without causing bumps and ridges.) Three successive layers of collodion were put on each tube. Three minutes from the time the first layer of a membrane was cast a second was added, and 5 min. later the third; after 6 min. more of rotation the tubes with the membranes were immersed in distilled ice-cooled water, which was changed repeatedly. There they were slowly rolled over every 30 see. for a few minutes so that the membranes would not lose their uniformity while in a semiplastic state. The use of ' ice water prevents bubble formation in the membranes. These membranes of high porosity were ready for oxidation after a few hours' soaking in several changes of distilled mater to remove all traces of alcohol and ether. T H E OXIDATION PROCEDURE
For oxidation the membranes mere immersed in 1.0 molar sodium hydroxide solution a t 25.0"C. =t 0.1"C. for periods varying from 8 to 14 min., timed exactly with a stop watch, depending upon the properties desired. The membranes were then placed in repeatedly changed water to soak out the base. After 5 min. of soaking the membranes mere'removed one after the other from the casting tubes, and the tubes and membranes were rinsed thoroughly with distilled water, following which each membrane was replaced on its original tube. After several hours of soaking in water the membranes were found to be free from traces of alkali.
PERMSELECTIVE COLLODION MEMBRANES
57
T H E DRYING OF T H E MEMBRANES
The drying of the membranes was carried out in a humidistat constructed from a large Pyrex vacuum desiccator. The central opening of the desiccator cover carried a tightly fitting bearing provided with a rotatable shaft which carried a pulley a t its upper end. The lower part of this shaft was provided a t its middle with two large propellers to agitate the air in the desiccator, and a small glass stirrer at its lower end which dipped into about 500 ml. of saturated salt solution a t the bottom of the desiccator. A wire screen rack provided with dowel pins circled the periphery of the desiccator's interior, a few inches above the solution. The casting tubes carrying the membranes were set on these pins just beyond the reach of the propeller blades, which provided vigorous air movement. Membranes were prepared in the humidity range from about 30 to 60 per cent relative humidity. With appreciably higher humidities the porosity of the resulting membranes was too high for the purpose on hand; drying at lower humidities may cause many membranes to crack and yields membranes which, on account of their density, were not desirable for the present purpose. However, membranes dried a t humidities higher than 60 per cent or lower than 30 per cent may be useful for certain purposes. The solutions of sodium bromide, potassium carbonate, and calcium chloride giving relative humidities of 58 per cent, 43 per cent, and 31 per cent, respectively, were kept saturated by an excess of the solid phase. The temperature of the humidistats was kept in the range of 22" to 28°C. Equilibrium between the moisture in the membranes and the salt solutions was established in all instances inside of 10 hr. if the air was circulated in the humidistat; routinely the membranes were dried in the humidistat for 12 hr. with stirring of the air. After being dried in the humidistat, the membranes were soaked in water for a few hours. This swells the membranes slightly (2) and makes them less brittle; they now can be removed without much effort from the mandrels. For easy handling the glass-clear, perfectly smooth membranes were tied with linen thread to glass rings which just fitted inside the open end of the membrane bag. The membranes are always kept wet. Any further drying without support would shrink the membranes detectably and profoundly change their essential characteristics; they are stored in water to which a crystal of thymol has been added as preservative. The thickness of these membranes is about 30 p . -4LCOHOL SWELLING O F PERMSELECTIVE MEMBRANES
The swelling of permselective collodion membranes was accomplished by placing water-wet membranes mounted upon glass rings into aqueous ethyl alcohol solutions of various concentration (1, 4). Swelling equilibrium is reached, a t concentrations between 50 and 75 per cent ethanol, inside of 3 hr. Longer immersion results in a gradual decrease of the electrical activity of the membranes, presumably by preferential dissolution of some of the oxidized collodion molecules which determine the electrical properties of the membrane. With these concentrations the actual swelling time was set a t 3 hr. At concentrations between 75 and 85 per cent ethanol, immersion for 3 hr. weakens the
58
€L4RRY P . GREGOR AND KARL SOLLNER
membranes too much, in some instances even partially dissolving them; therefore with these higher alcohol concentrations the time of immersion of the membranes was arbitrarily set at half an hour. (The alcohol concentrations given further below in the tables refer to milliliters of absolute alcohol per 100 ml. of solution.) After the proper period of immersion the membranes are soaked in water for several hours to remove the alcohol and to stabilize them. They are kept in the water to which a crystal of thymol has been added, as in the former case. In the course of the first day or two these membranes still change slightly; after this they have assumed their final characteristics, which change very little over considerable periods. T H E DESIGNATION O F T H E VARIOUS TYPES O F MEMBRANES
To characterize the special procedure of preparation of the various membranes which can be prepared according to the method outlined above, it is necessary to define the time of oxidation (in 1.0 M sodium hydroxide), the humidity of the atmosphere in which the membrane mas dried on its mandrel, and the concentration of the alcohol solution used for its swelling, if ‘this latter procedure was employed. Thus a membrane oxidized for 10 min., dried a t 43 per cent relative humidity, and swelled in 65 per cent alcohol is designated as: Ox 10 Hum 43 - Alc 65. I11 The water content of collodion membranes cannot be taken as a direct measure of their functional pore space. Some mater is “bound” by the collodion and the rest is functionally available only insofar as it is accessible to a particular solute under consideration (2). Nevertheless, in a family of closely related membranes, such as the various types of permselective collodion membranes, the water content is a sound indicator of their structural interrelationship. The volume per cent water content of membranes is a somewhat better measure of their porosity than the more easily measurable weight per cent water content; spatial relationships in these structures can be visualized more easily on a volume than on a weight basis (2). The volume per cent water content can be easily calculated from the dry and wet weights of the membranes if assumptions are made concerning the specific gravity of the solid membrane material and of the water in the membrane. Any such calculated values, of course, contain some fictitious elements, such as the neglect of the compression of water when it becomes bound (2). Thus the density of water was taken as 1.00, and the value 1.66 g. per milliliter was assumed for collodion (2). The weight per cent and volume per cent water content were calculated on the basis of the wet weight and wet volume, respectively, of the membranes. For a determination of water content, a mater-wetted membrane bag was cut open lengthwise, and its half-spherical bottom and the upper rim were removed. The remaining flat membrane was quickly blotted dry of surface water, immediately placed in a weighing bottle, and its weight determined. It was then
59
PERMSELECTIVE COLLODION MEMBRANES
placed over concentrated sulfuric acid in a desiccator and dried until constant weight was reached. The results of some of these determinations are given in table 1. Table l(a) shows the influence of various degrees of oxidation on the water content of permselective collodion membranes dried at the same relative humidity; table l(b) the influence of drying a t different relative humidities on the water content of membranes of identical degree of oxidation; and table l(c) the influence of alcohol swelling on membranes Ox 10 - Hum 43. TABLE 1 T h e injluence of ( a ) the period of oxidation, ( b ) the relative drying humidities, and (c)alcohol swelling o n the water content of some permselective collodion membranes WATER CONTENT MEMBRANE
B y weight (experimental)
I
B y volume (calculated)
~~
(a) Influence of period of oxidation Ox Ox Ox Ox Ox
I
0-Hum43 8-Hum43 10 - H u m 43 12 - Hum 43 14 - Hum 43 ~
per cent
)e7 cent
7.1 10.4 12.0 12.3 12.7
11.3 16.2 18.4 18.9 19.5
~~
(b)’ Influence of relative drying humidities
1
Ox 12 - Hum 31 Ox 12 - Hum 43 Ox 12 Hum 58
-
Ox 10 - Hum Ox 10 - Hum Ox 10 - Hum Ox 10 - Hum
12.0 12.3 12.9
43 43 - Alc 50 43 Alc 65 43 - Alc 80
12.0 16.1 19.4 27.5
-
1
18.5 18.9 19.7
18.4 24.1 28.5 38.6
Table 1 does not require much comment. Longer oxidation time, higher drying humidity, and higher concentrations of the swelling alcohol all make for more porous membranes. The great consistency of the influence of the different variables is evident.
IV To ascertain the reproducibility and controlled variability of the basic electrochemical properties of the various types of permselective membranes three types of measurements were made. For each membrane we determined (a) the
