Ultrafein filters as osmotic membranes

JOURNAL OF POLYMER SCIENCE VOL. XXXIIJ, PAGES 417-427 (1958)

Ultrafein Filters as Osmotic Membranes

M. F. VAUGHAN, Department of Scientific and Industrial Research, National Chemical Laboratory, Teddington, Middlesex, England

I. INTRODUCTION

The manufacturers indicate that the ultrafein filters (Membranfilter- gesellschaft, Gottingen, Germany) are made for experiments with aqueous solutions and are not suitable for use with organic solvents. However, we have found that these membranes can be used in other solvents, in par- ticular, benzene, and that in this solvent they are more selective than the corresponding ultracella filters which are widely used in the osmometry of nonaqueous solutions. With the best ultrafein filters, allerfeinst, molecular weights of the order of 2000 have been determined.

11. APPARATUS

The measurements were carried out in modified Zimm-Myerson osmome- ters2 which have been developed in this laboratory. The thick-walled glass cells of the osmometers have an internal diameter of 2.5 cm. and a volume of approximately 4 ml. The measuring capillaries have an internal diameter of 0.5 mm. and an effective length of approximately 12 em.

All membranes used in this work were stored in a 20% aqueous ethanol solution and before use mere conditioned to benzene by successive immer- sions for periods of not less than one hour in ( I ) ethanol, (2) a 50/50 ethanol benzene mixture, and (3) benzene.

It was found necessary, in some cases, to use methoxymethyl nylon gas- kets on the outer side of the membraiie in order to assemble a leak-free osmometer .

Measurements were carried out in a tank thermostatically controlled at 30' f 0.005'C.

111. PROCEDURE

In order to characterize the membranes, measurements of (a) the solvent permeability and (b) the selectivity were made.

(a) Permeability. An estimat,e of the solvent permeability has been made by measuring the rate of fall of the meniscus in the osmometer measuring capillary, under a head of 10 em. of benzene. Since the OS-

mometers are made to a standard pattern, the results obtained in this way should be comparable.

417

418 M. F. VAUGHAN

(b) Selectivity. The selectivity of the membranes has been investigated by measuring the osmotic pressures developed with solutions of materials of decreasing molecular weight. For this purpose, materials having a wide range of molecular weights are particularly useful and the following benzene solutions were used.

A 1.0% (w/w) solution of a heterodisperse “commercial” polystyrene (P.S.) having a number-average molecular weight (a,) of approximately 40,000.

A 0.1% (w/v) solution of a heterodisperse “ionic” polystyrene having a a, of approximately 4000. This polymer was prepared by adding drops of an ether solution of boron trifluoride to monostyrene at room tem- perature until an explosive polymerization occurred.

(3) A 0.025% (w/v) of pentaerythritol tetrastearate (P.E.T.S) with M , of 1200.

Before filling, the osmometers were rinsed out once or twice with the solution under test and between each rinsing were left out of the solvent for periods of up to 30 minutes. The number of rinsings necessary does not appear to be critical and the results in Table I indicate that one is sufficient.

(1)

(2)

TABLE I Effect of Rinsing Procedure on Observed Osmotic Pressure”

Number of rinsings Time of each rinsing, min. Equilibrium osmotic pressure, cm.

1 1 2

10 30 10

3 61 3 56 3 55

0.025y0 Benzene solutions of PETS at 30°C.; UFF/AF 655/3.

I n these measurements, the static method was employed, in most cases both the “rising” and “falling” equilibrium heads being measured. In the former case the meniscus was initially drawn below zero, and in the latter, where possible, i t was pushed to a height at least double the equilibrium rising head. The time for attaining: equilibrium was up to six days, de- pending on the membranes and the solutions used.

IV. RESULTS

Comparison of Ultrafein Filters with Ultracella Filters

A series of experiments were carried out a t 30°C. in benzene with ultracella filters (UCF) and ultrafein filters (UFF), the three grades fein (F), feinst, and allerfeinst (AF) of each t,ype being used. As there were com- paratively large variations in the properties of each class of membrane, the selection of results shown in Table I1 are those for membranes considered typical of their kind. In the case of the ultracella filters allerfeinst (UCF/ AF), several were found to be virtually impermeable.

ULTKAFElN FlLTERS AS OSMOTIC MEMBRANES 419

TABLE I1 Comparison of Ultrafein and Ultracella Filters"

Selectivity

Permeability 1.0% Commercial 0.10% 10 nic 0.025% P.E.T.S. fall from 10 P.S. P.S. em. head of

benzene, Max. O.P., Time, Max. O.P., Time, Max. O.P., Time, Membrane cm./30 min. em. hr. cm. hr. cm . hr .

~~~ ~~ ~ ~~ ~ ~ ~

Ultracella filter

Ultracella filter Allerfeinst (UCF/AF) 1.23 5.74 (rib 21 3.10(r)b 23 2.45 ( r ) b p d 23

Fein (UCF/F) 6.75 3.93 (r)b 3 0.74(r)' 1.5 - -

5.71 (f)" 27 2.94 (f)" 7 1.26 ( f ) " s d 71 Ultrafein filter

Allerfeinst 1.32 8 .95( r )* 24 8 .03( r )b 22 5.23 (r)b 47 (UFF/AF) 8.81 (f)" 46 7.91(f)" 71 5.17(f)" 23

~~

a Benzene solutions at 30°C. r = rising osmotic head. f = falling osmotic head. 0.050% solution.

The expected decrease in solvent permeability and increase in selectivity in progressing from the fein grades to the allerfeinst was observed and, in addition, i t mas found that, with each grade, the ultrafein filters were more selective than the corresponding ultracella filters. This is shown by the higher osmotic pressures developed with each test solution and also by the fact that the pressures were more stable with time.

In this connection it should be pointed out that, although the pressures have been described as equilibrium values, in some cases there was a tend- ency for the heads to decline because of solute d i f f~s ion .~ This accounts for the fact that the rising head, which was measured first, sometimes has a higher value than the falling head, which was measured subsequently without refilling.

Limit of Selectivity of Ultrafein Filters, Allerfeinst As apparently stable, almost theoretical, osmotic pressures were developed

by membranes UFF/AF 257/2 with solutions of P.E.T.S. (M. W. 1200)

TABLE I11 Measurements with Pentaerythritol Tetrastearate"

~ ~ ~~ ~ ~

Osmotic pressures Concentra-

C, % (w/v) h,, em. Time,hr. h,, cm. Time,hr. ho, cm. h,,, cm. ho/hth tion Rising Falling Observed Theoretical

0.020 4.32 72 4.46 23 4.32 4.92 0.88 0.025 5.23 47 5.17 23 5.20 6.15 0.85 0.030 6.02 26 6.26 23 6.10 7.38 0.83 0.040 8.30 71 8.14 22 8.30 9.84 0.84

a Benzene solutions a t 30'C.; UFF/AF 257/2.

420 M. F. VAUGIIAN

5 0 TIME (hours)

7 5

I

Fig. 1. 0.025y0 (w/v) Pentaerythritol tetrastearate ( M , = 1200) in benzene a t 30°C.; UFF/AF 257/2.

TIME (hours)

Fig. 2. 0.0200/, (w/v) Tristearin ( M , = 801) in benzene a t 30°C.; WFF/AF 257/2.

IJLTKAF'EIN FILI'EKS AS OSMOTIC MEMBRANES 421

of different coiicentratioiis (see Table III), further experiments were carried out to see if it would retain even smaller molecules, e.g., tristearin (ILI. W. 891) and sucrose octaacetate (M. W. 679). With solutions of these compounds (see Table IV and Figs. 1, 2 , and 3 ) the ratio of observed to theoretical osmotic pressure decreased with decreasing molecular weight as did the stability of the heads developed.

TABLE IV Measurements with Low Molecular Weight Compounds"

So 1 u t i o n Concentrrt- Osmotic pressures

Molecular tion, Time, Observed Theoretical Solute weight, M % (n/v) hr. hot cm. h t h , cm. ho/ht,

Pentaerythritol tetrastearate 1,200 0 025 47 5 20 6 15 0 85

Tristearin 89 1 0 020 28 4 50 6 A3 0 69 Tristearin 89 1 0 050 24 10 07 16 57 0 61 Sucrose octaacetate 679 0 025 47 0 34 10 86 0 03 Sucrose octaacetate 679 0 10 23 6 50 43 44 0 15

Benzene solutions a t 30°C. with UFF/AF 257/2.

It would appear that these ratios are further examples of the reflection coefficients, predicted by Stavermans from his treatment of membrane phenomena from the standpoint of irreversible thermodynamics. On this theory, the reflection coefficient would be unity for molecules completely retained by a membrane and zero for the molecules which are completely permeable.

It seems reasonable to conclude, therefore, that the most selective UFF/AF membranes retain molecules of molecular weight > 1500, restrict the diffusion of molecules of molecular weight 1500-500, and are freely permeable to molecules of molecular weight < 500. Further support for this view is to be found in Figure 4 and Table V, in which measurements on a fraction of a cationic polystyrene are summarized. The value of an = 2,460 is in good agreement with an estimate of gn 2,000-3,000 based on viscosity measurements (polymer and viscosity data kindly supplied by Dr. D. C. Pepper, Trinity Ccllege, Dublin).

TABLE V Determination of the A?, of Cationic Polystyrene Fraction P1/14"

Sumber-average molecular -~ Osmotic pressures

Concentration Rising Falling ._ Mean weiqht, C, 70 (n/v) h,, cm. Time, hr. h,, cm. Time,hr. h, cm. Bn

2 92 0 0250 2 80 187 :3 03 I I

0 0196 5 76 $4 (i '21 71 2460 0 0750 9 18 99 8 99 GT 9 09

r-

(I Benzcne solutions at 30°C.; UFF/AF %57/3.

422 M. F. VAUGHAN

Fig. 3. 0.10% (w/v) Sucrose octaacetate ( M , = 679) in benzene at 30°C.; UFF/AF 257/2.

TABLE VI Variations in Properties of UFF/AF Membranes"

Permeability, Selectivity fall from 10 l.Oyo Conimercial 0.10% Ionic cm. head of P.S. P.S. 0.025% P.E.T.S.

Max. O.P., Time, Max. O.P., Time, Max. O.P., Time, h f ~ d ~ a n e UFF/AF cm./30 min. cm. hr . cm. hr. cm. hr.

benzene,

257/3 257/2

1056/1 655/3 655/2

457/Al 457/A2 457/A3

457/Bl 457/B2

Membranes purchased April 1955

0.62 8.66 90 8.71 65 5.74 98 1.32 8.86 24 8.03 22 5.23 47 0.85 8.23 43 6.23 45 3.26 72 2.37 6.80 22 6.46 22 3.61 46 1.41 6.49 24 5.02 30 3.15 30

Membranes purchased April 1957, Series A 1.03 5.93 24 6.36 43 4.30 72

4.96 98 0.27 9.03 136 3.57 92 0.30 7.30 130 - -

Membranes purchased April 1957, Series B

1.33 5.97 24 4.31 43 2.34 49 5.60 64 0.51 8.97 43

- -

- -

a Benzene solutions a t 30°C.

ULTRAFEIN FILTERS AS OSMOTIC MEMBRANES 423

TIME (hours)

Fig. 4. 0.0496% (w/v) Cationic polystyrene fraction P1/14 (a,, = 2460) in benzene at 3OOC.; UFF/AF 257/3.

TABLE VII Stability of UFF/AF Membranes"

Permeability, fall from 10 Selectivity cm. head of 1.0% Commercial P.S. 0.02570 P.E.T.S

Time, benzene, Osmotic pressure, Time, Osmotic pressure, Time,

ntY .mercial P.S. 0.025% P.E.T.S . .

lure, Time, Osmotic pressure, Time, months cm./30 min. cm. hr . cm. hr .

0 1 3

5-6 9

11

0 3

3.0 0.60 0.69 0.78 0.48 0.60

0.62 0.51

UFF/AF 1056/1

5.45 46 6.45 48 6.44 48 8.23 42 8.02 96 7.92 50

UFF/AF 257/3

8.66 90 - -

3.54 3.52 -

5.36 5.79

96 96 -

76 96

~

Benzene solutions a t 30°C.

424 M. F. VAUGIIAN

Variations in Membrane Properties

The nonuniform properties of the UFFIBF membranes are shown by the results in Table V1. The differences in solvent permeability may be due to slight variations in the osmometers used, but the differences in selectivity must be due to variations in the pore sizes of the membranes. As i t is difficult to produce membranes of exactly similar properties, it seems likely that these variations occur during manufacture.

The results shorn further that, while the average selectivity of membranes in different batches of ultrafein filters allerfeinst varies, nevertheless each batch appears to contain some very selective membranes.

Life Time of Membranes

Once conditioned to benzene, no major alteration in the properties of the membranes appears to take place, a t least over a period of several months,

TABLE; VIII Determinations in Different Solvents a t 30°C.

Selectivit,y

Permeability, fall from 10 cm. head of

benzene, cm./30 min

0.27

2.11

0.177

2.45

0.97

Osmotic pressures

Rising Falling Ob- Theorct- Concen- -~ served ical tration, h,, Time, h,,, Time,

Solute yo cm. hr. cm. hr.

Toluene (UFF,/AF 457/A2)

Commercial 1.0 8.85 120 8.80 167

Pentaerythritol 0.025 5.50 99 5.50 98 polystyrene

tetrastearate

Methyl Ethyl Ketone (UFF/F 457/4)

Commercial 1 . 0 4.19 48 4.24 48 polystyrene

tetrasteara te Pentaerythritol 0.025 0.21 42 - -

Ethanol (UFF/AF AB)

Polysarcosine 0.088 4 74 264 4.95 79 dime thylamide

(UFF/F 457/3)

Polysarcosine 0.044 2.32 T 5 2.04 24

0.088 4.10 50 4.03 22 0.132 6.48 75 6.41 18 0.176 8.40 48 8.25 19

dimethy lamide

Water (UFF/.AF 457/A6)

Carbowaxcom- 0.10 4.20 48 4.23 126 pound4000 0.15 6.88 42 7.25 117

ho , cm

h m cm.

8 83

5 30

4.23

0 21

4 74

2 32

4 10 6 45 8 40

4 21 7 04

8.77"

6.15

8. 77"

6.15

4.91

2.45

4.91 7.36 9.82

6.94 10 41

a Benzene value UFF/AF 257/3).

IJLTRAI~ISIN FIL'1'ERS AS OSMO'I'LC MEMBRANES 425

:ind aiiy slight chmges that do occur :we i l l the direction of greater selectivity (Table VII). It would appeal, therefore, that the membranes are relatively unaffected by the exposure to drying conditions which occurs during the filling procedure, when the osmometers are left out of the solvent for periods of up to 30 minutes. Longer periods of exposure, however, do lead to changes, as has been observed with ultracella filters.6

Other Solvents

Osmotic measurements have been made with the TJFF/AF membranes in the following solvents (see Table VIII).

Toluene. The results show that the osmotic properties in toluene are very similar to those in benzene.

Methyl ethyl ketone. Osmotic measurements were made with a pair of UFF/F membranes which had been conditioned to methyl ethyl ketone directly from benzene. However, the membranes mere rather soft and difficult to use.

Chloroform. Membranes conditioned to this solvent directly from benzene were too soft to use.

Dimethylformamide. The membranes dissolved in this solvent. Ethanol. Almost the correct osmotic pressure was obtained with one

concentration of an alcoholic solution of a monodisperse polysarcosine dimethylamide, an = 5,800 from kinetic analysis,' although i t took a very long time to develop. Apparent equilibrium was established faster with UFF/F membranes, but the osmotic heads were appreciably lower than the theoretical values.

Stable osmotic heads were developed with aqueous solutions of Carbowax compound 4000 (The Carbide and Carbon Chemicals Com- pany), although the value of ATn calculated from these results, 5800, is much higher than that stated by the manufacturers, i.e., 3000-3700.

Water.

V. CONCLUSIONS

These experiments have shown that ultrafein filters are useful membranes for osmotic pressure measurements, not only with aqueous solutions, but also with certain organic solvents, such as benzene, toluene, and ethanol. In the case of the best UFF/AF membranes, it has been concluded from experiments with solutions of pentaerythritol tetrastearate (M.W. 1200) and lower molecular weight compounds, that in benzene these membranes retain molecules of M. W. > 1500, and an an of 2460 has been determined.

With benzene solutioiis of uiifractionated polystyrene samples, it has been shown that the osmotic pressures developed with these UFF/AF membranes are appreciably higher than those developed with ultracella filters. Since these latter membranes have been widely used for the determination of values of an in investigations of the kinetics of polymerization, it would appear that the use of UFF/AF membranes in such experiments should lead to more accurate evaluations of the rate constants.

426 M. F. VAUGEIAN

We are indebted to The Distillers Co., Ltd. for the gift of the commercial polystyrene, to Dr. F. Rushman of the Paint Research Station, Teddington, for supplying the puri- fied pentaerythritol tetrastearate, and to Dr. D. G. H. Ballard and Dr. C. H. Bamford for preparing the sample of polysacosine dirnethylamide. The author also wishes to thank Miss S. A. Monksfield and Miss H. M. Sankey who carried out much of the experimental work. This work forms part of the program of the National Chemical Laboratory and is published by permission of the Director.

References

1. Frank, H. P., and Mark, H., J . Polymer Sci., 17,7 (1955). 2. Zimm, B. H., and Myerson, I., J . Am. Chem. Soc., 68,911 (1946). 3. Staverman, A. J., Pals, D. T. F., and Kruissink, Ch. A., J . Polymer Sci., 23, 57

4. Alvbng, F., and Samuelson, O., J . Polymer Sci., 24,353 (1957). 5. Staverman, A. J., Rec. Irav. chim., 70,344 (1951). 6. Harness, A. A., J . Polymer Sci., 24,492 (1957). 7. Ballard, D. G. H., and Bamford, C. H., I’TUC. Roy. Soc. (London), A223,495 (1954).

Synopsis

(1957).

The use of ultrafein filters as osmotic membranes has been investigated in various solvents. It has been found that, at 30°C., measurements can be made in benzene, toluene, ethanol, water, and, with difficulty, methyl ethyl ketone, but not in chloroform or dimethylformamide. In benzene it has been found that all grades of the membranes are more selective than the corresponding ultracella filters and that, with the allerfeinst grade, molecular weights of the order of 2000 can be determined. With solutions of pentaerythritol tetrastearate (M.W. l200), tristearin (M.W. 891), and sucrose octaacetate (M.W. 679), the osmotic pressures developed were less than the theoretical values. The ratios of observed to theoretical osmotic pressure decreased with molecular weight, and these ratios appear to be examples of the reflection coefficients predicted by Staverman.

RCsumC L’utilisation de filtres ultrafins comme membranes osmotiques a Btk 6tudi6 dans

diffkrents solvants. On a trouvB que les mesures effecturkes b 30°C peuvent &re faites dans le benzbne, tolube, Bthanol, l’eau, mais sont plus difficiles dans la butanone, mais impossibles dans le chloroforme ou la dimethylformamide. Dans le benzbne, on a trouvk que toutes les sortes de membranes sont plus skiectives que les filtres ultracelles correspondants, et qu’avec les membranes lea plus fines des poids mol6culaires de 2.000 peuvent &re dkterminks. Avec des solutions de tBtrast6arate de pentahythrite (P.M. 1,200), de tristkarine (P.M. 891) et l’octoac6tate de sucrose (P.M. 679) Ies pressions osmotiques sont plus faibles que les valeurs thkoriqiies. Les rapports de pressions osmotiques expkrimentales e t thboriques dkcroissent avec le poids molbculaire; ces rapports semblent &re des exemples des coefficients de r6flecxion pr4vus par Staverman.

Zusammenfassung Es wurde die Anwendung von Ultrafein-Filtern als osmotische Membrane in ver-

schiedenen Losungsmitteln untersucht. Bei 30“ Konnen Messungen in Benzol, Toluol, hhanol , Wasser und, mit einiger Schwierigkeit, in Methylathylketon, nicht aber in Chloroform oder Dimethylformamid ausgefuhrt werden. Es zeigte sich, dass alle Fein- heitsgrade der Membranen in Benzol selektiver wirken als die entsprechenden Ultra- cellafilter und dass mit den “Allerfeinst” Filtern Molekulargewiclite von etwa 2000 bestimmt werden konnen. Bei Losungen von Pentaerythrit-Tetrastearat (M.G. 1,200),

ULTRAFEIN FILTERS AS OSMOTIC MEMBRANES 427

Tristearin (M.G. 891) und Sucrose-octa-acetat (M.G. 679) war der ausgebildete osmotische Druck Kleiner als der theoretsiche Wert. Das Verhaltnis des beobachteten zum theoretischen osmotischen D ruck nahm mit dem Molekulargewicht ab und dieses Ver- haltnis scheint ein Beispiel fur den von Staverman vorausgesagten Reflexionskoeffizien- ten zu sein.

Received December 13,1957

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Ultrafein filters as osmotic membranes