Preparation and Characterization of Cellulose Acetate Membranes for Osmosedimentation

Introduction

Approach to sedimentation equilibrium under gravity or in a centrifuge is much faster than usual if the solution or dispersion under study is contained in one compartment of a dialysis cell, while the other is filled with solvent. This is a recent finding (1 -5) which opens the way t o a number of new concentration and separation techniques, applicable t o polymer solutions and t o colloidal dispersions. For example, we have demonstrated that it is possible to spin down a dextran dissolved in water, just by using a dialysis cell fitted in a tabletop cen- trifuge tube holder, a t 3000 rpm (4). A solution of the same dextran, centri- fuged under similar conditions but contained within a standard test tube, did not show any tendency t o sediment, in agreement with well-known and experimen- tally verified theoretical predictions.

In this kind of sedimentation experiment (which we call osmosedimentation) it is essential t o have the dialysis cell fitted with membranes which reject the solute but are highly solvent-permeable. The rate of approach to equilibrium in osmosedimentation and in “normal” sedimentation is highly dependent on the membrane permeability and values in the 10’’ - 10’’ molecules cm-’ s-l J-’ mol range are needed for practical purposes (3). Moreover, osmosedimentation devices require large amounts of membranes, in various sizes and shapes. On the other side these membranes are never subjected to large pressure differences, for which reason they d o not have to be mechanically strong.

These requirements are rather unique and we were thus prompted t o develop methods t o prepare cellulose acetate membranes suitable for osmosedimentation work. These methods and evaluation data on the membranes obtained are de- scribed in this article.

Experimental

The membranes were cast from cellulose acetate (Carlo Erba, 53% acetyl con- tent,M,isc = 3 X 10‘) dissolved in acetic acid-acetone-water. The solution was spread with a glass rod on a piece of window glass, fitted with stretches of ni- chrome wire t o give the desired film thickness. Solvent was allowed t o evapo- rate for 2 min in the laboratory’s atmosphere and the filmcovered glass was im- mersed in distilled water a t room temperature. Membranes were kept immersed in water a t 2 4 ° C .

For protein retention measurements, membrane disks (12.8 cm2 each) were cut and mounted on dialysis cells made of Plexiglas.

One compartment was filled with 0.1% gamma globulin solution in 0.1M NaCl and the other with O.1M NaCl aqueous solution. After 24 h at 3-5°C the cell contents were withdrawn and solution absorbances were read in a PMQ-I1

-

Journal of Polymer Science: Polymer Letters Edition, Vol. 21, 49-55 (1983) 0 1983 John Wiley & Sons, Inc. CCC 0360-6384/83/010049-07$01.70

50 POLYMER LETTERS EDITION

';u 0.03 E

-?

E \

' C .- - E 0.02

2 QOI

X

0 40 80

%ACETONE (m/m) Fig. 1. Effect of percent acetone in the solvent mixture on the water flux (A)

and gamma globulin retention (0) from a 0.1% solution in 0.1M aqueous NaCI. The casting solution was 12% in cellulose acetate and the membranes, tested under 1 m of H20 and in dialysis cells, were annealed at 60°C for 1.5 h.

t

0 8 I0 12

% ACETATE (dm)

Fig. 2. Effect of cellulose acetate concentration in the casting solution on water flux (A) and retention (0) of a 0.1% gamma globulin solution in 0.1M aqueous NaC1. The solvent mixture was 3 1% acetone, 41% acetic acid, and 28% water. The membranes were annealed at 60°C for 1.5 h and tested under a 1 -m H20 column head.

Zeiss spectrophotometer. The retention was taken as the ratio between dialyzed solution and initial solution absorbances.

Water permeability was determined by fitting the membranes in Plexiglas dialysis cells or in Millipore Swinnex 47 filter holders and measuring water (or solution) fluxes under the given pressures. Gamma globulin retention under

POLYMER LETTERS EDITION 5 1

A

02 Q3 0.4 Q5

WIRE THICKNESS/ mm

00

a9 ‘1

60 0 I- z w I- W (z

20

Fig. 3. Effect of the wire thickness on retention (0 ) and flux (A) of a 0.1% gamma globulin solution in 0.1M aqueous NaCI, under 2 atm. The casting solu- tion was 10% cellulose acetate, 37% acetic acid, 29% acetone, and 24% water.

I 0 100 XK)

TIME/min

Fig. 4. Effect of compaction under 1 m of H20 on membranes with (0) and without (A) previous annealing treatment. The casting solution was 12% cellu- lose acetate, 26% water, and 62% acetic acid.

flow was measured by pumping (2 atm) a 0.1% solution over the filter, collecting the first 5 mL of effluent, and measuring its absorbance.

Results

In our earlier work (2), membranes were cast from cellulose acetate solutions in acetic acid-water. They showed appropriate permeabilities t o water but re- tention was satisfactory for very high-molecular-weight solutes, only [Blue Dex- tran (4), PTFE latex (2), colloidal silver (S)] . Thus, we have examined the be- havior of casting solutions using acetone-acetic acid-water as the solvent mix- ture.

Acetone concentration has a major effect on the membrane permeability, as

5 2 POLYMER LETTERS EDITION

0 33 40

TIME/ min

Fig. 5. Effect of compaction under 2 atm, on membranes cast using solutions of different acetone contents: (0) 0%, (m) 12.5%, (A) 32.8%, (0) 52.1%. The membranes were annealed at 60°C for 1.5 h .

shown in Figure 1. On the other side, it increases solute (gamma globulin) reten- tion. From this figure, we can see that ca. 98% gamma globulin is retained by membranes having permeabilities of the order of lo-' g-' cm-' s mol, well in the range required for osmosedimentation experiments.

Membranes can be prepared using various concentrations of cellulose acetate in the casting solution. Below ca. 7% acetate the membranes obtained are too fragile; above 12% casting is rendered difficult by the excessive viscosity of the solution. The results of water permeability measurements and gamma globulin retention (Fig. 2 ) given show that lower cellulose acetate concentrations yield membranes with higher permeabilities but lower retentions. In this case, a de- crease in retention from 98.7 t o 93.9% is accompanied by a 16-fold increase in permeability.

The effect of the thickness of the casting solution layer used in membrane preparation was also examined. Solution thicknesses in the 0.5 1-0.20-mm range were used, yielding coagulated membranes 0.2-0.07 mm thick, for a range of solution concentrations. Typical data are reported in Figure 3 showing that membranes from thicker castings are more water-permeable and less solute-reten- tive than thinner ones.

Membrane compaction under a water head pressure of 1 m is easily noticeable. Measurements performed with aged (by heating to 60°C for 1.5 h , under water) and nonaged membranes are shown in Figure 4. It is clear that annealing reduces water permeabilities considerably but nonaged membranes are certainly much more susceptible t o compaction. Membrane compaction was also observed at higher pressure ( 2 atm). Data in Figure 5 show that permeability decreases due t o Compaction by 30-5076, a behavior similar t o that observed under 0.1-atm head pressure. From this, we conclude that permeable, collapsible structures in the membrane are rather weak, being already affected at a low pressure. More- over, compression-decompression cycles d o not give superimposable measure-

POLYMER LETrERS EDITION 53

Q4

P E c! Oc

E

- - E > Q2

3 LL

0 0 I 2

PRESSURE /atm

Fig. 6. Hysteresis effect on membrane compaction going from (0) 2 to 0 atm, (m) 0 to 2 atm, and (A) 2 to 0 atm again. The casting solution was 12% cellulose acetate, 26% water, and 62% acetic acid, and the membranes were annealed at 60°C during 1.5 h.

20 40

96 ACETONE ( W/W)

Fig. 7. Effect of acetone content in the solvent mixture on the retention of 0.7% gamma globulin solution in 0.1M aqueous NaCl, in dialysis cells under 1950 rpm during 24 h. The casting solution was 12% cellulose acetate and the membranes were annealed at 60°C for 1.5 h.

ments, as observed in Figure 6. Thus, to some extent the membranes recover themselves from compression if they are brought back to low pressure.

To check the membranes’ retention characteristics in osmocentrifugation ex- periments, they were fitted to dialysis cells, where one side was filled with 0.7% gamma globulin and the other with solvent (0.1M aqueous NaCI). The dialysis

54 POLYMER LETTERS EDITION

cells were spun at 1950 rpm for 24 h. The amounts of gamma globulin retained in these experiments are plotted as a function of acetone in the casting solvent in Figure 7. These results confirm that the membranes are sufficiently retentive t o be used in centrifugation experiments.

Discussion

In this work, we have observed an increase in retention and a decrease in per- meability of cellulose acetate membranes cast from solution in water-acetic acid-acetone, when the concentration of the latter is increased. This effect can be associated to the higher volatility of acetone, leading to higher cellulose ace- tate concentrations in the superficial layer of the cast film. According to Kest- ing model for membrane formation (6-9), acetone can allow formation of small- er pores in the membrane filtering layer, due to its good solvent characteristics and concurrent tendency t o the formation of smaller supramolecular aggregates. A result which may be understood using the same arguments is the effect of polymer concentration in the casting solution on membrane properties: dilute cellulose acetate solutions give highly permeable but low retentive membranes, due to the difficulty t o originate dense surface layers.

The effect of the casting solution thickness on the membrane properties is more interesting: Thicker films did not allow the formation of denser mem- branes, probably due to solvent transport from beneath the surface, during evap- oration. Indeed, one should attempt t o use still thinner casting f ims, provided tear resistance is not insufficient for handling.

The experiments on membrane compaction and on thermal aging showed that highly permeable but unstable structures are obtained in the membranes cast using low acetone concentrations. This is partly avoided by using higher concen- trations, a behavior which can again be ascribed t o the presence of denser super- ficial layers and (concurrently) thinner porous layers.

The membranes described in this work are suitable for osmocentrifugation experiments. Work along this line is currently in progress.

Acknowledgments

FG acknowledges FAPESP Grant 8l/0170-0 and CNPq Grant 4015 14/80. SPN is a FAPESP fellow.

References

( I ) F. Galembeck, J. F . Rodrigues, P. R. Robilotta, E. A. Pinheiro, I . Joekes,

(2) F. Galembeck, P. R . Robilotta, E. A. Pinheiro, 1. Joekes, and N. Ber-

(3) F. Galembeck, I . Joekes, and E. A.Pinheiro, An. Acad. Bras.Cienc., 53,

and N . Bernardes, An. Acad. Bras. Cienc., 51, I73 (1979).

nardes, J . Phys. Chem., 84, 112 (1980).

25 (1981).

POLYMER LETTERS EDITION 55

(4) F. Galembeck, Communication to the 10th NERM/ACS, Potsdam, NY, 1980.

(5) A. T. N. Pires, I . Joekes, S. P. Nunes, and F. Galembeck, Communication to IUF’AC-MACRO 82, Amherst, MA, 1982.

(6) R. E. Kesting, in “Reverse Osmosis and Synthetic Membranes,” S. Sour- irajan, Ed., National Research Council of Canada, Ottawa, 1977, p. 89.

(7) S. Sourirajan and B. Kunst, in “Reverse Osmosis and Synthetic Mem- branes,” S. Sourirajan, Ed., National Research Council of Canada, Ottawa, 1977, p. 129.

(8) 0. Kutowy and S. Sourirajan, J . Appl. Polym. Sci., 19,1449 (1975). (9) L. Pageau and S. Sourirajan, J. Appl. Polym. Sci., 16,3185 (1972).

Suzana Pereira Nunes Fernando Galembeck

Institute of Chemistry Universidade Estadual de Campinas CP 6 154, Campinas SP Brazil

Received June 25, 1982 Accepted July 15, 1982