New Characterization Methods for Asymmetric Ultrafiltration Membranes

15 New Characterization Methods for Asymmetric Ultrafiltration Membranes

Downloaded by UNIV OF BATH on March 15, 2016 | http://pubs.acs.org Publication Date: December 12, 1985 | doi: 10.1021/bk-1985-0269.ch015

C. A. SMOLDERS and E. VUGTEVEEN Twente University of Technology, Department of Chemical Technology,P.O.Box 217, 7500 AE Enschede, The Netherlands Three new methods to characterize the pore structure and pore size distribution in the top layer of asymmetric membranes have been developed or refined in our laboratory during the past few years: a) the gas adsorption/desorption method, b) thermoporometry and c) selective permeation (fractional rejection). Pore size distributions are determined from the hysteresis loop in gas adsorption/desorption isotherms and from calorimetric measurements by the shift in the melting (or freezing) peak for a phase transition of water inside the pores. The determination of the fractional rejection properties is done by permeation experiments of a macromolecular solute with a broad molecular weight distribution (MWD). The MWD of permeate and feed are compared and translated into a fractional rejection curve. The comparison of results obtained from these three independent methods for some characteristic membranes gives an indication of the strength and weakness of each of the methods studied.

The use of u l t r a f i l t r a t i o n (UF) membranes for the separation of d i s solved molecules of d i f f e r e n t size and nature has seen an increased interest i n recent years. Depending on their pore s i z e , membranes can be used i n a variety of f i e l d s , such as removal of particulates from a i r , f i l t r a t i o n of c o l l o i d a l suspensions, treatment of product streams i n the food and beverage industry, recovery of useful material from coating or dyeing baths i n the automobile and t e x t i l e industries and treatment of i n d u s t r i a l waste waters (\_ 2). UF membranes also serve as supports for u l t r a t h i n reverse osmosis (composite) membranes. Asymmetric u l t r a f i l t r a t i o n membranes consist of a thin, dense top layer (the skin), which i s responsible f o r the s e l e c t i v e reject i o n of solute molecules, and a more open, porous substructure that does not affect the membrane performance negatively. The most important c h a r a c t e r i s t i c s of these membranes are the 9

0097-6156/85/0269-0327$06.00/0 © 1985 American Chemical Society Lloyd; Materials Science of Synthetic Membranes ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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MATERIALS SCIENCE OF SYNTHETIC MEMBRANES

thickness of the top layer (hydrodynamic resistance) and the pore structure (mean pore size and pore size d i s t r i b u t i o n ) of the skin. In order to develop and subsequently use the most appropriate membrane for a certain application, one has to determine the features just men tioned above, using independent characterization methods. Amongst these are: - pure water flux and/or gas permeability C3,4_) ; - the c r i t i c a l a i r pressure (the bubble pressure) method (5); - electron microscopy (3,6-8); - molecular weight cut-off measurements. However, only a few methods can be used to characterize the pores i n the skin of the membrane. Although none of the techniques mentioned can give a complete description of the skin structure, a reasonable impression can be obtained by a combination of techniques. In our laboratory three new methods have been developed or r e fined during the past few years and w i l l be described below. These methods are: a) the gas adsorption/desorption method (9); b) thermoporometry (10); c) selective permeation ( f r a c t i o n a l rejection) (11). The gas adsorption/desorption. The determination of pore size and pore size d i s t r i b u t i o n from gas adsorption/desorption isotherms i s known from other types of adsorbents: a hysteresis loop occurs between the adsorption and desorption curves when a f u l l isotherm i s measured. This has been explained as being due to c a p i l l a r y condensation i n the pores of the adsorbent. As the pressure i s reduced, the adsorbate does not evaporate as readily from the c a p i l l a r i e s as i t does from a f l a t surface due to a lowering of the vapour pressure over the concave me niscus formed by the condensed vapour i n the pores. The lowering of the vapour pressure (p) f o r a c y l i n d r i c a l c a p i l l a r y of radius r ^ i s given by the Kelvin equation: -2yV RT In — = ρ *o

r, k

cos Θ

(1)

Here p i s the saturated vapour pressure of the system at temperature T[K], γ and V L are the surface tension and the molar volume of the ad sorbate i n l i q u i d form, R i s the molar gas constant and Θ the angle of contact between the l i q u i d and the walls of the pore. For nitrogen ad sorption/desorption at l i q u i d nitrogen temperature (77 K) i t leads t o : Q

=

r

k

zAil

(2)

10 log(p/p ) Q

The pore radius ( r ) can be calculated by p

r

p

= r

f c

• t

(3)

i n which t i s the thickness of the adsorbed layer of vapour i n the pores. A l l experiments were carried out with a Carlo Erba Sorptomatic, model 1800. The method of Barrett, Joyner and Halenda (12) which was

Lloyd; Materials Science of Synthetic Membranes ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


15.

Asymmetric Ultrafiltration Membranes

SMOLDERS AND VUGTEVEEN

329

refined i n 1964 by Dollimore and Heal (13) was used to calculate the pore sizes from the isotherms. A complete description of the method w i l l be given by Bodzek et al. (9). The method has already been employed for polymeric membranes by several authors (14-16). Although there are some l i m i t a t i o n s for using this technique, f o r example, c y l i n d r i c a l pores are assumed and the membranes have to be dried without damaging the pore structure before the measurements can s t a r t , results were obtained f o r UF mem branes made from d i f f e r e n t polymeric materials (Cellulose Acetate (CA), Poly-2,6-dimethyl-l,4-Phenylene Oxide (PPO) and some other non c e l l u l o s i c materials). Excellent results have been obtained f o r PPO membranes. In Figure 1 complete adsorption/desorption isotherms are given f o r PPO membranes, made from casting solutions containing 9% and 10% polymer (by weight). From the hysteresis loop the cumulative pore volume and the pore size d i s t r i b u t i o n are calculated, and these are shown i n Figure 2. From the d i f f e r e n t i a l pore volume versus pore radius graph i t can be seen that pores with about 2 nm radius are present i n both membranes. Furthermore, increasing the polymer concentration i n the casting solution leads to higher pore volume. The abrupt change i n the desorption branch of PPO membranes (Figure 1) indicates a narrow pore size d i s t r i b u t i o n i n con trast to the desorption isotherms of CA, Polysulfone (PSf) and Polya c r y l o n i t r i l e (PAN) membranes, which w i l l be shown i n one of our f u ture papers (9). There we w i l l discuss our experimental results i n more d e t a i l . Thermoporometry. This method i s based on the observation that the equi librium conditions of s o l i d , l i q u i d and gaseous phases of a highly dispersed pure substance are determined by the curvature of the i n t e r f a c e ^ ) (10,17). In the case of a l i q u i d ( i n this work, pure water) contained i n a porous material (the membrane), the s o l i d - l i q u i d i n t e r face curvature depends closely on the size of the pores. The s o l i d i f i cation temperature therefore i s d i f f e r e n t i n each pore of the material. The s o l i d i f i c a t i o n thermogram can be 'translated' into a pore size d i s t r i b u t i o n of the membrane with the help of the equations derived by Brun (17). For c y l i n d r i c a l pores, with water inside the pores, i t leads to the following equations: during s o l i d i f i c a t i o n : r

and during melting

=

^ Λ Τ ^ + 0.57

: r = —-r^p,m Δ1

+0.68

(4)

(5)

In these equations r i s the pore radius [nm] and ΔΤ i s the extent of undercooling [K]. A l l the calorimetric experiments were performed by cooling the samples with maximal speed (320 K/min) to 210 R and subsequently heating (after equilibrium was reached). So Equation 5 was used f o r c a l c u l a t i n g the pore r a d i i . A Perkin Elmer d i f f e r e n t i a l scanning ca lorimeter, model DSC I I , was used. With this apparatus, pore r a d i i from 2 to 20 nm can be determined. About 50 mg of membrane material (including water) was used f o r each experiment and a heating rate of 1.25 K/min gave reproducible r e s u l t s . After a thermogram was obtained, i t was analyzed with the help p



330


Figure 1.

Nitrogen adsorption and desorption isotherms f o r polyphenylene oxide (PPO) membranes. (PP0-9 and PPO-10 indicate the polymer concentration (wt. %) i n the casting solution).



15.

SMOLDERS AND VUGTEVEEN

20

Figure 2.

40


60

80

100

120

140

331

.160

Pore Radius [Â]

Cumulative pore volume and pore size d i s t r i b u t i o n f o r PPO membranes, calculated from gas adsorption/desorption isotherms.


332


of a computer program i n order to obtain the cumulative pore volume vs. pore radius i n i n t e g r a l or i n d i f f e r e n t i a l form. More detailed i n formation w i l l be given i n the near future by Vugteveen et al. (10). Several membrane materials were analysed (for example PPO and PSf). In Figure 3 the cumulative pore volume ( V ) and pore size d i s t r i b u t i o n (dV/dr ) are given f o r some PPO membranes (casting solu t i o n 10% polymer by weight). In this figure the values 0.15 and 0.20 refer to the casting thickness (mm). The dependency of the pore volume on the casting thickness i s obvious. Assuming a l l the measured pores are i n the skin layer and assuming the porosity (ε) being constant, an increase i n pore volume means that the skin becomes thicker. Similar results were found by Broens et al. (16) by means of the gas adsorp tion/desorption method. Furthermore, one can see from this figure that the shape of the d i f f e r e n t i a l pore size d i s t r i b u t i o n (dV/dr ) does not change s i g n i f i c a n t l y : most pores were between 1.5 and 4 nm i n s i z e , and the mean pore radius remains p r a c t i c a l l y constant. Therefore, we may conclude that PPO membranes have a narrow pore size d i s t r i b u t i o n . Experiments with UF membranes made of polysulfone (P3500, Union Carbide) leads to a quite d i f f e r e n t thermogram as shown i n Figure 4. In this figure two thermograms are given, one for PSf and one f o r PPO. The thermogram of the PSf membrane does not return to the baseline. Hence no d i s t i n c t i o n can be made between the pores i n the skind and pores i n the supporting layer. I t i s assumed that pores below the skin of PSf membranes gradually increase i n size and are cone-shaped. c u m

p


p

Selective permeation. In order to come to a more precise characteriza t i o n of the r e j e c t i o n properties of an UF membrane we developed ameth od (11) i n which the molecular weight d i s t r i b u t i o n (MWD) of macromole cules present i n the permeate i s compared tp the one present i n the feed. For these experiments a macromolecular solute i s used with a broad MWD; For example Polyethylene Glycol (PEG) 100 000 or mixtures of various PEGs. Comparing the MWD of the feed with the MWD of the permeate, the f r a c t i o n a l r e j e c t i o n (Rj^) i s defined as follows:

„ *M.

1

_

w_. -w (1-R --) Μ. - , M. _ overall i,feed ι,permeate w~ M. ι,feed

( 6 )

£

i n which w^. i s the weight f r a c t i o n of a certain molecular weight Equation 6 can be derived from:

M^.

C -C M. . M.

C i n combination with C ^ = C.w ^ M

M

and R e r a l l " O V

1 _

where C j ^ i s the

concentration of macromolecules with molecular weight M^, and C i s the t o t a l i n i t i a l concentration of the macromolecular solute. The samples of permeate and feed were analysed using the high performance l i q u i d chromatographic/low-angle laser l i g h t scattering (HPLC/LALLS) method. The columns used were TSK (Toyo Soda) G4000PW and



15.

SMOLDERS A N D VUGTEVEEN

0

Figure 3.

20


40

60 Pore Radius

. 80 [A]

100

Cumulative pore volume and pore size d i s t r i b u t i o n f o r PPO membranes, calculated from the thermograms. (The numbers 0.15 and 0.20 indicate the casting thickness (nm) during preparation of the membranes).


M A T E R I A L S SCIENCE O F SYNTHETIC M E M B R A N E S


334


15.

S M O L D E R S A N D V U G T E V E E N


335


G3000PW ( i n s e r i e s ) , detection took place with a d i f f e r e n c i a l r e f r a c tometer (Brice Phoenic, λ = 633 nm) and the LALLS apparatus KMX-6 of Chromatix, according to the method described by McConnell (18). Most experiments were performed at laboratory temperature using an Amicon low pressure c e l l model 40IS with an e f f e c t i v e membrane area of 37.4 cm^. An operating pressure of 0.3 MPa was used. The i n i t i a l feed concentration was 1000 ppm. More d e t a i l s can be found i n one of our future papers by Vugteveen et al. (11). A comparison of the f r a c t i o n a l r e j e c t i o n (R^^) with the 'clas s i c a l ' cut-off curve, which can be seen i n the upper part of Figure 5

10

3

4

5

10

10 Molecular weight

Figure 5.

( I n t r i n s i c ) f r a c t i o n a l r e j e c t i o n (R^.) and ( i n t r i n s i c ) c l a s s i c a l r e j e c t i o n (R) curves for a PSf membrane, using PEG 100 000 as solute.


336

M A T E R I A L S SCIENCE O F SYNTHETIC M E M B R A N E S 1

T

f o r a P S f m e m b r a n e , seems t o s h o w t h a t t h e cut-off value i s reached a t l o w e r m o l e c u l a r w e i g h t . The m a g n i t u d e o f d i f f e r e n c e c a n be e x p l a i n e d b y t h e f a c t t h a t u n t i l n o w , n o c o r r e c t i o n h a s b e e n made f o r the i n f l u e n c e o f the c o n c e n t r a t i o n p o l a r i z a t i o n phenomenon. T a k i n g t h i s i n t o account, a quite d i f f e r e n t curve i s obtained (lower part of F i g u r e 5). H o w e v e r , i n t h i s c a s e , t h e o p e r a t i n g c o n d i t i o n s l e a d t o a h i g h c o n c e n t r a t i o n p o l a r i z a t i o n a t t h e membrane i n t e r f a c e . Another p o i n t o f i n t e r e s t i s that the f r a c t i o n a l r e j e c t i o n c u r v e , l i k e the ' c l a s s i c a l c u t - o f f c u r v e , a l s o depends on the t y p e o f m a c r o m o l e c u l a r s o l u t e u s e d a s i s s h o w n i n t h e u p p e r p a r t o f F i g u r e 6. T h e


1

F i g u r e 6.

( I n t r i n s i c ) f r a c t i o n a l r e j e c t i o n curves for a PSf membrane b a s e d o n d i f f e r e n t k i n d s o f m a c r o m o l e c u l a r solutes.


15. S M O L D E R S A N D V U G T E V E E N


337

PEG mixture consists of equal parts (by weight) of PEG with the f o l lowing molecular weights: 3000, 6000, 10 000, 20 000, 40 000 and 100 000 at a t o t a l concentration of 0.1%. The dextran solution of equal parts of Dextran T10 and T500 with molecular weight of 10 000 and 500 000 respectively, also at a t o t a l concentration of 0.1%. How ever, also i n this case the operating conditions are not well chosen as can be seen i n the lower part of Figure 6. In future work (11) we w i l l show experiments i n which the influence of concentration p o l a r i zation i s not so overwhelming. Together, f r a c t i o n a l r e j e c t i o n curve (R^.) and t r a d i t i o n a l cut o f f curve w i l l give more information on the r e j e c t i o n c h a r a c t e r i s t i c s of u l t r a f i l t r a t i o n membranes.


Acknowledgments The authors wish to express t h e i r gratitude to M. Bodzek, H.J.C. te Hennepe, H. Boertien, G. Grooten and G. van de Ridder. They a l l con tributed to this work by performing the numerous experiments. Parts of this work w i l l be used by one of the authors (EV) as f u l f i l l m e n t of the requirements of the Ph.D. degree at Twente University of Tech nology, Enschede, The Netherlands. This work w i l l also be published i n the form of three separate a r t i c l e s to be submitted to the Journal of Membrane Science, giving more detailed information. Legend of symbols

L

t o t a l ( i n i t i a l ) concentration of macromolecular solute [g cm~3] concentration of macromolecules with molecular weight M^ [g cm~3] vapour pressure of the system at temperature Τ (Κ) saturated vapour pressure of the system at temperature Τ

M£

Ρ Po

( κ )

gas constant, 8.313

[ J mol

1 -I Κ ]

f r a c t i o n a l r e j e c t i o n of a certain molecular weight M^ 'overall' r e j e c t i o n of a macromolecular solute radius of the c y l i n d r i c a l c a p i l l a r y [nm] pore radius [nm] temperature [K] extent of undercooling [K] thickness of the adsorbed layer of vapour i n the pores [nm] molar volume of the adsorbate [cnr* mol ] weight f r a c t i o n of a c e r t a i n molecular weight M^

R

overall *k

Τ ΔΤ t V w. L

M

surface tension of the adsorbate [dyne cm ^] angle of contact between the l i q u i d and the walls of the pore [°]

γ Θ

Subscripts M£ f ρ

= = =

molecular weight (M) f r a c t i o n i feed permeate


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MATERIALS SCIENCE O F SYNTHETIC M E M B R A N E S


Literature Cited 1. Michaels, A.S. Chem. Eng. Progr. 1968, 64, 45. 2. Porter, M.C.; Michaels, A.S. Chemical Technology 1971, 1, 56; 1971, 1, 248; 1971, 1, 440; 1971, 1, 663; 1972, 2, 56. 3. Kesting, R.E. In "Synthetic Polymer Membranes", McGraw-Hill Inc., New York, 1971. 4. Yasuda, H . ; Tsai, J . T . J . Appl. Polym. Sci. 1974, 18, 805. 5. Jacobs, S. F i l t r . Separ. 1972, 9, 525. 6. Riley, R . L . ; Gardner, J . O . ; Merten, U. Desalination 1966, 1, 30. 7. Riley, R . L . ; Gardner, J . O . ; Merten, U. Science 1964, 143, 801. 8. Merin, U . ; Cheryan, M. J . Appl. Polym. Sci. 1980. 25, 2139. 9. Bodzek, M.; Vugteveen, E.; Heskamp, H . ; Noordegraaf, D.; Smolders, C.A. submitted to J . Membrane Sci. 10. Vugteveen, E.; te Hennepe, H . J . C . ; Bargeman, D.; Smolders, C.A. submitted to J . Membrane Sci. 11. Vugteveen, E.; Bargeman, D.; Smolders, C.A.; to be submitted to J . Membrane Sci. 12. Barrett, E . P . ; Joyner, L . G . ; Halenda, P.P. J . Amer. Chem. Soc. 1951, 73, 373. 13. Dallimore, D.; Heal, G.R. J . Appl. Chem. 1964, 14, 109. 14. Ohya, H . ; Imura, Y . ; Moriyama, T . ; Kitaoka, M. J . Appl. Pol. Sci. 1974, 18, 1855. 15. Ohya, H . ; Konuma, J.; Negishi, Y. J . Appl. Pol. Sci 1977, 21, 2515. 16. Broens, L.; Bargeman, D.; Smolders, C.A. Proc. 6th Int. Symp. Fresh Water from the Sea 1978, Vol. 3, 165. 17. Brun, M.; Lallemand, Α.; Quinson, J.F.; Eyraud, Ch. Thermochimica Acta 1977, 21, 59. 18. McConneil, M.L. American Laboratory 1978, May. RECEIVED August 6, 1984


New Characterization Methods for Asymmetric Ultrafiltration Membranes

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