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The mechanism for formation of “skinned” membranes
Desalinuriun
THE
- Elsevier Publishing Company,
MECHANLSM
II. EQU!LIBRfUM
H~drona~rics-~sraef
FORMATlON
- Printed in The Netherlands
OF “SKINNED”
PROPERTIES AND OSMOTIC FLOWS DETERMINING
M. A. FROMMER, (RLweived
FOR
Amsterdam
I. FEINER, Ltd.,
Kiriur
0. KEDEM Weizmznn.
AND
Rehov5r
MEMBRANES
MEMBRANE STRUCTURE
f<. BLOCH (Israel)
August 13 1969; received in revised form October 31. 1969)
SUMMARY
The critical water concentrations required for initiating precipitation of cellulose acetate from 209; solutions in acetone, dioxane, acetic acid, triethyl phosphate (TEP), dimethyi formamide (DMF) and dimethyl sulfoxide (D.MSO) have heen determined. The direction and magnitude of volume Bow as well as the contribution of solvent and water to this osmotic flow through a porous cellulose acetate membrane separating pure water from a 2 : 3 water-solvent mixture have been measured. A mechanism based on the assumption that the density of each layer of the membrane is determined by the concentration of the corresponding layer of the cast polymer solution at the precipitation point has been suggested. The voiume concentration of the polymer at the precipitation point has been correlated to: a. the concentration of non solvent required for precipitating cellulose acetate b. the direction and magnitude of osmotic flows of liquids into or out of the cast solution during teaching INTRODUCTiON
The procedure for preparing composite structure (“skinned”) membranes from cellulose acetate may be divided into the following major stages: 1. casting the polymer solution, 2. exposure of the cast layer of polymer solution to the air, 3. precipitation of the polymer and leaching out the solvent in an ice-water bath, and 4. annealing of the membrane in hot water. Various mechanisms have been suggested for explaining skin formation. They were summarized by Lonsdale (I). Ah these mechanisms are essentially based upon the same fundamental assumption, namely that the density of each layer of the membrane is determined by the concentration of the poIymer solution at precipitation. This means that a dense, non-porous skin forms when the polymer precipitates from a highly concentrated solution, whereas a porous membrane results from precipitation from a dilute solution. Desahbation,
7 (1970)
393402
394
hf.
A.
FROMMER et al.
It has been suggested (I, 2) that during the “drying period” the soIvent evaporates and the upper layer of the cast solution concentrates. The immersi,m of the cast plate inzan ice-water bath causes instantaneous precipitation and. since the conatntration of the upper layer of the i3ast solution is higher, the density of the precipitate4 membrane in that region should afso be higher. The annealing step is assumed to tighten the structure of the skin. This mechanism imp&z that rkinned membrane; can be furmed only from casting solutions containing a vofatite s&vent, and that the skin may be Iocated only at the r+cast solution interface. Both of these assumptions were invalidated
by the observations described in the first paper of this series (3). When studying the structure and properties of cellulose acetate membranes cast from binary solutions, it was found that: membranes cast from acetone or dioxane are homogeneous and dense; from triethyl phosphate homogeneous and porous; from acetic acid, composite with skin on the air-side surface. and from dimethy suIfoxide or dimethyl formamide. with skin on the supported side. Since evaporation of votatilr solvents during air exposure can no longer account excIusiveIy for the formation of composite structure membranes, a more thorough study of other stages of the preparation procedure is required, In this paper we shall correIate the porosity of ceflulose acetate membrrtnes cast from binary solutions and leached immediately in water to: 1. the concentration of water required for precipitating
from the casting solution 2. the direction and magnitude
the celIuIose acetate
of osmotic flows taking place during Ieaching
EXPERIMENTAL
Ma3eriaIs Cellulose acetate Eastman type E-398-3 - was dried at 105°C in an evacuated desiccator and stored in vactmm. Acetone, dioxnne, acetic acid and dimethy formamide were of analytic& grade purity. Triethyl phosphate and dimethy sulfoxide were redistilled from technical reagents. Porous celluiose acetate
membranes used for measurin’g the direction and magnitude of osmotic flows were cast from 20% dried E-398-3 ceIIufose acetate in TEP, Ieached imme~ateIy
in an ice water bath, annealed 30 min. at 80°C and stored in water. Annealing was performed in order to eliminate pinholes and imperfections possibly existing in the non-annealed membrane. Ttitiated water and Cx4 labeled acetic acid were obtained from New Engfand Nuclear Corp. and C” IaheIed acetone was manufactured by the Radio ChemicaI Centre.
The concentration of water required for precipitation of cellulose acetate was determined by adding a known amount of water-solvent
from 20% solution
MECHANISM
mixture
acetate.
FOR
FORMATION
OF “SKINNED”
tc an erlenmeyer containing The erlenmeyer was sealed
MEMBRANES II
395
a known weight of carefully dried cellulose immediately after the addition of the warer-
solvent solution and the mixture was stirred thoroughly with a magnetic stirrer for four days. The exact water content in the liquid mixture was determined by the KarLFiscSer technique. utilizing a “hfetrohm” E 408A Karl-Fischer tittator and “F:uka” reagents. The formation of a turbid precipitate and the existence of two disringuishable phases were taken as indication for phase separation. me initial direction and magnitude of volume flow through a porous cellulose acetate membrane separating pure water from a 2: 3 (weight ratio) water:solvent soiution were measured at (30.0 -i_ O.l)C in a po!ypropylene cell described in Fig. 1. The volume of each half-cell was 80 ml. and the open area of
.-s .’ I) 2) 3) 4) 5)
,
Fig. I. Cell for measuring osmotic Capillary t&e hoider Cell body, Polypropytme Washer *o* rings ‘0’ rings
6) ‘0’ rings 7) Capilinry tube
volume Rows 8) Wing nut 9) Cell base IO) Liquid leveling screw t 1) Locking screw 12) Hypodermic syringe 13) Spiral stirring blade
14) Porous membrane
the membrane was 12.6 cm’. The 2 : 3 water : solvent ratio is the lowest permissib!e ratio for the acetic acid, dioxane and acetone systems. IMembranes equilibrated with sohrtions having higher solvent content considerably swell or even dissolve in the solution. The tota! volume flow through the membrane J, is the vector net flows of water Jw and organic solvent Js through it, i.e., J,
sum
of the
= J,. -I- Js
The magnitude
of solvent
flow into the pure water compartment
can be determined
Desalination, 7 (1970) 39342
396 from
hf. A.
measurements
and magnitude
of the rate of carbon-14
of the net water
labeled
flow can be computed
solvent from
momim ei al.
flow. The direction the rate of tritium
labeled water flow in each direction. These measurements were performed at the same temperature of (30.0 f 0.1)X in a diffusion celi shown in Fig. 2. In this cell the open area of the membrane
was 6.6cm2
Fig. 2. Diffusion cell 1) Air inlet. tube, sample port
about 20 mt.
7) Half cell 8) Washer 9) Spiral stirringblade 10) Magnetic stirring bar 11) Lcjcking screw 12) Wang nut
2) Spacer
3) 4) 5) 6)
and each half-ceil contained
‘0’ ring ‘0’ ring Cell body. teflon Porous membrane
RESULTS
Table for initiating
1 summarizes
the experiments
on the water concentration
required
phase separation
in a 200/, cellulose acetate solution, and the data on flows through a porous cellulose acetate pure water from a 2 : 3 water :soivent mixture. Column 2 the results of our study on the structure and properties of
the direction and magnitude
membrane separating of Table I summarizes
of osmotic
membranes cast from 20% binary solutions and leached immediately in water (3). Since in 20% polymer solutions it is very difkult to distinguish between a viscous gelatinous solution and a swollen precipitate, we cannot give more accurate figures for the critical water content needed for initiating phase separation. The figures given are, however, meaningful within 2% error. Initial rates of volume flows through a porous cellulose acetate membrane separating pure water and a water:solvent mixture were computed from the change with time of the liquid DesalinaZion, 7 (1970) 393-402
MECHANISM
TABLE
FOR FORMATION
MEMBRANES II
397
I
STRUCTURE
OF
CENTRATION AND
OF “SKINNED”
CELLULOSE
REQUIRED
DIRECTION
AND
4 membrane
Acetone
Dioxanc Acetic acid TEP DMF DMSO
_..._ ~--
Homogecneous very dense Homogencous dense Skin, air side Homogeneous porous Skin glass side ..
--
MEMBRANES
FREClPlTATlNG
MAGNITUDE
Sirucrare
Sulwnr
ACEXATE FOR
OF OSMOTIC
Approx. water cant. at precip. poinr o/0
CASI-
FROM
CELLULOSE FLOWS
20%
ACETATE
THROUGH
BINARY FR0.W
A POROUS
SOLUTIONS.
20%
WATER
BINARY
MEMBRANE.
Initial rates of solrrnre and &aid flows r~roi~g~ a porous CA. membrane separathg pare water from a 2~3 w-let : solvert mixfare, at (30.0&0.1) “C. Ffmes are given in anits of (nricrolire+cm”-. sec. ‘100 p) V&r.re pux Water influx SoCvent orujhx +* Direction
Magiritade
28
Solution to water
0.12.
-0.04*
30
..
0.08
0.06
0.14
31
Notdetectable ..
-
> 0.06
0.13
0.07
0.07
Water to solution ..
0.08
0.16
O-08
0.12
0.27
10 12 15
.--
--.--
CON-
SOLUTIONS,
--.-
.--
0.16+
0.15
-_.__
* The sum of the acetone flux and water flux as measured by tritiated water and C1a, labelled acetone Rows are higher than the volume flux measured in the capillary tubing cell. More accurate measurements of osmotic flows, confirming the possibility of negative anomalous osmosis in this system will soon be published. l * The solvent outflux in the systems containing dioxane, TEP, DMFand DMSO were not measured directly. but computed from the volume fh~x and the tritiated water Rows into and out of the solution.
fronts in the capillary tubings liquid menisci in the capillary
of the osmotic celi. The velocity of movement of the tubings was constant for approximately 30 minutes.
The initial rates of water and solvent flows were determined
in a similar
manner. One chamber of the diffusion cell was filled with a non labeled liquid (water or water-solvent mixture) and at zero time a liquid containing tritiated water or Cl4 labeled organic solvent was added to the other chamber. Ten samples were taken at approximately 3 min. intervals. The water and organic solvent Radiation was ffows were cahzulated from the H3 and C’* radiation respectively. measured with the aid of a Packard Tricarb Liquid Scintillation Spectrometer. It will be shown in the Discussion that the most meaningful measure for solvation power to be correlated with the structure of cellulose acetate membranes cast from 20”,/, solution and leached in water, is the concentration of water required for precipitation of the polymer from such concentrated solutions. The data Desalination, 7 (1970) 393-402
shown in Table 1 enable us therefore to subdivide the six solvents used for casting membranes into three groups according to their “solvation power” for cellulose acetate. TEP is the poorest solvent for cellulose acetate and only CCL10:; water is required for precipitating the polymer. DMF and DMSO are somewhat better solvents than TEP since 12-15°/0 water is required for initiating phase separation. Acetone. dioxanc and acetic acid are the best solvents in this series of liquids, forming a clear solution even if the water content is 28% or higher. The data of Table I also enable us to subdivide rhe six solvents into three groups according to the directic n of osmotic volume flow through a porous cellulose acetate membrane separating pure water from a 2 : 3 water :soiven~ mixture. When acetone or dioxane comprise the organic solvent in the watersolvent mixture. there is a net volume flow out of the solution compartment into the water compartment. whereas when DMF or DMSO are present in the solution. rhe volume of water flowing into the solution compartment is higher than the volume of soIvent flowing out of it. When TEP or acetic acid are contained in the binary solvent-water solution, no volume flow to either direction could be detected, within experimental error.
Various authors (I, 4, 19, 20) tried to correlate data on the interactions of ceIlulose acetate with various solvents with the structure and porosity of cellulose acetate membranes cast from these solvents. Reviewing the literature on chemical physical properties of cellulose acetate in solutions (4-18) one finds that the extent of solvent polymer interaction changes considerably with the concentration of the polymer in solution. Thus rhe FIory-Huggins polymer-solvent interaction parameter-X, changes considerably with the volume fraction+, of cellulose
Fig. 3. Flory-Hums solvent-polymer interaction Parameter-X,. volume fraction of the polymer-qk, for various cellulose acetate systems.
as a function
Desalination,
OF the
7 ( 1970) 393402
MECHANISM
0.5
-
0
E
,
I
OF “SKINNED”
I
I
I
i
I 0.2
i
MEMBRANES
399
It
Osmotic 00t0
\
-0s X?
FOR FORMATION
\
\
--I5 F \
-1.0
\
i l
-2.0
0
0.1
0.3
$2 Fig. 4. Values of Flary-Huggins solvent polymer interaction parameter-X1, acetate acetic acid system plotted against the volume fraction of the polymer-6
for thecellutosc:
acetate in solution.
This is shown in Fig. 3 (reproduced from Ref. 6) for sulutions of ceflulose acetate in acetone, methyl acetate, pyridine and dioxane, and in Fig. 4 (reproduced from Ref. f3) for solutions of cellufose acetate in acetic acid. This can be exemplified further by the marked change in the number of solvent mole-
cules (acetone, dioxane, pyridine, anifint, methyl acetate, ~ceto~itrile~ associated with each glucose residue with the concentration of cellulose acetate in solution (8, 9). The influence of non-solvent on the solvation power of Iiquids for cellulose acetate changes also with the concentration of the polymer whereas the addition of water to a dilute solution of cetlulosr
in soiution.
Thus,
acetate in acetone when added to concentrated
improves soIvation. water acts as a non-solvent solutions (4). Considering the con~~uration of cel!uIose acetate molecufes in solution, one finds that the polymeric chains of cellulose acetate are rather extended and not flexible, Viscosity is therefore not a good measure for solvent-poIymer interactions (7). A better simple measure for the solvation power of various solvents for cellulose acetate turned out to be the amount of non-solvent (ir-hexane) required for initiating phase-separation (7. II). A similar order of solvation power for cellulose acetate was found by osmotic pressure (Flop-Hu~ins polymer-solvent interaction parameter) and initial phase separation measurements. Viscosity data yield a different order of salvation-power (fl)_ it is obvious from the above discussion that the most meaningful measure of solvent-polymer interactions to be correlated with the structure of cellulose acetate membranes cast from 20*/, solution and leached in water would be the concentration of water needed for precipitating ceIIulose acetate from such concentrated solutions. We are fzow able to suggest a mechanism which is based on the assumption that the density of each membrane is determined by the concentration of the
corresponding layer of polymer solution at the precipitation point. The volume concentration of the polymer at the precipitation point is determined by: (a) the concentration of non-solvent (i.e. water) required for precipitating cellulose acetate (b) the direction and magnitude of flows of Iiquids into and out of the cast solution. ‘If, during leaching there is a net volume flow out of the cast polymer solution, this solution concentrates prior to precipitation and therefore yields a dense membrane. On the other hand, if the volume flow from the cast solution into the leaching water is zero or even negative (i.e. the amount of water penetrating into the cast solution is higher than the amount of organic solvent leaving it). the cellulose acetate will precipitate from a 20y0 or even smaller polymer concentration and the resultant membrane will therefore be porous. Examining again the data of Table I one can find that the behavior displayed by the acetone, dioxane and TEP systems is in full arcordance with this suggested mechanism. The formation of a homogeneous and dense membrane from acetone and dioxane can clearly bc ascribed to the fact that in these systems the polymer solution concentrates prior td precipitation and to the fact that a high concentration of water is required for initiating phase separation_ The analysis of the relative contributions of each liquid component to the total volume flow may explain even finer differences between the structure of membranes cast from binary solutions. Thus, in the case of acetone. the volume flow from the solution to water is exclusively due to the solvent outflux. in the case of dioxane the direction of volume flow is also from solution toward water. However, there is also a considerable net water flow into the solution. Translating these observations in terms of processes occurring during the leaching stage, one finds that in the case of acetone the po&mer solution concentrates without precipitation since no non-solvent penetrates into the cast solution. In the case of dioxane, the composition of the mixed soIvent changes continuously during leaching and the polymer solution may concentrate onIy until the concentration of (he penetrating water reaches the precipitation point, after which the situation is frozen. Indeed. comparing the porosities of these two membranes as reflected by their water content, one finds that membranes cast from acetone and leached immedia:eiy are much denser than those cast from dioxane. Simiiarly in the case of TEP one can attribute the fact that membranes cast from this solvent and leached immediately are homogeneous and porous to the fact that the cast polymer solution does not concentrate prior to precipitation and that only a retatively iow concentration of water is required for precipitating the cellulose acetate. Let us explain now the formation of skinned membranes in terms of this mechanism, Aceiic acid represents an intermediate case with no noticeable volume flux, but with high water content needed for precipitation. The formation of a dense polymer layer on top of the cast solution may result from evaporation of the Desalination,
7 (1970) 393402
MECHANISM FOR FORMATION OF “SKINNED”
XIEXIBRANES
II
401
sol.gent during air-exposure prior to leaching (2). Indeed it is found that a cast celluIose acetate solution in acetic acid loses about 2.7 mg/cm” during 10 minutes of evaporation at ambient conditions. However, when the cast solution is immersed in an ice water bath, no volume flow to either direction takes place. This means that cellulose acetate precipitates from approximately ZOO/;;polymer soIution, and the substructure supporting the skin is therefore porous_ How can we now account for the formation of skin on the supported surface? In the DMF and DMSO systems the water concentration required for initiating phase separation is relatively small. and the direction of osmotic flow during leaching is from the water into the cast solution. Neither DMF nor DMSO are volatile. When solutions of cellulose acetate in these solvents are exposed to the ambient atmosphere prior to leaching, there is no weight loss even after IO minutes of “evaporation” and in the case of DMSO the weight of the cast solution even increases with time. Under these conditions no dense layer can form at the air cast solution interface as a consequence of evaporation_ However, there still exists the possibility that the solvent will flow out of the bottom layer of the cast solution and the poiymer solution wiil concentrate at that region prior to precipitation. We do not as yet have any experimental evidence for this hypothesis and the reasons for formation of skin on the “wrong” side must further be studied. In principle. however, the dependence of osmotic flow on composition might give rise to such a phenomenon. In conclusion. our suggested mechanism has been esperimentally established. We have shown that: 1. A correlation exists between the structure of the membrane and the solvation power of the casting solvent, as expressed by the amount of water required for precipitation. 2. There is a correlation between the direction and magnitude of the osmotic volume flows taking place during the [caching phase and the nature of the obtained membrane. ACKNOWLEDGMENT
The authors wish to thank the Office of Saline Water, U.S. Department of the Interior, for Grant No. 14-OI-OtlOl-1706 supporting this work. REFERENCES
1. H. K. LOXSVALE, in D~sahttion by Reverse Osmosis, U. MERTES. (Editor) M.I.T. Press. Cambridge, Mass., 1966. 1. W. BANKS ASD A. SHARPLES. J. Appi. C&m., 16(1966) 28,94. 3. R. BLCXH ASD M. A. FRONIIIER. Desuiittatiott. 7( 1969)259. 4. C. W. SALTONSCALL, F. C. BURNETT. JR.. W. S. HIGI.EY, W. M. Ktw AND A. L. VINCENT, Office of Saline Water, Res. Develop. Progs. Rept. No. 232, Jan. 1967. DesalinaHort, 7 (1970) 393-402
402
Xl. A. FRC’MMER
t?t Ot.
5. W. R. MOORE AND R. SHUTTLEWORTH, Polwwr Letters, 2( 1964) 89. W. R. MOORE AND R. SHUI-TLEXORIH, J. Poiymcr Sri., Al(l963) 733. W. R. MOORE ASD J. RUSSELL. J. Cohid SC:.. S(1953) 243. W_ R. MOORE, J. Chem. SW. (London). Special Pubkation 23 (1968) 185. W. R. MOORE, J. Pu&~nrer Sci.. C1641967) 57. IO. W. R. !&XIRE AND f3. M. ~IDSWELL. J. Poher Sci.. 29(1958) 37. 11. W. R. MOORE, J. A. EPSTEIN. A. M. BROWN ASD B. M. TIDSLVELL.J. Po&merSci.. 2311957) 23. IZ?. W. J. BAZXX& AND H. MARK, J. P&-s_ CofloiLf C&-m., 51( 1947’) 58. 13. 1. K.*w-nr, J. Po&mer Sri_, 32(1958) 425. 14. P. J. FLOIZY, 0. K. SPURR JR. AND D. K. CARPENTER. J. Po~wner Sti.. 27( ,19%) 231. 15. H. J. PXU_LIPP AND C. F. BJORK, J. Polymer Sri.. 6(1950) 549. 16. R. S. STEIN AND R. DOIZ. J. Am. Chef??. SW.. 68(1936) 159. 17. JEFFRIES.Trans. Furuc.?uySm., 53(1957) 1952. 18. W. R. MOORE ABD J. RUSSELL. .I. P&mcr Sri.. 18(1955) 63. 19. R. E. Kmtxc. M. E~GDAHL ASD W. STONE. JR., J. Macromd Sci., A3( 1%9) 157. 20. R. KmsG. J. Appl. PoIwncr Sci.. 9(1965) 663. 6. 7. 8. 9.
Desalinution,
7 (1970) 393-402
THE
- Elsevier Publishing Company,
MECHANLSM
II. EQU!LIBRfUM
H~drona~rics-~sraef
FORMATlON
- Printed in The Netherlands
OF “SKINNED”
PROPERTIES AND OSMOTIC FLOWS DETERMINING
M. A. FROMMER, (RLweived
FOR
Amsterdam
I. FEINER, Ltd.,
Kiriur
0. KEDEM Weizmznn.
AND
Rehov5r
MEMBRANES
MEMBRANE STRUCTURE
f<. BLOCH (Israel)
August 13 1969; received in revised form October 31. 1969)
SUMMARY
The critical water concentrations required for initiating precipitation of cellulose acetate from 209; solutions in acetone, dioxane, acetic acid, triethyl phosphate (TEP), dimethyi formamide (DMF) and dimethyl sulfoxide (D.MSO) have heen determined. The direction and magnitude of volume Bow as well as the contribution of solvent and water to this osmotic flow through a porous cellulose acetate membrane separating pure water from a 2 : 3 water-solvent mixture have been measured. A mechanism based on the assumption that the density of each layer of the membrane is determined by the concentration of the corresponding layer of the cast polymer solution at the precipitation point has been suggested. The voiume concentration of the polymer at the precipitation point has been correlated to: a. the concentration of non solvent required for precipitating cellulose acetate b. the direction and magnitude of osmotic flows of liquids into or out of the cast solution during teaching INTRODUCTiON
The procedure for preparing composite structure (“skinned”) membranes from cellulose acetate may be divided into the following major stages: 1. casting the polymer solution, 2. exposure of the cast layer of polymer solution to the air, 3. precipitation of the polymer and leaching out the solvent in an ice-water bath, and 4. annealing of the membrane in hot water. Various mechanisms have been suggested for explaining skin formation. They were summarized by Lonsdale (I). Ah these mechanisms are essentially based upon the same fundamental assumption, namely that the density of each layer of the membrane is determined by the concentration of the poIymer solution at precipitation. This means that a dense, non-porous skin forms when the polymer precipitates from a highly concentrated solution, whereas a porous membrane results from precipitation from a dilute solution. Desahbation,
7 (1970)
393402
394
hf.
A.
FROMMER et al.
It has been suggested (I, 2) that during the “drying period” the soIvent evaporates and the upper layer of the cast solution concentrates. The immersi,m of the cast plate inzan ice-water bath causes instantaneous precipitation and. since the conatntration of the upper layer of the i3ast solution is higher, the density of the precipitate4 membrane in that region should afso be higher. The annealing step is assumed to tighten the structure of the skin. This mechanism imp&z that rkinned membrane; can be furmed only from casting solutions containing a vofatite s&vent, and that the skin may be Iocated only at the r+cast solution interface. Both of these assumptions were invalidated
by the observations described in the first paper of this series (3). When studying the structure and properties of cellulose acetate membranes cast from binary solutions, it was found that: membranes cast from acetone or dioxane are homogeneous and dense; from triethyl phosphate homogeneous and porous; from acetic acid, composite with skin on the air-side surface. and from dimethy suIfoxide or dimethyl formamide. with skin on the supported side. Since evaporation of votatilr solvents during air exposure can no longer account excIusiveIy for the formation of composite structure membranes, a more thorough study of other stages of the preparation procedure is required, In this paper we shall correIate the porosity of ceflulose acetate membrrtnes cast from binary solutions and leached immediately in water to: 1. the concentration of water required for precipitating
from the casting solution 2. the direction and magnitude
the celIuIose acetate
of osmotic flows taking place during Ieaching
EXPERIMENTAL
Ma3eriaIs Cellulose acetate Eastman type E-398-3 - was dried at 105°C in an evacuated desiccator and stored in vactmm. Acetone, dioxnne, acetic acid and dimethy formamide were of analytic& grade purity. Triethyl phosphate and dimethy sulfoxide were redistilled from technical reagents. Porous celluiose acetate
membranes used for measurin’g the direction and magnitude of osmotic flows were cast from 20% dried E-398-3 ceIIufose acetate in TEP, Ieached imme~ateIy
in an ice water bath, annealed 30 min. at 80°C and stored in water. Annealing was performed in order to eliminate pinholes and imperfections possibly existing in the non-annealed membrane. Ttitiated water and Cx4 labeled acetic acid were obtained from New Engfand Nuclear Corp. and C” IaheIed acetone was manufactured by the Radio ChemicaI Centre.
The concentration of water required for precipitation of cellulose acetate was determined by adding a known amount of water-solvent
from 20% solution
MECHANISM
mixture
acetate.
FOR
FORMATION
OF “SKINNED”
tc an erlenmeyer containing The erlenmeyer was sealed
MEMBRANES II
395
a known weight of carefully dried cellulose immediately after the addition of the warer-
solvent solution and the mixture was stirred thoroughly with a magnetic stirrer for four days. The exact water content in the liquid mixture was determined by the KarLFiscSer technique. utilizing a “hfetrohm” E 408A Karl-Fischer tittator and “F:uka” reagents. The formation of a turbid precipitate and the existence of two disringuishable phases were taken as indication for phase separation. me initial direction and magnitude of volume flow through a porous cellulose acetate membrane separating pure water from a 2: 3 (weight ratio) water:solvent soiution were measured at (30.0 -i_ O.l)C in a po!ypropylene cell described in Fig. 1. The volume of each half-cell was 80 ml. and the open area of
.-s .’ I) 2) 3) 4) 5)
,
Fig. I. Cell for measuring osmotic Capillary t&e hoider Cell body, Polypropytme Washer *o* rings ‘0’ rings
6) ‘0’ rings 7) Capilinry tube
volume Rows 8) Wing nut 9) Cell base IO) Liquid leveling screw t 1) Locking screw 12) Hypodermic syringe 13) Spiral stirring blade
14) Porous membrane
the membrane was 12.6 cm’. The 2 : 3 water : solvent ratio is the lowest permissib!e ratio for the acetic acid, dioxane and acetone systems. IMembranes equilibrated with sohrtions having higher solvent content considerably swell or even dissolve in the solution. The tota! volume flow through the membrane J, is the vector net flows of water Jw and organic solvent Js through it, i.e., J,
sum
of the
= J,. -I- Js
The magnitude
of solvent
flow into the pure water compartment
can be determined
Desalination, 7 (1970) 39342
396 from
hf. A.
measurements
and magnitude
of the rate of carbon-14
of the net water
labeled
flow can be computed
solvent from
momim ei al.
flow. The direction the rate of tritium
labeled water flow in each direction. These measurements were performed at the same temperature of (30.0 f 0.1)X in a diffusion celi shown in Fig. 2. In this cell the open area of the membrane
was 6.6cm2
Fig. 2. Diffusion cell 1) Air inlet. tube, sample port
about 20 mt.
7) Half cell 8) Washer 9) Spiral stirringblade 10) Magnetic stirring bar 11) Lcjcking screw 12) Wang nut
2) Spacer
3) 4) 5) 6)
and each half-ceil contained
‘0’ ring ‘0’ ring Cell body. teflon Porous membrane
RESULTS
Table for initiating
1 summarizes
the experiments
on the water concentration
required
phase separation
in a 200/, cellulose acetate solution, and the data on flows through a porous cellulose acetate pure water from a 2 : 3 water :soivent mixture. Column 2 the results of our study on the structure and properties of
the direction and magnitude
membrane separating of Table I summarizes
of osmotic
membranes cast from 20% binary solutions and leached immediately in water (3). Since in 20% polymer solutions it is very difkult to distinguish between a viscous gelatinous solution and a swollen precipitate, we cannot give more accurate figures for the critical water content needed for initiating phase separation. The figures given are, however, meaningful within 2% error. Initial rates of volume flows through a porous cellulose acetate membrane separating pure water and a water:solvent mixture were computed from the change with time of the liquid DesalinaZion, 7 (1970) 393-402
MECHANISM
TABLE
FOR FORMATION
MEMBRANES II
397
I
STRUCTURE
OF
CENTRATION AND
OF “SKINNED”
CELLULOSE
REQUIRED
DIRECTION
AND
4 membrane
Acetone
Dioxanc Acetic acid TEP DMF DMSO
_..._ ~--
Homogecneous very dense Homogencous dense Skin, air side Homogeneous porous Skin glass side ..
--
MEMBRANES
FREClPlTATlNG
MAGNITUDE
Sirucrare
Sulwnr
ACEXATE FOR
OF OSMOTIC
Approx. water cant. at precip. poinr o/0
CASI-
FROM
CELLULOSE FLOWS
20%
ACETATE
THROUGH
BINARY FR0.W
A POROUS
SOLUTIONS.
20%
WATER
BINARY
MEMBRANE.
Initial rates of solrrnre and &aid flows r~roi~g~ a porous CA. membrane separathg pare water from a 2~3 w-let : solvert mixfare, at (30.0&0.1) “C. Ffmes are given in anits of (nricrolire+cm”-. sec. ‘100 p) V&r.re pux Water influx SoCvent orujhx +* Direction
Magiritade
28
Solution to water
0.12.
-0.04*
30
..
0.08
0.06
0.14
31
Notdetectable ..
-
> 0.06
0.13
0.07
0.07
Water to solution ..
0.08
0.16
O-08
0.12
0.27
10 12 15
.--
--.--
CON-
SOLUTIONS,
--.-
.--
0.16+
0.15
-_.__
* The sum of the acetone flux and water flux as measured by tritiated water and C1a, labelled acetone Rows are higher than the volume flux measured in the capillary tubing cell. More accurate measurements of osmotic flows, confirming the possibility of negative anomalous osmosis in this system will soon be published. l * The solvent outflux in the systems containing dioxane, TEP, DMFand DMSO were not measured directly. but computed from the volume fh~x and the tritiated water Rows into and out of the solution.
fronts in the capillary tubings liquid menisci in the capillary
of the osmotic celi. The velocity of movement of the tubings was constant for approximately 30 minutes.
The initial rates of water and solvent flows were determined
in a similar
manner. One chamber of the diffusion cell was filled with a non labeled liquid (water or water-solvent mixture) and at zero time a liquid containing tritiated water or Cl4 labeled organic solvent was added to the other chamber. Ten samples were taken at approximately 3 min. intervals. The water and organic solvent Radiation was ffows were cahzulated from the H3 and C’* radiation respectively. measured with the aid of a Packard Tricarb Liquid Scintillation Spectrometer. It will be shown in the Discussion that the most meaningful measure for solvation power to be correlated with the structure of cellulose acetate membranes cast from 20”,/, solution and leached in water, is the concentration of water required for precipitation of the polymer from such concentrated solutions. The data Desalination, 7 (1970) 393-402
shown in Table 1 enable us therefore to subdivide the six solvents used for casting membranes into three groups according to their “solvation power” for cellulose acetate. TEP is the poorest solvent for cellulose acetate and only CCL10:; water is required for precipitating the polymer. DMF and DMSO are somewhat better solvents than TEP since 12-15°/0 water is required for initiating phase separation. Acetone. dioxanc and acetic acid are the best solvents in this series of liquids, forming a clear solution even if the water content is 28% or higher. The data of Table I also enable us to subdivide rhe six solvents into three groups according to the directic n of osmotic volume flow through a porous cellulose acetate membrane separating pure water from a 2 : 3 water :soiven~ mixture. When acetone or dioxane comprise the organic solvent in the watersolvent mixture. there is a net volume flow out of the solution compartment into the water compartment. whereas when DMF or DMSO are present in the solution. rhe volume of water flowing into the solution compartment is higher than the volume of soIvent flowing out of it. When TEP or acetic acid are contained in the binary solvent-water solution, no volume flow to either direction could be detected, within experimental error.
Various authors (I, 4, 19, 20) tried to correlate data on the interactions of ceIlulose acetate with various solvents with the structure and porosity of cellulose acetate membranes cast from these solvents. Reviewing the literature on chemical physical properties of cellulose acetate in solutions (4-18) one finds that the extent of solvent polymer interaction changes considerably with the concentration of the polymer in solution. Thus rhe FIory-Huggins polymer-solvent interaction parameter-X, changes considerably with the volume fraction+, of cellulose
Fig. 3. Flory-Hums solvent-polymer interaction Parameter-X,. volume fraction of the polymer-qk, for various cellulose acetate systems.
as a function
Desalination,
OF the
7 ( 1970) 393402
MECHANISM
0.5
-
0
E
,
I
OF “SKINNED”
I
I
I
i
I 0.2
i
MEMBRANES
399
It
Osmotic 00t0
\
-0s X?
FOR FORMATION
\
\
--I5 F \
-1.0
\
i l
-2.0
0
0.1
0.3
$2 Fig. 4. Values of Flary-Huggins solvent polymer interaction parameter-X1, acetate acetic acid system plotted against the volume fraction of the polymer-6
for thecellutosc:
acetate in solution.
This is shown in Fig. 3 (reproduced from Ref. 6) for sulutions of ceflulose acetate in acetone, methyl acetate, pyridine and dioxane, and in Fig. 4 (reproduced from Ref. f3) for solutions of cellufose acetate in acetic acid. This can be exemplified further by the marked change in the number of solvent mole-
cules (acetone, dioxane, pyridine, anifint, methyl acetate, ~ceto~itrile~ associated with each glucose residue with the concentration of cellulose acetate in solution (8, 9). The influence of non-solvent on the solvation power of Iiquids for cellulose acetate changes also with the concentration of the polymer whereas the addition of water to a dilute solution of cetlulosr
in soiution.
Thus,
acetate in acetone when added to concentrated
improves soIvation. water acts as a non-solvent solutions (4). Considering the con~~uration of cel!uIose acetate molecufes in solution, one finds that the polymeric chains of cellulose acetate are rather extended and not flexible, Viscosity is therefore not a good measure for solvent-poIymer interactions (7). A better simple measure for the solvation power of various solvents for cellulose acetate turned out to be the amount of non-solvent (ir-hexane) required for initiating phase-separation (7. II). A similar order of solvation power for cellulose acetate was found by osmotic pressure (Flop-Hu~ins polymer-solvent interaction parameter) and initial phase separation measurements. Viscosity data yield a different order of salvation-power (fl)_ it is obvious from the above discussion that the most meaningful measure of solvent-polymer interactions to be correlated with the structure of cellulose acetate membranes cast from 20*/, solution and leached in water would be the concentration of water needed for precipitating ceIIulose acetate from such concentrated solutions. We are fzow able to suggest a mechanism which is based on the assumption that the density of each membrane is determined by the concentration of the
corresponding layer of polymer solution at the precipitation point. The volume concentration of the polymer at the precipitation point is determined by: (a) the concentration of non-solvent (i.e. water) required for precipitating cellulose acetate (b) the direction and magnitude of flows of Iiquids into and out of the cast solution. ‘If, during leaching there is a net volume flow out of the cast polymer solution, this solution concentrates prior to precipitation and therefore yields a dense membrane. On the other hand, if the volume flow from the cast solution into the leaching water is zero or even negative (i.e. the amount of water penetrating into the cast solution is higher than the amount of organic solvent leaving it). the cellulose acetate will precipitate from a 20y0 or even smaller polymer concentration and the resultant membrane will therefore be porous. Examining again the data of Table I one can find that the behavior displayed by the acetone, dioxane and TEP systems is in full arcordance with this suggested mechanism. The formation of a homogeneous and dense membrane from acetone and dioxane can clearly bc ascribed to the fact that in these systems the polymer solution concentrates prior td precipitation and to the fact that a high concentration of water is required for initiating phase separation_ The analysis of the relative contributions of each liquid component to the total volume flow may explain even finer differences between the structure of membranes cast from binary solutions. Thus, in the case of acetone. the volume flow from the solution to water is exclusively due to the solvent outflux. in the case of dioxane the direction of volume flow is also from solution toward water. However, there is also a considerable net water flow into the solution. Translating these observations in terms of processes occurring during the leaching stage, one finds that in the case of acetone the po&mer solution concentrates without precipitation since no non-solvent penetrates into the cast solution. In the case of dioxane, the composition of the mixed soIvent changes continuously during leaching and the polymer solution may concentrate onIy until the concentration of (he penetrating water reaches the precipitation point, after which the situation is frozen. Indeed. comparing the porosities of these two membranes as reflected by their water content, one finds that membranes cast from acetone and leached immedia:eiy are much denser than those cast from dioxane. Simiiarly in the case of TEP one can attribute the fact that membranes cast from this solvent and leached immediately are homogeneous and porous to the fact that the cast polymer solution does not concentrate prior to precipitation and that only a retatively iow concentration of water is required for precipitating the cellulose acetate. Let us explain now the formation of skinned membranes in terms of this mechanism, Aceiic acid represents an intermediate case with no noticeable volume flux, but with high water content needed for precipitation. The formation of a dense polymer layer on top of the cast solution may result from evaporation of the Desalination,
7 (1970) 393402
MECHANISM FOR FORMATION OF “SKINNED”
XIEXIBRANES
II
401
sol.gent during air-exposure prior to leaching (2). Indeed it is found that a cast celluIose acetate solution in acetic acid loses about 2.7 mg/cm” during 10 minutes of evaporation at ambient conditions. However, when the cast solution is immersed in an ice water bath, no volume flow to either direction takes place. This means that cellulose acetate precipitates from approximately ZOO/;;polymer soIution, and the substructure supporting the skin is therefore porous_ How can we now account for the formation of skin on the supported surface? In the DMF and DMSO systems the water concentration required for initiating phase separation is relatively small. and the direction of osmotic flow during leaching is from the water into the cast solution. Neither DMF nor DMSO are volatile. When solutions of cellulose acetate in these solvents are exposed to the ambient atmosphere prior to leaching, there is no weight loss even after IO minutes of “evaporation” and in the case of DMSO the weight of the cast solution even increases with time. Under these conditions no dense layer can form at the air cast solution interface as a consequence of evaporation_ However, there still exists the possibility that the solvent will flow out of the bottom layer of the cast solution and the poiymer solution wiil concentrate at that region prior to precipitation. We do not as yet have any experimental evidence for this hypothesis and the reasons for formation of skin on the “wrong” side must further be studied. In principle. however, the dependence of osmotic flow on composition might give rise to such a phenomenon. In conclusion. our suggested mechanism has been esperimentally established. We have shown that: 1. A correlation exists between the structure of the membrane and the solvation power of the casting solvent, as expressed by the amount of water required for precipitation. 2. There is a correlation between the direction and magnitude of the osmotic volume flows taking place during the [caching phase and the nature of the obtained membrane. ACKNOWLEDGMENT
The authors wish to thank the Office of Saline Water, U.S. Department of the Interior, for Grant No. 14-OI-OtlOl-1706 supporting this work. REFERENCES
1. H. K. LOXSVALE, in D~sahttion by Reverse Osmosis, U. MERTES. (Editor) M.I.T. Press. Cambridge, Mass., 1966. 1. W. BANKS ASD A. SHARPLES. J. Appi. C&m., 16(1966) 28,94. 3. R. BLCXH ASD M. A. FRONIIIER. Desuiittatiott. 7( 1969)259. 4. C. W. SALTONSCALL, F. C. BURNETT. JR.. W. S. HIGI.EY, W. M. Ktw AND A. L. VINCENT, Office of Saline Water, Res. Develop. Progs. Rept. No. 232, Jan. 1967. DesalinaHort, 7 (1970) 393-402
402
Xl. A. FRC’MMER
t?t Ot.
5. W. R. MOORE AND R. SHUTTLEWORTH, Polwwr Letters, 2( 1964) 89. W. R. MOORE AND R. SHUI-TLEXORIH, J. Poiymcr Sri., Al(l963) 733. W. R. MOORE ASD J. RUSSELL. J. Cohid SC:.. S(1953) 243. W_ R. MOORE, J. Chem. SW. (London). Special Pubkation 23 (1968) 185. W. R. MOORE, J. Pu&~nrer Sci.. C1641967) 57. IO. W. R. !&XIRE AND f3. M. ~IDSWELL. J. Poher Sci.. 29(1958) 37. 11. W. R. MOORE, J. A. EPSTEIN. A. M. BROWN ASD B. M. TIDSLVELL.J. Po&merSci.. 2311957) 23. IZ?. W. J. BAZXX& AND H. MARK, J. P&-s_ CofloiLf C&-m., 51( 1947’) 58. 13. 1. K.*w-nr, J. Po&mer Sri_, 32(1958) 425. 14. P. J. FLOIZY, 0. K. SPURR JR. AND D. K. CARPENTER. J. Po~wner Sti.. 27( ,19%) 231. 15. H. J. PXU_LIPP AND C. F. BJORK, J. Polymer Sri.. 6(1950) 549. 16. R. S. STEIN AND R. DOIZ. J. Am. Chef??. SW.. 68(1936) 159. 17. JEFFRIES.Trans. Furuc.?uySm., 53(1957) 1952. 18. W. R. MOORE ABD J. RUSSELL. .I. P&mcr Sri.. 18(1955) 63. 19. R. E. Kmtxc. M. E~GDAHL ASD W. STONE. JR., J. Macromd Sci., A3( 1%9) 157. 20. R. KmsG. J. Appl. PoIwncr Sci.. 9(1965) 663. 6. 7. 8. 9.
Desalinution,
7 (1970) 393-402