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Aqueous—organic membrane bioreactors. Part I. A guide to membrane selection
139
Journal of Membrane Scrence, 71 (1992) 139-149
Elsevler Science Pubhshers B V , Amsterdam
Aqueous-organic membrane bioreactors. A guide to membrane selection
Part I.
A M Vaidya”, G. Bell” and P.J. Hallingb ‘Department of Chemccal Engglneermg, Strathclyde Unrversrty, McCance Buddmng, Richmond Street, Glasgow Gl 1XS (Scotland) bDepartment of BLosclence and Biotechnology, Strathclyde Unrverstty, McCance Burldmng, RLchmond Street, Glasgow Gl 1XQ (Scotland)
(Received October 30,1991, accepted m revised form March 17,1992)
Abstract The influence of membrane pore structure on the ease with which an aqueous-orgamc interface can be maintained m the plane of the membrane of a two-phase membrane reactor 1s &cussed Four factors affecting the pressure required to cause breakthrough of the non-wetting phase have been identified (1) membrane pore size, (u) asymmetry of membrane pore structure, (m) the placement of the wetting liquid when an asymmetnc membrane 1s used, and (iv) a change m the wetting charactenstlcs of the membrane as the reactlon progresses The fourth factor 1s part~ularly Important smce two phase blocatalytic reactions frequently involve surface active reactants and/or products It 1s shown that the ideal pattern of surfactant-membrane mteractlons - which 1s reflected by the desired direction of change m the angle of contact between the wettmg liquid and the membrane - depends on the second and third factors An expenment 1s suggested to assess the importance of the vanous factors and a set of rules of thumb have been presented to assist m the selection of membrane matinal and type The unportance of correctly ldentifymg the wetting liquid when an ampluphdlc membrane polymer 1sused has been pomted out Keywords
two-phase biocatalyns; membrane through, enzymatic synthesis, theory
Introduction The considerable advantages of aqueous-organic two-phase systems for many biocatalytx processes have been described in several recent reviews [l-3]. When all the reactants can be supplied in the organic phase, and all the prodCorrespondence to A M Valdya, Department of Chemical Engmeermg, Strathclyde Umversity, McCance Bulldmg, Richmond Street, Glasgow Gl lXS, Scotland
bioreactor,
surfactant-membrane
mteractlon,
break-
ucts removed from the same phase, it is possible to design various types of reactors with the aqueous phase confined to the surfaces of the catalyst particles. However, in a number of potential applications, access is required to the aqueous phase as well, e.g. because at least one of the reactants or products is only appreciably soluble in the aqueous phase [ 41. Emulslficatlon is an obvious technique to accomplish this, but the problems of post-reaction phase sepa-
0376-7388/92/$05 00 0 1992 Elsevler Science Publishers B V All nghts reserved
A M Vasdya et al /J Membrane Scl 71(1992) 139-149
140
TABLE 1 Two-phase blocatalytlc even m bold letters
reactlon
systems mvolvmg surface active components
The surface active components
have been
Reference
Polar reactant
Non-polar reactant
Product
Enzyme
Alcohol (methanol, glycerol) Water
Fatty acid (Laurie, Oleic) Triglyceride (olive oil) Alcohol (Hexadecyl, Menthol) Long chain alcohols Phospatidyl choline
Ester”
Llpase
2
Fatty acid Ester”
Llpase
7
Mlcroblal e&erase
10
Alkylbetaglucoside Phosphatidyl glycerol
Glycosldase
11
Phosphohpase
12
Acid (acetic, succmic ) Glucose Glycerol
“Glycerol monoesters
and succmate
esters are surface actwe
ration can seriously limit its practical utility
[51
The posslblhty of using a selectively wetted membrane to bring about interfacial contact between bulk aqueous and organic phases has been demonstrated [ 6-91. Hoq et al. [ 6,7] used microporous polypropylene membranes for this purpose A slight positive pressure, of the order of 20 cm of water (ca. 0.02 bar), was apphed on the aqueous phase to keep the interface m the plane of the membrane. Pronk et al. [8] pointed out the advantages of using hydrophilic ultrafiltration (UF) membranes m such an apphcation The Cuprophan membranes employed by these workers permitted the use of pressures as high as 1 bar for interface maintenance. Neither of these two groups of workers have commented on the criteria used by them for the selection of the membrane material Some of the products produced during the course of such reactions are frequently surface active. Some of the starting materials are surface active as well The blocatalysts used also possess some surface activity. These surface active components can change the wetting behavior of the mem-
brane, and make maintenance of the interface in the membrane difficult Table 1 lists several examples of two phase biocatalytic reaction systems and the surface active components involved. The criteria to be considered while selecting membrane materials in order to minimize the effects of the surfactant are discussed m this paper. The manner m which membrane pore structure can affect this choice is also discussed Theory Figure 1 shows the interface formed between two immiscible liqmds withm an arbitrarily shaped pore m a sohd wetted by one of the two hqulds. Evidently, a positive pressure must be applied to the non-wettmg liquid in order to maintain this interface. If this pressure is sufficiently large, displacement of the wetting liquid can occur and this will eventually result m the breakthrough of the non-wetting hqmd. Stable reactor operation can only be achieved d the uutial breakthrough pressure, dP,,, is reasonably high, and remains so during the course
A M Vatdya et al/J
_------
---_------__ _-------__ --------__
-
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Membrane Scz 71(1992) 139-149
----_-
--Weltmg Liquid _ _
-___
-----_
--------____ --------__ ------____ -----__
_
___---------
Fig 1 Llquld-liquid interface formed mslde an arbltrarlly shaped membrane pore
Wettmg Liquid ----
__
-_-I
wm/ ----. 8 -IYe
of the reaction. A change m the content of surface active components, as a result of reaction, can lead to a change m the breakthrough pressure Ideally, one would hke to find a membrane for which dP,, remained constant, or mcreased, durmg the course of the reaction The factors which mfluence the magmtude of dP,, will be identified m this section. The effect of surface active components at the various mterfaces will then be discussed and the pomts to be considered while selectmg a membrane for use m a two phase membrane reactor will be hlghhghted. Fmally, a set of rules of thumb will be presented to assist the process of membrane selection In order to identify the factors which determme the magmtude of dP,, it is useful to examme three special forms of the general equation of capillarlty [ 131
AP,
=2gwn - r sin (P+ f%,)
(2)
2own - r sln(P-%,I
(3)
AP,, =
Equation ( 1) is the Laplace-Young equation
Fig 2 Liquid-liquid interface formed inside a cyhndrlcal membrane pore The inset shows the angle of contact, O,.,,, between the wetting liquid and the membrane
[ 131 and gives the breakthrough pressure when the pore is of cylindrical cross-section (Fig. 2). Equations (2) and (3) have been derived by Adam [ 141 to describe the behavior of memscl m tapering capillaries, as shown m Figs 3 (A) and 3 ( B ) , respectively. When /3= 90 ‘, eqns. ( 2 ) and (3 ) reduce to eqn. ( 1) An exammatlon of these three equations reveals that the magmtude of the breakthrough pressure is determined by four factors (1) The pore sue In all three cases the breakthrough pressure is inversely proportional to the pore radius. This implies that the mamtenance of a stable hquldliquid interface would be easier with an UF membrane than with a microfiltration (MF) membrane, which has pore diameters larger by several orders of magnitude. (2) The pore geometry Any change m the content of surface active components m the system, as a result of reac-
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A M Valdya et al/J
_-_--
_-A--
---- - Wetbng Llqutd ______----
----
=.f --B
_____-__-_--_-____------
--Qwm: --_---------
---
B
---_------- __--------
--___----______---_---
increase or decrease. Clearly, the nature of the change will depend on the shape of the pore, and this m turn will affect the stability of an interface formed in a given pore. Two types of asymmetric membrane have been identified by Kestmg [ 151. (1) Skinless MF membranes. (ii) Skinned UF membranes. A model skmless MF membrane is shown m Fig. 3, while Figs 4 and 5 show the two possible structures for a model skinned membrane. These two model structures will be referred to as cylindrically skinned and conically skinned membranes, respectively
----
__----
Membrane Scl 71(1992) 139-149
(A)
(3) The placement of the lrquldphases
_---
Wetting Llquld
--_
_
_
Fig 3 Llquld-liquid interface at the point of mclplent breakthrough m the pore of an Ideal skinless MF membrane The inset shows the contact angle, &,,, and the angle j3 formed by the mchned walls of the pore with the radial direction (A) Wetting hquld placed on the open side of the pore (B) Interface formed by movement of the nonwetting liquid through the open end of the pore
tlon, may be reflected by a change m the contact angle, 13,~ As a result of this the trigonometric terms in eqns (l)-(3) can either
When a membrane is asymmetric, the equation to be used to calculate the breakthrough pressure is determined by the placement of the liquid phases With the arrangement shown m Fig. 3 (A), eqn. (2) must be used to calculate the breakthrough pressure With the non-wetting liquid on the dense side of a skinned membrane - as shown in Figs 4 (A) and 5 (A) - eqn. (1) can be used to determine dPb when the membrane is cyhndrically skinned and eqn ( 2 ) when the membrane is conically skinned. There is no easily apparent reason for not using the liquid phase locations shown in Figs. 3(B)5(B). Creating such interfaces offers the posslblhty of increasing the interfacial area which can be obtained with a given surface area of the membrane. It is logical to assume that the breakthrough pressure can still be calculated by applying eqns. ( 1) - ( 3 ) using the pore radius at the point of mclplent breakthrough. However, it must be borne m mmd that the interfaces of Figs. 3 (B ) -5 (B ) have been formed by moving an interface uutially located at the open end of the pore, where the pore radius is considerably greater Such interfaces can be set m motion at comparatively low pressures Once a moving interface has been created, its shape and the
A M Valdya et al/J
143
Membrane SCL 71(1992) 139-149
1-z_--L__.J _------_---_-_ ---- Wenmg Lqud ~
_< -a -
-
-
----
Wetting Llquld
-_
-=e wm ---
(‘V
W
__-------_ _--__-------_ ---
-----
---
Wetting Liquid _
---
---
Wettfng Llquld :
-
-
63 Fig 4 Liquid-hqmd interface at the point of mclplent breakthrough m the pore of a cyhntically skinned membrane The inset shows the contact angle (A) Wetting hqmd placed on the open side of the pore (B ) Interface formed by movement of the non-wetting hquld through the open end of the pore
contact angle, O,,, both undergo a change. This phenomenon has been reported by several workers [ 16-191 By applying the Le Chatlier prmclple Rose and Hems [ 191concluded that the contact angle would always change in such
Fig 5 Llquld-hquld Interface at the point of mclplent breakthrough m the pore of a comcally skinned membrane The mset shows the contact angle, O,, and the angle /3 formed by the wall of the comcal pore, m the skm of the membrane, with the radml dxectlon (A) Wetting hquld placed on the open side of the pore (B) Interface formed by movement of the non-wettmg hquld through the open end of the pore
a manner as to resist the movement of the interface. Thus would indicate that the Interfaces shown m Figs 3 (B )-5 (B ) will be stable. How-
144
A M Valdya et al/J
ever, experimental verification of interfacial behavior will have to be done to ascertam the feasibility of placmg the non-wetting liquid on the open side of an asymmetric membrane. Till such mformation is available it can only be said that breakthrough pressures calculated using eqns ( 1 )- (3 ) are liable to be misleading. Ideal contact angle behavior deduced from these equations could also be incorrect (4) Interfaczal effects It is evident from eqns (l)- (3), that the breakthrough pressure is directly proportional to the interfacial tension of the hquid-liquid Interface, o,, However, there are two other mterfaces m a two phase membrane reactor which can slgmficantly affect its stability, namely. (1) The interface between the wetting hquid and the membrane. (11) The interface between the non-wetting hquid and the membrane. The changes induced at these interfaces in the presence of surface active materials, must also be considered when selecting a membrane for use in a two-phase reactor The importance of doing this is best illustrated by considermg a symmetric membrane. When a stable interface exists in the pores of such a membrane, the force balance described by the Young-Dupre equation [ 131 can be used to write 0
-
cos&,, = Imn CJwn
%rn
(4)
Combining eqns. ( 1) and (4 ) yields an expression relatmg the two membrane-liquid interfacial tensions and the breakthrough pressure
Equation (5) is merely a way of lookmg at the effects of changes in a,, and &,, - discussed earlier m terms of eqn. (1) - which provides a
Membrane Scr 71(1992) 139-149
better insight mto the possible consequences of surfactant adsorption Surfactant adsorption As was pointed out m the preceding section, there are three interfaces m a two-phase membrane reactor. Any surfactant - reactant, product or biocatalyst - present in the system can adsorb at all three interfaces This adsorption will invariably result m a lowermg of the mterfacial tension at that interface. The consequences of this decrease will be pointed out m this section. (1) The lzquzd-lzquzdznterface
Equations (l)-(3) indicate that dP,, is directly proportional to the hquld-liquid mterfacial tension, a,,. Clearly, any lowermg of this quantity IS undesirable. However, very little control can be exercised over surfactant adsorption at the liquid-liquid interface The use of a suitable solvent for the organic phase can help to mmimize the reduction m a,,, but this may not always be feasible Even when a solvent can be used the post-reaction solvent reunwelcome introduces covery step complications. (2) The lzquzd-membrane znterfaczal tenszons From eqn. (5) it is evident that the direction of change m the breakthrough pressure as a result of surfactant adsorption depends on the relative extent of adsorption at the two hqmdmembrane interfaces. Preferential adsorption at the interface between the membrane and the non-wetting liquid will result in a decrease m dP,. If the surfactant adsorbs selectively at the wetting liquid-membrane interface, dP,, will increase More generally, it can be stated that surfactant adsorption at the liquid-membrane mter-
145
A M VaLdya et al /J Membrane Scl 71(1992) 139-149
faces will result m a change in the angle of conand the tact, &,, between the membrane wetting liquid. It is not immediately obvious that this angle can change in both directions. However, an examination of eqn. (4) will reveal that this can - at least, in prmclple happen. Membrane selection The points to be considered while selecting membranes for use m two-phase reactors will be outlined m this section based on the discussions presented earlier
(2) Contact angle behavior It has been pointed out earlier that the presence of surface active components leads to a decrease m dP, as a result of adsorption at the liquid-liquid interface. This effect can be combatted, to some extent, by selectmg a membrane material for which the contact angle, SW,,, changes in such a manner as to minimize the decrease in dP,,. Table 2 summarizes the nature of the desired change m &,, for the different pore shapes and liquid phase locations, described earlier. (3) Factors affectzng adsorptwn
(I) UF versus MF The small pore sizes m ultrafiltration membranes offer the advantage of a much higher breakthrough pressure. It would be difficult to use a microporous membrane if the breakthrough pressure were very small, as found by Hoq et al. [ 61. This is particularly so when microporous hollow fibers are used. A simple calculation, based on the Hagen-Poiseullle equation [ 201, will show that if it were desired to pump a viscous substrate (such as olive oil which has a viscosity of 0.084 Pa-set at room temperature [ 211) through the lumen of a typical commercially available microporous hydrophobic hollow fiber module (with a total length of 25 cm and fibers with an internal dlameter of 190pm) the lumen side pressure drop across the module would be of the order of 0 01 bar - for a residence time of one hour. As Pronk et al. [ 81 have commented, the pressure control required for interface maintenance would have to be extremely accurate. Slight variations m the trans-membrane pressure drop, or an accidental increase m the lumen side pressure downstream of the reactor, would be sufficient to cause movement of the interface. The generation of a surface active product would further complicate matters.
The desired patterns of contact angle behavior, as a result of surfactant adsorption, outlined above require preferential adsorption of surfactant at the wetting liquid-membrane mterface when a decrease in &, is desired. Conversely, when an increase m 0,, is desired the surfactant must selectively adsorb at the mterface between the non-wetting liquid and the membrane. There are several factors which determine the extent of the adsorption of a surfactant at a solid-liquid interface which have been discussed m some detail by Fowkes [ 221. It is extremely unlikely, if not impossible, that all the details of surfactant-membrane interaction can be determined without taking recourse to experiment. However, the following observations can serve as a useful guide: (1) Fane [23] and Jonsson and Jonsson [24] have reported that surfactants m aqueous solution tend to adsorb more strongly on to hydrophobic membrane surfaces (11) The Lundelms rule [ 251 states that there is an inverse relationship between the extent of adsorption of a solute and its solubllity. Table 3 illustrates the utility of these observations The partition coefficient for the surfactant between the organic and aqueous phases was
146
A M Vatdya et al/J
Membrane Set 71(1992) 139-149
TABLE 2 Ideal contact angle behavior, for membranes with the specified ideal pore structures, m the presence of surfactants Membrane Type
Wettmg phase placement
Reference figure
Reference equation
Ideal contact angle behavior
Symmetric
N/A
2
1
Small, and decreasing
Skinless MF
Open side Tight side
3(A) 3(B)
2 3”
Cylindrically skinned
Open side
4(A)
1
Small and decreasing
Skinned side
4(B)
1”
b
Open side Skmned side
5(A) 5(B)
2 3”
Conically skmned
(~+&,)=900
b
(/3+e,,)=900 b
“As indicated m the text the apphcablhty of these equations ISdoubtful bIf the specified reference equations can be used, a small and decreasing f?,,.,would be desirable TABLE 3 The effect of surfactants on the contact angle and the vamous mterfaclal tensions for the system Nylon-6-water-heptane/ chloroform-s&a&ant The contact angles and mterfaclal tensions have been extracted from Smolders [ 261 Membrane material
Surfactant concentration moles/hter
System
RVUI
Gl (mN/m)
Gm - %nl (mN/m)
Nylon-6
0 3x 10-4
1 1
70” 60”
30 10
10 26 5 00
Nylon-6
0 2x10-6 55x10-6
2” 2 2
10” 10” 71”
52 4 40 32 4
5160 39 39 10 55
“Extrapolated values w water, m Nylon 6, n heptane/chloroform System 1 Chloroform-water-dodecyl pyrldmmm bromide System 2 Heptane-water-dodecyl pyrldmmm bromide
found to be 27 for system 1 and very close to zero for system 2. The figures m the last column of Table 3, calculated using the YoungDupre equation, indicate a much faster dechne in a,, - pointing to preferential adsorption at the heptane-membrane mterface. It will also be seen that both a decrease and an increase in the contact angle is possible as a result of sur-
factant adsorption. The results of Table 3 have been cited purely as an example. The applicability of the drop-weight method, used by Smolders [ 261, for the estimation of the interfacial tensions of surfactant solutions is questionable. The unreliability of the closely related drop-volume method has been clearly demonstrated by Carroll and Doyle [ 271.
A M Vatdya et al/J
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Membrane See 71 (1992) 139-149
(4) Membrane mated It has been indicated above that making the correct choice of membrane material for a given reaction will require some experimental work. This is best done by measuring the variation in breakthrough pressure for a given membrane as the concentration of surfactant m the system changes. However, before embarking on such experiments three other points must be noted. (1) Membrane wetfng characterlsttcs A wide variety of purportedly hydrophilic membranes are marketed by various companies For instance, several manufacturers supply Nylon-66 membranes which are claimed to be hydrophilic PVDF and PS membranes of a hydrophrhc character are produced by Gelman Sciences. Nylon-66 with its polar amide linkages, and the non-polar intervening polymethylene groups, must clearly have an amphlphlhc character. The chemical structures of PVDF [ 281 and the various polysulfones [ 291 suggest that these materials should be mtrinslcally hydrophobic. Since the majority of membrane filtration applications involve aqueous solutions it is useful to impart a measure of hydrophilicity to such membrane materials by surface or monomer modification Keurentjes et al. [ 301 carried out X-ray photoelectron spectroscopy (XPS) of IRIS PVDF membranes made by RhBne-Poulenc Their results are reproduced m Table 4. Clearly, some form of surface modification, accounting for the presence of oxygen and mtrogen atoms near the surface, was used for imparting hydrophiliclty to the membrane Again, it would be more correct to call such membranes amphiphihc m that if they are uutlally immersed m air they may be readily wetted by either a polar or a non-polar liquid. The type of liquid which will preferentially wet such membranes m the competitive environment of the
TABLE 4 Atomic ratios found by XPS measurements on IRIS PVDF membranes Results of KeurentJes et al [30] The theoretlcally expected and experlmentally determmed atomic ratios of the specified elements have been gven at two depths of analysis Atom
C F 0 N
Theory
Depth of analysis
50 50 0 0
two liquid phase membrane ily apparent.
5nm
lnm
57 6 29 1 77 56
60 6 26 1 79 53
reactor is not read-
(u) The effect of bwcatalysts The enzymes used in many of the reactions described in Table 1 are themselves surface active It is generally believed that the surface activity of the enzyme will be of little importance m the presence of a strongly surface active material. For instance, the breaking of oil-m-water emulsions stabilized by proteins has been ascribed to the displacement of the protein by smaller molecules with higher spreading pressures [31]. The inhibition of hpase activity during the hydrolysis of triglycerides has been imputed as being caused by the displacement of the enzyme by the non-ionic surfactant produced during the reaction [ 321. (~1) The effect of other components Although only one of the components of the reaction system may be strongly surface active the overall behavior of the system may be sigmticantly altered by the presence of other components which also possess some surface actlvity. For instance, ethanol and laurlc acid are produced as a result of the hydrolysis of ethyl laurate Although laurlc acid is the more strongly surface active product, the ethanol -
148
which will pass mto the aqueous phase - can bring about some change m the wettability of the membrane by the aqueous phase. Clearly, the results of a simple breakthrough pressure test can only be used as a gmde to membrane selection. Conclusions The criteria which must be considered while choosmg a membrane for use in a two-phase membrane reactor have been discussed m this paper Based on these discussions the followmg set of rules of thumb can be drawn up: ( 1) Any combination of membrane material and structure which leads to a small breakthrough pressure requirement should be avoided In practice this means that breakthrough pressures should be of the order of several tenths of a bar, or more. As a general observation, UF membranes will offer higher breakthrough pressures because of their smaller pore size (2 ) The wetting characteristics of the membrane will change as the composition of the system changes during the course of the reaction. This may be reflected by a change in the breakthrough pressure. This variation should be studied while selecting membrane material and type (3) It should be borne in mmd that pore asymmetry can lead to an asymmetry m breakthrough pressure In particular, it must be noted that placing the non-wettmg liquid on the open side of an asymmetric membrane can give rise to movmg interfaces whose breakthrough behavior is difficult to predict. (4) With some membrane polymers, the effects of their amphiphihcity and pre-treatment must be properly understood before determmmg the nature of the phase to be used as the wetting hqmd. This is best accomphshed by doing two sets of breakthrough pressure exper-
A M Valdya et al /J Membrane SCL 71(1992) 139-149
iments usmg both the liquids mvolved in the role of the wettmg liquid Finally, it must be stressed that mterpretation of results of breakthrough pressure measurement based on the discussions presented in this paper must be done with caution, smce idealized membrane structures have been used as the bases for these discussions. Experimental results of breakthrough pressure experiments lllustratmg the utihty of these rules will be presented m a separate paper. List of symbols Ah r 8wm P
u wn 0 wm u nm
breakthrough pressure (Pa) pore radms at the three phase lure of contact (m) angle of contact between wetting liquid and membrane acute angle formed between wall of a tapering capillary and the radial direction - see Figs. 3,4 and 5. mterfacial tension at the hqmd-liquid interface (N/m) mterfacial tension at the wetting hqmd-membrane interface (N/m) mterfacial tension at the non-wetting hqmd-membrane interface (N/m)
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30
31 32
F.A. Holland, Fluid Flow for Chemical Engineers, Edward Arnold Ltd, London, 1973. Viscosities of liquids, in: R.C. Weast et al. (Eds.), Handbook of Chemistry and Physics, 70th edn., CRC Press, Boca R&on, FL, 1990, p. F-44. F.M. Fowkes, Surfactants and Adsorption, in: R.L. Patrick (Ed. ) , Treatise on Adhesives and Adhesion, Marcel Dekker, New York, NY, 1967, pp. 327-442. A.G. Fane, Ultrafiltration: factors influencing flux re.jection, in: R.J. Wakeman (Ed.), Progress in Filtration and Separation, Elsevier, Amsterdam, 1986, pp. 101-179. A.-S. Jlinsson and B. Jiinsson, The influence of nonionic and ionic surfactants on hydrophobic and hydrophilic ultrafiltration membranes,J. Membrane Sci., 56 (1991) 49. M.D. Jaycock and G.D. Parfitt, Chemistry of Interfaces, Ellis Horwood Ltd, 1981, pp. 259-260. C.A. Smolders, Wetting of polymers: Contact angles of aqueous surfactant solutions on polar polymers in oil, in: Wetting, SC1 Monograph No. 25, Sot. Chem. Ind., London, 1967, pp. 318-327. B.J. Carroll and P.J. Doyle, Differences in surface tensions of non-ionic surfactant solutions as measured by the drop-volume and Wilhelmy-plate methods, J. Chem. Sot., Faraday Trans. I, 81 (1985) 2975. N.J. Ballintyn, Polymers containing sulfur, in: H.F. Mark et al. (Eds.), Kirk-Othmer Encyclopaedia of Chemical Technology, 3rd edn, Wiley and Sons, New York, NY, 1982, Vol. 18, p. 832. R.L. Johnson, Tetr~uroethylene copolymers with ethylene in: H.F. Mark et al. (Eds.), Kirk-Othmer Encyclopaedia of Chemical Technology, 3rd edn., Wiley and Sons, New York, NY, 1982, Vol. 11, p. 35. J.T.F. Keurentjes, J.G. Harbrecht, D. Brinkman, J.H. Hanemaaijer, M.A. Cohen Stuart and K. van% Riet, Hydrophobicity measurements of microfiltration and ultrafiltration membranes, J. Membrane Sci., 47 (1989) 333. J.L. Moilliet, B. Collie and W. Black, Surface Activity, E & F.N. Spon Ltd, London, 1961, pp. 266-268. E.P. Jorolan and B.W. Janicki, Influence of some nonionic surfactants on pancreatic lipase activity, Proc. Sot. Exptl. Biol. Med., 120 (1965) 313.
Journal of Membrane Scrence, 71 (1992) 139-149
Elsevler Science Pubhshers B V , Amsterdam
Aqueous-organic membrane bioreactors. A guide to membrane selection
Part I.
A M Vaidya”, G. Bell” and P.J. Hallingb ‘Department of Chemccal Engglneermg, Strathclyde Unrversrty, McCance Buddmng, Richmond Street, Glasgow Gl 1XS (Scotland) bDepartment of BLosclence and Biotechnology, Strathclyde Unrverstty, McCance Burldmng, RLchmond Street, Glasgow Gl 1XQ (Scotland)
(Received October 30,1991, accepted m revised form March 17,1992)
Abstract The influence of membrane pore structure on the ease with which an aqueous-orgamc interface can be maintained m the plane of the membrane of a two-phase membrane reactor 1s &cussed Four factors affecting the pressure required to cause breakthrough of the non-wetting phase have been identified (1) membrane pore size, (u) asymmetry of membrane pore structure, (m) the placement of the wetting liquid when an asymmetnc membrane 1s used, and (iv) a change m the wetting charactenstlcs of the membrane as the reactlon progresses The fourth factor 1s part~ularly Important smce two phase blocatalytic reactions frequently involve surface active reactants and/or products It 1s shown that the ideal pattern of surfactant-membrane mteractlons - which 1s reflected by the desired direction of change m the angle of contact between the wettmg liquid and the membrane - depends on the second and third factors An expenment 1s suggested to assess the importance of the vanous factors and a set of rules of thumb have been presented to assist m the selection of membrane matinal and type The unportance of correctly ldentifymg the wetting liquid when an ampluphdlc membrane polymer 1sused has been pomted out Keywords
two-phase biocatalyns; membrane through, enzymatic synthesis, theory
Introduction The considerable advantages of aqueous-organic two-phase systems for many biocatalytx processes have been described in several recent reviews [l-3]. When all the reactants can be supplied in the organic phase, and all the prodCorrespondence to A M Valdya, Department of Chemical Engmeermg, Strathclyde Umversity, McCance Bulldmg, Richmond Street, Glasgow Gl lXS, Scotland
bioreactor,
surfactant-membrane
mteractlon,
break-
ucts removed from the same phase, it is possible to design various types of reactors with the aqueous phase confined to the surfaces of the catalyst particles. However, in a number of potential applications, access is required to the aqueous phase as well, e.g. because at least one of the reactants or products is only appreciably soluble in the aqueous phase [ 41. Emulslficatlon is an obvious technique to accomplish this, but the problems of post-reaction phase sepa-
0376-7388/92/$05 00 0 1992 Elsevler Science Publishers B V All nghts reserved
A M Vasdya et al /J Membrane Scl 71(1992) 139-149
140
TABLE 1 Two-phase blocatalytlc even m bold letters
reactlon
systems mvolvmg surface active components
The surface active components
have been
Reference
Polar reactant
Non-polar reactant
Product
Enzyme
Alcohol (methanol, glycerol) Water
Fatty acid (Laurie, Oleic) Triglyceride (olive oil) Alcohol (Hexadecyl, Menthol) Long chain alcohols Phospatidyl choline
Ester”
Llpase
2
Fatty acid Ester”
Llpase
7
Mlcroblal e&erase
10
Alkylbetaglucoside Phosphatidyl glycerol
Glycosldase
11
Phosphohpase
12
Acid (acetic, succmic ) Glucose Glycerol
“Glycerol monoesters
and succmate
esters are surface actwe
ration can seriously limit its practical utility
[51
The posslblhty of using a selectively wetted membrane to bring about interfacial contact between bulk aqueous and organic phases has been demonstrated [ 6-91. Hoq et al. [ 6,7] used microporous polypropylene membranes for this purpose A slight positive pressure, of the order of 20 cm of water (ca. 0.02 bar), was apphed on the aqueous phase to keep the interface m the plane of the membrane. Pronk et al. [8] pointed out the advantages of using hydrophilic ultrafiltration (UF) membranes m such an apphcation The Cuprophan membranes employed by these workers permitted the use of pressures as high as 1 bar for interface maintenance. Neither of these two groups of workers have commented on the criteria used by them for the selection of the membrane material Some of the products produced during the course of such reactions are frequently surface active. Some of the starting materials are surface active as well The blocatalysts used also possess some surface activity. These surface active components can change the wetting behavior of the mem-
brane, and make maintenance of the interface in the membrane difficult Table 1 lists several examples of two phase biocatalytic reaction systems and the surface active components involved. The criteria to be considered while selecting membrane materials in order to minimize the effects of the surfactant are discussed m this paper. The manner m which membrane pore structure can affect this choice is also discussed Theory Figure 1 shows the interface formed between two immiscible liqmds withm an arbitrarily shaped pore m a sohd wetted by one of the two hqulds. Evidently, a positive pressure must be applied to the non-wettmg liquid in order to maintain this interface. If this pressure is sufficiently large, displacement of the wetting liquid can occur and this will eventually result m the breakthrough of the non-wetting hqmd. Stable reactor operation can only be achieved d the uutial breakthrough pressure, dP,,, is reasonably high, and remains so during the course
A M Vatdya et al/J
_------
---_------__ _-------__ --------__
-
141
Membrane Scz 71(1992) 139-149
----_-
--Weltmg Liquid _ _
-___
-----_
--------____ --------__ ------____ -----__
_
___---------
Fig 1 Llquld-liquid interface formed mslde an arbltrarlly shaped membrane pore
Wettmg Liquid ----
__
-_-I
wm/ ----. 8 -IYe
of the reaction. A change m the content of surface active components, as a result of reaction, can lead to a change m the breakthrough pressure Ideally, one would hke to find a membrane for which dP,, remained constant, or mcreased, durmg the course of the reaction The factors which mfluence the magmtude of dP,, will be identified m this section. The effect of surface active components at the various mterfaces will then be discussed and the pomts to be considered while selectmg a membrane for use m a two phase membrane reactor will be hlghhghted. Fmally, a set of rules of thumb will be presented to assist the process of membrane selection In order to identify the factors which determme the magmtude of dP,, it is useful to examme three special forms of the general equation of capillarlty [ 131
AP,
=2gwn - r sin (P+ f%,)
(2)
2own - r sln(P-%,I
(3)
AP,, =
Equation ( 1) is the Laplace-Young equation
Fig 2 Liquid-liquid interface formed inside a cyhndrlcal membrane pore The inset shows the angle of contact, O,.,,, between the wetting liquid and the membrane
[ 131 and gives the breakthrough pressure when the pore is of cylindrical cross-section (Fig. 2). Equations (2) and (3) have been derived by Adam [ 141 to describe the behavior of memscl m tapering capillaries, as shown m Figs 3 (A) and 3 ( B ) , respectively. When /3= 90 ‘, eqns. ( 2 ) and (3 ) reduce to eqn. ( 1) An exammatlon of these three equations reveals that the magmtude of the breakthrough pressure is determined by four factors (1) The pore sue In all three cases the breakthrough pressure is inversely proportional to the pore radius. This implies that the mamtenance of a stable hquldliquid interface would be easier with an UF membrane than with a microfiltration (MF) membrane, which has pore diameters larger by several orders of magnitude. (2) The pore geometry Any change m the content of surface active components m the system, as a result of reac-
142
A M Valdya et al/J
_-_--
_-A--
---- - Wetbng Llqutd ______----
----
=.f --B
_____-__-_--_-____------
--Qwm: --_---------
---
B
---_------- __--------
--___----______---_---
increase or decrease. Clearly, the nature of the change will depend on the shape of the pore, and this m turn will affect the stability of an interface formed in a given pore. Two types of asymmetric membrane have been identified by Kestmg [ 151. (1) Skinless MF membranes. (ii) Skinned UF membranes. A model skmless MF membrane is shown m Fig. 3, while Figs 4 and 5 show the two possible structures for a model skinned membrane. These two model structures will be referred to as cylindrically skinned and conically skinned membranes, respectively
----
__----
Membrane Scl 71(1992) 139-149
(A)
(3) The placement of the lrquldphases
_---
Wetting Llquld
--_
_
_
Fig 3 Llquld-liquid interface at the point of mclplent breakthrough m the pore of an Ideal skinless MF membrane The inset shows the contact angle, &,,, and the angle j3 formed by the mchned walls of the pore with the radial direction (A) Wetting hquld placed on the open side of the pore (B) Interface formed by movement of the nonwetting liquid through the open end of the pore
tlon, may be reflected by a change m the contact angle, 13,~ As a result of this the trigonometric terms in eqns (l)-(3) can either
When a membrane is asymmetric, the equation to be used to calculate the breakthrough pressure is determined by the placement of the liquid phases With the arrangement shown m Fig. 3 (A), eqn. (2) must be used to calculate the breakthrough pressure With the non-wetting liquid on the dense side of a skinned membrane - as shown in Figs 4 (A) and 5 (A) - eqn. (1) can be used to determine dPb when the membrane is cyhndrically skinned and eqn ( 2 ) when the membrane is conically skinned. There is no easily apparent reason for not using the liquid phase locations shown in Figs. 3(B)5(B). Creating such interfaces offers the posslblhty of increasing the interfacial area which can be obtained with a given surface area of the membrane. It is logical to assume that the breakthrough pressure can still be calculated by applying eqns. ( 1) - ( 3 ) using the pore radius at the point of mclplent breakthrough. However, it must be borne m mmd that the interfaces of Figs. 3 (B ) -5 (B ) have been formed by moving an interface uutially located at the open end of the pore, where the pore radius is considerably greater Such interfaces can be set m motion at comparatively low pressures Once a moving interface has been created, its shape and the
A M Valdya et al/J
143
Membrane SCL 71(1992) 139-149
1-z_--L__.J _------_---_-_ ---- Wenmg Lqud ~
_< -a -
-
-
----
Wetting Llquld
-_
-=e wm ---
(‘V
W
__-------_ _--__-------_ ---
-----
---
Wetting Liquid _
---
---
Wettfng Llquld :
-
-
63 Fig 4 Liquid-hqmd interface at the point of mclplent breakthrough m the pore of a cyhntically skinned membrane The inset shows the contact angle (A) Wetting hqmd placed on the open side of the pore (B ) Interface formed by movement of the non-wetting hquld through the open end of the pore
contact angle, O,,, both undergo a change. This phenomenon has been reported by several workers [ 16-191 By applying the Le Chatlier prmclple Rose and Hems [ 191concluded that the contact angle would always change in such
Fig 5 Llquld-hquld Interface at the point of mclplent breakthrough m the pore of a comcally skinned membrane The mset shows the contact angle, O,, and the angle /3 formed by the wall of the comcal pore, m the skm of the membrane, with the radml dxectlon (A) Wetting hquld placed on the open side of the pore (B) Interface formed by movement of the non-wettmg hquld through the open end of the pore
a manner as to resist the movement of the interface. Thus would indicate that the Interfaces shown m Figs 3 (B )-5 (B ) will be stable. How-
144
A M Valdya et al/J
ever, experimental verification of interfacial behavior will have to be done to ascertam the feasibility of placmg the non-wetting liquid on the open side of an asymmetric membrane. Till such mformation is available it can only be said that breakthrough pressures calculated using eqns ( 1 )- (3 ) are liable to be misleading. Ideal contact angle behavior deduced from these equations could also be incorrect (4) Interfaczal effects It is evident from eqns (l)- (3), that the breakthrough pressure is directly proportional to the interfacial tension of the hquid-liquid Interface, o,, However, there are two other mterfaces m a two phase membrane reactor which can slgmficantly affect its stability, namely. (1) The interface between the wetting hquid and the membrane. (11) The interface between the non-wetting hquid and the membrane. The changes induced at these interfaces in the presence of surface active materials, must also be considered when selecting a membrane for use in a two-phase reactor The importance of doing this is best illustrated by considermg a symmetric membrane. When a stable interface exists in the pores of such a membrane, the force balance described by the Young-Dupre equation [ 131 can be used to write 0
-
cos&,, = Imn CJwn
%rn
(4)
Combining eqns. ( 1) and (4 ) yields an expression relatmg the two membrane-liquid interfacial tensions and the breakthrough pressure
Equation (5) is merely a way of lookmg at the effects of changes in a,, and &,, - discussed earlier m terms of eqn. (1) - which provides a
Membrane Scr 71(1992) 139-149
better insight mto the possible consequences of surfactant adsorption Surfactant adsorption As was pointed out m the preceding section, there are three interfaces m a two-phase membrane reactor. Any surfactant - reactant, product or biocatalyst - present in the system can adsorb at all three interfaces This adsorption will invariably result m a lowermg of the mterfacial tension at that interface. The consequences of this decrease will be pointed out m this section. (1) The lzquzd-lzquzdznterface
Equations (l)-(3) indicate that dP,, is directly proportional to the hquld-liquid mterfacial tension, a,,. Clearly, any lowermg of this quantity IS undesirable. However, very little control can be exercised over surfactant adsorption at the liquid-liquid interface The use of a suitable solvent for the organic phase can help to mmimize the reduction m a,,, but this may not always be feasible Even when a solvent can be used the post-reaction solvent reunwelcome introduces covery step complications. (2) The lzquzd-membrane znterfaczal tenszons From eqn. (5) it is evident that the direction of change m the breakthrough pressure as a result of surfactant adsorption depends on the relative extent of adsorption at the two hqmdmembrane interfaces. Preferential adsorption at the interface between the membrane and the non-wetting liquid will result in a decrease m dP,. If the surfactant adsorbs selectively at the wetting liquid-membrane interface, dP,, will increase More generally, it can be stated that surfactant adsorption at the liquid-membrane mter-
145
A M VaLdya et al /J Membrane Scl 71(1992) 139-149
faces will result m a change in the angle of conand the tact, &,, between the membrane wetting liquid. It is not immediately obvious that this angle can change in both directions. However, an examination of eqn. (4) will reveal that this can - at least, in prmclple happen. Membrane selection The points to be considered while selecting membranes for use m two-phase reactors will be outlined m this section based on the discussions presented earlier
(2) Contact angle behavior It has been pointed out earlier that the presence of surface active components leads to a decrease m dP, as a result of adsorption at the liquid-liquid interface. This effect can be combatted, to some extent, by selectmg a membrane material for which the contact angle, SW,,, changes in such a manner as to minimize the decrease in dP,,. Table 2 summarizes the nature of the desired change m &,, for the different pore shapes and liquid phase locations, described earlier. (3) Factors affectzng adsorptwn
(I) UF versus MF The small pore sizes m ultrafiltration membranes offer the advantage of a much higher breakthrough pressure. It would be difficult to use a microporous membrane if the breakthrough pressure were very small, as found by Hoq et al. [ 61. This is particularly so when microporous hollow fibers are used. A simple calculation, based on the Hagen-Poiseullle equation [ 201, will show that if it were desired to pump a viscous substrate (such as olive oil which has a viscosity of 0.084 Pa-set at room temperature [ 211) through the lumen of a typical commercially available microporous hydrophobic hollow fiber module (with a total length of 25 cm and fibers with an internal dlameter of 190pm) the lumen side pressure drop across the module would be of the order of 0 01 bar - for a residence time of one hour. As Pronk et al. [ 81 have commented, the pressure control required for interface maintenance would have to be extremely accurate. Slight variations m the trans-membrane pressure drop, or an accidental increase m the lumen side pressure downstream of the reactor, would be sufficient to cause movement of the interface. The generation of a surface active product would further complicate matters.
The desired patterns of contact angle behavior, as a result of surfactant adsorption, outlined above require preferential adsorption of surfactant at the wetting liquid-membrane mterface when a decrease in &, is desired. Conversely, when an increase m 0,, is desired the surfactant must selectively adsorb at the mterface between the non-wetting liquid and the membrane. There are several factors which determine the extent of the adsorption of a surfactant at a solid-liquid interface which have been discussed m some detail by Fowkes [ 221. It is extremely unlikely, if not impossible, that all the details of surfactant-membrane interaction can be determined without taking recourse to experiment. However, the following observations can serve as a useful guide: (1) Fane [23] and Jonsson and Jonsson [24] have reported that surfactants m aqueous solution tend to adsorb more strongly on to hydrophobic membrane surfaces (11) The Lundelms rule [ 251 states that there is an inverse relationship between the extent of adsorption of a solute and its solubllity. Table 3 illustrates the utility of these observations The partition coefficient for the surfactant between the organic and aqueous phases was
146
A M Vatdya et al/J
Membrane Set 71(1992) 139-149
TABLE 2 Ideal contact angle behavior, for membranes with the specified ideal pore structures, m the presence of surfactants Membrane Type
Wettmg phase placement
Reference figure
Reference equation
Ideal contact angle behavior
Symmetric
N/A
2
1
Small, and decreasing
Skinless MF
Open side Tight side
3(A) 3(B)
2 3”
Cylindrically skinned
Open side
4(A)
1
Small and decreasing
Skinned side
4(B)
1”
b
Open side Skmned side
5(A) 5(B)
2 3”
Conically skmned
(~+&,)=900
b
(/3+e,,)=900 b
“As indicated m the text the apphcablhty of these equations ISdoubtful bIf the specified reference equations can be used, a small and decreasing f?,,.,would be desirable TABLE 3 The effect of surfactants on the contact angle and the vamous mterfaclal tensions for the system Nylon-6-water-heptane/ chloroform-s&a&ant The contact angles and mterfaclal tensions have been extracted from Smolders [ 261 Membrane material
Surfactant concentration moles/hter
System
RVUI
Gl (mN/m)
Gm - %nl (mN/m)
Nylon-6
0 3x 10-4
1 1
70” 60”
30 10
10 26 5 00
Nylon-6
0 2x10-6 55x10-6
2” 2 2
10” 10” 71”
52 4 40 32 4
5160 39 39 10 55
“Extrapolated values w water, m Nylon 6, n heptane/chloroform System 1 Chloroform-water-dodecyl pyrldmmm bromide System 2 Heptane-water-dodecyl pyrldmmm bromide
found to be 27 for system 1 and very close to zero for system 2. The figures m the last column of Table 3, calculated using the YoungDupre equation, indicate a much faster dechne in a,, - pointing to preferential adsorption at the heptane-membrane mterface. It will also be seen that both a decrease and an increase in the contact angle is possible as a result of sur-
factant adsorption. The results of Table 3 have been cited purely as an example. The applicability of the drop-weight method, used by Smolders [ 261, for the estimation of the interfacial tensions of surfactant solutions is questionable. The unreliability of the closely related drop-volume method has been clearly demonstrated by Carroll and Doyle [ 271.
A M Vatdya et al/J
147
Membrane See 71 (1992) 139-149
(4) Membrane mated It has been indicated above that making the correct choice of membrane material for a given reaction will require some experimental work. This is best done by measuring the variation in breakthrough pressure for a given membrane as the concentration of surfactant m the system changes. However, before embarking on such experiments three other points must be noted. (1) Membrane wetfng characterlsttcs A wide variety of purportedly hydrophilic membranes are marketed by various companies For instance, several manufacturers supply Nylon-66 membranes which are claimed to be hydrophilic PVDF and PS membranes of a hydrophrhc character are produced by Gelman Sciences. Nylon-66 with its polar amide linkages, and the non-polar intervening polymethylene groups, must clearly have an amphlphlhc character. The chemical structures of PVDF [ 281 and the various polysulfones [ 291 suggest that these materials should be mtrinslcally hydrophobic. Since the majority of membrane filtration applications involve aqueous solutions it is useful to impart a measure of hydrophilicity to such membrane materials by surface or monomer modification Keurentjes et al. [ 301 carried out X-ray photoelectron spectroscopy (XPS) of IRIS PVDF membranes made by RhBne-Poulenc Their results are reproduced m Table 4. Clearly, some form of surface modification, accounting for the presence of oxygen and mtrogen atoms near the surface, was used for imparting hydrophiliclty to the membrane Again, it would be more correct to call such membranes amphiphihc m that if they are uutlally immersed m air they may be readily wetted by either a polar or a non-polar liquid. The type of liquid which will preferentially wet such membranes m the competitive environment of the
TABLE 4 Atomic ratios found by XPS measurements on IRIS PVDF membranes Results of KeurentJes et al [30] The theoretlcally expected and experlmentally determmed atomic ratios of the specified elements have been gven at two depths of analysis Atom
C F 0 N
Theory
Depth of analysis
50 50 0 0
two liquid phase membrane ily apparent.
5nm
lnm
57 6 29 1 77 56
60 6 26 1 79 53
reactor is not read-
(u) The effect of bwcatalysts The enzymes used in many of the reactions described in Table 1 are themselves surface active It is generally believed that the surface activity of the enzyme will be of little importance m the presence of a strongly surface active material. For instance, the breaking of oil-m-water emulsions stabilized by proteins has been ascribed to the displacement of the protein by smaller molecules with higher spreading pressures [31]. The inhibition of hpase activity during the hydrolysis of triglycerides has been imputed as being caused by the displacement of the enzyme by the non-ionic surfactant produced during the reaction [ 321. (~1) The effect of other components Although only one of the components of the reaction system may be strongly surface active the overall behavior of the system may be sigmticantly altered by the presence of other components which also possess some surface actlvity. For instance, ethanol and laurlc acid are produced as a result of the hydrolysis of ethyl laurate Although laurlc acid is the more strongly surface active product, the ethanol -
148
which will pass mto the aqueous phase - can bring about some change m the wettability of the membrane by the aqueous phase. Clearly, the results of a simple breakthrough pressure test can only be used as a gmde to membrane selection. Conclusions The criteria which must be considered while choosmg a membrane for use in a two-phase membrane reactor have been discussed m this paper Based on these discussions the followmg set of rules of thumb can be drawn up: ( 1) Any combination of membrane material and structure which leads to a small breakthrough pressure requirement should be avoided In practice this means that breakthrough pressures should be of the order of several tenths of a bar, or more. As a general observation, UF membranes will offer higher breakthrough pressures because of their smaller pore size (2 ) The wetting characteristics of the membrane will change as the composition of the system changes during the course of the reaction. This may be reflected by a change in the breakthrough pressure. This variation should be studied while selecting membrane material and type (3) It should be borne in mmd that pore asymmetry can lead to an asymmetry m breakthrough pressure In particular, it must be noted that placing the non-wettmg liquid on the open side of an asymmetric membrane can give rise to movmg interfaces whose breakthrough behavior is difficult to predict. (4) With some membrane polymers, the effects of their amphiphihcity and pre-treatment must be properly understood before determmmg the nature of the phase to be used as the wetting hqmd. This is best accomphshed by doing two sets of breakthrough pressure exper-
A M Valdya et al /J Membrane SCL 71(1992) 139-149
iments usmg both the liquids mvolved in the role of the wettmg liquid Finally, it must be stressed that mterpretation of results of breakthrough pressure measurement based on the discussions presented in this paper must be done with caution, smce idealized membrane structures have been used as the bases for these discussions. Experimental results of breakthrough pressure experiments lllustratmg the utihty of these rules will be presented m a separate paper. List of symbols Ah r 8wm P
u wn 0 wm u nm
breakthrough pressure (Pa) pore radms at the three phase lure of contact (m) angle of contact between wetting liquid and membrane acute angle formed between wall of a tapering capillary and the radial direction - see Figs. 3,4 and 5. mterfacial tension at the hqmd-liquid interface (N/m) mterfacial tension at the wetting hqmd-membrane interface (N/m) mterfacial tension at the non-wetting hqmd-membrane interface (N/m)
References P J Halhng, Blocatalysls m multi-phase reactlon mixtures contammg organic hqulds, Blotechnol Adv , 5 (1987) 47 J S Dordlck, Enzymatic catalysis m monophaslc organic solvents, Enzym Mlcrob Technol , 11 (1989) 194 K D MukherJee, Llpase-catalyzed reactions for modification of fats and other lip&, Blocatal , 3 (1990) 277 MD Lilly, A J Brazier, MD Hocknull, A C Wllhams and J M Woodley, Bloloecal conversions mvolvmg water-msoluble organic compounds, m C Laane, J Tramper and M D Lilly (Eds ), Blocatalysls m Orgamc Media, Elsevler, Amsterdam, 1987, pp 3-20 J M Woodley, Design of blocatalytlc processes mvolvmg two liquid phase enzymatic catalysis, Ph D Thesis, Umverslty of London, 1988
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7
8
9
10
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12
13 14 15 16
17
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M.M. Hoq, T. Yamane and S. Shimizu, Continuous hydrolysis of olive oil by lipase in a microporous hydrophobic membrane bioreactor, J. Am. Oil. Chem. Sot., 62 (1986) 1016. M.M. Hoq, M. Koike, T. Yamane and S. Shimizu, Continuous hydrolysis of olive oil by lipase in a microporous hydrophobic hollow fiber bioreactor, Agric. Biol. Chem., 49 (1985) 3171. W. Pronk, P.J.A.M. Kerkhof, C. van Helden and K. van’t Riet, The hydrolysis of triglycerides by immobilized lipase in a hydrophilic membrane reactor, Biotechnol. Bioeng., 32 (1988) 512. M. Rucka and B. Turkiewicz, Hydrolysis of sunflower oil by means of hydrophobic membrane with lipolytic activity, Biotechnol. Lett., 11 (1989) 167. T. Omata, N. Iwamoto, T. Kimura, A. Tanaka and S. Fukui, Stereoselective Rhodotorula minuta var. texenis cells in organic solvent, Eur. J. Appl. Microbial. Biotechnol., 11 (1981) 199. E.N. Vulfson, R. Patel, J.E. Beecher, A.T. Andrews and B.A. Law, Glycosidases in organic solvents. I. Alkyl-beta-glucoside synthesis in a water-organic twophase system, Enzym. Microb. Technol., 12 (1990) 950. L.R. Juneja, N. Hibi, T. Yamane and S. Shimizu, Repeated batch and continuous operations for phosphatidyl-glycerol synthesis from phosphatidylchol~e with immobilized phospholipase-D, Appl. Microbial. Biotechnol., ‘27 (1987) 146. A.W. Adamson, The Physical Chemistry of Surfaces, 5th edn., Wiley-Interscience, New York, NY, 1990. N.K. Adam, Principles of penetration of liquids into solids, Discuss. Faraday. Sot., 3 (1948) 5. R.E. Kesting, Synthetic Polymeric Membranes, 2nd edn., Wiley-Interscience, New York, NY, 1985. P. Van Remoortere and P. Joos, The kinetics of wetting: The motion of a three-phase contact line in a capillary, J. Colloid Interface Sci., 141 (1991) 348. T.E. Mumley, C.J. Radke and M.C. Williams, Kinetics of liquid/liquid capillary rise, J. Colloid-Interface Sci., 109 (1986) 398. G.E.P Elliott and A.C. Riddiford, Dynamic contact angles and rates of adsorption, Nature, 195 (1962) 795. W. Rose and R.H. Heins, Moving interfaces and contact angle rate dependency, J. Colloid Sci., 17 (1962) 39.
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