journal of MEMBRANE

SCIENCE

ELSEVIER Journal of Membrane Science 114 (1996) 39-50

Flux enhancement by hydrophilization of thin film composite reverse osmosis membranes

Ashish Kulkarni, Debabrata Mukherjee ~, Will iam N. Gill Howard P, lsermann Department of Chemical Engineering. Rens.wlaer Polytechnic Institute, Troy, NY 12180. USA

Received 19 April 1995; revised 5 September 1995; accepted 1 October 1995

Abstract

Protic acids, including hydrofluoric, hydrochloric, sulfuric, phosphoric and mtric acids, are studied as hydrophilizing agents for modifying the surfaces of thin film composite (TFC) reverse osmosis membranes. HR95PP and HR98PP membranes on exposure to various concentrations of these acids increase in flux up to an order of magnitude without any loss in ion-rejection. The flux enhancement is about an order of magnitude in HR98PP and a factor of two in high flux HR95PP membranes.

Surface characterization with contact angle measurements indicate an increase in hydrophilicity of the membrane surface. At solvatable sites along the polymer chain, reactions causing partial hydrolysis may be responsible for the increase in hydrophilicity resulting in the observed flux increase. Exposure to mild solvents like ethanol and 2-propanol (isopropyl alcohol, IPA) also increases the flux with no loss in rejection; in fact a significant increase in rejection is obtained in some cases. Selective dissolution and elimination of defects probably are responsible for the increased rates of transport,

Exposure to a mixture of acid and ethanol caused an increase in flux with no loss in rejection within experimental error. This method of treatment with protic acids and alcohols could be used as a viable post treatment method in developing

high flux and high rejection membranes of the future.

Keyword.s: Flux enhancement; Reverse osmosis; Chemical surface modification; Composite membranes; Skin

1. In t roduc t ion

Hydrophilization of membranes has received a lot of attention in the last few years [1-22]. Hy- drophilization of hydrophobic surfaces combines im- proved separation characteristics with strength and chemical resistance. With ultrafiltration and microfil-

tration membranes it results in increased resistance

* Corresponding author. Present address: Schoeller Technical Papers Inc., Pulaski, NY

13142, USA.

to fouling. Most of the published work on surface modification is related to ultrafiltration or microfil- tration membranes.

Polyethylene microfiltration membranes were hy- drophilized using ternary mixtures of e thanol- water-inorganic acids [1]. The membranes showed

an increase in initial flux and lower flux decline

compared to fresh untreated membranes. Treatment with sulfonating agents like chlorosulfonic acid [2] caused an increase in flux or water permeability of polyethylene membranes. Use of sulfonating agents like propane sultone in the presence of Friedel-Crafts

0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0376-7388(95)00271-5

40 A. Kulkarni et a l . / Journal of Membrane Science 114 (1996) 39-50

catalysts [3,4] increase the rejection (decrease in molecular weight cut off) with no change in flux for polysulfone ultrafiltration membranes. Matsuda et al. [5] produced hydrophilic polyolefin membranes by sulfonating them.

Low temperature plasmas also have been used extensively for modifying the surface of ultrafiltra- tion and microfiltration membranes [6-8]. Shimo- mura et al. [6] used helium oxygen plasma to pro- duce acrylonitrile reverse osmosis membranes. Clarotti et al. [12] hydrophilized polysulfone mem- branes with plasma containing a mixture of ethylene oxide and perfluorohexane. Wang et al. [13] altered polypropylene surfaces in the presence of ammonia to make them more hydrophilic. Membranes made of polyetherimide, allylamine and poly(dimethyl silox- ane) also were modified using low temperature plas- mas [9-11]. Sedath et al. [14] modified polysulfone ultrafiltration membranes using fluorine gas to de- crease the rate of fouling. Fluorine and oxygen added to the surface seems to account for the increase in hydrophilicity and decreased fouling rate.

Almost all of these methods involve chemical modification of the membranes. A disadvantage of this approach is that, usually, the flux increase is accompanied by some damage to the porous struc- ture and a decrease in rejection of the membrane or vice versa.

Very recently aqueous fluorinating agents such as hydrofluoric acid were used [15,16] to modify the properties of reverse osmosis membranes. Dramatic flux increases up to an order of magnitude were obtained without any loss in ion rejection. The ob- served flux increase was attributed to partial fluori- nation along with surface etching.

Another kind of hydrophilization is a temporary one, which is achieved by wetting the membranes with water-soluble solvent. Subsequently, capillary repulsive forces of the hydrophobic membranes cease to counteract the water flux. The basic disadvantage of this method, in which the chemical structure does not change, is that water flux decreases with time due to gradual leaching of the hydrophilizing agent by the water permeating through the membrane [1].

In the present work we hydrophilize thin film composite reverse osmosis membranes to improve permanently both their pure water flux and ion-rejec- tion by treating them with binary solutions of acids

in water, alcohol in water and ternary solutions of acid, alcohol and water. This is a logical generaliza- tion of our previous work [15] where we used hydro- fluoric acid (HF) which is very corrosive and toxic. Here we use less toxic chemicals to obtain large and permanent improvements in performance.

In our previous work, XPS (X-ray photoelectron spectroscopy) and SEM (scanning electron mi- croscopy) were used to characterize the morphologi- cal changes on the surface of the membrane. Here, the membranes after treatment have been character- ized by contact angle measurements to quantify changes obtained in hydrophilicity. These results could be combined with our previous results to explain the observed phenomena.

Tensile strength measurements also were per- formed to determine the change in the ability of the membrane to operate under high pressures after the chemical treatment.

Currently, efforts are underway to use other tech- niques like ATR-FTIR (attenuated total reflection- Fourier transform infrared) spectroscopy to study the changes on the surface of thin film composite re- verse osmosis membranes. This technique cannot be used directly because it penetrates about 0.6 /xm deep into the membrane sample. The skin of these membranes is usually 0.1 to 0.2 /xm and in order to get clear information on changes in membrane struc- ture, the skin has to be separated from the polysul- fone support layer.

2. Experimental

2.1. Membranes

The HR95PP and HR98PP thin film composite reverse osmosis membranes studied here are very widely used and were obtained from Niro-Hudson Inc (DDS Denmark distributor). These membranes are very similar [24] to FT-30 membranes (manufac- tured and marketed by Filmtec Corp.,) in their chem- ical and operational properties: HR95PP is similar to FT-BW30 and HR98PP to FT-SW30HR. We used FF-30 membranes previously [15]. All of these mem- branes are produced by the interfacial reaction of aromatic diamines in the aqueous phase with triacyl chlorides in the solvent phase [23]. The skin is

A. Kulkarni et al. / Journal of Membrane Science 114 (1996) 39-50 41

reported to be fully aromatic polyamide containing carboxyl groups which may become negatively charged at approximately neutral pH [24]. The chem- ical structure of the HR98PP is shown in Fig. 1.

2.2. Chemical treatment

The HR95PP and HR98PP membranes, without masking on any face, were dipped in binary solutions of water and hydrofluoric acid, hydrochloric acid, phosphoric acid, nitric acid, sulfuric acid, ethyl alco- hol or isopropyl alcohol (2-propanol) under con- trolled conditions of temperature for various periods of time. Experiments also were conducted with membranes exposed to ternary solutions of acids, alcohol and water. In all cases the membranes were thoroughly washed with deionized water before mea- suring their transport characteristics.

P~.M~ ' rE I IEl~ ' r

ROTA METER

Fig. 2. Schematic of experimental setup used for transport mea- surements.

2.3. Transport measurements

A schematic of the reverse osmosis apparatus is shown in Fig. 2 and consists of a single cell, flat sheet, closed loop recycle system in which the pip- ings, fittings and the cell were made of SS316. A diaphragm pump (Pulsafeeder 7660) was used for recycling the salt solution (NaCI, 0.5 wt%) through the system at 350 psi unless otherwise indicated to be 250 psi and the feed tank temperature was main- tained constant at 24°C. The conductivities of the feed, reject and permeate samples were measured with a conductivity meter (Cole-Parmer).

The membranes were taken out of the solutions after various intervals of time, and rinsed with deion- ized water. Then the permeate flux and rejection of salt were measured. The rejection was defined as:

Cp R = I - - - (1)

CR

where C e and C R are bulk permeate and retentate salt concentrations respectively. The data on flux and rejection were determined by multiple replications and the standard deviations of the observations were determined to be 5% for flux and 0.5% for rejection.

2.4. Characterization

Contact angle measurement is one of the most sensitive methods for obtaining information on sur- face wetting [25-27] and here we correlate contact angle measurements on heterogeneous membrane surfaces with wettability. Membranes are precleaned and allowed to soak in water for some time. Octane/water /membrane interfaces [28] are formed by immersing membranes in a glass observation cell containing octane-saturated water, and releasing oc- tane drops beneath the solid surface from a syringe. As octane has a lower density than water the octane

[ o i o c

I Fig. 1. Chemical structure of HR98PP membranes.

42 A. Kulkarni et a l . / Journal of'Membrane Science 114 (1996) 39-50

75

65

~S.5 E

4s

35

~ u 10% HCI o

tic

36% HC!

,36% H C I ~ 7.4"/. S~dturJc

4 - - ' - -

18% llCl

I

0.99

0.911

o.97

;7

0.96

0.94

30 0 5 10 15 20 2$

Time of exposure (days)

Fig. 3. Flux and rejection of HR95PP membranes exposed to sulfuric acid and hydrochloric acid for various periods of time.

drops rise, touch the solid surface and form the interface. Contact angles on both sides of the bubble are recorded using a Zeiss macroscope with a video camera to capture the image on a video tape and the video picture is used with Aldus eye software to make a still picture. The drop is magnified at least 200 times before the measurement is made.

The strength of the membrane under high pressure was measured with an Instron 4202 tensile test ma- chine with a 200 lb. load cell using a crosshead speed of 0.1 in/min. Membranes were cut into standard size in the form of dog bones, 7.5 inches long, 0.75 inches wide at the end and 0.5 inches wide at the center. The center part is 4.5 inches long

80.0 • , • , . , . ,

70.0

6o.o

411,0

30.0 ~ _T._ ~ 1% itF.n~alT~01~i

Note: Flux enhancement ~ with HF concentration i i

o.o o'.0 31o ,.o 91o 1 .o ,s.o Time of exposure (days)

Wig. 4. Flux of HR95PP membranes (at 350 and 250 psi) exposed to 15 and 1% HH for various periods of time.

A. Kulkarni et al . / Journal of Membrane Science 114 (1996) 39-50

11.990 , . . . . . . • , • ,

43

g

0.980

0.970

0.960

0.950

~ 350 psi

J ~ _ _ ~ _ _ _ . . ~ I%HF ~r~n at 250 psi ~1

Note: Increase in rejection with increase in HF concentration

i ' i . . . . 0.940 0 0 3.0 6 0 9.0 12.0 Time of exposure (days)

Fig. 5. Rejection of HR95PP membranes (at 350 and 250 psi)

15.0

exposed to 15 and 1% HF for various periods of time.

and the wide ends are 1.5 inches long on each end. The thickness of the membranes was determined to be 0.005 inches from cross sectional SEMs (not shown here).

All the measurements were made on the mem- brane samples under wet conditions as they are wet under the operational conditions. Temperature was maintained at about 24°C during the course of mea- surement. Thorough washing of samples was done to remove the traces of chemicals from the exposed samples.

3. Results and discussion

3.1. Transport measurements

Extraordinarily large improvements in the perfor- mance of membranes can be obtained depending on

the solutions, concentrations and times of exposure used. First the results for HR95PP membranes are presented and then the results for HR98PP will be given.

3.1.1. Treatment of HR95PP membranes Figs. 3-5 and Table 1 represent the results ob-

tained by chemical treatment of HR95PP mem- branes.

3.1.1.1. Binary solutions. Fig. 3 shows the increased flux obtained with HR95PP membranes after expo- sure to binary solutions of HC1 or H 2 S O 4 in water. The flux through the membrane increases from 35 to about 52 1/m 2 h on exposure to 36% (10 M) HCI. In the case of 24% (2.5 M) H 2 S O 4 the flux increased to 42 l / m 2 h. With 18% (5 M) HC1 it increased to about 38 l / m 2 h and then levels off to the flux of fresh membrane after 21 days of exposure. The

Table 1 Flux and rejection of HR95PP membranes exposed to ternary solutions of 5% acids and 50% ethanol and 45% water for I day

Fresh membrane

Membranes treated with 50% ethanol, 45% water and

Hydrochloric Nitric Sulfuric Phosphoric acid 5% acid 5% acid 5% acid 5%

Flux ( l /m 2 h) 35.92 42.78 43.41 47.50 52.51 Rejection 0.9780 0.9792 0.9686 0.9655 0.9695

44 A. Kulkarni et al. /Journal of Membrane Science 114 (1996) 39-50

60

O.99 SO M% IgtJumoJ

• o .gs 40 -o

18% tiC1

'~ ~0% Igtbanol .~' ~" ~ ~ 0.97

2o% su~tu~

0.96 20

0.95 !0

o ' I ' I ' I ' I ' I ' I ' 0.94

0 2 4 6 8 10 12

Time of exposure (days)

Fig. 6. Flux and rejection of HR98PP membranes exposed to hydrochloric acid, sulfuric acid and ethanol binary solutions in water for

various periods of time.

~ 7O

3O

20

tO

0

! lOO%, g~anoJ 1 ] 120

' t ~ llO 9% IIg

o.~ I00

~ ,.~ 70% Nitric -- .- . .-0

0.9

70%. Nitric --4 0.85

49%. HF ~ - . . . . 4 0.8

10o% Ethano l 0.75

i i t J i i i I i 1 i , i i i i i ~ t J i i i i i i i 0 . 7 i i i

Time of exposure (seconds)

Fig. 7. Flux and rejection of HR98PP membranes exposed to concentrated nitric acid, hydrofluoric acid and 100% ethanol for very short

periods of time. Note the almost instant decrease in rejection for nitric acid.

A. KulkaJrni et al. / Journal of Membrane Science 114 (1996) 39-50 45

90

80

70

60

iEso

30

20

l0

5% nitric-50% elkanoG4S%water

5 % s u l f u r k m i x t u r e

5% HCI-M% ethanoi-45% w a t e ~

8% EICI- 50% ,I1"--"u..2% water ©

-.@

5%mlfuric-50% ¢thlmol-45%water

$% HCI mixtnr¢

5% nitric mizture

l P

i I I I I i I I i I 0 1 2 3 4 5 6 7 8 9

T i m e of exposure (days)

I

0.99

0.98

0.97

0.96 ~

0.95 ~

0.94

0.93

0.92

0.91

10

Fig. 8. Flux and rejection of HR98PP membranes exposed to ternary solutions of acid and alcohol for various periods of time.

70

6 0

50

f. ~4o

I~ 3o

20

I0

15% HF- lw.~.k

100% IPA- 1.5 days

1

0.95

0.9

0.85

i i 0.8

f reS l l ~ A A A A - - - - A A~ - - A &

0 i i i ~ I i i I I I ' il , , i |i ' i i li , i , II i , , li i

0 20 40 60 so ioo 120 140

R u n t ime (hours)

Fig. 9. Flux and rejection of HR98PP membranes under dynamic experimental run conditions after membrane was exposed to various concentrations of HF and IPA.

46 A. Kulkarni et a l . / Journal of Membrane Science 114 (1996) 39-50

rejection (Fig. 3) of the membrane does not decrease. In fact, it increases from 97.5 to about 99% in some cases at the same time the flux increases. Figs. 4 and 5 show the flux and rejection, respectively, at two pressures (350 and 250 psi) for HR95PP membrane after exposure to 1% (0.5 M) HF and 15% (7.5 M) HF. In this case, the flux at 350 psi goes up from 58 to about 75 l / m 2 h with rejection increasing from 96 to 97.5%. The flux at 250 psi goes up from 32 to 45 l / m 2 h with rejection going up from 95.5 to 97%. On exposure to 1% (0.5 M) HF there is no signifi- cant increased flux but the rejection goes up slightly. This indicates that higher concentrations of HF cause higher flux increases.

The fluxes of the fresh (untreated) membranes in Figs. 4 and 5 are for different membranes from those in Fig. 3 but they both were supplied under the brand name of HR95PP by the vendor.

3.1.1.2. Ternary solutions. Table 1 shows the results of similar studies conducted by exposing HR95PP membranes to a ternary solution of acids and alco- hols in water for a short period of time (I day). In this case an increase in flux of about 30-40% also was obtained with a negligible loss in rejection.

3.1.2. Treatment of HR98PP membranes Figs. 6 - 9 and Table 2 give the results obtained

with HR98PP membranes after chemical treatment.

3.1.2.1. Binary solutions. Fig. 6 shows the flux and rejection of these membranes when exposed to ethanol, sulfuric acid or hydrochloric acid binary solutions in water. On exposure to 50% ethanol the flux goes up dramatically from about 10 to about 32 l / m 2 h without any loss in rejection and similar behavior is exhibited on exposure to 20% (2.04 M)

Table 2 Flux and rejection of HR98PP membranes exposed to 5% acids for 2 days

Fresh Membranes treated with

membrane Nitric Phosphoric Sulfuric

acid acid acid

Flux ( l / m 2 h) 9.19 19.58 24.67 31.92 Rejection 0.9774 0.9843 0.9849 0.9782

sulfuric acid. But the optimum time of exposure is different in these cases.

Results of studies conducted with 5% acids in water, to determine the effect of low concentrations are tabulated in Table 2. As seen in Figs. 4 and 5, increased HF concentration caused higher flux. Therefore we increased the acid concentrations and decreased the exposure time. The results are shown in Fig. 7. Exposure to 49% (24.5 M) HF in water, even for 5 s, increases the flux from 5 to about 37 l /m 2 h without any loss of rejection. Similar result was obtained with 100% ethanol in which case the flux increased to 18 l / m 2 h. Exposure to 70% (11 M) nitric acid gives very different results; the flux increases from 5 to about 65 l /m 2 h but the rejec- tion drops drastically indicating that the membrane is hydrolyzed.

These studies of exposure to concentrated acids for very short periods of time give dramatic im- provements which can be utilized commercially for post treatment after membrane fabrication.

3.1.2.2. Ternary solutions. Fig. 8 indicates the flux and rejection, respectively, of HR98PP membranes after exposure to a ternary solution of acid and alcohol in water for periods of time up to 9 days. In all cases membranes were exposed to solutions con- taining 5% acid, 50% ethanol and 45% water. The flux increased from 10 to about 42 l / m 2 h without any loss in rejection and in some cases with an increase in rejection. The major changes occurred in the first 1-3 days of exposure.

A few of the exposed membranes were run up to 130 h and the rejection and flux of treated and untreated membranes were monitored continuously. Fig. 9 illustrates the large permanent increase in flux as indicated by the steady state values following an initial compression and rearrangement of the mem- brane polymer. The results in Fig. 9 also show a sharp increase to about 99% rejection. Large steady state flux increases by factors of three to five were obtained depending on the treatment. One clear re- sult from Fig. 9 is that the HR98PP membrane exposed to 2-propanoi gives 99.8% rejection consis- tently which is unusually high for any membrane operating at 350 psi. Furthermore, the flux for this membrane (Fig. 9) after treatment, is more than double that of the fresh, unexposed one.

A. Kulkxlrni et al. / Journal o f Membrane Science 114 (1996) 39-50 47

Exposure to HF also shows exactly the same pattern with an even higher steady state increase in flux.

Fig. 9 also indicates that the exposed membranes take longer to attain steady state than fresh (unex- posed) membrane. At this point it is speculated that the treatments proposed here cause both physical and chemical changes in the membrane polymeric net- work and hence it takes longer for the network to adjust to give constant (steady state) transport prop- erties.

3.2. Contact angle measurements

The contact angles for octane-membrane-water systems are tabulated for fresh and exposed mem- branes in Table 3. Contact angles were measured on both sides of the bubble surface for at least six bubbles at different places on each sample. There- fore, each reported value is an average of at least twelve independent measurements. The results indi- cate that on exposure to HF the contact angles increase which suggests a decrease in hydrophobicity

. / ~ II / ~ . II 1 / ~ 11 / ~ . II / , , ~ . , - - ~ c c ~ - -~ ~ c ~ -

COON I -n

I 1 ,=.,o. H X + H20 = F = e l

H~ ° O X - = H2PO4

= No~

__) ~o.. ,o--x o -] F V " ~ ' - ~ - o

GOOH

I X = HSO 4

H X + HzO = F = CI

H3* O X - = H2PO4

= NO}

i

C .,~----O

I

°L)

Fig. 10. The scheme of reactions indicating increase in surface charge partial hydrolysis on the surface of membrane.

48 A. Kulkarni et aL /Journal of Membrane Science 114 (1996) 39-50

Table 3 The contact angles of fresh and exposed (15% HF) HR98PP membranes

Time of exposure Contact angle (days) (degrees)

0 135.75 _+ 2 1 141.5_+2 7 148.5_+2

or an increase in hydrophilicity (wettability) of the membrane. And from our previous results [15,16] it was observed that the flux and rejection of these membranes increase gradually in the first 7 days and this increase is attributed to an increase in the hy- drophilicity as determined by contact angle measure- ments.

3.3. Tensile strength measurements

Table 4 compares the result of tensile strength measurements on the fresh (untreated) HR98PP membranes with those of the membranes exposed to HF and IPA. The values reported are the average of at least three independent measurements on different samples. The results indicate that the modulus of the treated membranes is only slightly less than the fresh membranes. Therefore, the chemical modification does not have a detrimental effect on the strength and high pressure performance of the membrane.

3.4. Discussion

Based on our transport and contact angle mea- surements, we propose that an increase in hy- drophilicity is causing the increase in flux through the membranes without any loss in ion-rejection. Because of the presence of hydrogen bonding on the surface of the membrane it appears that acids in the

presence of water react on these solvatable sites generating more charge there. But increasing dramat- ically the concentration of acids (with the exception of HF) causes complete hydrolysis of the polymer chain, which results in decreased rejection as seen for nitric acid in Fig. 7. This concentration at which complete hydrolysis occurs is different for each acid. The scheme of reactions is shown in Fig. 10.

Therefore, it appears that partial hydrolysis causes increased hydrophilicity and flux upon treatment with acids. Similar behavior has been obtained before when polyamide dope was prepared in solutions of acids [29-31] before casting the membranes. How- ever, the properties observed in our study were affected significantly by the type of acid used. The larger anionic groups such as nitrate, sulfate or phos- phate do not show the increase in rejection observed with hydrochloric or hydrofluoric acids. Also, in- creasing the concentration and time of exposure must be controlled to avoid total hydrolysis of polyamides resulting in a breakdown of the polymeric structure. Strong acids are well known solvents for polyamides.

The increase in flux observed on treatment with ethanol may be attributed to its mild solvent charac- teristics with respect to polyamides. Hildebrand solu- bility parameters are a very good indicators of the ability of certain organic solutions to act as solvents or swelling agents for membranes [32]. The value of the solubility parameter of ethanol is 26.6 MPa ~/2, 23.6 MPa ~/2 for Isopropyl Alcohol (IPA) and 23 MPa ~/2 for the membrane [33] which suggests that ethanol and IPA are swelling agents indicates the level of their interaction with the membrane [34].

On exposure to ethanol the membrane swells and washing off the ethanol removes small molecular fragments making the membrane a more porous structure. The skin of the membrane swells more than that of the support layer which is seen from the way the membrane folds in ethanol solution and water. Based on the Brown theory of latex formation

Table 4 Tensile strength measurements for fresh and chemically treated HR98PP membranes

Membrane Displacement at Stress at Elongation at point Tensile breakpoint (in) breakpoint (psi) breakpoint (%) modulus (psi)

HR98PP (untreated) 0.7500 3420 18.75 152 333 HR98PP (treated, 15 wt% HF for 4 days) 0.6624 3080 16.56 150600 HR98PP (treated, 40 wt% IPA for 18 h) 0.8060 3610 20.15 148 900

A. Kulkxwni et a l . /Journal of Membrane Science 114 (1996) 39-50 49

[35,36], it is suggested that the imperfections or defects in the membranes are removed which results in an increase of rejection. The removal of small molecular fragments because of partial dissolution in alcohols along with the elimination of defects be- cause of compression effects makes the membrane smoother and thinner. This combination of defect elimination and partial dissolution causes an increase in both the flux and rejection. The acids and alcohol together in a water solution cause both partial hydro- lysis and skin modification, resulting in both a high flux and high rejection membrane.

4. Conclusions

A novel and simple method for improving the flux and rejection properties of thin film composite reverse osmosis membranes has been demonstrated here. Flux enhancement up to an order of magnitude is possible with this type of treatment. The HR98PP and HR95PP membranes used in this investigation are representative of a wide variety of those that are commercially available. Consequently, one would expect similar results with other commercially avail- able thin film composite reverse osmosis mem- branes. Ethanol, 2-propanol, hydrofluoric acid and hydrochloric acid improve the flux with no loss in rejection. The other acids cause the flux to increase with some loss in ion rejection properties. The treat- ment method demonstrated here can be applied eas- ily for improving the flux and rejection for many applications including desalination and waste treat- ment. Indeed the large flux increases obtained would reduce the surface area needed and the size of the units which would make RO systems significantly more cost effective.

Acknowledgements

The authors wish to gratefully acknowledge the financial support of New York State Energy Re- search and Development Authority (NYSERDA) for this work. We also thank Dow Denmark and Niro Hudson Inc for the membrane samples used during the course of this research.

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