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Study on the interaction between membranes and organic solutes by the HPLC method
Desalination, 71 (1989) 107-126 Elsevier Science Publishers B.V., Amsterdam -
107 Printed in The Netherlands
Study on the Interaction Between Membranes and Organic Solutes by the HPLC Method JI JIANG, SUN MINGJI, FE1 MINLING and CHEN JIAYAN* Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian (China), Tel.: 86-411-331641, Telex: 86436 DZCO CN (Received March 23,1988)
SUMMARY
Interactions between three membrane materials and about 90 organic solutes have been studied by the high-performance liquid chormatography method. Three relationships characterizing the interactions, which are related to membrane performance, have been obtained by means of both thermodynamics and reverse osmosis experiments. The relationship between interaction and structure of polymer and organic solutes is discussed based on the partition coefficient and Gibbs free energy obtained from high-performance liquid chromatography experiments. The results may be useful for the molecular design of membrane materials. Keywords: interaction, membrane, reverse osmosis, HPLC, thermodynamics.
INTRODUCTION
Interactions between membrane and separated compounds are closely related to membrane performance [ 1,2]. A clear understanding of interactions between membrane and separated compounds will be greatly significant to molecular design and selection of membrane materials, elucidation of transport mechanism through synthetic membranes and development of membrane applications. Therefore, interactions between three membrane materials and about 90 organic solutes have been studied by the high-performance liquid chromatography (HPLC ) method. Sourirajan et al. [3] have obtained significant results by using the HPLC method to study membranes. However, something in their work needs to be improved because the pure polymer powder was used as a stationary phase, and therefore the dead reten*Present address: Shenzhen Membrane Separation and Biotechnology China.
OOll-9164/89/$03.50
0 1989 Elsevier Science Publishers B.V.
Laboratory, Shenzhen,
108
tion volume of the liquid chromatography (LC) column could not be determined precisely. They have used the solute with the least retention volume as a reference, but unfortunately, the solute with the least retention volume is not always the same one for different polymers, and therefore it is difficult to compare the results with one another. EXPERIMENTAL
Polysulfonamide (PSA) was purchased from the Shanghai Cellulose Company, polybenzimidazolone (PBIL) was supplied by the Department of Chemistry of Jinan University, and the aromatic polyamide (PA) prepared by our institute. The other compounds used were all reagent grade. An HPLC Pump BT 3020 of Biotronik and a differential refractometer R403 of Waters were used and data were processed automatically by means of a computer. The LC column was a 1 m long stainless steel tube (4 mm I.D., 8 mm O.D.). The temperature of the column and the detector was controlled at 38 ? 0.1 ‘C by circulating water. Distilled water was used as a mobile phase, the flow rate was fixed at 1 cm3/min, and quartz powder of 200-300 mesh coated with an about 1000 A thick polymer film was used as a stationary phase. The polymer was about 0.1-0.3 wt.% of the quartz powder which was proved to be chemically inert. Some results are listed in Table I; the dead retention volume’ of the column was 7.976 cm3, with the error being less than 0.90%. THEORY
It is necessary to introduce the fundamental theory used in this paper because another, different theory has been used in this field [ 31. The chromatographic partition coefficient K, in liquid-solid systems is defined by K, =m”/(n’/V,)
(1)
where m” is the concentration of solute in the stationary phase expressed in terms of number of moles of the solute adsorbed per gram of polymer, n’ is the number of moles of the solute in the mobile phase and V, is the volume of the mobile phase. The capacity ratio k is defined by the ratio of the net retention time (tn - t,) of a solute to the retention time t, of a non-adsorbed substance k= (tR -tJltm=(VR-Vm)/Vm
(2)
where tR and VR are the total retention time and volume of the solute, respectively. Actually, the value of k represents the ratio in which a given amount of solute is distributed between the stationary and the mobile phase, i.e.,
109 TABLE I Retention volume of organic solutes’ Solute
VR (cm3)
Methanol Glycerol Furan 1,4-Dioxane 2-Chloroacetic acid Phenol Benzyl alcohol l-B&an01 Dimethyl sulfoxide Resorcinol Pyrrole N-Methyl-apyrrolidone Cyclohexanol Pyridine
7.950 8.000 8.030 7.967 8.005 7.967 7.973 8.002 7.967 8.017 7.998 7.957
Average value
7.976
Error (%)
7.905 7.927
<0.900
‘Stationary phase is 250-300 mesh quartz powder.
k=
amount of solute in the stationary phase amount of solute in the mobile phase
(3)
It follows from Eqn. (3) that
k=K, W/V,,,
(4)
where W is the weight of stationary phase (polymer). Combining Eqns. (2) and (4) gives K=(V,-VA/W
(5)
Let us consider the process of adsorption as a reversible reaction in which one mole of solute is transferred from the mobile phase into the stationary phase under isothermal and isobaric conditions. Denoting the solute (i) in the mobile phase and stationary phases by i(m) and i (s), respectively, the reaction can be represented by i(m)-+i(s)
(6)
AGs=fis-h
(7)
where kci.and km are the chemical potentials of the solute in the stationary
110
and mobile phase, respectively. The chemical potentials can further be expressed by h, = gS +RZlna”
(8)
j.&=&+RZlnU’
(9)
where & , ,&, , a” and a’ are the standard chemical potentials and activities of the solute in the respective phases. Hence, AG, is given by
where the difference & - ,L& is the standard Gibbs free energy of adsorption and will be denoted by AG: . Eqn. (10) also holds for systems which are out of adsorption equilibrium (e.g., the leading or trailing part of the chromatographic zone), and the negative value of AG, is actually the driving force of adsorption. At equilibrium (approximately in the center of the zone), AG, = 0 and the standard Gibbs free energy of adsorption is AGZ = -RZ’ln(u”/u’),
(11)
where the subscript eq indicates that equilibrium solute activities are considered. The ratio (a” /a’ )eq can be considered as a thermodynamic distribution constant. It is readily apparent that this thermodynamic distribution constant is not a priori identical with the conventional chromatographic partition coefficient KS. For the activity (a) of the solute in either phase
a=f If” where f is the actual fugacity
(12)
of the solute and f” is the fugacity under chosen standard conditions. Further, in order to define a relationship between the activity and concentration of the solute in the given phase, a reference state (the state of unit activity coefficient) has to be chosen. Hence, the chromatographic and thermodynamic distribution constants can be related to each other only through the appropriate choice of the standard and reference states for the solute in both phases. It should be noted that the numerical values of the thermodynamic distribution constants and, consequently, of the AGZ values calculated from it, depend on the choice of the standard and reference states. It is therefore necessary that these states be specified unequivocally whenever numerical data are presented on the thermodynamic properties depending on the above choice. The following choice for the standard and reference states will be made in this instance: for the solute standard state in the stationary phase, one mole of solute per gram of absorbent (m” = 1) at T and p of the system; for the solute standard state in the mobile phase, pure solute at T andp of the system; for the reference state for the solute in the stationary
111
phase, infinitely low deposition of solute on the adsorbent (polymer) surface (ri,+ 1 for m” + 0) at T and p of the system; and for the reference state for the solute in the mobile phase, infinitely dilute solution of solute ( rh+ 1 for x’ -to) at T andp of the system. Thus
fi, =ri,k”m” fi, =r,h’x’ f ;s=k” f &,=h’
(15) (16)
& = (fis/f L)/ (f-,/f &) =%mm”/FimX’
(17)
(13) (14)
where k” is a proportionality constant, h’ is Henry’s law constant, Fi, and Fi, are the solute activity coefficients in the stationary and mobile phases, respectively, and x’ is a mole fraction of the solute in the mobile phase. Both Fi, and Fi, approach unity under usual conditions, so that K th=mr’/x’
(18)
and dGi= -RZ’ln(m”/x’)
(19)
As n’/V m=x’p,/M, where pm and M, are density and molecular weight of mobile phase, respectively, it can be readily shown that KS= m” M,.Jx’pm
(20)
and AG: = -Rfln(K,p,IM,)
(21)
KS= (M,/p,)ew(
(22)
-AGWT)
It is difficult for one gram of polymer to adsorp more than one mole of solute under usual conditions, so that for most of the organic solutes AGZ will be positive. Based on some membrane dependent models, such as the solution diffusion model [4],
W=l+
(eK&,l~)
(l/q)
(23)
where F is the rejection, e is the porosity of the membrane, D,, is the diffusion coefficient, cl is the effective membrane thickness and q is the volume flux, it can be concluded that the larger the partition coefficient of a solute, that is, the stronger the interactions between the membrane material and the solute, the smaller the solute rejection. On the other hand, the stronger the interactions between membrane and water, the more will the volume flux and the
112 TABLE II Relationship between solute rejection and interactions of PA membrane with solutes Solute
Benzyl alcohol 2-Methoxy-benzylalcohol
rs (o/o)
Formula
CH20H
Q-0
g3*o
CH20H 83.0 0CH3
1,4-Dioxane
99.0
K 16.9
BK K DzO 0.777
ca.16.Sb
ca.0.777
9.83
0.449
AG: (kJ*mol-‘) 0.152
Y&o-AG $?J*mol-‘) -0.654
+ 1.56
-2.07
1.67
-2.17
2.55
-3.05
“UO 1,3-Dioxane
98.8
ca. 9.83b ca.0.449
6”
a
1,4-Butanediol
CH2CH2CH2CH2
CH2CHCH2CH3 bH
Sodium chloride
NaCl
9.44
0.432
AH
AH
1,2-Butanediol
g5’5
96.0
ca.9.44b ca.0.42
bH
99.0
6.72
0.308
“These data are taken from Ref. (1); the highest rejection at appropriate conditions is used. bEstimated values based on the results of solute with similar structure.
rejection increase. It should be emphasized that the ratio of strength of interactions between membrane and solute to the strength of interactions between membrane and water determines the membrane separation properties. This strength ratio can readily be estimated by the ratio of solute partition coefficient to that of heavy water, that is KB/KDzo.Because heavy water can easily be detected when using water as the mobile phase, KDzocan be used to express interactions between membrane material and water. We have proved that in this case the isotope effect because of exchange of hydrogen with deuterium may be neglected. The retention volume of water and heavy water has been proved to be the same by using ethyl alcohol and acetone as the respective mobile phases. The following scheme and relationships further demonstrate the idea: Solute (m) -+ Solute (s ) D20(m)+D20(s) KJKnzo = exp 1W&
-AR 1IRTI
(24)
113 TABLE III Relationship
between solute rejection and interactions
Solute
of PBIL membrane
K.
r’
Formula
K.
(%)
0
CH20H
0
CH20H
0-
Benzylalcohol
2-Methoxybenzylalcohoi
K-0
with solutes
AG:
AC;, - AG:
(kJ*mol-‘)
(kJ*mol-‘)
-2.99
3.89
- 0.057
0.999
0.392
0.506
1.14
0.559
0.399
12.4
0.974
0.967
12.7
1.00
16.0
51.2
4.50
55.0
ca.57.2b
ca.4.50b
97.5
18.4
1.47
95.0
ca.18.4b
ca.1.47b
96.9
15.5
1.22
95.7
ca.15.5b
ca.1.22b
100.0
14.5
99.5
Q0CH3
1,4-Dioxane “L.-I0
1,3-Dioxane
1,4-Butanediol CHzCHzCHZCHz AH
1,2-Butanediol
bH
Triton
OH
CH2CHCH2CH3 bH
OCHzCHz)
H17Ce
Sodium chloride
NaCl
Heavy water
DxC
g,,oOH
“These data are taken from Ref. ( 1); the highest rejection obtained at appropriate bEstimated values based on the results of solutes with similar structure.
Membrane
material
G//////////l//
conditions
-0.069
is used.
““7/ cpolymer
1
Therefore, we have three cases:
(III)K,/Kn,,
< 1, AG;,o -AG:
y#g$%%Jgr’-
If the membrane material interacts more strongly with water than with the solute, relationship (III) is satisfied, and water is preferably adsorped on the surface, which causes an increase in the rejection of the membrane to the solute. AGho - AGZ < 0 further shows that water has a larger tendency to transfer
114 TABLE IV Interaction parameters between three RO membrane materials and aliphatic compounds Organic solute
Name
Polysulfonamide
Formula
PK
K
BK K DzO
AC
(kJ.mol-‘)
AC%,,-AC (kJ.mol-‘)
Methanol
CH 30H
2.89
0.745
4.72
Ethanol
CH 3CHZOH
3.96
1.02
3.88
0.0160
6.25
1.61
2.72
1.23
6.16
1.59
2.76
1.19
3.25
0.918
3.02
l-Propanol 2-Propanol l-Butanol
CH3CH2CH20H
CH 3CHCH3 OH
12.6
CH3CH2CH2CH20H
-0.775
Formaldehyde
CH20
2.93
0.756
4.66
Acetaldehyde
CH 3CH0
4.46
1.15
3.58
0.362
Acetone
cH 36%
5.61
1.45
2.99
0.951
3.25
0.915
3.03
Methylethyl ketone
b
12.6
CHsCH2CCH3
-0.717
‘d
Formic acid Acetic acid
HCOOH
3.15
5.26
1.36
3.16
0.784
CHICOOH
4.15
4.97
1.28
3.31
0.637
Propionic acid
CH3CH2COOH
4.87
2.35
0.606
5.23
Acrylic acid
H,C=
4.25
4.63
1.19
3.49
0.454
n-Butyric acid
CHJ(CHZ)$OOH
4.81
5.30
1.37
3.14
0.805
n-Hexanoic acid
CH3(CH~L&ooH
4.88
21.2
5.46
-0.415
4.36
Methylamine
CH3NH2
10.7
11.6
2.99
1.13
2.81
n-Butylamine
CH3(CH2)3NH2
10.8
23.0
5.94
-0.632
4.58
Acetonitrile
CH3CEN
5.26
1.36
3.16
0.782
Formalamide
HCNH,
3.40
0.878
4.28
- 0.333
5.27
1.36
3.15
0.792
6.64
1.71
2.56
1.38
7.32
1.89
2.31
1.64
2.89
1.22
2.72
CHCOOH
- 1.29
‘d
N,N-Dimethylformalamide N,N-Dimethylacetamide
HCN(CH~)~
8 CH3CN(CH312
‘d
Acetic acid ethyl ester
CH,?
Acetic acid ethylene ester
CH3C-0-CH=CHZ
-
o -
CH2CH3
0
R
11.2
115
Polybenzimidalone
Aromatic polyamide
+~&&&Y&+
~~@!&&f
0 KS
n
n K &
AG: (kJ.mol-‘)
AG&o-AG: (kJ.mol-‘)
Ks
AK K DsO
AGZ (kJ.mol-‘)
AGAzo -AC (kJ.mol-‘)
13.2
1.03
0.810
0.0880
6.94
0.318
2.46
-2.97
15.1
1.19
0.459
0.439
8.13
0.372
2.06
-2.56
18.2
1.43
- 0.0280
0.926
0.497
1.31
-1.81
0.413
1.79
-2.29
0.483
1.38
- 1.88
10.9 9.03
26.3
2.07
- 0.980
1.88
15.8
1.25
0.332
0.566
9.06
0.414
1.78
-2.28
14.8
1.16
0.512
0.386
9.71
0.444
1.60
-2.10
16.4
1.29
0.238
0.660
11.0
0.503
1.28
- 1.78
11.3
0.518
1.20
- 1.70
10.7
0.488
1.36
-1.86
10.6
15.2
1.20
0.433
0.465
18.2
1.43
-0.0240
0.922
9.56
0.437
1.64
-2.14
19.0
1.50
-0.145
1.04
6.13
0.280
2.79
- 3.29
20.4
1.61
-0.326
1.22
0.510
1.24
- 1.74
24.0
1.88
-0.741
1.64
0.424
1.72
-2.22
12.9
0.589
0.863
- 1.37
24.9
1.14
28.5
2.24
-1.19
2.09
11.2 9.28
- 0.840
0.338
_ 14.4
1.13
0.586
10.5
0.827
1.39
17.3
1.37
18.6
_
0.312 - 0.492
6.66
0.305
2.57
-0.308
0.0990
0.798
9.46
0.433
1.66
- 2.17
1.46
- 0.0820
0.980
8.88
0.406
1.83
-2.33
23.7
1.86
-0.713
1.61
13.3
1.41
0.494
1.32
- 1.82
0.733
0.125
10.8
(continued
ouerleaf)
116 TABLE IV (continued)
Organic solute
Name
Dimethyl sulfoxide AJ,N-Dimethylamine Diethylether Urea 2-Chloroethanol 2,2,2-Trichloroacetaldehyde 2Chloroacetic acid
Polysulfonamide
Formula
PK
K
8K K I320
A@? (kJ+mol-‘)
AG,o -AC (kJ.mol-‘)
1:
4.18
1.08
3.75
0.192
(CH3j2NH
4.39
1.13
3.62
0.320
KH3CH,),0
8.10
2.09
2.05
1.89
3.47
0.896
4.23
- .0.282
5.25
1.36
3.16
0.780
9.43
2.43
1.66
2.28
H3CSCH3
$1
H2NCNH2
FHzCHzoH Cl CC13CH0 H2CCICOOH
2.85
3.94
1.02
3.90
0.0400
2,2,2_Trichloro acetic acid
CClyXOH
0.10
4.44
1.15
3.60
0.345
l-Aminoacetic acid
FHpCOOH NH2
9.78
8.19
2.11
2.02
1.92
Methylene chloride Chloroform
CH2C12
4.02
0.369
3.58
15.6 39.8
HCCI,
Oxalic acid
HOOCCOOH
Malonic acid
HOOCCH
Succinic acid
HOOC(CH2)2COOH
Glutaric acid
HOOC(CH213COOH
Ethylene glycol
CH2CHZ bH
Glycerol
z COOH
FH2 CHCH2 I I
THZCHCH3
OH
1,3_Propanediol
5.98
1.23 4.19
8.57
2.21
1.91
2.04
2.83 5.69
5.10
1.32
3.24
0.704
4.16 5.61
7.93
2.05
2.11
1.84
4.34 5.41
7.27
1.88
2.33
1.62
4.41
1.15
3.58
0.361
4.19
1.08
3.74
0.202
3.98
1.03
3.87
0.0710
4.01
1.04
3.85
0.0910
5.01
1.29
3.28
0.661
bH
~H2C”zC”z OH
-2.03
bH
OH OHOH 1,2_Propanediol
10.3
OH
1,3-Butanediol CH2CH2CHCH3 OH AH
117
Aromatic polyamide
Polybenzimidalone
KS
8K K DzO
AG: (kJ.mol-‘)
AG,o -&I (kJ.mol-‘)
14.6
1.15
0.542
0.356
16.7
1.31
0.199
0.699
K
>K K DzO
6.99
0.320
AG;,, - AG
AG
(kJ.mol-‘)
(kJ.mol-‘)
2.45
- 2.95
_ 12.9
1.11
0.866
0.0320
9.33
0.427
1.70
-2.20
8.26
0.378
2.02
- 2.52
20.0
1.57
- 0.273
1.17
9.08
0.415
1.30
-1.80
25.3
1.99
-0.881
1.78
9.74
0.446
1.59
-2.09
13.4
1.06
0.759
0.139
36.2
1.66
- 1.81
1.30
17.6
1.38
0.0620
0.838
24.3
1.11
-0.772
0.269
21.3
1.67
-0.431
1.329
13.6
0.642
0.717
- 1.22
23.6
1.86
-0.705
1.60
12.9
0.590
0.863
-1.37
13.8
1.08
0.689
0.208
9.71
0.444
1.60
-2.10
14.2
1.12
0.606
0.292
9.74
0.446
1.59
-2.09
9.31
0.423
1.71
-2.21
15.0
1.18
0.475
0.422
9.58
0.438
1.63
-2.14
16.1
1.27
0.285
0.613
9.58
0.438
1.63
-2.14
(continued
over-leaf)
118 TABLE IV (continued) Organic solute
Name
Polysulfonamide
Formula
pK,
K,
1.36
3.16
0.788
5.77
1.49
2.92
1.02
5.45
1.41
3.07
0.877
(HOCH2CH212NH
9.15
2.36
1.74
2.21
(HOCH2CH2)jN
8.31
2.14
1.99
1.96
7H2 CH2 NH2 NH2
l,%PrOpane-
yH2yHCH3
diamine
NH2 NH2
1,5-Pentanediol 1,6-Hexanediol
CHZ(CHZ)~CHZ
Triton
ZCe$
Sodium chloride Heavy water Sebacic acid
A%,, - AG (kJ.mol-‘ )
OH
OH
Diethanolamine
AG (kJ.mol-‘)
5.26
1,4-Butanediol FHzCHzCHzlH2 Ethylenediamine
K K DzO
_
OH
bH
_
~Hz(CHZ)~FHZ 0CH2CH2)g,,00H
-
NaCl
2.49
0.643
5.08
- 1.14
o 2O
3.88
1
3.94
0
HOOC(CH2)eCOOH ,CH
HOOCH2C,
2COOH
‘NCH2CH2N’
EDTA
/ HOOCH2C
\ CH2COOH
-
from the mobile phase to the stationary phase. In case (II) have water and solute the same tendency to be adsorped on the membrane surface, and are the separation properties of the membrane mainly governed by a dynamic factor, i.e. diffusion. In case (I), it is unfavorable to increase the rejection only from a point of view of thermodynamics, but sometimes if KS/Kn2o is little larger than one, it is also possible to obtain a high rejection, because solutes have a larger molecular weight and size than water, and therefore diffusion is the predominant factor to govern the separation. If KS/Kn20 is much larger than one, the thermodynamic factor predominates the separation, and the rejection is usually small. The following experimental results seem to be in agreement with the above analysis.
119
Aromatic polyamide
Polybenzimidalone
K
8K K D20
AG
(kJ*mol-‘)
A&o-AC
(kJ.mol-‘)
15.5
1.22
0.392
0.506
14.7
1.16
0.519
0.379
28.5
2.25
- 1.19
2.09
20.8
1.64
-0.382
1.28
19.3
1.52
-0.178
1.08
26.5
2.09
-1.01
1.90
14.5
1.14
0.559
0.339
12.4
0.974
0.967
- 0.0690
12.7
1
0.989
K
9.44
8K
AG
AC;,, -AC
K DzO
(kJ.mol-‘)
(kJ.mol-‘)
0.432
1.67
-2.17
0.446
1.58
-2.09
0.562
0.990
-1.49
0.308
2.55
-3.05
_ 9.76
12.3
6.72
0
21.9
1
-0.502
0
_
32.1
1.47
- 1.50
0.993
11.2
0.512
1.23
- 1.73
RESULTS AND DISCUSSION
Some information about interactions between a PA membrane and organic solutes is given in Table II. The data in the third column are from Ref. 1. Besides for 2-methoxybenzylalcohol, the rejections of the PA membrane to these solutes are high. According to the above discussion, this could be suspected because all of these solutes agree well with relationship (III). Because of strong interaction between PBIL and benzyl alcohol, its partition coefficient is large and satisfies relationship (I) and thus the rejection is small, while NaCl satisfies relationship (III), and its rejection is up to 99.5% (see Table III). The theory and experimental results seem to be consistent. For the
120 TABLE V Interaction compounds
parameters
between
materials
and heterocyclic
and alicyclic
Polysulfonamide
Organic solute
Name
three RO membrane
Formula
K
AG&o - Af$
&
AG (kJ.mol-‘)
5.09
1.31
3.24
0.700
Tetrahydrofuran
8.12
2.10
2.04
1.90
Furfurylalcohol
8.21
2.12
2.02
1.93
Pyrrole
9.19
2.32
1.73
2.22
N-Methyl-2pyrrolidone Thiazol
5.40
1.39
3.09
0.851
10.7
2.56
1.34
2.61
Pyridine
10.8
2.79
1.31
2.64
4-Methylpyridine
11.5
2.97
1.15
2.80
2,4-Dimethylpyridine
10.4
2.69
1.40
2.54
2,4,6-Trimethylpyridine
13.8
3.56
0.683
3.26
Cyclohexanol
6.98
1.80
2.43
1.51
Cyclohexanone
6.99
1.80
2.43
1.52
Sulfolane
5.56
1.44
3.02
0.928
3.49
0.450
Furan
r”,
K,
I (kJ*mol-‘)
2-Methyl-5ethylpyridine
_
a-Aminopyridine 1,4-Dioxane
4.62
1.19
121
Aromatic polyamide
Polybenzimidalone
K
K K 020
9
AC
AG&o--AG:
(kJ.mol-‘)
(kJ.mol-‘)
0.123
0.774
A
K
ho
AC
(kJ*mol-‘)
AC”,,,- AG (kJ.mol-‘ )
0.642
0.642
- 1.15
0.436
1.65
-2.15
10.2
0.468
1.46
-1.97
10.0
0.458
1.51
-2.02
0.439
1.57
-2.07
12.7
0.580
0.907
- 1.41
2.87
16.7
0.764
0.193
- 0.695
0.618
17.2
0.758
0.124
- 0.626
2.31
18.7
0.854
0.0950
- 0.407
2.61
18.6
0.853
0.0910
0.412
27.2
1.24
17.2
1.35
25.7
2.02
- 0.923
1.82
18.7
1.47
- 0.0960
0.994
41.4
3.25
-2.15
3.05
17.9
1.41
0.0170
12.1
0.949
1.03
-0.134
38.5
3.03
- 1.97
16.2
1.27
31.0
2.44
-1.41
34.8
2.74
- 1.71
0.280
K
0.880
14.0 9.53
9.61
- 1.07
0.563
31.6
2.49
- 1.46
2.35
11.1
0.507
1.26
- 1.76
37.1
2.91
- 1.87
2.77
18.2
0.831
- 0.0230
-0.479
19.2
1.51
-0.169
1.07
8.99
0.468
1.79
-2.30
29.7
2.34
- 1.30
2.20
_
18.4
1.47
- 0.0570
0.999
9.83
0.449
1.57
- 2.07
122 TABLE VI Interaction
parameters
between three RO membrane
Organic solute
materials
and aromatic compounds
Polysulfonamide
Formula
Name
Phenol
PK
@.H
9.89
Aniline
K,
sK
AG&,, -AC (kJ.mol-‘)
KD~O
AG (kJ.mol-‘)
33.7
8.70
-1.61
17.2
4.43
0.124
3.82
2.86
0.739
4.72
- 0.778
4.18
1.08
3.75
0.192
9.75
2.52
1.57
2.37
21.9
5.65
- 0.503
4.45
38.4
9.91
- 1.95
5.89
20.4
5.25
-0.316
4.26
5.56
4.63
Nitrobenzene Sodium phenyl sulfonate Benzoic acid Benzyl alcohol Benzeldehyde Hydroquinone
10.4
Resorcinol
9.81
24.9
6.43
- 0.834
4.78
Pyrocatechol
9.85
24.4
6.31
-0.787
4.73
p-Amino
6.10
15.8
4.07
m-Amino aniline
4.90
21.3
5.50
-0.435
4.38
o-Amino aniline
4.50
11.3
2.93
1.19
2.76
m-Phathalic
3.54 4.60
0.908
4.19
-0.248
o-Phthalic anhydride
aniline
acid
3.52
acid
0.339
3.61
_
4.73
m-Methylaniline
27.0
6.98
-1.04
4.99
m-Nitrobenzoyl hydrazine p-Toluylsulfonic methyl ester
acid
Sulfosalicylic acid
11.7
3.03
1.10
2.85
31.3
8.07
- 1.42
5.36
NO2
2,4,6_Trinitrophenol
0” NO1
0.380
123
Polybenzimidalone
Aromatic polyamide
+[&&$&)j_
~~@h-~*~+ n
KS
sK Kn,o
AG (kJ.mol-‘)
ACgo -AC%’ (kJ.mol-‘)
Ks
sK K DzO
59.9
4.71
-3.11
4.01
16.0
0.732
39.4
3.10
-2.03
2.92
33.0
1.51
-1.56
1.06
45.8
2.10
-2.42
0.149
11.1
0.507
1.26
-1.76
0.125
-0.654
_ 57.2
4.50
16.2 1.27 40.5 3.19
A(% (kJ.mol-‘)
0.303
AC”,,, - AG (kJ.mol-‘)
-0.805
-2.99
3.89
17.0
0.777
0.273 -2.10
0.625 3.00
39.2
1.79
13.4
0.613
0.762
-1.26
0.436
-0.938
-2.01
1.51
45.3
3.56
-2.39
3.29
15.2
0.696
47.9
3.77
-2.53
3.43
22.4
1.02
61.8
4.86
-3.19
4.09
18.7
0.857 -0.104
-0.398
56.1
4.41
-2.94
3.84
15.4
0.703
0.411
-0.913
45.3
3.56
-2.39
3.29
13.2
0.602
0.810
-1.31
8.03 0.367
2.09
-2.59
1.35
-1.28
29.5
_
_
_
1.29
0.236
0.0590
0.776
_ 55.1
16.4
-0.561
2.52
9.79 0.448
0.662
-2.89
2.39
1.58
-2.08
11.5
0.528
0.149
-1.65
16.5
0.755
0.234
-0.726
124
other compounds in Table III, though the ratios of KS to KDzoare little larger than one, the rejections of the PBIL membrane to these solutes are high enough, based on the relationship D= (kT)l(Gnyr,)
(25)
where k, T, q and rA are the Boltzmann constant, absolute temperature, solution viscosity and solute radius, respectively. Perhaps a dynamic factor also plays a role, because the size and molecular weight of these solutes are larger than that of water. More detailed information about interactions between organic solute and the three membrane materials is shown in Table IV-VI. It is obvious from Table IV that the partition coefficients of the solutes increase with the increasing number of carbon atoms for alcohols, aldehydes, ketones and amines. When comparing acetone with urine, the partition coefficient of acetone becomes larger by substituting two hydrophilic -NH:! groups of urine by two hydrophobic -CH, groups. This is similar for formamide, N,N-dimethyl formamide and N,N-dimethyl acetamide, which indicates that increasing the number and hydrophobicity of hydrophobic groups of the solutes favors interactions between membrane materials and organic solutes, thus increasing the partition coefficients. When comparing propionic acid with acrylic acid, acetic acid ethyl ester with acetic acid ethylene ester, it can be seen that an increasing degree of unsaturation in the organic solutes favors the interactions between the PSA or PA membrane and the solutes. This is different for PBIL, which indicates that the interactions are closely related to the structures of both membrane materials and organic solutes. Induction effects of polar groups in the organic solutes on interactions between membrane materials and the solutes are also studied. When comparing ethyl alcohol with 2-chloroethanol, methylene chloride with chloroform, it can be seen that increasing the number of polar groups enlarges the partition coefficients, perhaps because the inductive effect of the polar -Cl group makes the -H in the molecule more positive, so that the solutes can form stronger hydrogen bonds with the membrane materials. Furthermore, when comparing acetaldehyde with 2,2,2_trichloroacetaldehyde, acetic acid with 2-chloroacetic acid and 2,2,2_trichloroacetic acid, it can be seen that the polar groups affect the interactions, but the extent of effect is different for different membrane materials, which also indicates that the interactions depend on the structural and chemical properties of polymer and organic solute. Interactions between membrane materials and solutes with more than one functional group are also investigated, and the results are given in Table IV. For some diols and diamines, their partition coefficients are quite near one another, which indicates that the effect of position isomerization of functional
125
groups on interactions between membrane materials and solutes are small because the a-bonds in the main chain of the solutes can rotate freely. The results of investigation of interactions between membrane materials and organic solutes as shown in Table V further demonstrate that the interactions strongly depend on structural and chemical properties of polymers and organic solutes. When comparing cyclohexanol with cyclohexanone, it shows that their partition coefficients for PSA are almost the same, but their partition coefficients for PBIL and PA are quite different. For the other solutes, such as furan, tetrahydrofuran and pyrrole, the partition coefficients for the different membrane materials are also quite different. In general, for pyridine and its derivates, an increasing number of alkyl groups causes an increase in their partition coefficients. However, this is not normal for PBIL, for which the partition coefficient of pyridine is the largest. For the other derivates, the more alkyl groups, the larger are the coefficients, that is, the more favorable are the interactions between the polymers and these solutes. Table VI shows the results of interactions between three membrane materials and aromatic compounds. Obviously, the aromatic compounds with electron-releasing groups, such as phenol, aniline and benzyl alcohol, have strong interactions with the membrane materials, while the others with electronwithdrawing groups, such as nitrobenzene, benzoic acid and sodium benzene sulfonate, have weak interactions with the materials. Because sulfosalicylic acid has an electron-releasing group -OH and two electron-withdrawing groups -COOH and -S03H, its partition coefficient is between those of the two kinds of aromatic compounds. It is easy to find that the partition coefficients of pyrocatechol, hydroquinone, resorcinol and three o,m,p-amino-anilines are quite different from one another, the largest difference being about 16. These results greatly differ from those of the corresponding aliphatic compounds shown in Table IV, which shows that an aromatic compound shows a stronger interacting effect of geometric matching with the membrane material, i.e. polymer, than the corresponding aliphatic compound. Only considering from the point of view of thermodynamics, there is a possibility of separating isomers with the membrane separation technique. It has also been found by analyzing the pK, and KS data of the solutes that the partition coefficients, i.e. the interactions, do not directly depend on the acidity of these solutes. Another important result is that for PA membrane material the partition coefficients of most organic compounds are smaller than that of water; 85% of the solutes satisfy with relationship (III), while 10% for PSA and only 3% for PBIL of the solutes satisfy with relationship (III). Based on the discussion given in the previous theoretical part and only considering the thermodynamics, it can be seen that the stronger the interactions between membrane materials and water are, the more favorable are both solute rejection and volume flux to increase. Therefore, PA membranes may be more appropriate in sense of increasing solute rejection than the other two membrane materials.
126 CONCLUSION
When using the HPLC method to investigate interactions between membrane materials and solutes, using quartz powder coated homogeneously by membrane materials (polymer ) as a stationary phase is better than using pure polymer powder, because for the former the dead volume of LC column can be easily determined precisely, but for the latter this is more difficult. Interactions between membrane materials and organic solutes strongly depend on the structure and chemical properties of polymers and organic solutes. Increasing the number and hydrophobicity of the hydrophobic groups of organic solutes favors interactions between membrane materials and organic solutes. Introducing electron-releasing groups in the aromatic compounds favors interactions between membrane materials and these compounds, while introducing electron-withdrawing groups in the aromatic compounds does not favor interactions between membrane materials and these compounds. Aromatic compounds have stronger interactions of geometric matching with three membrane materials containing aromatic rings than the corresponding aliphatic compounds have. Only considering the thermodynamics, the relative strength of interactions between membrane and solute to interactions between membrane and water governs the membrane performance. The information obtained from HPLC experiments on interactions between membrane materials and organic solutes, especially the three-simple relationships about Ks/Kn20, is useful for selecting membrane materials, designing macromolecules for use as membrane materials, understanding transport mechanism through synthetic membranes and developing membrane applications. It is worth noticing that the correlation of HPLC data with thermodynamic equilibrium data is an approximation. ACKNOWLEDGEMENTS
The financial support provided by the Chinese Natural Science Fund Committee (C.N.S.F.C. No. 85273) is gratefully acknowledged, and Miss Zhang Lehua, who has done part of the work, is also kindly acknowledged.
REFERENCES 1 2 3 4
J.Y. Chen and W. Pusch, J. Appl. Polym. Sci., 33 (1987) 1809-1822. W. Pusch and A. Walch, J. Membrane Sci., 10 (1982) 325-360. T. Matsuura, Y. Taketani and S. Sourirajan, Desalination, 38 (1981) 319-337. W. Pusch, Desalination, 59 (1986) 105-198.
107 Printed in The Netherlands
Study on the Interaction Between Membranes and Organic Solutes by the HPLC Method JI JIANG, SUN MINGJI, FE1 MINLING and CHEN JIAYAN* Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian (China), Tel.: 86-411-331641, Telex: 86436 DZCO CN (Received March 23,1988)
SUMMARY
Interactions between three membrane materials and about 90 organic solutes have been studied by the high-performance liquid chormatography method. Three relationships characterizing the interactions, which are related to membrane performance, have been obtained by means of both thermodynamics and reverse osmosis experiments. The relationship between interaction and structure of polymer and organic solutes is discussed based on the partition coefficient and Gibbs free energy obtained from high-performance liquid chromatography experiments. The results may be useful for the molecular design of membrane materials. Keywords: interaction, membrane, reverse osmosis, HPLC, thermodynamics.
INTRODUCTION
Interactions between membrane and separated compounds are closely related to membrane performance [ 1,2]. A clear understanding of interactions between membrane and separated compounds will be greatly significant to molecular design and selection of membrane materials, elucidation of transport mechanism through synthetic membranes and development of membrane applications. Therefore, interactions between three membrane materials and about 90 organic solutes have been studied by the high-performance liquid chromatography (HPLC ) method. Sourirajan et al. [3] have obtained significant results by using the HPLC method to study membranes. However, something in their work needs to be improved because the pure polymer powder was used as a stationary phase, and therefore the dead reten*Present address: Shenzhen Membrane Separation and Biotechnology China.
OOll-9164/89/$03.50
0 1989 Elsevier Science Publishers B.V.
Laboratory, Shenzhen,
108
tion volume of the liquid chromatography (LC) column could not be determined precisely. They have used the solute with the least retention volume as a reference, but unfortunately, the solute with the least retention volume is not always the same one for different polymers, and therefore it is difficult to compare the results with one another. EXPERIMENTAL
Polysulfonamide (PSA) was purchased from the Shanghai Cellulose Company, polybenzimidazolone (PBIL) was supplied by the Department of Chemistry of Jinan University, and the aromatic polyamide (PA) prepared by our institute. The other compounds used were all reagent grade. An HPLC Pump BT 3020 of Biotronik and a differential refractometer R403 of Waters were used and data were processed automatically by means of a computer. The LC column was a 1 m long stainless steel tube (4 mm I.D., 8 mm O.D.). The temperature of the column and the detector was controlled at 38 ? 0.1 ‘C by circulating water. Distilled water was used as a mobile phase, the flow rate was fixed at 1 cm3/min, and quartz powder of 200-300 mesh coated with an about 1000 A thick polymer film was used as a stationary phase. The polymer was about 0.1-0.3 wt.% of the quartz powder which was proved to be chemically inert. Some results are listed in Table I; the dead retention volume’ of the column was 7.976 cm3, with the error being less than 0.90%. THEORY
It is necessary to introduce the fundamental theory used in this paper because another, different theory has been used in this field [ 31. The chromatographic partition coefficient K, in liquid-solid systems is defined by K, =m”/(n’/V,)
(1)
where m” is the concentration of solute in the stationary phase expressed in terms of number of moles of the solute adsorbed per gram of polymer, n’ is the number of moles of the solute in the mobile phase and V, is the volume of the mobile phase. The capacity ratio k is defined by the ratio of the net retention time (tn - t,) of a solute to the retention time t, of a non-adsorbed substance k= (tR -tJltm=(VR-Vm)/Vm
(2)
where tR and VR are the total retention time and volume of the solute, respectively. Actually, the value of k represents the ratio in which a given amount of solute is distributed between the stationary and the mobile phase, i.e.,
109 TABLE I Retention volume of organic solutes’ Solute
VR (cm3)
Methanol Glycerol Furan 1,4-Dioxane 2-Chloroacetic acid Phenol Benzyl alcohol l-B&an01 Dimethyl sulfoxide Resorcinol Pyrrole N-Methyl-apyrrolidone Cyclohexanol Pyridine
7.950 8.000 8.030 7.967 8.005 7.967 7.973 8.002 7.967 8.017 7.998 7.957
Average value
7.976
Error (%)
7.905 7.927
<0.900
‘Stationary phase is 250-300 mesh quartz powder.
k=
amount of solute in the stationary phase amount of solute in the mobile phase
(3)
It follows from Eqn. (3) that
k=K, W/V,,,
(4)
where W is the weight of stationary phase (polymer). Combining Eqns. (2) and (4) gives K=(V,-VA/W
(5)
Let us consider the process of adsorption as a reversible reaction in which one mole of solute is transferred from the mobile phase into the stationary phase under isothermal and isobaric conditions. Denoting the solute (i) in the mobile phase and stationary phases by i(m) and i (s), respectively, the reaction can be represented by i(m)-+i(s)
(6)
AGs=fis-h
(7)
where kci.and km are the chemical potentials of the solute in the stationary
110
and mobile phase, respectively. The chemical potentials can further be expressed by h, = gS +RZlna”
(8)
j.&=&+RZlnU’
(9)
where & , ,&, , a” and a’ are the standard chemical potentials and activities of the solute in the respective phases. Hence, AG, is given by
where the difference & - ,L& is the standard Gibbs free energy of adsorption and will be denoted by AG: . Eqn. (10) also holds for systems which are out of adsorption equilibrium (e.g., the leading or trailing part of the chromatographic zone), and the negative value of AG, is actually the driving force of adsorption. At equilibrium (approximately in the center of the zone), AG, = 0 and the standard Gibbs free energy of adsorption is AGZ = -RZ’ln(u”/u’),
(11)
where the subscript eq indicates that equilibrium solute activities are considered. The ratio (a” /a’ )eq can be considered as a thermodynamic distribution constant. It is readily apparent that this thermodynamic distribution constant is not a priori identical with the conventional chromatographic partition coefficient KS. For the activity (a) of the solute in either phase
a=f If” where f is the actual fugacity
(12)
of the solute and f” is the fugacity under chosen standard conditions. Further, in order to define a relationship between the activity and concentration of the solute in the given phase, a reference state (the state of unit activity coefficient) has to be chosen. Hence, the chromatographic and thermodynamic distribution constants can be related to each other only through the appropriate choice of the standard and reference states for the solute in both phases. It should be noted that the numerical values of the thermodynamic distribution constants and, consequently, of the AGZ values calculated from it, depend on the choice of the standard and reference states. It is therefore necessary that these states be specified unequivocally whenever numerical data are presented on the thermodynamic properties depending on the above choice. The following choice for the standard and reference states will be made in this instance: for the solute standard state in the stationary phase, one mole of solute per gram of absorbent (m” = 1) at T and p of the system; for the solute standard state in the mobile phase, pure solute at T andp of the system; for the reference state for the solute in the stationary
111
phase, infinitely low deposition of solute on the adsorbent (polymer) surface (ri,+ 1 for m” + 0) at T and p of the system; and for the reference state for the solute in the mobile phase, infinitely dilute solution of solute ( rh+ 1 for x’ -to) at T andp of the system. Thus
fi, =ri,k”m” fi, =r,h’x’ f ;s=k” f &,=h’
(15) (16)
& = (fis/f L)/ (f-,/f &) =%mm”/FimX’
(17)
(13) (14)
where k” is a proportionality constant, h’ is Henry’s law constant, Fi, and Fi, are the solute activity coefficients in the stationary and mobile phases, respectively, and x’ is a mole fraction of the solute in the mobile phase. Both Fi, and Fi, approach unity under usual conditions, so that K th=mr’/x’
(18)
and dGi= -RZ’ln(m”/x’)
(19)
As n’/V m=x’p,/M, where pm and M, are density and molecular weight of mobile phase, respectively, it can be readily shown that KS= m” M,.Jx’pm
(20)
and AG: = -Rfln(K,p,IM,)
(21)
KS= (M,/p,)ew(
(22)
-AGWT)
It is difficult for one gram of polymer to adsorp more than one mole of solute under usual conditions, so that for most of the organic solutes AGZ will be positive. Based on some membrane dependent models, such as the solution diffusion model [4],
W=l+
(eK&,l~)
(l/q)
(23)
where F is the rejection, e is the porosity of the membrane, D,, is the diffusion coefficient, cl is the effective membrane thickness and q is the volume flux, it can be concluded that the larger the partition coefficient of a solute, that is, the stronger the interactions between the membrane material and the solute, the smaller the solute rejection. On the other hand, the stronger the interactions between membrane and water, the more will the volume flux and the
112 TABLE II Relationship between solute rejection and interactions of PA membrane with solutes Solute
Benzyl alcohol 2-Methoxy-benzylalcohol
rs (o/o)
Formula
CH20H
Q-0
g3*o
CH20H 83.0 0CH3
1,4-Dioxane
99.0
K 16.9
BK K DzO 0.777
ca.16.Sb
ca.0.777
9.83
0.449
AG: (kJ*mol-‘) 0.152
Y&o-AG $?J*mol-‘) -0.654
+ 1.56
-2.07
1.67
-2.17
2.55
-3.05
“UO 1,3-Dioxane
98.8
ca. 9.83b ca.0.449
6”
a
1,4-Butanediol
CH2CH2CH2CH2
CH2CHCH2CH3 bH
Sodium chloride
NaCl
9.44
0.432
AH
AH
1,2-Butanediol
g5’5
96.0
ca.9.44b ca.0.42
bH
99.0
6.72
0.308
“These data are taken from Ref. (1); the highest rejection at appropriate conditions is used. bEstimated values based on the results of solute with similar structure.
rejection increase. It should be emphasized that the ratio of strength of interactions between membrane and solute to the strength of interactions between membrane and water determines the membrane separation properties. This strength ratio can readily be estimated by the ratio of solute partition coefficient to that of heavy water, that is KB/KDzo.Because heavy water can easily be detected when using water as the mobile phase, KDzocan be used to express interactions between membrane material and water. We have proved that in this case the isotope effect because of exchange of hydrogen with deuterium may be neglected. The retention volume of water and heavy water has been proved to be the same by using ethyl alcohol and acetone as the respective mobile phases. The following scheme and relationships further demonstrate the idea: Solute (m) -+ Solute (s ) D20(m)+D20(s) KJKnzo = exp 1W&
-AR 1IRTI
(24)
113 TABLE III Relationship
between solute rejection and interactions
Solute
of PBIL membrane
K.
r’
Formula
K.
(%)
0
CH20H
0
CH20H
0-
Benzylalcohol
2-Methoxybenzylalcohoi
K-0
with solutes
AG:
AC;, - AG:
(kJ*mol-‘)
(kJ*mol-‘)
-2.99
3.89
- 0.057
0.999
0.392
0.506
1.14
0.559
0.399
12.4
0.974
0.967
12.7
1.00
16.0
51.2
4.50
55.0
ca.57.2b
ca.4.50b
97.5
18.4
1.47
95.0
ca.18.4b
ca.1.47b
96.9
15.5
1.22
95.7
ca.15.5b
ca.1.22b
100.0
14.5
99.5
Q0CH3
1,4-Dioxane “L.-I0
1,3-Dioxane
1,4-Butanediol CHzCHzCHZCHz AH
1,2-Butanediol
bH
Triton
OH
CH2CHCH2CH3 bH
OCHzCHz)
H17Ce
Sodium chloride
NaCl
Heavy water
DxC
g,,oOH
“These data are taken from Ref. ( 1); the highest rejection obtained at appropriate bEstimated values based on the results of solutes with similar structure.
Membrane
material
G//////////l//
conditions
-0.069
is used.
““7/ cpolymer
1
Therefore, we have three cases:
(III)K,/Kn,,
< 1, AG;,o -AG:
y#g$%%Jgr’-
If the membrane material interacts more strongly with water than with the solute, relationship (III) is satisfied, and water is preferably adsorped on the surface, which causes an increase in the rejection of the membrane to the solute. AGho - AGZ < 0 further shows that water has a larger tendency to transfer
114 TABLE IV Interaction parameters between three RO membrane materials and aliphatic compounds Organic solute
Name
Polysulfonamide
Formula
PK
K
BK K DzO
AC
(kJ.mol-‘)
AC%,,-AC (kJ.mol-‘)
Methanol
CH 30H
2.89
0.745
4.72
Ethanol
CH 3CHZOH
3.96
1.02
3.88
0.0160
6.25
1.61
2.72
1.23
6.16
1.59
2.76
1.19
3.25
0.918
3.02
l-Propanol 2-Propanol l-Butanol
CH3CH2CH20H
CH 3CHCH3 OH
12.6
CH3CH2CH2CH20H
-0.775
Formaldehyde
CH20
2.93
0.756
4.66
Acetaldehyde
CH 3CH0
4.46
1.15
3.58
0.362
Acetone
cH 36%
5.61
1.45
2.99
0.951
3.25
0.915
3.03
Methylethyl ketone
b
12.6
CHsCH2CCH3
-0.717
‘d
Formic acid Acetic acid
HCOOH
3.15
5.26
1.36
3.16
0.784
CHICOOH
4.15
4.97
1.28
3.31
0.637
Propionic acid
CH3CH2COOH
4.87
2.35
0.606
5.23
Acrylic acid
H,C=
4.25
4.63
1.19
3.49
0.454
n-Butyric acid
CHJ(CHZ)$OOH
4.81
5.30
1.37
3.14
0.805
n-Hexanoic acid
CH3(CH~L&ooH
4.88
21.2
5.46
-0.415
4.36
Methylamine
CH3NH2
10.7
11.6
2.99
1.13
2.81
n-Butylamine
CH3(CH2)3NH2
10.8
23.0
5.94
-0.632
4.58
Acetonitrile
CH3CEN
5.26
1.36
3.16
0.782
Formalamide
HCNH,
3.40
0.878
4.28
- 0.333
5.27
1.36
3.15
0.792
6.64
1.71
2.56
1.38
7.32
1.89
2.31
1.64
2.89
1.22
2.72
CHCOOH
- 1.29
‘d
N,N-Dimethylformalamide N,N-Dimethylacetamide
HCN(CH~)~
8 CH3CN(CH312
‘d
Acetic acid ethyl ester
CH,?
Acetic acid ethylene ester
CH3C-0-CH=CHZ
-
o -
CH2CH3
0
R
11.2
115
Polybenzimidalone
Aromatic polyamide
+~&&&Y&+
~~@!&&f
0 KS
n
n K &
AG: (kJ.mol-‘)
AG&o-AG: (kJ.mol-‘)
Ks
AK K DsO
AGZ (kJ.mol-‘)
AGAzo -AC (kJ.mol-‘)
13.2
1.03
0.810
0.0880
6.94
0.318
2.46
-2.97
15.1
1.19
0.459
0.439
8.13
0.372
2.06
-2.56
18.2
1.43
- 0.0280
0.926
0.497
1.31
-1.81
0.413
1.79
-2.29
0.483
1.38
- 1.88
10.9 9.03
26.3
2.07
- 0.980
1.88
15.8
1.25
0.332
0.566
9.06
0.414
1.78
-2.28
14.8
1.16
0.512
0.386
9.71
0.444
1.60
-2.10
16.4
1.29
0.238
0.660
11.0
0.503
1.28
- 1.78
11.3
0.518
1.20
- 1.70
10.7
0.488
1.36
-1.86
10.6
15.2
1.20
0.433
0.465
18.2
1.43
-0.0240
0.922
9.56
0.437
1.64
-2.14
19.0
1.50
-0.145
1.04
6.13
0.280
2.79
- 3.29
20.4
1.61
-0.326
1.22
0.510
1.24
- 1.74
24.0
1.88
-0.741
1.64
0.424
1.72
-2.22
12.9
0.589
0.863
- 1.37
24.9
1.14
28.5
2.24
-1.19
2.09
11.2 9.28
- 0.840
0.338
_ 14.4
1.13
0.586
10.5
0.827
1.39
17.3
1.37
18.6
_
0.312 - 0.492
6.66
0.305
2.57
-0.308
0.0990
0.798
9.46
0.433
1.66
- 2.17
1.46
- 0.0820
0.980
8.88
0.406
1.83
-2.33
23.7
1.86
-0.713
1.61
13.3
1.41
0.494
1.32
- 1.82
0.733
0.125
10.8
(continued
ouerleaf)
116 TABLE IV (continued)
Organic solute
Name
Dimethyl sulfoxide AJ,N-Dimethylamine Diethylether Urea 2-Chloroethanol 2,2,2-Trichloroacetaldehyde 2Chloroacetic acid
Polysulfonamide
Formula
PK
K
8K K I320
A@? (kJ+mol-‘)
AG,o -AC (kJ.mol-‘)
1:
4.18
1.08
3.75
0.192
(CH3j2NH
4.39
1.13
3.62
0.320
KH3CH,),0
8.10
2.09
2.05
1.89
3.47
0.896
4.23
- .0.282
5.25
1.36
3.16
0.780
9.43
2.43
1.66
2.28
H3CSCH3
$1
H2NCNH2
FHzCHzoH Cl CC13CH0 H2CCICOOH
2.85
3.94
1.02
3.90
0.0400
2,2,2_Trichloro acetic acid
CClyXOH
0.10
4.44
1.15
3.60
0.345
l-Aminoacetic acid
FHpCOOH NH2
9.78
8.19
2.11
2.02
1.92
Methylene chloride Chloroform
CH2C12
4.02
0.369
3.58
15.6 39.8
HCCI,
Oxalic acid
HOOCCOOH
Malonic acid
HOOCCH
Succinic acid
HOOC(CH2)2COOH
Glutaric acid
HOOC(CH213COOH
Ethylene glycol
CH2CHZ bH
Glycerol
z COOH
FH2 CHCH2 I I
THZCHCH3
OH
1,3_Propanediol
5.98
1.23 4.19
8.57
2.21
1.91
2.04
2.83 5.69
5.10
1.32
3.24
0.704
4.16 5.61
7.93
2.05
2.11
1.84
4.34 5.41
7.27
1.88
2.33
1.62
4.41
1.15
3.58
0.361
4.19
1.08
3.74
0.202
3.98
1.03
3.87
0.0710
4.01
1.04
3.85
0.0910
5.01
1.29
3.28
0.661
bH
~H2C”zC”z OH
-2.03
bH
OH OHOH 1,2_Propanediol
10.3
OH
1,3-Butanediol CH2CH2CHCH3 OH AH
117
Aromatic polyamide
Polybenzimidalone
KS
8K K DzO
AG: (kJ.mol-‘)
AG,o -&I (kJ.mol-‘)
14.6
1.15
0.542
0.356
16.7
1.31
0.199
0.699
K
>K K DzO
6.99
0.320
AG;,, - AG
AG
(kJ.mol-‘)
(kJ.mol-‘)
2.45
- 2.95
_ 12.9
1.11
0.866
0.0320
9.33
0.427
1.70
-2.20
8.26
0.378
2.02
- 2.52
20.0
1.57
- 0.273
1.17
9.08
0.415
1.30
-1.80
25.3
1.99
-0.881
1.78
9.74
0.446
1.59
-2.09
13.4
1.06
0.759
0.139
36.2
1.66
- 1.81
1.30
17.6
1.38
0.0620
0.838
24.3
1.11
-0.772
0.269
21.3
1.67
-0.431
1.329
13.6
0.642
0.717
- 1.22
23.6
1.86
-0.705
1.60
12.9
0.590
0.863
-1.37
13.8
1.08
0.689
0.208
9.71
0.444
1.60
-2.10
14.2
1.12
0.606
0.292
9.74
0.446
1.59
-2.09
9.31
0.423
1.71
-2.21
15.0
1.18
0.475
0.422
9.58
0.438
1.63
-2.14
16.1
1.27
0.285
0.613
9.58
0.438
1.63
-2.14
(continued
over-leaf)
118 TABLE IV (continued) Organic solute
Name
Polysulfonamide
Formula
pK,
K,
1.36
3.16
0.788
5.77
1.49
2.92
1.02
5.45
1.41
3.07
0.877
(HOCH2CH212NH
9.15
2.36
1.74
2.21
(HOCH2CH2)jN
8.31
2.14
1.99
1.96
7H2 CH2 NH2 NH2
l,%PrOpane-
yH2yHCH3
diamine
NH2 NH2
1,5-Pentanediol 1,6-Hexanediol
CHZ(CHZ)~CHZ
Triton
ZCe$
Sodium chloride Heavy water Sebacic acid
A%,, - AG (kJ.mol-‘ )
OH
OH
Diethanolamine
AG (kJ.mol-‘)
5.26
1,4-Butanediol FHzCHzCHzlH2 Ethylenediamine
K K DzO
_
OH
bH
_
~Hz(CHZ)~FHZ 0CH2CH2)g,,00H
-
NaCl
2.49
0.643
5.08
- 1.14
o 2O
3.88
1
3.94
0
HOOC(CH2)eCOOH ,CH
HOOCH2C,
2COOH
‘NCH2CH2N’
EDTA
/ HOOCH2C
\ CH2COOH
-
from the mobile phase to the stationary phase. In case (II) have water and solute the same tendency to be adsorped on the membrane surface, and are the separation properties of the membrane mainly governed by a dynamic factor, i.e. diffusion. In case (I), it is unfavorable to increase the rejection only from a point of view of thermodynamics, but sometimes if KS/Kn2o is little larger than one, it is also possible to obtain a high rejection, because solutes have a larger molecular weight and size than water, and therefore diffusion is the predominant factor to govern the separation. If KS/Kn20 is much larger than one, the thermodynamic factor predominates the separation, and the rejection is usually small. The following experimental results seem to be in agreement with the above analysis.
119
Aromatic polyamide
Polybenzimidalone
K
8K K D20
AG
(kJ*mol-‘)
A&o-AC
(kJ.mol-‘)
15.5
1.22
0.392
0.506
14.7
1.16
0.519
0.379
28.5
2.25
- 1.19
2.09
20.8
1.64
-0.382
1.28
19.3
1.52
-0.178
1.08
26.5
2.09
-1.01
1.90
14.5
1.14
0.559
0.339
12.4
0.974
0.967
- 0.0690
12.7
1
0.989
K
9.44
8K
AG
AC;,, -AC
K DzO
(kJ.mol-‘)
(kJ.mol-‘)
0.432
1.67
-2.17
0.446
1.58
-2.09
0.562
0.990
-1.49
0.308
2.55
-3.05
_ 9.76
12.3
6.72
0
21.9
1
-0.502
0
_
32.1
1.47
- 1.50
0.993
11.2
0.512
1.23
- 1.73
RESULTS AND DISCUSSION
Some information about interactions between a PA membrane and organic solutes is given in Table II. The data in the third column are from Ref. 1. Besides for 2-methoxybenzylalcohol, the rejections of the PA membrane to these solutes are high. According to the above discussion, this could be suspected because all of these solutes agree well with relationship (III). Because of strong interaction between PBIL and benzyl alcohol, its partition coefficient is large and satisfies relationship (I) and thus the rejection is small, while NaCl satisfies relationship (III), and its rejection is up to 99.5% (see Table III). The theory and experimental results seem to be consistent. For the
120 TABLE V Interaction compounds
parameters
between
materials
and heterocyclic
and alicyclic
Polysulfonamide
Organic solute
Name
three RO membrane
Formula
K
AG&o - Af$
&
AG (kJ.mol-‘)
5.09
1.31
3.24
0.700
Tetrahydrofuran
8.12
2.10
2.04
1.90
Furfurylalcohol
8.21
2.12
2.02
1.93
Pyrrole
9.19
2.32
1.73
2.22
N-Methyl-2pyrrolidone Thiazol
5.40
1.39
3.09
0.851
10.7
2.56
1.34
2.61
Pyridine
10.8
2.79
1.31
2.64
4-Methylpyridine
11.5
2.97
1.15
2.80
2,4-Dimethylpyridine
10.4
2.69
1.40
2.54
2,4,6-Trimethylpyridine
13.8
3.56
0.683
3.26
Cyclohexanol
6.98
1.80
2.43
1.51
Cyclohexanone
6.99
1.80
2.43
1.52
Sulfolane
5.56
1.44
3.02
0.928
3.49
0.450
Furan
r”,
K,
I (kJ*mol-‘)
2-Methyl-5ethylpyridine
_
a-Aminopyridine 1,4-Dioxane
4.62
1.19
121
Aromatic polyamide
Polybenzimidalone
K
K K 020
9
AC
AG&o--AG:
(kJ.mol-‘)
(kJ.mol-‘)
0.123
0.774
A
K
ho
AC
(kJ*mol-‘)
AC”,,,- AG (kJ.mol-‘ )
0.642
0.642
- 1.15
0.436
1.65
-2.15
10.2
0.468
1.46
-1.97
10.0
0.458
1.51
-2.02
0.439
1.57
-2.07
12.7
0.580
0.907
- 1.41
2.87
16.7
0.764
0.193
- 0.695
0.618
17.2
0.758
0.124
- 0.626
2.31
18.7
0.854
0.0950
- 0.407
2.61
18.6
0.853
0.0910
0.412
27.2
1.24
17.2
1.35
25.7
2.02
- 0.923
1.82
18.7
1.47
- 0.0960
0.994
41.4
3.25
-2.15
3.05
17.9
1.41
0.0170
12.1
0.949
1.03
-0.134
38.5
3.03
- 1.97
16.2
1.27
31.0
2.44
-1.41
34.8
2.74
- 1.71
0.280
K
0.880
14.0 9.53
9.61
- 1.07
0.563
31.6
2.49
- 1.46
2.35
11.1
0.507
1.26
- 1.76
37.1
2.91
- 1.87
2.77
18.2
0.831
- 0.0230
-0.479
19.2
1.51
-0.169
1.07
8.99
0.468
1.79
-2.30
29.7
2.34
- 1.30
2.20
_
18.4
1.47
- 0.0570
0.999
9.83
0.449
1.57
- 2.07
122 TABLE VI Interaction
parameters
between three RO membrane
Organic solute
materials
and aromatic compounds
Polysulfonamide
Formula
Name
Phenol
PK
@.H
9.89
Aniline
K,
sK
AG&,, -AC (kJ.mol-‘)
KD~O
AG (kJ.mol-‘)
33.7
8.70
-1.61
17.2
4.43
0.124
3.82
2.86
0.739
4.72
- 0.778
4.18
1.08
3.75
0.192
9.75
2.52
1.57
2.37
21.9
5.65
- 0.503
4.45
38.4
9.91
- 1.95
5.89
20.4
5.25
-0.316
4.26
5.56
4.63
Nitrobenzene Sodium phenyl sulfonate Benzoic acid Benzyl alcohol Benzeldehyde Hydroquinone
10.4
Resorcinol
9.81
24.9
6.43
- 0.834
4.78
Pyrocatechol
9.85
24.4
6.31
-0.787
4.73
p-Amino
6.10
15.8
4.07
m-Amino aniline
4.90
21.3
5.50
-0.435
4.38
o-Amino aniline
4.50
11.3
2.93
1.19
2.76
m-Phathalic
3.54 4.60
0.908
4.19
-0.248
o-Phthalic anhydride
aniline
acid
3.52
acid
0.339
3.61
_
4.73
m-Methylaniline
27.0
6.98
-1.04
4.99
m-Nitrobenzoyl hydrazine p-Toluylsulfonic methyl ester
acid
Sulfosalicylic acid
11.7
3.03
1.10
2.85
31.3
8.07
- 1.42
5.36
NO2
2,4,6_Trinitrophenol
0” NO1
0.380
123
Polybenzimidalone
Aromatic polyamide
+[&&$&)j_
~~@h-~*~+ n
KS
sK Kn,o
AG (kJ.mol-‘)
ACgo -AC%’ (kJ.mol-‘)
Ks
sK K DzO
59.9
4.71
-3.11
4.01
16.0
0.732
39.4
3.10
-2.03
2.92
33.0
1.51
-1.56
1.06
45.8
2.10
-2.42
0.149
11.1
0.507
1.26
-1.76
0.125
-0.654
_ 57.2
4.50
16.2 1.27 40.5 3.19
A(% (kJ.mol-‘)
0.303
AC”,,, - AG (kJ.mol-‘)
-0.805
-2.99
3.89
17.0
0.777
0.273 -2.10
0.625 3.00
39.2
1.79
13.4
0.613
0.762
-1.26
0.436
-0.938
-2.01
1.51
45.3
3.56
-2.39
3.29
15.2
0.696
47.9
3.77
-2.53
3.43
22.4
1.02
61.8
4.86
-3.19
4.09
18.7
0.857 -0.104
-0.398
56.1
4.41
-2.94
3.84
15.4
0.703
0.411
-0.913
45.3
3.56
-2.39
3.29
13.2
0.602
0.810
-1.31
8.03 0.367
2.09
-2.59
1.35
-1.28
29.5
_
_
_
1.29
0.236
0.0590
0.776
_ 55.1
16.4
-0.561
2.52
9.79 0.448
0.662
-2.89
2.39
1.58
-2.08
11.5
0.528
0.149
-1.65
16.5
0.755
0.234
-0.726
124
other compounds in Table III, though the ratios of KS to KDzoare little larger than one, the rejections of the PBIL membrane to these solutes are high enough, based on the relationship D= (kT)l(Gnyr,)
(25)
where k, T, q and rA are the Boltzmann constant, absolute temperature, solution viscosity and solute radius, respectively. Perhaps a dynamic factor also plays a role, because the size and molecular weight of these solutes are larger than that of water. More detailed information about interactions between organic solute and the three membrane materials is shown in Table IV-VI. It is obvious from Table IV that the partition coefficients of the solutes increase with the increasing number of carbon atoms for alcohols, aldehydes, ketones and amines. When comparing acetone with urine, the partition coefficient of acetone becomes larger by substituting two hydrophilic -NH:! groups of urine by two hydrophobic -CH, groups. This is similar for formamide, N,N-dimethyl formamide and N,N-dimethyl acetamide, which indicates that increasing the number and hydrophobicity of hydrophobic groups of the solutes favors interactions between membrane materials and organic solutes, thus increasing the partition coefficients. When comparing propionic acid with acrylic acid, acetic acid ethyl ester with acetic acid ethylene ester, it can be seen that an increasing degree of unsaturation in the organic solutes favors the interactions between the PSA or PA membrane and the solutes. This is different for PBIL, which indicates that the interactions are closely related to the structures of both membrane materials and organic solutes. Induction effects of polar groups in the organic solutes on interactions between membrane materials and the solutes are also studied. When comparing ethyl alcohol with 2-chloroethanol, methylene chloride with chloroform, it can be seen that increasing the number of polar groups enlarges the partition coefficients, perhaps because the inductive effect of the polar -Cl group makes the -H in the molecule more positive, so that the solutes can form stronger hydrogen bonds with the membrane materials. Furthermore, when comparing acetaldehyde with 2,2,2_trichloroacetaldehyde, acetic acid with 2-chloroacetic acid and 2,2,2_trichloroacetic acid, it can be seen that the polar groups affect the interactions, but the extent of effect is different for different membrane materials, which also indicates that the interactions depend on the structural and chemical properties of polymer and organic solute. Interactions between membrane materials and solutes with more than one functional group are also investigated, and the results are given in Table IV. For some diols and diamines, their partition coefficients are quite near one another, which indicates that the effect of position isomerization of functional
125
groups on interactions between membrane materials and solutes are small because the a-bonds in the main chain of the solutes can rotate freely. The results of investigation of interactions between membrane materials and organic solutes as shown in Table V further demonstrate that the interactions strongly depend on structural and chemical properties of polymers and organic solutes. When comparing cyclohexanol with cyclohexanone, it shows that their partition coefficients for PSA are almost the same, but their partition coefficients for PBIL and PA are quite different. For the other solutes, such as furan, tetrahydrofuran and pyrrole, the partition coefficients for the different membrane materials are also quite different. In general, for pyridine and its derivates, an increasing number of alkyl groups causes an increase in their partition coefficients. However, this is not normal for PBIL, for which the partition coefficient of pyridine is the largest. For the other derivates, the more alkyl groups, the larger are the coefficients, that is, the more favorable are the interactions between the polymers and these solutes. Table VI shows the results of interactions between three membrane materials and aromatic compounds. Obviously, the aromatic compounds with electron-releasing groups, such as phenol, aniline and benzyl alcohol, have strong interactions with the membrane materials, while the others with electronwithdrawing groups, such as nitrobenzene, benzoic acid and sodium benzene sulfonate, have weak interactions with the materials. Because sulfosalicylic acid has an electron-releasing group -OH and two electron-withdrawing groups -COOH and -S03H, its partition coefficient is between those of the two kinds of aromatic compounds. It is easy to find that the partition coefficients of pyrocatechol, hydroquinone, resorcinol and three o,m,p-amino-anilines are quite different from one another, the largest difference being about 16. These results greatly differ from those of the corresponding aliphatic compounds shown in Table IV, which shows that an aromatic compound shows a stronger interacting effect of geometric matching with the membrane material, i.e. polymer, than the corresponding aliphatic compound. Only considering from the point of view of thermodynamics, there is a possibility of separating isomers with the membrane separation technique. It has also been found by analyzing the pK, and KS data of the solutes that the partition coefficients, i.e. the interactions, do not directly depend on the acidity of these solutes. Another important result is that for PA membrane material the partition coefficients of most organic compounds are smaller than that of water; 85% of the solutes satisfy with relationship (III), while 10% for PSA and only 3% for PBIL of the solutes satisfy with relationship (III). Based on the discussion given in the previous theoretical part and only considering the thermodynamics, it can be seen that the stronger the interactions between membrane materials and water are, the more favorable are both solute rejection and volume flux to increase. Therefore, PA membranes may be more appropriate in sense of increasing solute rejection than the other two membrane materials.
126 CONCLUSION
When using the HPLC method to investigate interactions between membrane materials and solutes, using quartz powder coated homogeneously by membrane materials (polymer ) as a stationary phase is better than using pure polymer powder, because for the former the dead volume of LC column can be easily determined precisely, but for the latter this is more difficult. Interactions between membrane materials and organic solutes strongly depend on the structure and chemical properties of polymers and organic solutes. Increasing the number and hydrophobicity of the hydrophobic groups of organic solutes favors interactions between membrane materials and organic solutes. Introducing electron-releasing groups in the aromatic compounds favors interactions between membrane materials and these compounds, while introducing electron-withdrawing groups in the aromatic compounds does not favor interactions between membrane materials and these compounds. Aromatic compounds have stronger interactions of geometric matching with three membrane materials containing aromatic rings than the corresponding aliphatic compounds have. Only considering the thermodynamics, the relative strength of interactions between membrane and solute to interactions between membrane and water governs the membrane performance. The information obtained from HPLC experiments on interactions between membrane materials and organic solutes, especially the three-simple relationships about Ks/Kn20, is useful for selecting membrane materials, designing macromolecules for use as membrane materials, understanding transport mechanism through synthetic membranes and developing membrane applications. It is worth noticing that the correlation of HPLC data with thermodynamic equilibrium data is an approximation. ACKNOWLEDGEMENTS
The financial support provided by the Chinese Natural Science Fund Committee (C.N.S.F.C. No. 85273) is gratefully acknowledged, and Miss Zhang Lehua, who has done part of the work, is also kindly acknowledged.
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