2880
J. Phys. Chem. B 2008, 112, 2880-2887
In Situ Study of Diffusion and Interaction of Water and Mono- or Divalent Anions in a
Positively Charged Membrane Using Two-Dimensional Correlation FT-IR/Attenuated
Total Reflection Spectroscopy
Beibei Tang,† Peiyi Wu,*,† and H. W. Siesler‡
The Key Laboratory of Molecular Engineering of Polymers (Ministry of Education) and AdVanced Materials
Laboratory and Department of Macromolecular Science, Fudan UniVersity, Shanghai 200433,
People’s Republic of China, and Department of Physical Chemistry, UniVersity of Duisburg-Essen,
D 45117 Essen, Germany
ReceiVed: July 21, 2007; In Final Form: December 13, 2007
Two-dimensional (2D) correlation analysis based on time-resolved FT-IR/attenuated total reflection (ATR)
spectroscopy has been used to study the diffusion behavior of water and mono- or divalent anions in the
positively charged membranes of different charge density. In 2D FT-IR/ATR spectra, the splitting of the
water δ(OH) bending band in the spectral range 1700-1500 cm-1 indicates that there are three different
states of water in the positively charged membrane, that is, the water molecules forming strong or weak
hydrogen bonds with hydrophilic groups of the membrane and water molecularly dispersed with weaker
hydrogen bonds. The wavenumber difference of the δ(OH) band in the low- and high-charge-density membrane
indicates that water molecules form much stronger hydrogen bonds with hydrophilic groups in the highcharge-density membrane. The sequential order of the three water bands intensity changes shows that, in the
process of water diffusion into the high-charge-density membrane, the hydrogen-bonding interaction between
hydrophilic groups of the membrane and water molecules takes place gradually due to the highly crosslinked network structure of the membrane; in the process of water diffusion into the low-charge-density
membrane, the strong hydrogen-bonding interaction between hydrophilic groups of the membrane and water
molecules takes place instantaneously and this type of water easily diffuses due to the weak interactions
between the water molecules and the membrane polymer. Furthermore, the diffusion processes of the electrolyte
solution such as NaAc and Na2SO4 aqueous solutions in the positively charged membrane have also been
examined.
1. Introduction
Membrane filtration technology is now being used industrially
as an alternative to conventional separation methods due to its
effectiveness, energy saving, and operation convenience.1
Among various membrane processes, charged membranes
predominate due to the necessity of antifouling and ionic
rejection. With the increasing application in industry and the
rapid development of membrane technology, the study of
transport mechanisms in membranes has attracted much attention. Many researchers have made great efforts in the development and optimization of mathematical models with the purpose
of predicting membrane performance.2-6 In fact, the diffusion
process plays an important role in the transport mechanism of
solutes. Thus, it is of interest to investigate the diffusion process
in the membrane and the structural relations between the
membrane polymer matrix and the diffusant in order to
understand membrane permeability on a molecular level.
As a technique which is sensitive to interactions between
small molecules and a polymer, FT-IR/attenuated total reflection
(FT-IR/ATR) spectroscopy has proved to be a powerful method
for studying the diffusion behavior of small molecules in
polymer systems.7,8 This technique has been used in the study
* Corresponding author. Tel.: +86-21-55664034. Fax: +86-2165640293. E-mail: [email protected].
† Fudan University.
‡ University of Duisburg-Essen.
TABLE 1: Ion Exchange Capacity and Water Content of
the Positively Charged Membrane with Different
Post-treated Time by Immersion in TMEDA Aqueous
Solution
post-treated time
of membrane
(h)
ion exchange capacity (IEC)
(mmol/g dry membrane)
water
content
(%)
6
22
0.43
1.32
4.77
19.35
of many polymeric organic membranes. It was applied by
Murphy and Pinho for characterization of water in the active
layer of asymmetric membranes.9,10 Pereira and Yarwood
monitored the penetration of water into the sulfonated poly(ether sulfone) films by FT-IR/ATR and investigated the
interaction of water and polymer.1,11 Recently, they also studied
the interaction of water and electrolyte solutions in polymeric
membranes and discussed reasons for changes in the transport
coefficient due to ionic screening of interchain repulsions.12
However, to the best of our knowledge, a detailed study of the
dynamic diffusion and interaction of water and anions with
different valence in the positively charged membrane of different
charge density has not been reported in the literature.
Generalized two-dimensional (2D) IR spectroscopy proposed
by Noda13,14 has received much attention in recent years. It
emphasizes spectral features not readily observable in conventional one-dimensional (1D) spectra and also probes the specific
10.1021/jp075729+ CCC: $40.75 © 2008 American Chemical Society
Published on Web 02/20/2008
Diffusion of Water and Anions in Charged Membranes
J. Phys. Chem. B, Vol. 112, No. 10, 2008 2881
SCHEME 1: Reaction between BPPO Base Membrane and Tetramethylethylenediamine to Form a Positively Charged
Membrane
order of certain events taking place with the development of a
controlling physical variable. Many applications of 2D correlation spectroscopy have been reported such as temperaturedependent, concentration-dependent, and time-dependent spectral changes.15-18
In our group, much work has been performed on the diffusion
of water and small organic molecules by FT-IR/ATR and 2D
correlation analysis.19-23 In this article, we monitor the diffusion
process of water and electrolytes with different anion valence
in the positively charged membranes with different charge
density, by time-resolved FT-IR/ATR spectrometry. Using 2D
correlation analysis, the dynamic sorption and diffusion of water
in membranes with different charge densities is discussed.
Furthermore, the diffusion processes of electrolytes with monoor divalent anions in the positively charged membrane are also
investigated. Additionally, the role of the valence of the
electrolyte and the charge density of the membrane on water
diffusion will be investigated.
2. Experimental Section
2.1. Materials. Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)
of intrinsic viscosity 0.57 × 10-3 m3 kg-1 (i.e., 0.57 dL g-1) in
chloroform at 25 °C was obtained from the Institute of Chemical
Engineering of Beijing (China); bromine (from Shanghai
Lingfeng Chemical Reagent Co., Ltd.) was used for the
preparation of brominated PPO (BPPO) according to the method
reported in refs 24 and 25; and chlorobenzene and tetramethylethylenediamine (from Shanghai Chemical Reagent Co., Ltd.)
were used as a solvent for the polymer and as a functionalization
reagent for the base membrane, respectively. Na2SO4 and NaAc
(from Shanghai Chemical Reagent Co., Ltd.) were chosen as
the diffusants to examine the diffusion processes of the anions
with different valences in the positively charged membrane.
2.2. Membrane Preparation and Characterization. 2.2.1.
Membrane Preparation. Step 1. Base membrane: BPPO was
dissolved in chlorobenzene with a concentration of about 10
wt % to form a homogeneous solution. The solution was allowed
to stand overnight before casting to remove the air bubbles and
then cast at room-temperature on a glass plate by spreading it
between thin wires with a glass knife in order to control the
thickness of the film. After drying in air for about 12 h, the
glass plate was kept in the vacuum oven at 60 °C for about 12
h to remove residual solvent. Then the base membrane with a
thickness of approximately 0.02 mm was obtained.
Step 2. Positively charged membrane: To attach positive
charges to the membrane, a functional post-treatment of the base
membrane is necessary. In this study, tetramethylethylenediamine (TMEDA) was chosen as the amination reagent, because
it can react with the bromomethyl functionalities of BPPO not
only to produce quaternary ammonium groups but also to form
a cross-linking structure as shown in Scheme 1. In that case,
the degree of amination of the membrane can be increased and
the positive charge density of the membrane will be larger as
well. Therefore, the base membrane was immersed in tetra-
methylethylenediamine (TMEDA) aqueous solution with a
concentration of about 16 wt % at a temperature of about
35 ( 1 °C. Immersion times of 6 and 22 h corresponding to
low and high positive charge densities, respectively, were used
to investigate the influence of charge density of the membrane
on the diffusion processes of water and the electrolytes. The
functionalized membranes were washed with deionized water
repeatedly and then were kept in the vacuum oven at 60 °C for
about 24 h. The dry membranes were stored in a desiccator
before use.
2.2.2. Water Content. The water content (WR) of the
membranes was determined after equilibrating a sample of
membrane in chloride ion form (which is the counteranion of
the positively charged membrane and converted by immersion
membrane in 1 M NaCl solution) with deionized water at roomtemperature. The membrane samples were removed from the
water and weighed immediately after blotting the free surface
water. Then, they were dried for over 24 h at 60 ( 2 °C in an
oven. The water content is deduced from the difference in weight
between the wet (W1) and the dry (W2) membrane, expressed
in g H2O/g dry membrane (in Cl- form) based on the formula
WR ) (W1 - W2)/W2.
2.2.3. Ion-Exchange Capacity (IEC). The anion exchange
capacity of the membrane was measured by using the Mohr
method. Membranes were converted to the Cl- form in 1 M
NaCl, and then the membrane samples in the chloride form and
in equilibrium with deionized water were converted to the sulfate
form by leaching with a 0.5 M Na2SO4 solution. The chloride
ions released from the membranes were determined by a titration
of 0.01 M AgNO3 solution with K2CrO4 as an indicator (Mohr
method), and they were given as mmol g-1 dry membrane (in
Cl- form). The IEC value of each membrane is averaged from
values determined with at least three membranes, and the error
is no more than 6%.
The experimental results of ion-exchange capacity and water
content of the positively charged membranes with different posttreated times by immersion in TMEDA aqueous solution are
summarized in Table 1.
2.3. Diffusion Measurements by Time-Resolved FT-IR/
ATR Spectroscopy. Time-resolved FT-IR/ATR measurements
were performed at 25 °C using a Nicolet Nexus Smart ARK
FT-IR spectrometer equipped with a DTGS detector and a ZnSe
IRE crystal. The membrane-covered IRE crystal with a filter
paper above the sample membrane was mounted in an ATR
cell as shown in Figure 1, and the spectrum of the dry membrane
was collected as a background spectrum. And then without
moving the sample, distilled water was injected into the filter
paper while starting the data acquisition by a macro program.
The spectra were collected at a spectral resolution of 4 cm-1
by accumulating 32 scans. The measured wavenumber range
was 4000-650 cm-1. All original spectra were baselinecorrected using the Omnic 5.1 software.
2.4. Two-Dimensional Correlation Analysis. Several spectra
at equal time intervals and in a certain wavenumber range were
2882 J. Phys. Chem. B, Vol. 112, No. 10, 2008
Tang et al.
Figure 1. Schematic description of the FT-IR/ATR experimental
arrangement.
selected for 2D correlation analysis using the software “2D
Pocha”, which was composed by Daisuke Adachi (Kwansei
Gakuin University). The time-averaged 1D reference spectrum
is shown at the side and top of the 2D correlation maps for
comparison. In the 2D correlation maps, unshaded regions
indicate positive correlation intensities, while shaded regions
indicate negative correlation intensities.
2.5. Estimation of Diffusion Coefficient. For Fickian diffusion in polymers, eq 1 is given to estimate the effective
diffusion coefficient of water (or electrolyte) from FT-IR/ATR
spectra.
At
A∞
)1-
8γ
π[1 - exp(-2γL)]
∞
∑
n)0
{
Figure 2. ATR-time-resolved FT-IR/ATR spectra measured during
the sorption of water into a positively charged membrane with a posttreatment time of 22 h.
×
}
exp(g)[f exp(-2γL) + (- 1)n(2γ)]
(2n + 1)(4γ2 + f2)
(1)
where
g)
-D(2n + 1)2π2t
,
4L2
f)
(2n + 1)π
2L
In eq 1, At is the band absorbance of the FT-IR/ATR spectra
at time t, A∞ is the band absorbance at equilibrium, γ is the
penetration depth of the evanescent wave, L is the thickness of
the polymer membrane (invariable), and D is the diffusion
coefficient. The diffusion coefficient can be calculated by a
nonlinear curve fitting23 to eq 1 from the variation of the ν(OH) stretching band area versus time.
3. Results and Discussion
3.1. Influence of Positive Charge Density of the Membrane
on the Diffusion Process. 3.1.1. Water Diffusion. Time-resolved
spectra for the diffusion of water into the membrane with a posttreatment time of about 22 h are shown in Figure 2 (the spectra
for the water diffusion into the membrane with a post-treatment
time of 6 h is similar to that of a 22 h post-treatment time).
Assignments are shown in Table 2.26 The bands in the range of
3800-3000 cm-1 and 1700-1550 cm-1 representing the
stretching and bending vibration of the OH group in water,
gradually increased as a function of time when water diffused
into the membrane. However, the band at 1470 cm-1 which is
characteristic of the aromatic ν(CdC) stretching vibration
decreased with time. This is because the technique probes the
near membrane/substrate interface and any swelling results in
a decrease in band intensity as the average density of the
polymer membrane at the interface decreases.12 To reveal the
process of water diffusion into the positively charged membrane
with different charge density, several spectra at equal time
Figure 3. Asynchronous 2D FT-IR/ATR correlation spectra of sorbed
water in the membrane with a post-treatment time of 6 h in the
spectroscopic range of 1700-1550 cm-1.
TABLE 2: Vibrational Assignment for Water and Positively
Charged Membrane Based on Brominated PPO Polymer
wavenumber
(cm-1)
assignment
3800-3000
1700-1550
3080-2800
1579,1470
1380
1300
1180
OH stretching (ν(OH))
OH bending (δ(OH))
stretching alkane and aromatic C-H (ν(CH))
aromatic CdC stretching (ν(CdC))
symmetric bending CH3 (δs(CH3))
C-O-C, C-C bridge bond
in-plane CH bending (γip(CH))
intervals in the range of 1700-1550 cm-1 were selected for
2D correlation analysis, and the results are demonstrated in
Figure 3 (for the membrane with a post-treatment time of 6 h)
and Figure 4 (for the membrane with a post-treatment time of
22 h). It should be pointed out that only the asynchronous
contour maps are demonstrated in the following because all the
synchronous cross-peaks are positive in this study.
It can be seen from Figure 3, in the low-charge-density
membrane which was post-treated for 6 h by TMEDA, the wide
OH vibration band at 1700-1550 cm-1 is split into three
separate bands, V1, V2, and V3, located around 1672, 1647, and
1633 cm-1, respectively, which overlap in the 1D FT-IR/ATR
spectra. While in the high-charge-density membrane which was
post-treated for 22 h by TMEDA, the three split bands are
located around 1682, 1649, and 1618 cm-1, respectively (as
Diffusion of Water and Anions in Charged Membranes
Figure 4. Asynchronous 2D FT-IR/ATR correlation spectra of sorbed
water in the membrane with a post-treatment time of 22 h in the
spectroscopic range of 1700-1550 cm-1.
shown in Figure 4). It suggests that there are three different
states of water in the positively charged membrane, regardless
of the membrane charge density during the water diffusion
process. As is well-known, when the water molecules form
hydrogen bonds, the water OH bending vibration band will shift
to a higher wavenumber compared to that of free water.27,28
Thus, the bands at higher wavenumbers (1672 and 1682 cm-1)
indicate strong hydrogen-bonding interactions between hydrophilic groups of the membrane and the water molecules, while
the bands at lower wavenumbers (1647 and 1649 cm-1) reveal
weak hydrogen-bonding interactions between the hydrophilic
groups and the water molecules. The absorption at the lowest
wavenumber (1633 and 1618 cm-1) could belong to dispersed
water molecules with weak hydrogen-bonding interactions. It
is found that the OH bands of water at 1672 and 1647 cm-1
which are assigned to hydrogen-bonding interactions between
hydrophilic groups and the water molecules in the low-chargedensity membrane have considerably lower wavenumber than
the bands observed in the high-charge-density membrane (1682
cm-1and 1649 cm-1), indicating that the water molecules form
stronger hydrogen bonds with hydrophilic groups in the highcharge-density membrane than in the low-charge-density membrane. However, the OH band of water molecularly dispersed
with less hydrogen bonding in the low-charge-density membrane
at 1633 cm-1 occurs at a higher wavenumber position than in
the high-charge-density membrane (1618 cm-1). Thus, it is
assumed that the existence of hydrophilic groups of the charged
membrane reduces the interactions between water molecules,
resulting in a decrease in force constant and finally inducing a
red-shift in absorbance.
The sign of the asynchronous correlation peak Ψ(V1,V2) yields
information about the sequential order of intensity changes
between band V1 and band V2. According to the rules of Noda,13
if Φ(V1,V2) > 0, Ψ(V1,V2) is positive (unshaded area), band V1
varies before band V2 does, and if Ψ(V1,V2) is negative (shaded
area), band V1 varies after band V2 does. Where Φ(V1,V2)
represents the overall similarity between two separate spectral
intensity variations measured at different spectral variables, Ψ(V1,V2) may be regarded as a measure of dissimilarity of the
spectral intensity variations. In Figure 3 (water diffusion in the
membrane with low charge density), Ψ(1672,1647) is positive,
J. Phys. Chem. B, Vol. 112, No. 10, 2008 2883
suggesting that the band at 1672 cm-1 varies prior to the band
at 1647 cm-1, and the negative asynchronous bands at 1647/
1633 cm-1 reveal that the band at 1647 cm-1 varies after the
band 1633 cm-1. Furthermore, it can also be found that the band
at 1633 cm-1 varies after the band at 1672 cm-1. Then the
sequence of the spectral changes for water diffusion in the lowcharge-density membrane is as follows: 1672f1633f1647
cm-1. Similarly, the sequence of the spectral changes for water
diffusion in the high-charge-density membrane can be obtained
from Figure 4: 1649f1618f1682 cm-1.
The difference in the order of the intensity changes in the
range of the δ(OH) bending absorption during the process of
water diffusion into the membrane with different charge density
is attributed to the charge character of the membrane. When
water diffuses into the positively charged membrane, the
hydration of the hydrophilic group (-N+(CH3)3) occurs first.
For the low-charge-density membrane, the strong hydrogenbonding interaction between hydrated hydrophilic groups of the
membrane and water molecules will be formed first and this
type of water easily diffuses to the membrane side which is in
contact with the ATR element due to the low charge density of
the membrane. Then, the weaker hydrogen-bonding interaction
between molecularly dispersed water may be formed. To
continue the diffusion process in the membrane, the interactions
between water molecules and hydrated hydrophilic groups
weaken because of the hydrophobic characteristic of the lowcharge-density membrane and some strong hydrogen-bonded
water may be isolated. However, when water diffuses into the
membrane with high charge density, the weak hydrogen-bonding
interaction between hydrated hydrophilic groups of the membrane and water molecules will be formed at first because the
cross-linked network structure of the membrane increases
correspondingly when the membrane carries high charge density
by increasing immersion time in TMEDA solution as shown in
Scheme 1. Then, the weaker hydrogen-bonding interactions
between molecularly dispersed water may be formed. With the
increasing content of water molecules diffusing into the
membrane matrix, the strong hydrogen-bonding interactions
between hydrated hydrophilic groups of the membrane and water
molecules start gradually.
The diffusion coefficient of water from FT-IR/ATR spectra
was estimated. The fitting curve and analysis results are shown
in Figure 5 and Table 3, respectively. It is found that the
diffusion coefficient of water in the high-charge-density membrane is lower than that in the low-charge-density membrane,
which can be attributed to the strong interaction between water
molecules and hydrated hydrophilic groups of the high-chargedensity membrane as mentioned above.
3.1.2. NaAc Solution Diffusion. The diffusion process of NaAc
aqueous solution into the membrane with different charge
density has been examined. Time-resolved spectra for NaAc
solution diffusion into the high-charge-density membrane (with
a post-treatment time of about 22 h) are shown in Figure 6 (the
spectral shape for NaAc solution diffusion into the membrane
with a post-treatment time of 6 h is similar to that with a posttreatment time of 22 h). The characteristic ν(CdO) absorptions
of the COO- functionality of solid NaAc are localized around
1583 and 1421 cm-1, and they correspond to the antisymmetric
and symmetric stretching vibrations, respectively. Compared to
the antisymmetric stretching vibration, the intensity changes of
the COO- symmetric stretching vibrations is not sensitive to
our system, which limits its use for measuring the NaAc solution
diffusion process in the membrane. Therefore, the νas(COO-)
band (1700-1500 cm-1) was used for subsequent analysis in
2884 J. Phys. Chem. B, Vol. 112, No. 10, 2008
Tang et al.
Figure 5. Integrated intensity of ν(OH) versus time to give diffusion
curves for deionized water in the membrane with post-treatment times
of (a) 6 h and (b) 22 h.
Figure 6. FT-IR/ATR spectra of the diffusion process of 0.5 mol/L
NaAc solution into the positively charged membrane with a posttreatment time of 22 h as a function of time.
TABLE 3: Diffusion Coefficient of Water Estimated from
the Integral Intensity of the ν(OH) Band for Various
Solutions Penetrating into the Positively Charged Membrane
with Different Post-treatment Time
Figure 7. Asynchronous 2D FT-IR/ATR correlation spectra of NaAc
solution diffusion into the membrane with a post-treatment time of (a)
6 h and (b) 22 h in the spectroscopic range of 1700-1500 cm-1.
diffusion solution
post-treated time
of membrane
(h)
diffusion
coefficient
(10-9 cm2 s-1)
deionized water
deionized water
NaAc aqueous solution (0.5 M)
Na2SO4 aqueous solution (0.5 M)
6
22
22
22
2.34
1.84
6.17
4.07
tion between COO- groups or that there is interaction between
the COO- group and the positively charged membrane.
Moreover, according to the rule proposed by Noda, the
sequence of the spectral changes can be obtained from asynchronous 2D FT-IR/ATR correlation spectra by judgment of
the sign of the correlation peak, as shown in Figure 7. For NaAc
solution diffusion in the low-charge-density membrane, the
sequence is as follows: 1632f1676f1651f1551 cm-1. Similarly, the sequence of the spectral changes for NaAc solution
diffusion in the high-charge-density membrane is as follows:
1632f1649f1687 f1552 cm-1.
In contrast to the pure water diffusion in the positively
charged membrane, the water molecularly dispersed with weaker
hydrogen bonding first diffuses to the membrane side contacted
with the ATR element in the presence of NaAc independent of
the membrane charge density. It means that the existence of
NaAc reduces the interaction between hydrated hydrophilic
groups of the membrane and the water molecules and results
in a fast diffusion of molecularly dispersed water with weaker
our study. The results of 2D correlation analysis are demonstrated in Figure 7.
Note that except for the band of water, a new absorption band
was detected at 1551 cm-1 in the 2D asynchronous spectrum
of NaAc solution diffusion in the low-charge-density membrane
(it is observed at 1552 cm-1 in the high-charge-density
membrane) which can be attributed to the antisymmetric
stretching vibration of the COO- group. Compared to solid
NaAc, the νas(COO-) band in the FT-IR/ATR spectra of NaAc
solution diffused into the positively charged membrane shifts
to lower wavenumber, which indicates that there is an associa-
Diffusion of Water and Anions in Charged Membranes
J. Phys. Chem. B, Vol. 112, No. 10, 2008 2885
Figure 8. FT-IR/ATR spectra of the diffusion process of 0.5 mol/L
Na2SO4 solution into the positively charged membrane with a posttreatment time of 22 h as a function of time.
hydrogen-bonding interactions. This leads to an increase in the
water diffusion rate as shown in Table 3. It is mainly due to
the interactions between COO- groups and the positively
charged membrane which reduces the interaction of water
molecules and hydrophilic groups of the membrane.
3.2. Influence of the Anion Valence of the Electrolyte on
the Diffusion Process. To examine the influence of the anion
valence of the electrolyte on the diffusion process in the
positively charged membrane, time-resolved spectra for the
diffusion of Na2SO4 solution were measured and the result is
demonstrated in Figure 8. In our study, the membrane with a
post-treatment time of 22 h by TMEDA was used and compared
with the diffusion of pure water and monovalent electrolyte
(NaAc). The new absorption band around 1130 cm-1 can be
observed clearly due to the strong absorption of the ν(SdO)
band of Na2SO4. More information can be obtained from the
corresponding asynchronous correlation spectra (Figure 9). In
the range of the δ(OH) water band, two negative cross-peaks
(1687/1645 cm-1, 1645/1630 cm-1) can be observed in the
upper left triangle of Figure 9 a, indicating that the sequence
of spectral changes in the range of the water bending band for
Na2SO4 solution diffusion in the high-charge-density membrane
is as follows: 1630f1645f1687 cm-1. The sequential order
of the intensity change in the range of the water bending band
is similar to the sequence of diffusion of NaAc solution, which
demonstrates that the existence of salt reduces the interaction
between hydrated hydrophilic groups of the membrane and the
water molecules. Further evidence can be obtained from the
diffusion coefficient of water for various electrolyte solutions.
Table 3 shows the diffusion coefficient for water which was
estimated from the integral ν(OH) intensity (shown in Figure
10) in various solutions penetrating into the positively charged
membrane with a different post-treatment time. It is shown in
Table 3 that the diffusion coefficient of water increased in the
presence of electrolyte solution. However, the water diffusion
rate of NaAc solution is higher than that of Na2SO4 solution. It
is mainly due to the fact that the accelerated hydration rate of
the quaternary ammonium group in the positively charged
membrane reduces the water diffusion rate because the interaction between the SO42- group and the membrane is accelerated
due to the high anion valence of the SO42- group. Thus, the
diffusion coefficient of water for various solutions decreased
in the series NaAcfNa2SO4fwater.
Figure 9, parts b and c, respectively, shows the contour map
selected spectrum at the initial stage of diffusion and after a
period of time when diffusion takes place in the region of 17001550 cm-1 vs 1250-1050 cm-1 to correlate the band intensity
changes of water and the SO42- group at the different stages of
Figure 9. Asynchronous 2D FT-IR/ATR correlation spectra of Na2SO4 solution diffusion into the membrane with a post-treatment time
of 22 h in the spectroscopic range of (a) 1700-1550 cm-1, (b) 17001550 cm-1 vs 1250-1050 cm-1 in the initial stage of the diffusion,
and (c) 1700-1550 cm-1 vs 1250-1050 cm-1 after progressing
diffusion.
2886 J. Phys. Chem. B, Vol. 112, No. 10, 2008
Figure 10. Integrated intensity of the ν(OH) band versus time for (a)
NaAc and (b) Na2SO4 in the membrane with a post-treatment time of
22 h.
diffusion. Surprisingly, the change of the SO42- band intensity
is different at different stages of diffusion for Na2SO4 aqueous
solution. Initially (Figure 9 b), it can be found that the band at
1134 cm-1 varies after the band at 1645 cm-1. After a certain
period of diffusion (as shown in Figure 9c), the former varies
prior to the latter. The main reason can be attributed to the strong
interaction between the positive charge of the membrane and
the divalent anion of Na2SO4. At the start of diffusion, the SO42group has significantly lower diffusivity than the water weakly
hydrogen-bonded to hydrophilic groups because of strong
interaction between the SO42- group and the quaternary ammonium groups of the positively charged membrane. To
continue the diffusion process in the membrane, the content of
Na2SO4 solution diffusing into the membrane increased; the
interaction between the SO42 - groups and the membrane are
the reason for the movement of the SO42- groups across the
membrane. Thus, in the late stage of Na2SO4 diffusion, the
diffusivity of SO42- groups is faster than that of water weakly
hydrogen-bonded to hydrophilic groups.
4. Conclusions
In the presented study, FT-IR/ATR spectra were measured
as a function of time to study the diffusion of water and
electrolytes in a positively charged membrane. The influences
of charge density of the membrane and the valence of the anions
of the electrolytes on the diffusion processes were investigated.
The dynamic information and the state of water molecules
during the diffusion process of water or aqueous electrolyte
solutions can be explained from the analysis of 2D FT-IR/ATR
spectra. The following conclusions can be drawn from the
present study.
1. In the 2D correlation spectra, the original broad δ(OH)
water band is found to split into three separate bands, indicating
that there are three states of water in the positively charged
membrane during the water diffusion process regardless of the
membrane charge density. The three states of water are assigned
to water molecules forming strong or weak hydrogen bonds with
Tang et al.
hydrated hydrophilic groups of the membrane and to molecularly
dispersed water with weaker hydrogen bonds, respectively. The
wavenumber difference of the δ(OH) band between diffusion
in the low- and high-charge-density membrane is attributed to
the different strength of the interaction between water molecules
and the membranes of different charge density. The sequential
order of intensity changes of the three water bands indicates
that, in the process of water diffusion into the high-chargedensity membrane, the hydrogen-bonding interaction between
hydrated hydrophilic groups of the membrane and water
molecules is formed gradually due to the highly cross-linked
network structure of the membrane. When water diffuses into
the low-charge-density membrane, the strong hydrogen-bonding
interactions between hydrophilic groups of the membrane and
water molecules will be formed first and this type of water easily
diffuses across the membrane.
2. The presence of an electrolyte reduces the interaction
between hydrated hydrophilic groups of the membrane and water
molecules and results in a rapid diffusion of molecularly
dispersed water with weaker hydrogen bonding, which can be
attributed to the interaction of the anions and the positively
charged membrane which reduces the interaction of water
molecules and hydrophilic groups of the membrane.
3. The water diffusion rate of monovalent anion solution
(NaAc) is higher than that of divalent anion solution (Na2SO4).
It is mainly due to the fact that the accelerated hydration rate
of the quaternary ammonium group in the positively charged
membrane reduces the water diffusion rate because the interaction between SO42- group and the membrane is accelerated due
to the high anion-valence of the SO42- group.
4. For the diffusion of a high-valence anion (SO42-) into the
positively charged membrane, the diffusion process can be
divided into two steps. In the initial stage of diffusion, the SO42group has significantly lower diffusivity than the water weakly
hydrogen-bonded to hydrophilic groups because of the strong
interaction between SO42- anions and the quaternary ammonium
group of the positively charged membrane. To continue the
diffusion process in the membrane, the content of Na2SO4
solution diffusing into the membrane increased and the interaction between the SO42 - group and the membrane pull the SO42across the membrane easily, leading to a higher diffusivity of
SO42- than the water weakly hydrogen-bonded to hydrophilic
groups in the late stage of Na2SO4 diffusion.
Acknowledgment. The authors gratefully acknowledge the
financial support by the National Science of Foundation of China
(NSFC) (No. 20706009, No. 20573022, No. 20425415, and No.
20490220), the “Leading Scientist” Project of Shanghai (No.
07XD14002), the National Basic Research Program of China
(2005CB623800), the PHD Program of MOE (20050246010),
and the China Postdoctoral Science Foundation (20060400617).
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