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PolyamidePOSS hybrid membranes for seawater desalination: Effect of POSS inclusion on membrane properties Jun Hyuk Moon b , Anki Reddy Katha a,n , Shanthi Pandian a , Subramanya Mayya Kolake a , Sungsoo Han b,nn a Computational Simulations Group, SAIT-India, Bangalore 560093, India b Organic Electronics Materials Lab, Samsung Advanced Institute of Technology (SAIT), Gyeonggi-do 446 712, Republic of Korea article info Article history: Received 22 November 2013 Received in revised form 21 February 2014 Accepted 3 March 2014 Available online 12 March 2014 Keywords: Desalination Polyamide membrane POSS Molecular dynamic simulations Free volume theory abstract We have shown using both experiments and simulations that incorporation of polyoctahedral oligomeric silsequioxanes (POSS) into the active layer of polyamide (PA) thin lm composite membrane results in higher water ux and salt rejection than pure PA membranes. We report that the water ux increased by a factor of 1.32 and salt rejection increased from 99.0% to 99.62% for the PAPOSS. Molecular dynamics simulation was employed to study the effect of POSS on membrane polymerization, water and salt diffusion and free volume. Quantities, such as water diffusivity, water partition coefcient, fractional free volume and range of free volume size computed on the equilibrated congurations, increased on the inclusion of POSS in PA membranes. Moreover salt diffusivity was lower in PAPOSS than PA. Therefore from the simulation study, it was concluded that the introduction of POSS into PA membrane improved (a) porosity of composite membrane, (b) hydrophilicity and (c) charge on the membrane. These effects explain the increase of water ux and salt rejection in nanocomposite membranes. & 2014 Elsevier B.V. All rights reserved. 1. Introduction The large escalation of demand for fresh water due to rapid industrial growth, man-made pollution of natural sources and fast depleting ground water levels compounded by worldwide popula- tion explosion has made desalination a fast growing industry [1,2]. Desalination of brackish water and seawater is one of the many solutions to address water scarcity. Desalination is the process of removing salt and suspended solids from seawater in order to make it potable and can be achieved by using a number of techniques. Industrial desalination technologies mostly involve semi-permeable membranes based on reverse osmosis (RO), though focus on forward osmosis (FO) has increased substantially due to the conundrum of simultaneous solution to water and energy challenges. Any enabling technology/solution for large scale deployment of desalination plants would typically revolve around development of high performance membranes. Current technologies are based on polyamide (PA) thin lm composite membranes having three layers (active, support and non-woven fabric) that are typically characterized by two important proper- ties viz., water ux and salt rejection. Recently thin lm nanocomposite (TFN) RO membranes were developed by incorporating carbon nanotube, silica, TiO 2 , silicalite, MCM-41 silica nanoparticles, and zeolite nanoparticles into the PA layer and the membrane properties were studied [38]. In parti- cular, under the high pressure RO process, higher water perme- ability without any signicant loss of salt rejection was observed with zeolite embedded in the polyamide layer [811]. Similarly increase in water ux was also observed for zeolite incorporated membranes in FO mode [12]. Among the composite membranes mentioned above, there was an increase in either water ux or salt rejection. Moreover adhesion issues were also needed to be resolved in some membranes, which could play a role in the long-term performance of the membrane. Hence there is a need for nanoparticles to be embedded into PA to result in a membrane with superior performance both in terms of water ux and salt rejection and without any adhesion issues. Besides, incorporation of hybrid organicinorganic materials like polyhedral oligomeric silsequioxanes (POSS) into a polymer has received substantial attention for various applications. POSS-containing polymer com- posites have desirable thermal and mechanical properties, glass- transition temperatures, homopolymer blend toughness and dielectric constants compared against pristine polymers [1315]. In addition to tethering crosslinking systems and linear polymers with POSS hybrids, functionalized POSS copolymerization (with Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/memsci Journal of Membrane Science http://dx.doi.org/10.1016/j.memsci.2014.03.004 0376-7388/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Postal address: Samsung R&D Institute India-Bangalore Pvt. Ltd., TRIDIB, 65/2, Bagmane Tech Park, Byrasandra, CV Raman Nagar, Bengaluru 560093, India. Tel.: þ91 80 4181999x3558; fax: þ91 80 41819000. nn Corresponding author. Tel.: þ82 31 280 9354. E-mail addresses: [email protected] (A.R. Katha), [email protected] (S. Han). Journal of Membrane Science 461 (2014) 8995

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Page 1: Polyamide–POSS hybrid membranes for seawater desalination: Effect of POSS inclusion on membrane properties

Polyamide–POSS hybrid membranes for seawater desalination: Effectof POSS inclusion on membrane properties

Jun Hyuk Moon b, Anki Reddy Katha a,n, Shanthi Pandian a,Subramanya Mayya Kolake a, Sungsoo Han b,nn

a Computational Simulations Group, SAIT-India, Bangalore 560093, Indiab Organic Electronics Materials Lab, Samsung Advanced Institute of Technology (SAIT), Gyeonggi-do 446 712, Republic of Korea

a r t i c l e i n f o

Article history:Received 22 November 2013Received in revised form21 February 2014Accepted 3 March 2014Available online 12 March 2014

Keywords:DesalinationPolyamide membranePOSSMolecular dynamic simulationsFree volume theory

a b s t r a c t

We have shown using both experiments and simulations that incorporation of polyoctahedral oligomericsilsequioxanes (POSS) into the active layer of polyamide (PA) thin film composite membrane results inhigher water flux and salt rejection than pure PA membranes. We report that the water flux increased bya factor of 1.32 and salt rejection increased from 99.0% to 99.62% for the PA–POSS. Molecular dynamicssimulation was employed to study the effect of POSS on membrane polymerization, water and saltdiffusion and free volume. Quantities, such as water diffusivity, water partition coefficient, fractional freevolume and range of free volume size computed on the equilibrated configurations, increased on theinclusion of POSS in PA membranes. Moreover salt diffusivity was lower in PA–POSS than PA. Thereforefrom the simulation study, it was concluded that the introduction of POSS into PA membrane improved(a) porosity of composite membrane, (b) hydrophilicity and (c) charge on the membrane. These effectsexplain the increase of water flux and salt rejection in nanocomposite membranes.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

The large escalation of demand for fresh water due to rapidindustrial growth, man-made pollution of natural sources and fastdepleting ground water levels compounded by worldwide popula-tion explosion has made desalination a fast growing industry [1,2].Desalination of brackish water and seawater is one of the manysolutions to address water scarcity. Desalination is the process ofremoving salt and suspended solids from seawater in order tomake it potable and can be achieved by using a number oftechniques. Industrial desalination technologies mostly involvesemi-permeable membranes based on reverse osmosis (RO),though focus on forward osmosis (FO) has increased substantiallydue to the conundrum of simultaneous solution to water andenergy challenges. Any enabling technology/solution for largescale deployment of desalination plants would typically revolvearound development of high performance membranes. Currenttechnologies are based on polyamide (PA) thin film compositemembranes having three layers (active, support and non-woven

fabric) that are typically characterized by two important proper-ties viz., water flux and salt rejection.

Recently thin film nanocomposite (TFN) RO membranes weredeveloped by incorporating carbon nanotube, silica, TiO2, silicalite,MCM-41 silica nanoparticles, and zeolite nanoparticles into the PAlayer and the membrane properties were studied [3–8]. In parti-cular, under the high pressure RO process, higher water perme-ability without any significant loss of salt rejection was observedwith zeolite embedded in the polyamide layer [8–11]. Similarlyincrease in water flux was also observed for zeolite incorporatedmembranes in FO mode [12]. Among the composite membranesmentioned above, there was an increase in either water flux or saltrejection. Moreover adhesion issues were also needed to beresolved in some membranes, which could play a role in thelong-term performance of the membrane. Hence there is a needfor nanoparticles to be embedded into PA to result in a membranewith superior performance both in terms of water flux and saltrejection and without any adhesion issues. Besides, incorporationof hybrid organic–inorganic materials like polyhedral oligomericsilsequioxanes (POSS) into a polymer has received substantialattention for various applications. POSS-containing polymer com-posites have desirable thermal and mechanical properties, glass-transition temperatures, homopolymer blend toughness anddielectric constants compared against pristine polymers [13–15].In addition to tethering crosslinking systems and linear polymerswith POSS hybrids, functionalized POSS copolymerization (with

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/memsci

Journal of Membrane Science

http://dx.doi.org/10.1016/j.memsci.2014.03.0040376-7388/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Postal address: Samsung R&D Institute India-BangalorePvt. Ltd., TRIDIB, 65/2, Bagmane Tech Park, Byrasandra, CV Raman Nagar, Bengaluru560093, India. Tel.: þ91 80 4181999x3558; fax: þ91 80 41819000.

nn Corresponding author. Tel.: þ82 31 280 9354.E-mail addresses: [email protected] (A.R. Katha),

[email protected] (S. Han).

Journal of Membrane Science 461 (2014) 89–95

Page 2: Polyamide–POSS hybrid membranes for seawater desalination: Effect of POSS inclusion on membrane properties

poly (methyl methacrylate), polystyrene, polyimide and aromaticpolyamide) was also studied [16]. However, very few studies existon the utility of POSS containing PA membranes for seawaterdesalination. Moreover, studies providing in-depth understandingon the performance improvement due to POSS are still lacking.The present study focuses on the effect of incorporation of POSS,subnanometer sized particles in polyamide TFC membranes andtheir efficiency on salt rejection and water flux.

2. Experimental

Interfacial polymerization involving trimesoyl chloride (TMC),0.15 wt% in organic phase (Isopar-E) and 1,3 phenylene diamine(MPD), 3.4 wt% in aqueous phase was performed on the support ofpolysulfone ultrafiltration membrane (PS35, Sepro membranes) toobtain TFC membranes. For the synthesis of nanocompositemembranes, octa trimethyl ammonium(TMA) POSS (Hybrid plas-tics) was dispersed in the aqueous phase. The concentration ofPOSS was 5 mol% of MPD. Energy dispersive X-ray spectroscopy(EDXS) attached to scanning electron microscopy (Hitachi S-5500USR) was employed to verify the existence of POSS within themembrane. Further, the membrane was also characterized byTransmission Electron Microscopy (TEM) (JEM-2100F, JEOL) usingliquid nitrogen cooling holder (Gatan 613) under low dose condi-tion [17]. The TEM specimen was prepared by isolation of PA–POSSusing chloroform and subsequent transfer of separated PA–POSSonto carbon coated grid without staining [18]. The membraneperformance was evaluated through measuring permeate flux andsalt rejection using a cross flow filtration at operating pressureof 55 bar at 25 1C. The area of membrane in the test cell is72�37 mm2. The cross-flow rate we used was 3 L/min. The feedsolution used was 32,000 ppm NaCl solution at pH 6.5. Afteroperation for 24 h, the water flux per unit area per unit time(L m�2 h�1) was calculated from the volume of water collected.The salt rejection was calculated using conductivity differencebetween the feed solution and the permeated solution. Thecontact angles of sessile drops on the surfaces of membranes havebeen measured using KRUSS DSA100S. The zeta potential analysisof membranes has been performed at 0.001 M NaCl and pH 7 usingSurPASS from Anton Paar.

Fig. 1 shows SEM images of the membrane cross-section (a) andsurface topography (b) clearly indicating the porous nature of themembrane. The position at the red-cross indicated in Fig. 1c wasused for EDXS measurements of (d). The peak corresponding tosilicon in Fig. 1d clearly shows the presence of POSS within themembrane. Moreover, presence of POSS in the membrane was alsoascertained using TEM. As seen in Fig. 2, POSS particles are clearlyvisible due to the contrast difference between PA and POSS.Moreover, POSS is uniformly distributed throughout the mem-brane. The experimental results on water flux and salt rejectionare summarized in Table 1. A clear increase in water flux (factor of1.32) coupled with increase in salt rejection (99.0–99.62%) wasobserved. Moreover the measurements of surface potential andcontact angle reveal that there is an increase in the surface chargeand hydrophilicity of the membrane on inclusion of POSS. Thevalues reported in Table 1 are the mean values averaged over5 independent experiments for each category (PA and PA–POSS).We found that the standard deviations in the values of water flux,salt rejection, surface potential and contact angle are 77%,70.05%, 71 mV and 721 respectively. In addition to the above,we also performed experiments for two more concentrations ofPOSS (3 and 7 mol% of MPD). The water flux and salt rejectionvalues were found to be 38.5 LMH and 99.3% for 3 mol% and40 LMH and 99.3% for 7 mol%. Hence this experimental study

confirms that there is an optimal concentration of POSS, which is5 mol% of MPD to obtain the superior membrane performance.

3. Simulation

In the following, simulation methodology and results arediscussed to provide an understanding of the effect of POSSinclusion on water flux and salt rejection. The heuristic approachpreviously reported in the literature was employed to build thepolymeric and nanocomposite membranes [19,20]. We used thepotential energy function form as shown here for our moleculardynamics simulations.

V ¼ ∑bonds

kbðb�b0Þ2þ ∑angles

kθðθ�θ0Þ2þ ∑dihedrals

k∅½1þcosðn∅�δÞ�

þ ∑impropers

kωðω�ω0Þ2þ ∑nonbonded

ϵRminij

rij

� �12"

� Rminij

rij

� �6#þ qiqj

εrij

The first three terms represent the bonded interactions and thefourth term accounts for the impropers, that is out-of-planebending. Two types of non-bonded interactions namely van derWaals interactions represented by the 12-6 Lennard–Jones (LJ)potential and Coulombic interactions have been used in thepresent simulations. Here qi and qj are the partial charges onatoms labeled by i and j separated by a distance rij, and ε is thedielectric permittivity. The electrostatic interactions were com-puted using the Particle Mesh Ewald (PME) technique with a gridspacing of 1.0 Å. More details about the terms in the aboveequation can be found in [21] and references therein.

To obtain an atomistic model for polymeric membranes in arealistic time, we employed the heuristic approach. Initially themonomers as shown in Fig. 3 were placed in a simulation cell andcompressed to the target density of 1.38 g/cm3 (hydrated PAmembrane density). MPD and TMC were used to generate purePA and POSS residues were included to generate the PA–POSSmembrane. Based on the observations from experimental study,POSS is introduced into the simulation cell as a charged residueand the concentration is 5 mol% of MPD. All the monomers duringthe polymerization were allowed to move without any constraints.The polymerization was performed in NVT (constant volume andconstant temperature [300 K]) ensemble. The polymerizationproceeds by forming amide bonds as shown in Fig. 3(c) at every1 ps for the first 2 ns. The criterion for amide bond formation wasthat the distance between the nitrogen atom (N) of MPD andcarbonyl carbon (CE) of TMC should be less than 3.5 Å.

After this stage, an additional favorable interaction potentialwas applied between the unreacted sites of the monomers withthe continuation of polymerization for 500 ps. Subsequently, theunreacted monomers were deleted to get the final membrane forfurther molecular simulation study. The results of the membranepolymerization vis-a-vis evolution of the amide bonds and num-ber of monomers present in each fragment of the membrane wereanalyzed. Analysis showed that the largest fragment of pure PAmembrane had 865 monomers, while PA–POSS had 555 mono-mers.

Moreover the rate of polymerization was slow in PA–POSScompared to PA. This clearly suggests that the cross-linked poly-merization is hindered in the presence of charged POSS, thusresulting in fragments with smaller size than pure PA.

In order to obtain the water partition coefficient in themembranes, the final polymer structure obtained above is mergedwith pre-equilibrated water box (TIP3P model) followed byremoval of overlapping water molecules. The water box thickness

J.H. Moon et al. / Journal of Membrane Science 461 (2014) 89–9590

Page 3: Polyamide–POSS hybrid membranes for seawater desalination: Effect of POSS inclusion on membrane properties

used here is 1.5 times that of membrane thickness and themembrane normal is in the z-dimension. This combined systemof Fig. 3(e) was subjected to 20 ns NPT equilibration run usingNAMD [21] and the time step used in all the simulations was 1 fs.

The force-field parameters were chosen from the literature [20].Further 20 ns production run was used for computation of waterpartition coefficient [the ratio of water density inside the mem-brane to that of bulk water density] and found that this coefficientis roughly 35% higher in PA–POSS (0.403970.037) membranesthan PA (0.305270.031). Moreover to make sure that the resultsobtained were independent of the cell size used in simulations, wehad built the three polyamide membranes starting with 250, 500and 1000 monomers (MPD and TMC) using the heuristic approachand performed molecular dynamics simulations to determinewater partition coefficient. This coefficient was found to be 0.25for the membrane built by 250 MPD and 250 TMC monomers,whereas it was 0.30 for other two membranes. Hence it wassuggested that to have simulation results in agreement with

Fig. 1. SEM micrographs of PA–POSS membrane. (a) The cross-section and (b) the surface morphology of membrane, (c) cross section image with red-cross position used forEDXS and (d) EDXS spectra clearly indicating presence of silicon. (For interpretation of the references to color in this figure legend, the reader is referred to the web version ofthis article.)

Fig. 2. TEM micrograph of PA–POSS membrane.

Table 1Water flux (L m�2 h�1), percentage of salt rejection, surface potential (mV) andcontact angle (degree) for two experimental membranes.

Quantity PA PA–POSS

Water flux 33.71 44.57Salt rejection (%) 99.0 99.62Surface potential �10.58 �11.70Contact angle 39.1 35.2

J.H. Moon et al. / Journal of Membrane Science 461 (2014) 89–95 91

Page 4: Polyamide–POSS hybrid membranes for seawater desalination: Effect of POSS inclusion on membrane properties

experiment, the initial configuration for membrane polymershould have at least 500 monomers of each reactant.

In order to characterize the diffusion process of water and saltinside the membranes, the simulation setup was constructed usingthe membrane system in Fig. 3(e) sans the water boxes above andbelow the bulk membrane by inserting 10 ion pairs. Last 10 ns of a20 ns NVT run was used for analysis. First we will focus on waterdynamics in terms of diffusion, hydrogen bonds and the pathways.By tracking the center of mass position of water molecules as afunction of time, an estimate of the water diffusion coefficient (D)can be computed. The relation used for the calculation of diffusioncoefficient in the three dimensions is

D¼ ⟨Δr2CM⟩6t

ð1Þ

Δr2CM ¼ jrCMðtÞ�rCMð0Þj2 in Eq. (1) is the displacement of thecenter of mass and t is the time interval. The diffusion coefficient ofwater was found to be 0.3�10�5 cm2 s�1 and 0.5�10�5 cm2 s�1 inPA and PA–POSS membranes respectively, which is an order of mag-nitude less compared to bulk water diffusion. The diffusion coefficientreported here is averaged over three independent simulation trajec-tories each of 10 ns duration. We found that the error bars are notsignificant as the standard deviation is 73%; hence we reported themean values of diffusion coefficient here. In order to characterize thewater transport further, x–z positions of two randomly selected watermolecules from a trajectory of 10 ns (with positions saved at every10 ps) were plotted in Fig. 4 for both PA and PA–POSSmembranes. Thetrajectories in Fig. 4 shows evidence for jump–diffusion mechanism ofwater in agreement with the literature [22]. Hydrophilicity of themembrane can be understood by analyzing the hydrogen bondformation between membrane and water molecules. In the dynamicsof biopolymers and charged polymers in aqueous solutions, formationof hydrogen-bonds (H-bonds) plays an important role.

On the atomic scales, both inter- as well as intra-polymerH-bond formation can take place via the functional groups presentin the polymeric membranes. Here VMD [23] tool has beenemployed to calculate the number of hydrogen bonds between

water and polymeric membranes. Here we used the criterion thata H-bond is formed if the distance between the donor D andacceptor A is lesser than 3.0 Å, and the angle D–H–A is less than201. For a fixed volume of the membrane (125�103 Å3) PA formed223 hydrogen bonds (mean: 223 and standard deviation: 12) andPA–POSS formed 277 (mean: 277 and standard deviation: 13) withwater molecules. This clearly shows that the hydrophilicity of themembrane improved on inclusion of POSS.

In order to understand the effect of POSS inclusion on saltrejection, we analyzed the simulation trajectories to study iondynamics. Fig. 4 compares pathways of Naþ and Cl� ions in PA(Fig. 4(c)) and PA–POSS (Fig. 4(d)). As seen in Fig. 4, the diffusion ofions is much slower inside the membranes than in water. Quanti-tative analysis of the diffusion based on the ion motion inside themembranes suggests that PA–POSS membrane has lower iondiffusion coefficients than PA. The values of diffusion coefficientsof salt ions averaged over several trajectories of ions were found tobe 2.5�10�7 cm2 s�1 for PA–POSS and 3.1�10�7 cm2 s�1 for PAmembrane. This is one of the reasons for higher salt rejection inPA–POSS compared to PA. Moreover, considering that the ratio ofunreacted to reacted carboxyl groups in PA–POSS is marginallyhigher than that in PA, it contributes towards higher salt rejection.In order to provide further understanding on the ion dynamics andsolvation of ions within the membrane, pair correlation functionsbetween ions and water were computed. These pair correlationfunctions and the analysis of solvation behavior of ions revealedthat the membrane atoms were present in the solvation shell ofions apart from water molecules. In the case of sodium ion, oneoxygen atom of TMC or POSS was in the solvation shell. On theother hand, hydrogen atoms of MPD and TMC were present in thesolvation shell of chloride ion. This finding is in agreement withliterature [22]. In bulk aqueous salt solutions, it is generallyaccepted that sodium and chloride ions are surrounded by 6 and8 water oxygen atoms respectively, though some studies reportedthe deviation from these values [24,25]. Compared to the solvationshell of ions in the bulk aqueous solution, we clearly observe that,the solvation shell of the ions in the membrane is affected. This

Fig. 3. Monomers used in the simulation work: (a) MPD, (b) TMC, (c) amide-bond between MPD and TMC, (d) POSS, and (e) final polymeric membrane merged withwater box.

J.H. Moon et al. / Journal of Membrane Science 461 (2014) 89–9592

Page 5: Polyamide–POSS hybrid membranes for seawater desalination: Effect of POSS inclusion on membrane properties

solvation behavior also plays a role in the ion transport inside themembranes.

4. Free volume theory

The availability of free space in the membranes was used byseveral authors to characterize the membrane performance [26,27], asit provides a pathway for diffusion of molecules. Positron annihilationlifetime spectroscopy (PALS) is the preferred experimental method todetermine the free volume, although it may not provide the completedetails of the free volume voids. On the contrary, MD simulations canbe employed to compute the free volume size and morphology.Fractional free volume (FFV) [defined as (V�Vm)/V wherein Vm¼1.3Vw, V is the total simulation cell volume and Vw is the van derWaalsvolume of all the atoms in the membrane] is one of the measures offree volume. FFV values for PA and PA–POSS membranes were foundto be 0.325 and 0.435 respectively. As FFV is calculated over the size ofthe membrane, to gain more understanding on the free volume sizeand shape, we computed the free volume size distribution [28] andanalyzed the cross sections of the membranes. Fig. 5 shows the sizedistribution of the free volume for PA (red) and PA–POSS (green)membranes. As shown in Fig. 5, the fraction of larger sized pores ishigher in PA–POSS explaining the increase in water flux. Visualizationof free space in a slice of thickness 1.5 Å along the membrane normalis provided in the inset in Fig. 5, wherein the increase in size of freevolumes due to the inefficient packing of monomers, on inclusion ofPOSS, is clearly evident. This increase in free volume is responsible forhigher water flux.

Previous studies have employed solution–diffusion equationsbased on the free volume theory to relate the effect of FFV onwater permeability [29–31]. Here we employ similar methodologyto understand the effect of FFV on water permeability of PA and

PA–POSS membranes. Water flux (Jw) through a membrane can bewritten as

Jw ¼ PwðΔP�ΔπÞL

ð2Þ

where ΔP is the applied pressure, Δπ is the osmotic pressure of thefeed solution, L the membrane thickness and Pw is the perme-ability of water. The water permeability [26] as a function of FFV is

80

90

100

20 30 40 50

x(Å)

z(Å

)

90

100

110

30 40 50

100

110

120

130

15 25 35 45 70

90

110

130

40 50 60 70

Fig. 4. x–z Positions of two randomly selected water molecules in (a) PA and (b) PA–-POSS. x–z Positions of Naþ (red, green) and Cl� (blue, pink) in (c) PA and (d) PA–POSS.(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5. Pore size distribution and free volume visualization for PA (red) and PA–POSS (green) membranes. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

J.H. Moon et al. / Journal of Membrane Science 461 (2014) 89–95 93

Page 6: Polyamide–POSS hybrid membranes for seawater desalination: Effect of POSS inclusion on membrane properties

written as

Pw ¼ Ap exp�Bp

FAV

� �ð3Þ

where Ap and Bp are the constants which depend on the properties ofthe penetrant, feed composition and free volume necessary for thediffusion of penetrant and FAV is the fractional accessible volume.Assuming these constants are similar for both the membranes of thisstudy, the ratio of water flux in these two membranes can be writtenas

Pw of PA�POSSPw of PA

� exp1

FAVPA� 1FAVPA�POSS

� �ð4Þ

This ratio from the simulation study is 2.3009, whereasthe experimental ratio is 1.3222. The discrepancy in the ratiosobtained from simulations and experiment may be attributed tothe constants present in the equation. However this suggests thatthe free volume theory coupled with realistic atomistic models forthe membranes in study could provide valuable suggestions forthe improvement in the membrane performance.

Similarly salt passage (Tsalt) can be expressed as

Tsalt ¼Ps

Pw

1ðΔP�ΔπÞ �

1f w

R rmax

rwexpðSw=Vf f wÞf ðrÞdrR rmax

rsexpðð1� f wÞSs=Vf f wÞf ðrÞdr

ð5Þ

where Vf is the free volume (function of radius); fw is the fractionalfree volume accessible to water molecule; Ss is the cross-sectionalarea of hydrated salt ion; Sw is the cross-sectional area of watermolecule; Ps is the permeability of salt; rw is the radius of watermolecule; rs is the radius of hydrated salt ion and f(r) is thenormalized free volume distribution. The ratio of salt passagebetween PA and PA–POSS membranes is 15.13, which means thefree volume theory approach predicts well that salt rejection ofPA–POSS is higher than PA, though the pore size distribution ofPA–POSS is larger than PA as shown in Fig. 5. It is important toconsider fractional free volume as well as pore size distribution inorder to understand the salt rejection behavior. As shown in Fig. 4,the interaction between membrane and solvent/solute can play animportant role in the diffusion of solvent/solute. The free volumetheory does not consider the effects of these interactions, whichcould be the reason why there is discrepancy between the ratio ofsalt passage from experiments and the free volume theory.

5. Conclusion

To conclude, we have shown that the membrane performance canbe significantly improved by inclusion of nanoscale inorganic fillerPOSS into the active layer of polyamide thin film composite mem-brane. Our results clearly highlight that POSS is uniformly distributedthroughout the membrane and this results in higher water flux andsalt rejection. Molecular dynamics simulation further showed that,inclusion of POSS, resulted in increase in free volume, broader poresize distribution, water partition coefficient, hydrophilicity and chargeon the membrane. Moreover, water and salt diffusivity studies clearlyindicate that incorporation of POSS into PA membranes results inhigher water flux and salt rejection compared to pure PA membranecorroborating our experimental finding.

Acknowledgments

Anki Reddy would like to thank Foram Thakkar and Yun Luo ofArgonne for helpful discussions on membrane polymerization.

Nomenclature

D diffusion coefficientΔP applied pressureΔπ osmotic pressure of the feed solutionL membrane thicknessPw permeability of waterJw water fluxFAV fractional accessible volumeVf free volume (function of radius)fw fractional free volume accessible to water moleculeSs the cross-sectional area of hydrated salt ionSw cross-sectional area of water moleculePs permeability of saltrw radius of water moleculers radius of hydrated salt ionf(r) normalized free volume distribution

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