STUDIES ON THE SELF-DIFFUSION OF WATER ETHANOL MIXTURE IN CHITOSAN PERVAPORATION MEMBRANE WITH PULSE

Chinese Journal of polymer Science Vol. 20, No, 4,(2002), 317-323Chinese Journal ofPolymer ScienceC2002 Springer-VerlagSTUDIES ON THE SELF-DIFFUSION OF WATER ETHANOL MIXTURE INCHITOSAN PERVAPORATION MEMBRANE WITH PULSED FIELD GRADIENTNMR DATAJun Fang, Ji-cai Huang, Qun-hui Guo and Vitalij I. Volkova guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, ChinaKarpov institute of Physical Chemistry, 103064, Mascow, RussiaAbstract The self-diffusion of water, ethanol and water-ethanol mixtures in chitosan(CS)membranes crosslinked byaqucous H2SO4 solution and uncrosslinked membrane was measured using pulsed-field gradient(PFG)nuclear magneticresonance(NMR) spectroscopy to obtain the partial solubilities and self-diffusion coefficients. An attempt was made toexplain the transport properties of water and ethanol through the CS membrane. It was concluded that there are two types ofchannel water and ethanol diffusate transfer. the water was localized in the hydrophilic ionic region formed by the ionizedgroups, and the ethanol was localized in the hydrophobic amorphous network of the polymer. There was a good agreementbetween the separation factors estimated from PFG-NMR data and those obtained by pervaporation testingKeywords Chitosan membrane, Pervaporation, Pulsed-field gradient NMRINTRODUCTIONThe magnetic resonance methods, such as electron spin resonance(ESR), electron nuclear double resonance(ENDOR), and pulsed-field gradient nuclear magnetic resonance(PFG-NMR) are widely applied in theinvestigation of membrane polymeric matrix structure diffusion state and diffusion mobility on the microscopiclevel. The PFG-NMR method can directly observe the diffusion motion of molecules!, 2). Now, the PFG-NMRwater-in oil emulsions, polymers, solid foods and biological systems B-y-stems, such as gels, zeolite crystallinity,method has been used to investigate the molecular diffusion in various sysOnly recently in Russia and Japan, PFG-NMR has been used in the membrane field to study the submicron-structure of membrane transport channels with dimensions from 0.3 to 5.0 nm). PFG-NMR is a direct methodfor measuring the molecular and ionic self-diffusion coefficients in membranes. The main advantages of PFGnMr are that it can not only investigate the self-diffusion processes in a range from 10to 10 m, but alsopermits separation of the partial self-diffusion coefficients and solubilities in the case ofcomplicated seldiffusion processesFor pervaporation(PV), PFG-NMR is an idealmethod to measure the interaction between the permeantsand the polymer matrix and to determine their state of interaction. Therefore, the direct thermodynamic factors,which control the pervaporation process, can be understood. This information can help us to understand thetransport mechanism of the permeants, and to design suitable polymer membrane for the separation of mixturesThe application of chitosan(CS) to PV membrane materials has received much attention from manyresearchers since 1980s7. Chitosan PV membrane showed excellent permselectivity in the separation of ethanol-water mixture(, 3). The interactions occurring between the membrane and the polar solvents strongly affected themembrane separation properties(& 91中国煤化工Corresponding author: Jun Fang(F), E-mail: fangjun @mail gic. arReceived April 29, 2001; Revised August 15, 2001; Accepted August 20,CNMHG318J. fang et al.In this paper, the self-diffusion of water, ethanol and water-ethanol mixtures in chitosan membranecrosslinked by aqueous H2SO4 solution and uncrosslinked membrane was measured by PFG-NMR spectroscopEXPERIMENTSSelf-diffusion Coefficient MeasurementPFG-NMR measurements were carried out on a spectrometer which was constructed by the Molecular PhysicalDepartment of Kazan State University, Russiallo, l. The NMR frequency for the proton was 100MHz. Themeasurements were performed in a stimulated spin-echo mode. The spin-echo signal of H nuclear of diffusatemolecules was obtained if spin-spin T2 proton magnetic relaxation times of diffusate molecules was not shorterthan 10 s, the maximum value of pulse gradient amplitude g and the pulse duration were 40 T/m and 4 x 10respectively. It gives us the opportunity to measure the diffusion coefficient from 10-to 10- 4m?/s and to observethe amplitude echo signal changing over the four orders of magnitude. The latter is very important when the echoignal is non-exponentiaL. The process of measurement was largely computerized. The temperature varied from60c to 100C and was kept within to1 K. For molecules of diffusate undergoing unhindered isotropicBrownian motion, the evolution of spin-echo amplitude is described by the following equationA(24, 41, 8=(2, t1, 0)exp(r8& D,)where y is the proton geomagnetic(or magnetogyric)ratio, ta=(4-a)3 is the diffusion time. A is thebetween the gradient pulses. D, is the self-diffusion coefficient of diffusion in the i-th phase(or channel),of the calculation procedure of self-diffusion coefficients from the amplitude echo signal changing against g isalready described in our previous paperMePrChitosan(CS)was supplied by Toko Kasei Co Ltd (Japan)with a molecular weight of 8.0 x 10-1.0 x 10.Itsdeacetylation degree is 87 mol% and 75 mol%, respectivelyCS membrane was prepared as reported in the previous paper! The thickness of the membrane is aboutThe CS membranes were equilibrated with water or absolute ethanol or water-ethanol mixtures for threedays; the films and some feed solution were placed in glass NMR sample tubes(outside diameter was 7 mmwhich were sealedRESULTS AND DISCUSSIONSelf-Diffusion of Water in MembraneThe CS films equilibrated with different water content in the feed solution were investigated. The effects of theamount of water in the membrane on the self-diffusion coefficients of water for the crossslinked anduncrosslinked CS membranes are shown in Fig. l. The results indicated that only one self-diffusion coefficientfor moving water was obtained and the self-diffusion coefficient of water in the crosslinked membrane wasigher than that in the uncrosslinded membranes. This is attributable to the existence of a crosslinking structureof -NH,.SO4.. H,N. This structure has two effects. One is the formation of a three-dimensional structurewhich limits the degree of swelling and improves the mobility. The other is the existence of ionized aminogroups, which cause the preferential interaction with water. For crosslinked membrane, the sites of thepreferential interaction are formed and water permeates through the sites more readily. Therefore, water could bepreferentially bound to the ionized amino groups by hydrogen-bonding force or by forming hydration shells andtransport through the hydrophilic region by involving a high exchange rate between the free water andwater in the hydration shellsFigure 1 also shows that the self-diffusion coefficient of water increases with increasing water content in themembrane. when the content of water was lower than 50%. TL中国煤化工vater content in theembrane, the degree of swelling of the membrane increases,becomes wider andCNMHGelf-Diffusion of H2o-EtoH Mixture in Chitosan Membrane319the water mobility is enhanced. However, when the water content is higher than 50%, the self-diffusioncoefficient remains the same. The activation energy of the self-diffusion obtained in 50 wt% water content was21 kJ/mol, which is close to the bulk water self-diffusion activation energy. This indicates that the channel of themembrane at this water content level is large enough for water to diffuse freely. Onlyelf-diffusioncoefficient of water was obtained. It indicates that there is only one type of channel to be occupied by the watermolecules, or the water molecules in the channels are of different types but the exchange rates of the watermolecules between these channels are high as compared with the diffusion time(10-s)Sorption and Self-Diffusion of EthanolCS membranes were equilibrated with absolute ethanol(99.5 wt%). The swelling degree of the membrane wasfound to be 7 wt%10 wt% only, so the mobility of ethanol molecules was very low. Therefore, the spin-spinrelaxation time was too low(near 400 us at room temperature)in order to measure the self-diffusion coefficientSelf-diffusion of Water-Ethanol Mixtures in the CS Membraneigure 2 shows the results of self-diffusion of water and ethanol in the CS membranes, which have beenequilibrated with water-ethanol mixtures. These results indicate that there are two self-diffusion coefficients. Theone with lower self-diffusion coefficient is in the range of 10- m2/s. The population of this componentaccounts for 1/3 of the total amount of diffusate. The one with higher self-diffusion coefficient is above 10 m2/swhich was close to the self-diffusion coefficient of the whole of the water( 2. 1 x 10 "m/s at room temperature)The activation energy for the lower self-diffusion coefficients is 46 kJ/mol, but for the higher one is 21 kJ/mol10▲ Crosslinked▲ Crosslinked membranee Uncrosslinked membrane00.10203040.50.600.2Fig. 1 The effects of water content in the membraneFig. 2 The effects of water contents in ethanol-wateron the self diffusion coefficients of waterfeed solution on partial self-diffusion coefficients ofThe maximum value of iwate/membane is 50 wt%water(a, b,)and ethanol(a",b")a"),a) The membrane crosslinked by pH=0.5aqueous H2SO4 solutionb), b") The uncrosslinked membraneIn order to confirm whether the higher self-diffusion coefficto the self-diffusion of water ornot, D2O was used instead of ordinary water in the water-ethant中国煤化工 nt of water andethanol in the membrane was determined and found to be inYHCNMHGpopulations by320J Fang et alPFG-NMR. Thus it is concluded that in the Cs membrane the higher self-diffusion coefficient belongs to waterwhile the lower one belongs to ethanolFigure 2 also displays the effects of water contents in feed solution on the partial self-diffusion coeof water(Fig 2a, b )and ethanol(Fig. 2a",b") for the crosslinked and uncrosslinked CS membranes.indicate that the values of the self-diffusion coefficient of water( DsH o)for the crosslinked membrane are higherthan for the uncrosslinked one. However, for ethanol, the situation is reversed, the values of the self-diffusioncoefficient of ethanol(DEOH) for the crosslinked membrane are lower than for the uncrosslinked one. On theother hand, the self-diffusion coefficient of ethanol increases with increasing water contents in the feed solutionmore rapidly than the self-diffusion coefficient of water. This could be explained as following: For the ethanolcomponent, the ethanol diffusion transports through the hydrophobic amorphous network, so the ethanoldiffusion strongly depends on the mobility of the polymer chain segment. For crosslinked membrane or at lowwater content the polymer chain segment becomes less mobile, and the channel for ethanol diffusion alsobecomes smaller, so the self-diffusion coefficient of the ethanol is decreased. Increasing the content of water inthe membrane could improve the plasticization of polymer chain and increase its mobility, resulting in a rapidincrease of ethanol diffusion In the case of the water, it is mainly transport easily through the hydrophilic ionicpart region in CS membrane by high speed exchange between the ionized water in hydration shells and the freewater. Therefore, for crosslinked membrane, water diffusion becomes high, and the mobility of the polymer chainsegment exerts a smaller effect on the water transport than on ethanol. This is the main reason why the separationfactor(aw) for the crosslinked membrane is higher than that of theSolubility Selectivity, Diffusion Selectivity and Separation Factor of the Membrane from the Self-DiffusionPermeation of liquid across the pervaporation membranes was usually interpreted by a solution-diffusionmechanism. It has been pointed out that the separation factor could be calculated from the solubility selectivityand the diffusion selectivity. Spitzen et al. 2 defined the separation factor of pervaporation as followsa- asamM=DS H, O/Ds ENOHas=(c"H,o/c ETOH )(C'H 0/c'EtOHrespectively. Using the Wax 20, c'EtoH are the concentrations of water and ethanol in membrane and in the feed,where cmH, o, c"EtoH, chOter diffusion coefficient(D Ho ) water population in water channel PH, o), ethanoldiffusion coefficient(dErOH ) and ethanol population( PEtOH ) the separation factor could be calculated. Theresults of solubility selectivity, diffusion selectivity and the separation factor dependence on water contents infeed solution for crosslinked and uncrosslinked membranes are shown in Figs. 3, 4 and 5. The separation factorincreases with the increase of ethanol content in the feed. The separation factor which is calculated from the dataof self-diffusion coefficients and populations are very close to the separation factor of pervaporation experimentsas shown in our previous paper. From Figs. 3, 4 and 5, it is clear that the separation of water-ethanol mixturecould be achieved due to the preferential sorption and diffusion of water. Because of the existence of preferentialinteraction between ionized amino group in chitosan and water, the determination of the self-diffusion by thePFG-NMR gives us a good result to verify the permeation mechanism of water-ethanol in chitosan membrane中国煤化工CNMHGSel-Diffusion of H20-EtoH Mixture in Chitosan Membrane321am-DsHyODsEXOHl00中3 The effects of water contents in ethanolFig. 4 The effects of the water contents in ethr feed solution on the solubility selectivityater feed solution on diffusion selectivfor crosslinked and uncrosslinked membranescrosslinked and uncrosslinked membrane▲ CrosslinkedFig. 5 The effects of water contents in ethanol-water feed solution on separationactor for crosslinked and uncrosslinked membraneActivation Energies of EH.o and FEroH in CS Membrane Crosslinked with Diferent Amount of AqueousH,SO, Solution and the Uncrosslinked Membraneo and eero in CS membrane crosslinked with differentH中国煤化工 ution and theCNMHGJ. Fang et aluncrosslilnked membrane are listed in Table 1. The data indicate that the values of D, H,0, Ds EtOH and theactivation energy of EH, o and EToh are quite different, EETOH is always higher than EH, 0. This is the reasonwhy water and ethanol are localized in different parts of CS membrane. Water is localized in the hydrophilicionic moiety region which is formed by the ionized groups, while the ethanol is localized in the hydrophobicamorphous network of the polymer. The difference of EEro and EH,o is the lowest for the uncrosslinkedmembrane. Because an increase of the content of aqueous H_So4 in solution could decrease the flexibility of thepolymer, and diffusion of EtOH molecules become difficult. EErOH-EH,o increase with increase of the amountof H,SO4. These results are in good agreement with the data which were obtained from pervaporationexperiment. This is the main reason that the chitosan membrane could transport water preferentially to ethanol,and separate the water from the water-ethanol mixtures feed solution.Table 1. The self-diffusion coefficient of water Ds, o and ethanol D EoH and their activation energies EH o andEFroN in CS membranes crosslinked with different amount of H2 SO4 and for uncrosslinked membraneH20EEIoH-EH, o162厘021.027.61.5×10-161×10-1020.126.013×01l45.155×101021.723.440×10-144.04.1×10-018.8CONCLUSIONSThe self-diffusion of water, ethanol and water-ethanol mixture in chitosan membrane was investigated with thehelp of PFG-NMR spectroscopy. The partial self-diffusion coefficients and the population of water and ethanolin different channels of membrane were obtainedThe self-diffusion of water and ethanol for CS membrane equilibrated with water-ethanol mixtures werecalculated. The results indicate that there are two self-diffusion coefficients, the higher one belonging to waterand the lower one to ethanol. The self-diffusion of water(Dsr.o )for the crosslinked membrane is higher thanfor the uncrosslinked one however, for ethanol the situation is reversed.The solubility selectivity, diffusion selectivity and the separation factor dependence on water contents infeed solution for crosslinked and uncrosslinked membrane were obtained. The separation factor calculated fromthe data of self-diffusion coefficients and populations is very close to then factor obtained fropervaporation experiments. The determination of self-diffusion from PFG-NMR gives us a good result to verifyche permeation mechanism of water-ethanol in chitosan membrane.The values of DsH, o and DEOH, EH,o and EEroH are quite different,is always higher thanH,o. This is the reason why water and ethanol are localized in different parts of the CS membraneREFERENCESI Callaghan, P T, J. Phys., 1984, 37: 359Harriss, K R, Mills, R, Back, P.J. and Wabster, D.S., J. Magnet Reson, 1978, 29: 4733 Volkov, V I and Timashev, S F, Russ. J. 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