Membrane characterization using microscopic image analysis

Journal of Membrane Science 186 (2001) 85–96

Membrane characterization using microscopic image analysis

Isabelle Masselina, Laurence Durand-Bourlierb, Jean-Michel Laineb,Pierre-Yves Sizaretc, Xavier Chasseraya, Daniel Lemordanta,∗

a Laboratoire de Physicochimie des Interfaces et des Milieux Réactionnels (EA2098) Université de Tours,Faculté des Sciences et Techniques, Parc de Grandmont, 37200 Tours, Franceb Lyonnaise des Eaux, Cirsee, 38 av. Président Wilson, 78230 Le Pecq, France

c Laboratoire de Microscopie Electronique, Université de Tours, Faculté de Médecine, 2 bis Bd Tonnellé, 37032 Tours Cédex, France

Received 8 May 2000; received in revised form 16 November 2000; accepted 20 November 2000

Abstract

Five organic ultrafiltration membranes, made of different materials (PES, PVDF and PAN), have been visualized by afield emission scanning electron microscope (FESEM). Obtained images have been treated by the processing and analysisprogram NIH image and parameters such as, porosity (Ak), pore density (N), mean pore radius (rp) and pore size distributionhave been quantified for each membrane. The mean pore size obtained from image analysis agrees well with rejection data offluorescein isothiocyanate dextrans, using the Ferry’s law approximation. The membrane thickness1xhas been also measuredon images. Results allowed the comparison between theAk/1x values obtained from image analysis and theAk/1x valuesobtained by diffusion experiments. Results, combined with porosity and water permeability values, gave information on thematerial hydrophilicity. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Membrane characterization; Image analysis; Porosity to thickness ratio; Scanning electron microscopy; Pore size distribution;Rejection rate; Porosity; Pore density

1. Introduction

Scanning electronic microscopy is a great tool fordirect observation of objects down to about 1 nm insize [1] at 15 kV or 3 nm at 1 kV. Some tools, basedon image analysis, have already been developed tocharacterize porous media especially in the field ofgeology [2–6], metallurgy [7], ceramics and othermaterials [8,9].

In the membrane technology domain, the obser-vation of the surface or the section morphology iscommon. The method presented here allows the treat-

∗ Corresponding author. Tel.:+33-2-4736-6960;fax: +33-2-4736-6960.E-mail address:[email protected] (D. Lemordant).

ment and the analysis of digital images by the meanof a program, which allows pore counting and porousarea measurements.

The first part of this work concerns the analysisof the membrane surface of five asymmetric ultra-filtration membranes by the mean of a field emis-sion scanning electron microscope (FESEM). Afteracquisition, images are treated by the public domainimage processing and analysis program NIH Image(developed by the National Institute of Health, USA,Division of Computer Research and Technology).This program allows the determination of the porosity(Ak), the pore density (N), the mean pore radius (rp)and the pore size distribution of the membranes. Inaddition, the modal pore size has been then deducedfrom the pore size distribution.

0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0376-7388(00)00657-8

86 I. Masselin et al. / Journal of Membrane Science 186 (2001) 85–96

Nomenclature

Ak membrane surface porosity (%)C concentration (mol l−1)Lp hydraulic permeability

(m3 m−2 s−1 Pa−1 or l m−2 h−1 bar−1)M molecular mass (g mol−1)N pore density (m−2)r pore radius (m)R rejection rate (%)R0 true rejection (%)T threshold1x membrane thickness (m)φ rs/rp (−)

Subscriptsdif diffusionim imagep pore, porous or permeater relative to the retentates solute

The second part of this work concerns the measure-ment of the membrane thickness (1x) determinedfrom sliced samples of the same membranes. Resultsallowed the comparison betweenAk/1x values dedu-ced from image analysis and (Ak/1x)im from solutediffusion experiments (Ak/1x)dif exposed elsewhere[10].

2. Theoretical

2.1. Image analysis and measuring process

Images obtained at a chosen magnification areused to measure the porosity, the pore density, themean pore radius and the pore size distribution ofthe membrane. In this method, each pore must bedisconnected from its neighbors and is assumed to becircular. The original image, composed of 256 greylevels, is binarized at a chosen threshold levelT. Thepores are in black and the surface of the membraneis in white. The analysis of the binarized image isbased on the digitalization of each pore of the mem-brane surface followed by the measurement of thepore area. The pore density (number of pores per unit

area) is deduced from the number of labelized poreson a known surface. The porosity is the porous areaof the membrane, reported to the membrane area,seen on the image. The dispersion of the pore sizepopulation is characterized by the mean pore radiusand its standard deviation. The pore size distributionis calculated from the occurrence of pores in a givenradius interval. Here, calculations were performedwith a 0.5 nm interval. This value appeared to be agood compromise between the calculation precisionand the number of intervals. The distribution is thencharacterized by the following parameters

• The maximum frequencygives an information onthe gathering of data in the same radius interval.This means that it gives an idea on the confidencethat can be attributed to the modal pore interval andradius.

• The modal intervalis the pore radius interval forwhich the pore radii are the most frequent. A 10%variation around the maximum frequency value isaccepted in order to take into account wide distribu-tion. The center of this interval is calledthe modalpore radius(its frequency is assumed to be maxi-mum).

• The pore radiusdistribution extentis the maximumdifference between the superior boundary of thepore radius interval for which the frequency is min-imum (except zero), and the inferior boundary ofthe modal interval.

2.2. Membrane characterization based onpermeation data

2.2.1. Mean pore sizeThe rejection characteristics of the asymmetric

ultrafiltration membranes are determined by the mor-phology of the skin layer. The rejection rate of thesolute (R) is defined as

R = 1 − Cp

Cr(1)

where Cp and Cr are, respectively, the solute con-centrations in the permeate and in the feed solutions.As solute concentrations used are low, the effect ofconcentration polarization on rejection has not beenconsidered. At least as a first approximation, the

I. Masselin et al. / Journal of Membrane Science 186 (2001) 85–96 87

relationship between the solute size and the poreradius proposed by Ferry can be applied

R = [1 − (1 − Φ)2]2 (2)

In relation (2),Φ is the ratio of solute to pore radiusrs/rp. When rs is unknown, it can be deduced fromthe molecular mass [11] by the following empiricalrelation, established for proteins and other moleculesin the size range of 20–7× 106 g mol−1

rs = 0.038M0.4 (3)

rs is expressed in nm.

3. Experimental

3.1. Membranes

Asymmetric polyethersulfone (PES), polyvinili-denefluoride (PVDF) and polyacrylonitril (PAN)ultrafiltration membranes were supplied by Orelis(Rhodia Eco Services, France). The molecular weightcut off (MWCO) of the membranes used are reportedin Table 1. Before all experiments, membranes wererinsed with deionized water (resistivity≈ 15 M� cm),until the water permeability remained constant.

3.2. Microscopic images

Microscopic observation of the membrane surfacewas performed by using a FESEM, a LEO Gemini982 apparatus. After rinsing, samples were dried andcoated with a 5 nm layer of sputtered platinum. Theelectron beam was accelerated under a 2.00 or 3.00 kVtension. Images were digitized in 256 grey levels

Table 1Membranes under study

Material Manufacturereference

MWCO(kDa)

Referencein the text

Polyethersulfone (PES) 3028 30 PES303028 100 PES100

Polyvinilydenefluoride(PVDF)

3065 40 PVDF40

Polyacrylonitrile (PAN) 3038 40 PAN403050 50 PAN50

within a frame of 1024×1024 rectangular pixels withthe aspect ratio 5/4. The analysis program reads onlysquare pixels. As a consequence, the transfer of theimage from the microscope to the analysis computerneeded to resize the images into 1280× 1024 squarepixels in order to restore the ratio 5/4. After thisfirst step, the images underwent a series of differentoperations.

• They were spatially scaled according to the distancegiven by the microscope constructor and reportedin the image. A first treatment step was necessarybetween the image acquisition and digital imageanalysis [12,13]. It generally aims at enhancing theimage quality in terms of noise, contrast, imagerestoring in the case of images damaged by theimaging apparatus or of non uniform lightning dur-ing the image acquisition. Further operations arethen facilitated. In our case, a simple noise reduc-tion was realized on original images. The methodused was a frequential filtering which consisted inan image convolution using a nucleus of size 3× 3.Then, a mean grey level around the treated pixel wascalculated; thanks to the adjustment of judiciouscoefficients.

• After the calculation of the total surface area, theimage was thresholded at the chosen levelT. Thislevel corresponds to the grey value between 0 and255 for which the user considers that, compared tothe non-thresholded image, all the membrane poresare taken into account at a right size. The resultingbinary image, composed of only white and blackpixels, with pores represented in black, is displayedin Fig. 1. In order to evaluate the uncertainty re-sulting from the choice of the threshold valueT,its influence on the number of counted pores, themean grey density and the mean pore area has beenstudied in the case of the PES100 membrane at anominal magnification of 80,000×. All these pa-rameters exhibited a plateau value aroundT = 190,but the most pronounced one concerned the meanpore area for whichT = 190±10 as seen in Fig. 2.That is the reason why all values, pore density,porosity or pore size, are given with an uncertaintycorresponding toT ± 10.

• In order to determine the surface membrane poros-ity, the pore density and the pore size distribution,each pore was numbered and its area was calcu-


Fig. 1. Image pre-treatment operated on the PES100 membrane at a magnification of 80,000×.

lated. Particles with size inferior to five pixels wereignored to avoid the counting of little defaults. Theeffective size of the particles corresponding to fivepixels depends on the magnification. The surfacecalculation leads to the minimum particle radii of2.26, 1.41 and 1.13 nm, respectively, at 50,000×,80,000× and 100,000× (the scales are 0.559, 0.895and 1.115 pixels, respectively, per nm at 50,000×,80,000× and 100,000×). All these particle radiiare lower than the microscope resolution itself. Re-sults were then exported to a spreadsheet program

(Excel®) to be statistically treated as indicated inthe experimental section.

3.3. Solute rejection measurements

Experiments have been performed according to theX 45–103 AFNOR norm [14]. The solutes used forthis purpose were fluorescein isothiocyanate dextrans(FID) (Aldrich) of different molecular weights. The20 kDa FID (FID20) was chosen to test the PES30and PAN40 membranes and the 40 kDa FID (FID40)


Fig. 2. Mean pore area vs. threshold valuesT for the PES100 membrane.

for the PVDF40 and PAN50 membranes. The PES100membrane has not been characterized by this method.Solute concentrations were determined by light ab-sorption at 277 and 480 nm and the mean rejectionvalue from these two wavelengths is finally kept asthe result.

First, the solution under pressure passed through themembrane for 20 min in order to let enough time to theadsorption equilibrium of the solute at the membranesurface to be achieved. Then, four samples were taken:one in the feed solution (Cr1), two in the permeate(Cp1 andCp2), followed by one in the retentate (Cr2) inorder to verify that no significant variation of concen-tration, caused by adsorption or sampling happened.When the difference betweenCr1 andCr2 was negligi-ble, a mean value for rejection rateR was calculated.

3.4. Ak/∆x calculation from diffusion experiments

Diffusion of small molecules or ions (rs/rp � 1),performed at the pseudo-stationary state, is a conve-nient method to determine the porosity to thicknessratio. Accurate values ofAk/1x can be obtained byplotting solute concentration variations in the perme-ate versus time as previously exposed [10]. For thisstudy, diffusing solutes were salts (KCl, NaCl, KNO3,NaNO3), formamide and hydrochloric acid (HCl) in0.1 mol l−1 NaCl (used as a supporting electrolyte).

The use of NaCl solutions as supporting electrolyteis useful to achieve a better reproducibility by fixingthe ionic strength, and hence the charge density onthe pore walls.Ak/1x values, presented in this work,are the mean of experiments using all these solutes.

4. Results and discussion

4.1. Microscopic images

Some images of the membrane surfaces obtainedby FESEM at nominal magnification varying from80,000× to 100,000× are reported in Fig. 3. The poredensity, porosity and mean pore radius of the mem-branes under study are reported in Table 2. All valuesare referred to the nominal magnification used for im-age analysis. Values fluctuate with the magnificationused, but when the precision of the measurements istaken into account, these variations are not clearly sig-nificant. In addition, only slight variations were notedwhen the location of the image on the surface of themembrane varies.

The pore density of the PVDF40 and the PES100membranes are the lowest (2.1× 1014 and 2.3× 1014

pores per m2, respectively) and that of the PAN40 isthe highest (6.8 × 1014 pores per m2). The variationsin porosity, indicated in percentage, are larger than


Fig. 3. Images of the membranes: (a) PES30 (100,000×); (b) PES 100 (80,000×); (c) PAN40 (80,000×); (d) PAN50 (100,000×); (e)PVDF40 (80,000×).


Table 2Pore density (1014 m−2), porosity (%) and mean pore radius (nm) of membranes obtained at various magnifications

Membrane Magnification (×) Pore density Porosity (%) Pore radius

Mean valuea S.D.

PES30 30000 6.4± 1.7 3.6± 1.3 4.4± 0.2 1.950000 3.9± 1.0 3.9± 1.4 5.6± 0.3 2.380000 6.4± 1.9 4.6± 1.7 4.8± 0.3 2.2

100000 5.3± 1.2 2.7± 1.1 4.1± 0.3 1.9

PES100 80000 2.1± 0.5 1.3± 0.3 4.5± 0.1 1.980000 2.6± 0.5 1.7± 0.3 4.6± 0.04 1.9

PVDF40 50000 1.9± 0.6 11.4± 1.6 13.8± 0.1 8.450000 1.8± 0.4 16.4± 2.0 16.9± 0.2 10.850000 1.7± 0.2 14.2± 2.4 16.3± 0.8 9.880000 1.9± 0.3 10.1± 1.3 12.8± 0.5 9.180000 2.3± 0.8 13.8± 1.8 13.9± 0.2 7.7

PAN40 80000 6.1± 1.4 4.4± 1.7 4.8± 0.3 2.280000 7.4± 1.4 6.0± 2.2 5.1± 0.4 2.4

PAN50 100000 4.5± 1.8 2.8± 1.3 4.5± 0.2 2.0

a The uncertainty indicated is forT ± 10.

the variations in pore density or in pore size, as theminimum is 1.3% for the PES100 (at 80,000×) andthe maximum ranges from 10.1 to 16.4%, dependingon the magnification, for the PVDF40. This membraneexhibits, at the same time, a high porosity and a lowpore density and, as a consequence, its mean poreradius is larger and exceed by far what is expected fora 40 kDa MWCO. Moreover, it will be shown laterthat this membrane has a dispersed population in poresize. The PES100 presents the lowest porosity owingto a low pore density.

It is noticeable that various membranes of ap-proximately similar molecular weight cut-off showa remarkably diverse range of porosities. This is notsurprising when the coagulation process is used forfabrication. The porosity will depend strongly on thenature of the polymer and dissolving solvents used,but also on the processing conditions.

The mean pore radius depends more than the poredensity or the porosity on the magnification used.This might be due to the uncertainty on the sur-face calculation which depends on the magnificationwhereas it does not occur in the particles counting.Values reported in Table 2 show that, with the excep-tion of the PVDF membrane, the pore radius falls inthe range 4–4.5 nm for all investigated membranes.This is expected for membranes of similar MWCO

like the PES30, PAN40 and PAN50, but not for thePES100. The pore radii range from 0 to 14 nm forall the membranes except for the PVDF40 membranewhose pores are larger and for which the pore radiirange up to 54 nm.

It is worthy of note to notice that a membranewith such large pores will function in retaining ef-fectively 50 kDa protein molecules. This could beexplained by the hydrophobic interaction and/or byelectrical interactions between surface charges bornby the membrane and the protein. Hydrophobic in-teraction occurs when a hydrophobic surface is incontact with water. In the vicinity of the hydropho-bic surface, the structure of water is enhanced. Thiscontributes to restrict the transfer of the solvent (lowhydraulic permeability) and the permeation of largemolecules, like proteins, in the capillaries. In addi-tion, at neutral or basic pH, the signs of the electricalcharges born by the polymer and the protein areprobably both negative and enhanced rejection isexpected. Moreover, the presence of an ionic atmo-sphere around any charged particle will increase itsapparent diameter by a quantity approximately equalthe Debye reciprocal length. All these phenomenacould contribute to lower the MWCO of the PVDFmembrane when permeation of proteins is used for itsdetermination.


Fig. 4. Pore size distribution of the PES membranes using a 0.5 nm interval:PES30 (at 100,000×); PES100 (at 80,000×).

The pore size distributions are shown on the bargraphs reported in Figs. 4–7. All the parameters calcu-lated from these distributions are gathered in Table 3.All results corroborate the preceding observations:the pore size distributions are unimodal and havelog-normal evolutions. In Fig. 5 (PVDF40 membrane)

Fig. 5. Pore size distribution of the PVDF40 membrane (at 80,000×) using a 1 nm interval:h image 1;j image 2.

and Fig. 6 (PAN40 membrane) measurements carriedout at two different locations on the membrane surfaceare displayed. No significant variations are observedon the bar graphs. Although calculations were per-formed with a sampling interval of 0.5 nm, the graphin Fig. 5 represents the distribution of the PVDF40


Fig. 6. Pore size distribution of the acrylic PAN40 membrane, interval: 0.5 nm:image 1; image 2.

membrane with a sampling interval of 1 nm for clar-ity. In the case of the PES30 membrane (Fig. 4), thedistribution is clearly shifted toward lower pore radiusvalues as compared to the PES100, in agreement withthe mean pore radius values reported in Table 2.

For each distribution, the number of pore radiuscomprised between 0 and 1 nm is zero since the poreswith size inferior to five pixels are not taken intoaccount for the reason given in the experimental part.As observed in Table 3, the highest value for themaximum frequency is observed for the PES30 andPES100 membranes and the lowest for the PVDF40.The width of the modal interval is 0.5–1 for the PES30,PVDF40 and PAN50, but larger values are observedfor the PES100 (2.0) and the PAN40 (2.5). The modal

Fig. 7. Pore size distribution of the acrylic PAN50 membrane, interval: 0.5 nm.

pore radius values are relatively close to each oth-ers: between 1.75 (for the PVDF40) and 2.75 nm(for the PAN40). Mean pore and modal pore radiiare not correlated, but their meanings are different.The usefulness of the modal interval and the modalpore radius in the domain of membranes is question-able. On the opposite, the extent of the distribution isnaturally correlated to the maximum pore radius.

4.2. Solute rejection data

In order to compare the pore size values obtainedby image treatment to those given by the Ferry’s law,the rejection rate of FID has been determined for mostof the membranes under study.


Table 3Characterization of the pore size distribution in term of maximumfrequency, modal interval (nm), modal pore radius (nm) and extent(nm)

Membrane Maximumfrequency (%)

Modal interval→modal pore radiusb

Extent

PES30 13.9 [2–2.5]→ 2.25 8.5PES100 13.1 [1.5–3.5]→ 2.5 9.5

PVDF40a 8.3 [1.5–2]→ 1.75 535.4 [2–2.5]→ 2.25 48

PAN40a 11.9 [1.5–4]→ 2.75 11.511.2 [1.5–4]→ 2.75 12.5

PAN50 10.2 [2–3]→ 2.5 9.5

a Reported values correspond to two different samples.b Model intervals are expressed within [ ].

The influence of FID concentration (0.025 and0.05 g l−1) on rejection data has been briefly inves-tigated using the PAN40 and PAN50 membranes.The retention observed at 0.8 bar with the PAN40membrane is 18± 1.5% for the FID20 in the concen-tration range 0.025 to 0.05 g l−1. When the FID40 isallowed to permeate, at a pressure of 0.4 bar, throughthe PAN50 membrane the retention is 57± 1% whenthe concentration in the feed varies from 0.005 to0.05 g l−1. No significant influence of the concen-tration was observed, indicating that no adsorptionphenomena occurred under this condition. The effectof the solute concentration on the other materialswas not studied and a concentration of 0.05 g l−1

was finally retained for both solutes to accommodateapparatus sensibility and product consumption.

In order to minimize polarization concentration ef-fects, low transmembrane pressures have been used.Extrapolation atP = 0 gives the true rejection valueR0. R0 values are 97.8 ± 2.6% (FID20), 97.9 ± 1.8%

Table 5Membrane thickness,Ak/1x parameter (from image analysis and diffusion experiments) and permeability of membranes

Membrane Thickness,1x (mm) Ak/1x (cm−1) Permeability,Lp (l h−1 m−2 bar−1)

Image analysis Diffusion

PES30 136.5± 1.4 2.0 0.7 177PES100 79.1± 5.4 1.9 1.9 208PVDF40 125.3± 1.0 9.6 7.2 162PAN40 185.4± 3.5 2.8 9.7 413PAN50 185.8± 1.3 1.5 12.5 316

Table 4Mean pore radius,rp (nm), calculated from solute rejection dataor microscopic image analysis

Membrane Solute rejection Microscopic images

PES30 – 4.1± 0.3PVDF40 – 13.4± 1.1PAN40 5.05± 0.1 4.9± 0.4PAN50 3.8± 0.3 4.5± 0.2

(FID40), 35±0.3% (FID20) and 82.5±6.1% (FID40)for the PES30, PVDF40, PAN40 and PAN50 mem-branes, respectively. The mean pore radii calculatedfrom rejection results via Eqs. (2) and (3) and, fora comparison purpose, values determined by meanof image analysis are both reported in Table 4. Asthe rejectionR0 approaches 100% for the PES30 andPVDF40 membranes, an accurate determination ofthe pore radius was not possible by this method. Inthe case of the PAN40 and PAN50 membranes, thepore radii values obtained by the two methods are infair agreement.

4.3. Comparison between Ak/∆x values obtained bydiffusion experiments and image analysis

Comparison of the surface porosity to membranethickness ratioAk/1x, obtained from microscopic im-ages and from diffusion experiments can be achievedby use of the membrane thickness values determinedby image analysis and reported in Table 5. TheAk/1xvalues obtained by diffusion experiments (Ak/1x)difand by image analysis (Ak/1x)im are also reported inTable 5. Large discrepancies are observed inAk/1xvalues between the two methods, except for thePES100 and the PVDF40 membranes. The largestdifference is obtained for the PAN50 (ratio 1/8).


The observed discrepancies could originate in thesurface porosity termAk, in the1x value or both. Thesurface porosityAk determined by solute diffusion isdifferent from the visible surface porosity in the pres-ence of pores of non-uniform cross section. The sameis true for1x measurements. The membrane thicknessdetermined using microscopic imaging does not takeinto account neither the anisotropy of the membrane,nor the tortuosity of the pores. Moreover, although thispath is the same for two molecules of same size andshape, interactions between the molecule and the porewalls could interfere. Hydrophobic and hydrophilicinteractions occur with hydrophobic and hydrophilicsolutes, respectively. The membrane permeability re-ported in Table 5, gives an indication on the natureof the membranes used. The two hydrophilic acrylicmembranes have the highest water permeability andat the same time, theAk/1x values determined bydiffusion are larger than those determined by imageanalysis. The reverse is true for the hydrophobic PESand PVDF membranes: they have a reduced waterpermeability andAk/1x values from diffusion arelower or equal to those deduced from image analysis.The pathway of hydrophilic solutes (salts, urea, for-mamide) is then facilitated through hydrophilic acrylicmembranes and, as a consequence, the apparent1xvalues will be lower leading to higherAk/1x values.

5. Conclusions

The porosity, pore density, mean pore radius,pore size distribution, thickness andAk/1x param-eter of five asymmetric ultrafiltration membraneshave been determined by mean of image analysis.Values obtained from image analysis for the poreradius are found to be in good agreement with thosededuced from rejection experiments. On the oppo-site, theAk/1x values exhibit large variations from amembrane to another. These variations are probablyrelated to the tortuosity, the interconnection and thenon-uniform cross-section of the pores.

The image analysis is a pertinent method to measureall visualizable parameters such as the porosity, thepore density and the mean pore radius of a membranewhose pores are disconnected and respectively largerthan 2.26, 1.41 and 1.13 nm at 50,000×, 80,000× and100,000×. Such high resolutions are obtained as the

radius values are deduced from a surface area calcula-tion and not directly from the measurement of a lengthon the image. The membrane thickness can also beeasily determined as soon as some experimental pre-cautions have been taken. Drying is the main problemto be resolved in order to preserve the integrity of thepolymer surface, especially for hydrophilic materials.As the determination of theAk/1x parameter is notstraightforward, the use of complementary methods,such as, diffusion experiments and image analysis areof great help. Information, given by measurements,must be analyzed with care.

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Membrane characterization using microscopic image analysis