Characterisation and quantification of membrane surface properties using atomic force microscopy: A comprehensive review

DES-12244; No of Pages 15 Desalination xxx (2014) xxx–xxx

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Characterisation and quantification of membrane surface properties using atomic force microscopy: A comprehensive review Daniel Johnson a, Nidal Hilal a,b,⁎ a b

Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Swansea SA2 8PP, United Kingdom Qatar Environment and Energy Research Institute (QEERI), Qatar Foundation, Doha, Qatar

H I G H L I G H T S • • • • •

This review covers the technique of AFM and its application to the characterisation of membrane surfaces. AFM’s wide range of application makes it a versatile tool for the study and characterisation of membrane surfaces. AFM is of great value in assessing the fouling propensity of surfaces under different conditions. It is a high resolution imaging technique up to sub-nanometre resolution. Measurements can be made in ambient and liquid environments.

a r t i c l e

i n f o

Article history: Received 24 July 2014 Received in revised form 20 August 2014 Accepted 23 August 2014 Available online xxxx Keywords: Atomic force microscopy Membrane Characterization Fouling Biofouling Desalination

a b s t r a c t In recent years the technique of atomic force microscopy (AFM) has demonstrated a versatility in the characterisation of surface morphology and interaction forces. It is a high resolution imaging technique, using a sharp probe as a stylus to feel the surface in three dimensions to sub-nanometre resolution and which can work in air or liquid environments with no special surface preparation. It can also be used to probe the interaction forces between opposing surfaces, detecting both long range and adhesive interactions. It’s wide range of application make it a versatile tool for the study and characterisation of membrane surfaces, not just through production of high resolution imaging, but through the production of quantitative data such as surface roughness, surface pore size and pore size distribution and quantification of foulant – membrane interaction forces. This latter is of great value in assessing the fouling propensity of surfaces with different foulants and under different conditions. This review covers the technique of AFM and its application to the characterisation of membrane surfaces. © 2014 Elsevier B.V. All rights reserved.

1. Introduction With an ever expanding global population and growing industrial capacity, access to clean water is due to become an increasingly critical issue, both for creating safe drinking water and in the provision of adequate supplies for agricultural and industrial consumption, [1]. Recent projections suggest as many as 3.9 billion people worldwide are expected to be living in severely water stressed areas by 2030 [2]. Whilst this is a global problem with the most severe cases likely to occur in arid countries and developing nations, access to water is also a potential problem for developed countries. Deficits may be reduced by the desalination of brackish and saline water and by the recycling and reuse of municipal, domestic and industrial wastewater leading to a reduced demand on ⁎ Corresponding author at: Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Swansea SA2 8PP, United Kingdom. E-mail address: [email protected] (N. Hilal).

natural freshwater sources. One family of technologies which is of crucial importance in the production of safe and clean water from contaminated and otherwise non-potable sources are membrane filtration based technologies including nanofiltration (NF), reverse osmosis (RO), forward osmosis (FO), membrane bioreactors (MBR) and membrane distillation (MD) [3]. However, fouling of membranes used in such processes by inorganic, organic and biological foulants is the primary barrier to their more efficient and effective operation. Fouling leads to a catastrophic decrease in membrane flux causing a need for increased trans-membrane pressures to maintain water flow rates, chemical pre-treatment of feed-water, and downtime of water treatment plants to allow membrane cleaning. Fouling by microbiological organisms is considered to be the greatest cause of flux decline and loss of salt retention in reverse osmosis membranes [4]. For seawater RO alone, only one of many membrane treatment processes, the market for pre-treatment chemicals to prevent biofouling was estimated at $500 million in 2011 and is expected to rise to almost $1 billion by 2016 [5]. For MBR it has been estimated that 30-50% of

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Please cite this article as: D. Johnson, N. Hilal, Characterisation and quantification of membrane surface properties using atomic force microscopy: A comprehensive review, Desalination (2014), http://dx.doi.org/10.1016/j.desal.2014.08.019

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power consumption costs are due to the need to aerate the membranes to mitigate biofouling [6], a significant amount if this technology is to be more widely used to treat industrial and domestic effluent. All of these effects serve to increase the operating costs and power consumption of water treatment facilities using membrane based technology and to a decrease in the operating lifetimes of the membranes used. One major route to prevention or reduction of fouling of filtration membrane surfaces is by the modification of the physico-chemical properties of those membrane surfaces to increase foulant rejection and to reduce adhesive interaction forces, either by creating new experimental membranes with new properties or by the modification of the surfaces and active layers of existing commercially available membranes. Since its invention was first reported in 1986 [7], the atomic force microscope (AFM) has proven itself to be a useful and versatile tool in the field of surface characterisation. As an imaging technique it is capable of creating three dimensional representations of surface down to sub-nanometre scale resolution, with even atomic resolution possible under the right conditions. It is capable of imaging not only in vacuum, but under ambient conditions and liquid environments, allowing surfaces normally operational in aqueous environments to be viewed under a variety of conditions. This is of great interest when investigating membrane surfaces, because this means that the effects of such things as solution pH, ionic strength etc. on membrane surface morphology can be investigated. The AFM instrument itself is centred around a probe consisting of a sharp tip mounted on the end of a flexible micro-cantilever. By raster scanning this probe tip over a surface the topography of the sample can be profiled. In addition because of the mechanical nature of the imaging process, interaction forces between probes and samples can be measured as a function of separation distance. This is of great advantage when the probe is chemically functionalised or replaced by a colloidal particle, because this allows direct and quantitative measurement of foulant membrane interactions to be measured.

2. AFM operational principles At its core the AFM consists of a sharp probe mounted upon a flexible micro-cantilever arm. A diagram showing the typical set-up of and AFM instrument is shown in Fig. 1. These are typically fabricated from silicon or silicon nitride, depending upon the particular application to which they are intended. The probe tip is most often in the form of a cubic

based pyramid or cone, although some tips may have a spike of high aspect ratio or a carbon nanotube mounted at the end for specialist high resolution imaging applications, particularly if the sample to be imaged contains steep sided pits. Cantilevers are most often manufactured from Si, for tapping mode in air and non-contact mode probes, or from silicon nitride for most other applications. Typically the upper surface of the cantilever is often coated with a thin metallic layer to increase its reflectivity, usually with Au or Al. Some SEM images of typical probe tips and cantilevers are shown in Fig. 2. For typical commercially manufactured AFM set-ups the cantilever is moved in three dimensions by means of a piezo crystal which either the probe (tip-scanning configuration) or sample (sample scanning) is mounted upon. For most AFMs a single piezo crystal in the form of a hollow cylinder is used. By applying voltages across the crystal it can be made to flex or extend in all three dimensions. Less commonly a tripod configuration is used, where movement is achieved by extension and retraction of three separate piezo crystals, each mounted along one of the axes of movement. When the sharp probe tip interacts with a surface the forces exerted cause the cantilever to flex, with the deflection linearly proportional to the magnitude of the force felt by the tip. For the majority of AFM instruments this deflection is detected using an optical lever system, with a laser reflected from the back of the cantilever onto a position sensitive photodiode, typically divided into four sections. Any change in the deflection of the cantilever, either from flexing along the long axis or torsionally around the long axis, will cause a change in the position of the laser spot upon the photo-detector. The difference in the signal received by the top two versus the bottom two quadrants will give the deflection (in V or A) of the lever along the long axis, whilst the difference in signal between the left two and right two quadrants will allow detection of torsional or lateral bending. When the probe has come into contact with the surface of interest it may be raster scanned across that surface. As the sharp probe encounters topographic features it will be caused to bend, allowing a three dimensional picture of the surface form to be built up. The next section will describe image acquisition and the major imaging modes in more detail.

2.1. AFM imaging modes A large number of imaging modes are now available, providing a range of information about the sample surfaces, in addition to providing

Fig. 1. Diagrammatic representation of the set-up of a typical AFM. The probe tip is mounted on the underside of the apex of a flexible microcantilever. Deflection of this microcantilever is detected by an optical lever consisting of a laser reflected off the back of the cantilever onto a quadrant photodiode.

Please cite this article as: D. Johnson, N. Hilal, Characterisation and quantification of membrane surface properties using atomic force microscopy: A comprehensive review, Desalination (2014), http://dx.doi.org/10.1016/j.desal.2014.08.019

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Contact Mode Non-contact Mode Tapping Mode

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due to sample deformation and stick-slip movement of the probe over the surface. Deformation for the sample surface will necessarily lead to increased contact area between the probe and sample, reducing the maximum resolution which may be achieved. When soft and therefore easily deformable samples are to be examined dynamic modes of imaging are usually preferable. 2.3. Tapping mode (intermittent contact) mode

Fig. 2. Representation of the force regimes under which the three basic AFM imaging modes occur: contact mode occurs in the repulsive regime where hard contact is maintained between the imaging tip and the sample; intermittent contact repeatedly engages and disengages with the sample surface, but maintaining lower interaction forces with the surface; non-contact mode operates in the attractive regime, feeling the attractive van der Waals forces between tip and sample, enabling profiling of the surface with minimal surface deformations.

detailed three dimensional topographies. In the interests of brevity we will describe only the most common imaging modes here, contact mode, tapping (or intermittent contact) and non-contact mode. Fig. 2 shows the force regimes under which these three modes occur are illustrated. Here the interaction forces between tip and sample are sketched as a function of separation distance, sketched over a plot of LennardJones potential, with repulsive forces shown as positive. 2.2. Contact mode Contact mode is the simplest and most basic imaging mode available. During imaging the probe tip is in constant contact with the sample surface and thus operates in the repulsive regime (Fig. 2). Most commonly, the sample is imaged with a constant force maintained between the probe and the sample. As the probe encounters different heights in the imaged surface the cantilever becomes deflected away from the set-point. The set-point is the deflection value inputted by the user into the control software prior to making tip-sample contact. When the sample surface is first approached the sample and tip are moved towards each other using a stepper motor until the set-point is reached. The deflection is detected and an electronic feedback loop is used to adjust the height of the cantilever chip, using the scanner piezos, to restore the cantilever bending to its original value (the set-point). The height with which the probe is adjusted is recorded as the height variable (z) in the scanned image. One alternative imaging method which may be employed with smooth hard surfaces is to keep the probe chip at a constant height and instead use the measured deflection of the cantilever as the height signal. In addition to height other signals may be recorded, including deflection (an error signal due to delay in the feedback mechanism) and lateral (or torsional) bending of the lever. This lateral bending measurement can be especially useful for recording frictional force maps of the imaged surface [8,9]. Due to its relatively simple operation contact mode may be preferred when imaging a relatively smooth and hard surface. However, it has several drawbacks when imaging soft or rough surfaces, for which reason more advanced imaging modes have been developed. This is particularly a problem when imaging soft polymer membranes [10] or loosely adhered or friable foulant layers. When the probe traverses the surface lateral forces can occur from the probe meeting steep edges, or from frictional and adhesive forces, which may lead to wear damage to the probe or sample. This leads to a decrease in image resolution

Tapping mode (also known as intermittent contact mode or amplitude modulation AFM, AM-AFM) is a dynamic imaging technique which was developed to overcome the inherent limitations of contact mode [11–13]. With this mode of operation the cantilever is allowed to oscillate at close to its resonant frequency sufficiently close to the sample surface such that it repeatedly makes contact and disengages. Imaging frequencies are typically on the order of several tens of kHz. The interaction of the probe tip with the surface is monitored by observing the oscillatory amplitude, which will be restricted by increasingcontact with the sample surface. By adjusting the height of the probe chip to maintain this signal at a constant, whilst raster scanning over the surface, the surface topography may be built up. In addition to height, other signals may be recorded, including the amplitude or phase signals, which may show surface features with greater clarity than the height image alone. In particular the phase signal is of great interest. This phase signal is the difference between the drive frequency and the actual oscillation frequency, which will change as the probe encounters not only different surface geometry, but also is affected by adhesive and mechanical properties of the surface [14]. As a result phase images may show contrast between surface domains with different material properties. However, as the phase data is affected by a complex interplay between a number of parameters including tip-sample adhesion, long range interaction forces, scan speed, load force, topography and elastic compliance of the sample, it is difficult to extract quantitative information [15]. 2.4. Non-contact mode For non-contact mode (also termed frequency modulation or FM mode) the cantilever is oscillated at a smaller amplitude than for tapping mode. Long range interactions, such as van der Waals and electrostatic forces, occur between atoms in the probe tip and the sample which causes a detectable shift in the frequency of the cantilevers oscillations. Adjusting the height of the probe relative to the sample using an electronic feedback mechanism allows the height information to be built up during raster scanning of the sample surface [16]. Because the probe does not make hard contact with the sample surface, operating in the attractive range (see Fig. 2), the interaction area between the surface and the probe tip is minimised, allowing potentially for very high resolution which can be as high a true atomic resolution under appropriate conditions [17]. However, to achieve the best images with the highest resolution using non-contact mode is a greater technical challenge than with tapping mode. 2.5. Effect of probe Tip geometry on imaging – common imaging problems The geometry of the tip of the probe interacting directly with the surface of interest is of major importance when imaging at high resolution, as well as when carrying out nano-indentation and force distance measurements. The ability of the AFM depends upon both the sharpness of the imaging asperities of the probe tip and upon it’s aspect ratio, the latter being important hen imaging samples with steep sided features. Interaction of the probe geometry with surface asperities can lead to several common imaging problems. For instance, damage to the probe tip or contamination can directly affect the quality of images in a number of ways. Firstly a tip which is blunted from wear or addition of a contaminating layer will suffer

Please cite this article as: D. Johnson, N. Hilal, Characterisation and quantification of membrane surface properties using atomic force microscopy: A comprehensive review, Desalination (2014), http://dx.doi.org/10.1016/j.desal.2014.08.019

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from lower resolution in the x,y plane, as well as a greater prominence of tip convolution effects. Tip convolution arises when asperities on the imaged surface are sharper than the probe. In effect the probe ceases to image the surface and is itself imaged by these sharp asperities [18]. Such a situation is usually quite obvious, where a repeating pattern of the same shapes appearing all over the image, which represent repeated images of the probe tip. Another common imaging problem arising from damaging to the probe tip is multiple imaging. When the probe tip is damaged instead of a single sharp imaging asperity there may be two or more. This results in faulty images where sharp surface features appear multiple times adjacent to each other. Again this is often quite obvious making the images appear to have “double-vision”. These conditions can only be remedies by discarding the probe and starting again.

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2.6. Quantification of interaction force

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A particularly useful application of AFM, and one of particular relevance to the assessment of the resistance of membranes to fouling, is its use as a quantitative sensor of interfacial forces. The use of AFM as a sensor of interaction forces has been utilised in the study of many different materials and conditions including the nano- and micromechanical properties of materials [19–24]; surface adhesion forces [24–29]; long range van der Waals and electrostatic forces [30–33] and the bonding strengths of bio-molecules [34–36]. Interaction force data is typically presented as a plot of force versus probe–sample separation distance. Raw data for the interaction force is obtained as cantilever deflection, presented in the units of the photo-detector (commonly in V, occasionally in nA). The probe sample separation distance is not directly measured, it must instead be calculated from the cantilever deflection and the distance moved by the piezo in the z direction. A typical force-interaction curve, both raw data and force versus separation distance is shown in Fig. 3. The red trace shows the deflection of the cantilever as it approaches the surface, whilst the retraction of the cantilever from the surface is shown in blue. At large separation distances interaction forces between the probe tip and surface are minimal and a net force of zero is measured (point 1, both traces), its “free-level” (assuming no hydrodynamic drag forces on the cantilever are significant). As the probe-sample separation decreases long range interactions between probe and sample cause the cantilever to be deflected towards or away from the sample. If attractive forces are sufficiently strong they will overcome the restoring force of the cantilever, causing the probe to rapidly jump into contact with the surface. As the probe makes hard contact with the surface the cantilever is deflected upwards linearly (point 2). As the cantilever is retracted it follows the same profile as the approach whilst still in contact (although some situations, such as plastic deformation of the sample and interaction of the probe with surface roughness features may cause some hysteresis). If there are adhesion forces present then the probe tip will maintain contact with the surface, causing a downwards deflection of the cantilever, until sufficient force is applied via the cantilever spring to cause detachment. This is seen as a hysteresis between the approach and retract traces around the region of initial contact (point3). The force at which the probe tip detaches from the surface is typically recorded as the adhesion force for this particular interaction. Due to variation in the forces measured, particularly between different points on the sample surface, a large number of interaction cycles may need to be recorded to assess interactions between two materials. When both the sample and probe are hard and undergo no appreciable deformation under the forces experienced when in contact, then the movement of the piezo in the z direction will be equal to the deflection of the cantilever. As a consequence, dividing the deflection values (in V) by the slope of the contact region of the force interaction curve will provide force values in terms of the deflection distance of the cantilever (in nm). Subsequent multiplication of the deflection (nm) by the force

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Fig. 3. Example force distance curves. a) shows raw uncorrected data showing cantilever deflection in volts versus distance moved by piezo actuator. b) show corrected data to show actual force versus probe sample separation distance.

constant of the cantilever will convert the deflection into actual force values. In addition, subtracting the deflection distance from the zpiezo displacement for each point will allow determination of the distance travelled by the cantilever. After this step the contact slope should become vertical. The contact point is commonly set as the point of zero separation when presenting the data as force versus separation. One must be careful to consider that factors such as polymer and fluid over-layers and interaction forces may make determination of the point of zero-separation difficult. Fig. 3a shows the force curve in its raw unprocessed form as a plot of deflection in V versus zdisplacement in nm. Fig. 3b shows the same data after correction to show the actual force vs. separation. Fig. 4 shows part of the approach curve as a standard silicon nitride imaging probe tip approaches a Cyclopore membrane in water with different concentrations of dissolved NaCl [37]. At all concentrations the interaction is repulsive (positive force) at all separation distances. It can be seen that the range and magnitude of the repulsive forces increases as would be expected from standard DLVO theory [37]. By replacing the sharp scanning probe tip of a conventional AFM cantilever with a micron scale particle a colloidal probe may be created. Such probes are of great interest in the field of membrane fouling because probes can be created using materials which typically foul membranes, or by coating a microsphere with foulant chemicals. These probes can then be used to carry out force measurements to assess the adhesion forces between typical foulants and membrane surfaces, under various environmental conditions.

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Fig. 4. Repulsive forces between a silicon nitride imaging tip and a Cyclopore membrane in varying NaCl concentration solutions. Greater NaCl concentration leads to reduced repulsive forces due to charge shielding effects. Figure reproduced from Bowen et al. [37] with permission from Elsevier.

The use of colloidal probes was first reported by Ducker and colleagues [38,39] who used a 3.5 μm diameter silica sphere to measure its interactions with a polystyrene surface as a function of solution ionic strength and pH. By measuring the long range Derjaguin-LandauVerwey-Overbeek (DLVO) forces under different conditions it is possible to estimate the ability of a membrane surface to reject attachment of a colloidal foulant to the surface, whilst measurement of adhesion forces can give information about the strength of foulant-membrane attachments. This is of particular use when modifying membrane surfaces to resist fouling. From adhesion measurements it can be seen whether a membrane is likely to be fouling resistant using only a small sample of membrane without the need for fabrication of larger amounts of membrane which may be expensive. For example Hilal et al. [40] measured adhesion forces for PES membranes both unmodified and modified with 2dimethyl-aminoethylmethacrylate (qDMAEMA). It was found that the modification reduced the adhesion forces using a silica sphere from 39.5 mN/m to 24.8 mN/m, suggesting that the modified membrane would be more resistant to fouling by silicate particulates. 3. Visualisation and characterisation of membrane surfaces Imaging of the surfaces of samples of interest primarily involves obtaining high quality scans of surface topography, which consist of a three-dimensional array of height information. From this data a number of measures describing surface morphology, including surface roughness parameters, pore size and the pore size distribution, as well as showing in fine detail the morphology of fouled and unfouled membrane surfaces and the effects of chemical modification of membrane surfaces [10,41–47]. 3.1. Characterisation of surface roughness Surface roughness plays a major role in the modulation of fouling of membrane surfaces due to the effect of roughness on the surface area available to foulants. In general it has been observed that the greater the degree of surface roughness the greater the surface area available for foulants to adhere to, so much effort has been put into reducing fouling by reducing surface roughness. However, the influence of surface roughness is not simple and the amount of surface area available for fouling by particulates can depend upon the interplay between shape and size of surface topography and the size scale of the foulant particulates themselves and their own surface roughness. In effect the greater the area of contact between the foulant and the membrane the

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greater adhesive forces between the two will be, with roughness a parameter which will greatly affect this. Surface roughness is essentially the variation in height of an area of the surface of interest, and the numerous surface roughness parameters which may be quoted in the literature are merely statistical descriptions of this variation in height. Most common measures are the roughness average (Ra or Sa) and the root mean squared surface roughness (RMS, Rq or Sq). Where measurements are made from a line scan, the two dimensional roughness measure is abbreviated to the form Rx. Three dimensional measures taken over and area of the surface are of the form Sx. Roughness can provide valuable information for the study of fouling on membrane surfaces, including quantification of fouling of a membrane surface, as the clean membrane is often much smoother than layers of fouling particulates. However, the interplay between surface fouling and roughness is complex and the effects of roughness on membrane fouling can vary depending on the foulant membrane interaction. The most common roughness parameters found in the literature are the average roughness (Sa) and root mean squared roughness (RMS or Sq). The roughness average is the mean absolute deviation from the mean sample height: Sa ¼

n X m   1 X   Z  mn j¼1 i¼1 ij

ð1Þ

Here Zij is the height of an individual pixel, m and n are the number of pixels in the x and y directions respectively. RMS roughness may be obtained from the scanning data by the following relationship: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u m X n   u 1 X 2 Sq ¼ t Z xi ; y j mn i¼1 j¼1

ð2Þ

It should be noted that the magnitude of both of these measurements are dependent upon the size of the scan it is calculated from [10,47-49]. As larger scan sizes are used then larger surface features may be apparent, leading to a greater range of height values in a particular image. As these two measures of surface roughness are statistical descriptions of height variation within a scan it follows that increased scan size will tend to lead to an increase in these roughness parameters. In practice this is a problem for all surfaces, and effectively means that roughness values are only meaningful when comparing values obtained from scans of identical x and y dimensions. Slightly lower RMS roughness values for membrane surfaces have been noted when imaging in tapping mode than those measured for the same samples with contact mode [46]. This is likely due to the relatively sharper tip typically used for Si tapping mode levers, compared with the Si3N4 contact mode levers, providing a slightly different imaging resolution. Roughness can also be described by a ratio of the actual surface area compared with the area of the projected two dimensional plane of the image. This may be presented as a percentage, but if presented as a simple decimal then it is Wenzel’s roughness value [50], which can be related to the effect of roughness on observed contact angle [50–54]. Measurements using this measure have found that whilst it is less affected by the image size than Sa and Sq, there is a general decrease in its value over a range of increasing image sizes [10]. Another measure of surface roughness which may be applied to the study of surfaces is the fractal dimension, which has been previously reported in the literature to be scale invariant [10,55,56], however it has not been applied much as of yet to the study of membrane surfaces. Other measures of surface roughness are skewness and kurtosis, both of which describe the deviation of the height distribution from a normal, or Gaussian, one. The skewness, Sk represents the deviation of the pixel height distribution in the AFM image from a Gaussian or

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normal distribution. The skewness can be determined from the height distribution of the image pixels by the following relationship [57]:

Sk ¼

3 X Z i −Z avg

"

ð4Þ ðN−1Þ  Sq If the height distribution is a Gaussian distribution then the value of skewness will be zero. If the distribution has a positive tail the value will be positive and negative for a negative tail. Essentially, for surfaces containing small sharp asperities, the value is likely to be positive, for a pitted surface (such as for an otherwise flat surface containing pores) it will be negative. Kurtosis is another statistical measure of the surface shape, which gives information on how sharply data is distributed about the mean value: kurtosis ¼

n 1 1X 4 Zj 4n Sq j¼1

of the data, by fitting the following equation to the histogram of pore diameters [62]:

ð5Þ

Essentially, for surfaces with a positive value of kurtosis the height distribution for the imaged surface has a sharp peak with wide tails, for a negative kurtosis values the peak has a broad flattened peak with narrow tails. The normal or Gaussian distribution has a kurtosis value of zero. For a thorough characterisation of the roughness of a particular surface it is often best to make scans at a range of different sizes, with preferably a few orders of magnitude difference between the smallest and greatest. It is also important that the roughness parameters are obtained after the images have been processed to remove any background slope from the image which is otherwise likely to produce significant errors. This is a feature typically included as standard in instrument software. When comparing values for different images it is thus necessary to ensure that all have been processed in the same way. 3.2. Measurement of surface pore size From a scan of the surface of a porous polymer membrane the dimensions of individual pores can be measured to allow calculation of the mean surface pore size. Providing that enough pores are visible in the image, the surface pore size distribution can also be calculated. For example, Fig. 4 shows a 3D, or perspective, representation of a Cyclopore microfiltration membrane surface [58,59]. Images are shaded by feature height with lower features dark and higher ones lighter, showing pores as dark patches on the surface. Many pores are readily apparent, with enough present to measure a pore size distribution for this image (histogram on right). The average pore size here is approximately 200 nm in diameter. It should be noted that the pore sizes measured in this manner may show some disagreement with those obtained through other methods, such as the membrane transport method or porosimetry techniques [60,61]. These techniques effectively measure the minimum size of the pores, which may lie inside the membrane material. However, because the AFM method examines the membrane surface it only provides information about the size of the pore openings, which may be much larger than the minimum size of the pore. In addition convolution effects may be present due to interaction between the geometry of the probe tip and the nanoscale surface structures. This may lead to the apparent widening of features standing proud of the membrane surface and narrowing of small pits. These convolution effects will become more severe when the size of the features being examined approach the size of the imaging aperity present at the probe tip. One must always be aware of the potential for these effects when studying a membrane surface at fine resolution using AFM and when comparing AFM obtained pore-sizes with those obtained through other techniques. Given a sufficiently large number of observed pore-sizes, the pore size distribution can be obtained, assuming a log-normal distribution

 2 # dp 1 %f ¼ % f max exp − 2 ln X0 2σ

ð3Þ

where %f is frequency, %fmax is the maximum frequency, σ is the standard deviation, dp is measured pore diameter and X0 is the modal pore diameter. The monitoring of changes to the pore size and pore size distribution is one way of monitoring changes to the surface structure of membranes in response to modification. Figs. 5 and 6 show four different membrane surfaces and histograms showing the related pore size data from these images for polyether sulfone (PES) membranes, both unmodified and modified by quaternary 2dimethyl-aminoethylmethacrylate (qDMAEMA) for the purposes of minimising biofouling [40,63,64]. For the four surfaces shown in Fig. 5 the successive degrees of modification (DM) are 0, 202, 367 and 510 μg cm− 2. As can be seen the appearance of the surfaces is greatly affected by the modification with pores less and less evident, as can be seen from the distributions shown in Fig. 5. The initial unmodified membrane showed a wide pore size distribution with a σ value of 0.56 μm. Grafting of qDMAEMA leads to a narrowing of the PSD and a shift of the fit to smaller pore sizes. It was also noted that as the DM was increased the surface roughness also increased to a marked extent [63,64]. 3.3. Development of novel membranes Imaging of the membrane surface using AFM has great potential to aid the membrane technologist during membrane development by monitoring changes to the physical structure of the surface and by quantitative measurement of parameters including roughness [65]. For instance Shiraz et al. [66] studied electrospun polystyrene membranes which were modified by a contact heating method. After heating in a vacuum oven the electrospun membranes showed a decrease in surface roughness (for RMS, roughness average and peak to valley range), had a more uniform and more circular pore geometry. At the same time the membranes became more hydrophobic (as measured by water contact angle), which contradicts the behaviour predicted by Wenzel’s theory [50,67] for homogeneous surfaces, where increased surface roughness is expected to lead to increased measured contact angle. The authors explained this by a combination of the capillary effect and the Cassie-Baxter effect [68]. Here the smaller pores in the less rough heat treated membrane remain filled with air, rather than fill with water, leading to a surface which is more hydrophobic overall. Surface skewness and kurtosis of the image height distributions were also considered. Negative skewness values were observed for the non-heat treated membrane which became less negative to positive for the heat treated membrane (depending on image scan size), reflecting the smaller pore and thicker fibers observed with AFM and SEM. In addition the kurtosis values for the heat treated membrane were lower, reflecting the flatter surface of this membrane. The surface modification of reverse osmosis membranes intended for seawater desalination to reduce fouling was studied by Matin et al. [69]. The effect of the addition of a polymer layer to a commercial aromatic polyamide RO layer on the surface morphology was investigated using AFM techniques. Changes in surface roughness were found to be negligible, suggesting little physical change in the surface profile. This was put down to the initiated chemical vapour deposition method used to apply the polymeric overlayer leaving a thin, smooth and even film. Similarly, when PVDF membranes were modified with polyethylene glycol by Chang et al. [70] the pore-structures were observed to become increasingly covered with high molecular weight poly ethylene glycol. The surface was also seen to increase in roughness as the

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Fig. 5. PES membranes imaged by AFM before and during modification by qDMAEMA a) unmodified; b) degree of modification (DM) 202 μg cm−2; c) DM = 367 μg cm−2 d) DM = 510 μg cm−2.

amount of modification increased. However the surface roughness decreased markedly once the over-layer became complete. When studying the effect of imaging mode and environment for hydrophobic and hydrophilic PES based membranes globular features were observed when using tapping mode in liquid on a moderately hydrophobic membrane (water contact angle 72°), which were not seen in either the tapping mode in air or contact mode in liquid or in any instance for a similar hydrophilic membrane [10]. Tapping mode and contact mode images taken in water are shown in Fig. 7a and 7b respectively. Whilst identical underlying features in the surface are visible in both images, these repeated globular features are not present in the contact mode image. These features are relatively circular in shape, resembling spherical caps and are approximately 0.1 μm across. A close up 3D representation of a single one of these features is shown in Fig. 8 a). One possible explanation of the features is the presence of small pockets of air adhering to the surface. Such features have been previously described in the literature on a range of hydrophobic surfaces and termed ‘nanobubbles’ [71–75]. Nanobubbles have been extensively described, with a number of reviews present in the literature [76–78]. Their presence on a hydrophobic surface and their absence on the same surfaces when imaged in air or with contact mode are both indications that the features observed on the hydrophobic PES membrane may be explained by the presence of nanobubbles. The equation of a circle was fitted to the cross section of one of the putative nanobubbles using a least squares fitting approach (Fig. 8b). The close fit (R2 = 0.89) shows that the bubble is in the shape of a spherical cap [71]. From this fit the contact angle of the bubble (angle in liquid and external to the bubble) may be calculated to be approximately 165°. This is more than twice the measured macroscopic contact angle of 72.6°. The reason for this difference is unknown, but a study reported Borkent et al. [71] noted that all previous investigations of nanobubbles using AFM had reported contact angles of between approximately 150° and 170°,

independent of the macroscopic contact angles for the materials examined. It was suggested that the reason for the discrepancy is the sensitivity of the method to organic contamination at the sample surface. In addition mechanical deformation of the air bubbles may occur during scanning, which will affect their observational profile. The existence of nanobubbles on hydrophobic surfaces has been used to explain the presence of the long range (N10 nm) hydrophobic force [79–81]. These interactions have implications in a number of situations, including the fouling of filtration membranes by hydrophobic organic particulates and in the separation of minerals using froth flotation [72,82,83]. These forces are probably better described as capillary forces [84], rather than true hydrophobic forces which occur at shorter ranges and have been attributed to interactions between overlapping solvation zones [84–86]. Nanobubbles have implications for the imaging of hydrophobic membranes using AFM due to their obscuring of the underlying surface. Whilst the contact mode images were inferior to the tapping mode images when in air, the presence of the nanobubbles caused the tapping mode in water images of the hydrophobic membrane to show the membrane surface features less clearly than the contact mode images. It should be noted that degassing of aqueous solutions prior to imaging will most likely reduce the size and hence effect of the nanobubbles, as observed in a number of other studies [80, 81]. However, maintaining the degassed state of imaging liquid for sustained periods of time may be impractical. Hendren et al. [87] studied the surface modification of nanostructured ceramic filtration membranes. Alumina anodisc membranes were modified by surface treatments with several polymers to increase surface hydrophobicity and were also compared with commercially available TF-200 membranes. AFM was used as part of a battery of techniques to assess changes to surface structure and morphology. Line profiles from the AFM scans were used to measure pore sizes and surface

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Fig. 6. Pore size distributions for images shown in Fig. 5: a) unmodified; b) DM 202 μg cm−2; c) DM = 367 μg cm−2 d) DM = 510 μg cm−2.

roughness was monitored by measuring maximum peak to valley distance (Rmax). A study of forward osmosis membranes by Niksefat et al. [88] examined the effect of the additions of SiO2 nanoparticles on morphology and performance. The modified membrane had a significantly rougher surface than unmodified membranes when comparing 5 x 5 μm images for several roughness factors including Sa, Sq and mean peak to valley distance. Surface roughness is often correlated with membrane performance factors. For instance Rana et al. [89] found a positive linear relationship between Ra and molecular weight cut-off when studying the performance of modified PES membranes when filtering aqueous PEG and PEO solutions, a relationship which has

been observed by other researchers investigating other systems [90–92]. This was explained as being due to membrane structural nodules formed from polymer aggregates being more loosely packed under certain casting conditions leading to both a greater roughness and greater molecular weight cut-off [89,93]. The same authors also noted an inverse correlation between Ra versus the static contact angle, which is in agreement with Wenzel’s theory of wettability which suggests that for a homogenous surface the measured contact angle will be decreased by an increase in surface roughness [50,67]. Kochkodan et al. [94] used a layer by layer method to modify commercial polyamide (PA) nanofiltration membranes. AFM was used to study the changes in surface morphology of the membranes. For all

Fig. 7. Globular features only seen with tapping mode in water on hydrophobic surface (a) – not with contact mode in water (a) or with either mode. No such features could be observed for any mode or with hydrophilic membranes. Images reproduced from Johnson et al [10] with permission from Elsevier.

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Fig. 8. 3D view of single ‘nanobubble’, alongside a line profile through the same feature (red dots) fitted with the equation of a circle (dashed line) showing that this feature approximates a spherical cap. Image reproduced from Johnson et al. [10] with permission from Elsevier.

membranes surface roughness progressively reduced with an increasing number of added layers. Fig. 9 shows AFM scans of NF-90 nanofiltration membrane (Dow) both unmodified and with 4, 8 and 12 treated layers. As can be seen the prominent surface structure of the membrane seen in the unmodified sample image are progressively lost becoming less distinct as roughness decreases. For this membrane the RMSA roughness decreased from approx. 28–10 nm for untreated and 12 treatments respectively.

3.4. Characterisation of fouling of filtration membranes using AFM One of the major fouling types encountered when desalinating saline or brackish water, for membrane separation based techniques as well as others, is fouling by dissolved inorganic species, or scaling, and there have been several studies of this phenomenon as a result. For instance Mi and Elimelech [95] studied scaling by dissolved silica and its reversibility when using forward osmosis membranes. Force

Fig. 9. AFM images of NF90 membrane: a) unmodified membrane; and layer-by-layer modified samples with a different number of PSScoMA/PEI layers: b) 4 layers; c) 8 layers; d) 12 layers.

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measurements were utilised to elucidate the mechanisms by which membranes become fouled by silica. These measurements were carried out between a silica particle probe and cellulose triacetate (CA) and polyamide (PA) membranes in presence and absence of dissolved silica. In the absence of dissolved silica the measured adhesion forces were larger when using the CA membrane than measured for the PA membrane. This was speculated as to be due to the difference in hydrogen bond forming abilities between the two different membrane types, which is important in forming adhesion between silica particulates and membrane surface [96]. Fig. 10, reproduced from [95] illustrates the greater density of hydroxyl groups on the CA membrane compared with the PA membrane, which are important for forming hydrogen bonds with corresponding hydroxyl groups on the silica particle. This difference explains the greater adhesion force reported for the CA membrane. When measurements were carried out in the presence of 4.2 mM dissolved silica the measured adhesion forces were much greater for both membranes, with adhesion being measured at markedly greater separation distances than in the absence of dissolved silica (Fig. 11). The explanation for this greater extension, which usually is the result of extension of material on the probe or surface of interest, was that silica gel was forming at the interface between the membranes and the silica probe. Due to interaction with surface hydroxyl groups silica is likely to condense on both the membrane and silica particle surfaces. In this instance greater forces were measured for interactions with the PA membrane than with the CA membrane, suggesting that the gel layer is more strongly bonded to this membrane than the CA membrane. This was presumably due to the expected greater interaction area between the gel and membrane surface due to a higher degree of roughness. When desalinating brine and brackish water the primary scalants are dissolved mineral salts including Ca, Mg and Si ions. The effect of dry out of membranes on their scaling was studied by GuillenBurrieza et al. [97] in conditions which simulate membrane distillation of sea water. To increase understanding of the surface phenomena related to the salt scaling of these membranes both imaging and force measurement investigations were carried out. Surface RMS roughness of unfouled PTFE membranes were greater than that of the fouled membranes. Conversely PVDF membranes became rougher after fouling. This reflected the different observed structures of the two different membrane types. The PTFE membrane had a much more open and less dense fibrous structure. When the PVDF membrane was fouled salt crystals were deposited on the surface of the membrane, leading to an increase in RMS roughness. With the PTFE membranes however, the salt crystals were deposited within the open membrane structure, eading to a flatter surface and decreased surface roughness. Further investigations were carried out into adhesion forces measured between a CaCO3 particle probe and membrane surface (Fig. 12). This approach

was previously used to simulate initial scaling between CaCO3 and metal surfaces and generated information on the effectiveness of antiscalants [98]. Adhesion forces between the probe and the PVDF membranes were higher than that measured with the PTFE membranes. This suggested an inverse correlation between unfouled surface roughness and measured adhesion forces. This was attributed to increased roughness leading to a decreased contact area between the CaCO3 crystal and the membrane surface. Similar contact angle values measured for the two unfouled membrane types rules out differences in wetting as a significant factor. However, the authors did not rule out differences in chemical interactions for the two membranes for contributing to differences in the measured adhesion forces. The measured adhesion forces also correlated well with the degree of fouling of the membranes during MD tests [97]. It was suggested that the more stable attachment of microcrystals suspended in the feed-water to the PVDF membranes surfaces compared with the PTFE surfaces as shown by the greater adhesion forces, will likely produce more favourable conditions for the initiation of fouling by mineral salts as nucleation sites for further crystal growth will be provided by attached microcrystals. Another major category of foulant which may affect membrane systems is fouling by dissolved organic materials. These can be the breakdown product of plant matter, such as humic and fulvic substances, or include proteins, fatty acids and polysaccharides, many of which may be found as extracellular polymeric substances (EPS) produced by microorganisms either as excretions or due to cell lysis. Due to their importance there have been numerous studies to elucidate fouling mechanisms and effects on membrane surfaces. Usually such substances are found in solution alongside suspended microbial cells, either as their extraction products, such as in the case of biofilms, or incidental to their presence, such as in river water. As a result fouling by microorganisms and by organic matter will often occur simultaneously. Adhesion measurements between actual living cells and surfaces are reported in the literature. The first instance of this was reported by Bowen and colleagues [99] who mounted a yeast cell (S. cerevisia) as a colloidal probe and compared the adhesion forces measured against two different membranes with those measured using a colloid probe coated with BSA. Measurement of the adhesion of cells to membrane surfaces may provide more meaningful information on biofouling of membranes than simple analogues of cells, due to the complexity of biological attachment to surfaces. However, there are some practical problems which have prevented the adoption of this technique becoming widespread. This includes the compression of the cell probe making quantification of forces more problematic. Simple surface modifications of colloidal probes have been previously used in an attempt to simulate bacterial adhesion to surfaces. A carboxylated latex particle probe was used by Herzberg et al. [100] to simulate interactions between a

Fig. 10. Force measurements and schematic of silica fouling mechanism between a silica particle and CA and PA membranes in absence of dissolved silica. Reproduced from [95] with permission from Elsevier.

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Fig. 11. Force measurements and schematic of silica fouling mechanism between a silica particle and CA and PA membranes in presence of dissolved silica. Reproduced from [95] with permission from Elsevier.

bacterial cell and reverse osmosis membranes fouled by EPS in the presence and absence of calcium carbonate in the feed solution. During approach only repulsive forces were measured with unfouled membrane, both in the presence and absence of Ca2+ ions, which was found to be due to electrostatic double layer repulsion. When EPS fouling was present an attractive “jump-in” of the probe onto the membrane surface was observed when Ca2+ was added to the solution. These jump-in events occur when the gradient of attractive forces become greater than the restoring force of the AFM cantilever [101]. It was concluded that this was due to binding of Ca2+ to carboxylate groups both on the surface of the probe and in the fouling layer [100]. This result was similar to measurements reported by Li and Elimelech using carboxylated particles measured against humic acid fouled membranes [102]. Also, when calcium ions were added the measured adhesion force was seen to increase dramatically, whether the EPS fouling layer was present or not. This increase was particularly marked when an EPS layer was present. For some measurements when EPS was present in solution no measureable adhesion was seen. This was attributed to interaction of the Ca2 + ions with carboxylate groups in the EPS layer. The same type of probe was also used to demonstrate a reduction in adhesion forces for a PVDF membrane when modified with a layer of grafted copolymer, with no attractive forces observed when the membrane was immersed in 100 mM NaCl solution [103]. Similar work by Bernstein et al. [104] also used a carboxylated probe to assess the fouling resistance of RO membranes with surfaces modified with graft polymerization using a number of the probe to the membrane surface were dominated by electrostatic double layer forces, as seen when the solution pH was changed to alter the charged state of the interacting

Fig. 12. SEM image of a CaCO3 particle used as a probe for adhesion force tests with PVDF and PTFE membranes used for membrane distillation.

surfaces. Adhesion forces were largely due to electrostatic mechanisms, hydrophobic/hydrophilic interaction and possible hydrogen bonding, with the explanations of being different for the different membranes, demonstrating the complexity of foulant membrane interaction mechanisms. Tansel and co-workers [105] used AFM to study the fouling of reverse osmosis membrane surfaces by EPS, which perform an important role in the attachment of bacteria during fouling [42,106]. A soft overlayer was observed to form over a sub-layer which consisted of discrete molecular units which were firmly attached both to each other and the membrane surface after less than one day of cross-flow filtration. The over-layer deposits were loosely adhered and could be removed by the action of the AFM tip. The authors also noted evidence of membrane compaction, with the imprint of the membrane spacer visible through the membrane after filtration [105]. Similar studies were carried out by Su et al. [107] who investigated the effects of EPS on membrane surface morphology. PES, PVDF and CA membranes were treated with a solution containing EPS obtained from activated sludge. All membranes showed an increase, with the relatively smooth CA membrane showing the greatest increase. Ang and Elimelech [108] studied the effect of feed solution chemistry on fouling of reverse osmosis membranes by fatty acids. Adhesion force measurements were carried out under conditions identical to those used for parallel cross flow filtration experiments, with measurements carried out using a CML particle probe in the presence of dissolved octanoic acid, which served as an analogue for fatty acids. When pH was not adjusted measured adhesion forces were insensitive to the Ca2+ concentration. When pH was increased then increased Ca2+ concentration decreased adhesion, which was deduced as being due to a reduction in the hydrophobicity of surfaces due to Ca2+ binding. The solution pH itself also affected the magnitude of adhesion forces due to its effects on the dissociation state of the octanoic acid, with adhesion forces being greater at higher pH values. The charged dissociated molecules which are more populous at higher pH values were speculated to be adsorbed onto the CML probe in a configuration which favoured greater hydrophobic interactions. The observed behaviour of the adhesion forces correlated closely with the fouling behaviour observed during cross-flow filtration tests. Evans et al. [109] used the colloidal probe technique to assess the efficacy of membrane cleaning on fouling by polyphenols present in black tea. Using a silica microsphere coated with a model polyphenol adhesion force measurements were carried out with regenerated cellulose ultrafiltration membranes. It was found that adhesion forces were greater for virgin and fouled then cleaned membranes than for fouled membranes. Pre-treatment of the membrane using sodium hydroxide reduced adhesion forces compared with the virgin and cleaned membrane, suggesting the benefits of membrane pre-treatment prior to filtration use. To assess the fouling potential of dissolved proteins on polymer membranes Bowen and colleagues [110] used a silica microsphere

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with an adhering layer of BSA to measure force distance curves between the probe and two different membrane types (ES 4040 and XP117). Significant adhesion forces were measured for both membranes, which were much higher than seen for silica alone. From this it would be safe to conclude that the fouling of membranes by silica particulates would be likely to be more severe and more rapidly occurring in the presence of dissolved proteins. This is a likely situation when filtering water from natural sources and wastewater, where protein components of EPS will be present. Other groups have focussed on the effects of EPS substances specifically. The chemical and physical processes involved in the fouling of a cellulose acetate (CA) forward osmosis membrane by alginate, BSA and humic acid (HA) were investigated in a comparative study by Mi and Elimelech [111]. A CML probe was used as a proxy for these foulants due to the large number of hydroxyl groups present on both these foulants and on the CML probe surface. The presence of Ca2+ ions was found to promote the adhesion of the CML probe by twofold in the presence of alginate, which was attributed to the formation of intermolecular bridges formed by the Ca2 + ions. On the other hand, when measurements were made in BSA solution there was no effect on measured adhesion forces by the presence of Ca2+. This was attributed to the low concentration of carboxyl groups on the surface of BSA, which reduces the number of cross-links which can be formed by the Ca2 + ions. For HA solutions, Ca2+ increased the measured adhesion force in a similar manner to that observed with alginate, but to a lesser extent. The authors reported that the measured adhesion force exhibited a strong correlation with the rate of fouling of membranes during filtration with these organic foulants. It was concluded that adhesion force can serve as a good indicator of fouling propensity. When using a CML probe to compare two different forward osmosis membranes and their resistance to alginate deposition [112], it was observed that distributions of adhesion values were much wider for the PA membrane than

for the CA membrane, which was attributed to the PA membrane having a more heterogeneous surface. The mean adhesion forces in this case showed no correlation with the relative fouling trends observed during membrane flow tests. It was reported that the mean adhesion force did not appear to be a good measure of fouling propensity when comparing different membranes. This was in contrast to the study of solution chemistry discussed earlier, where the mean adhesion force had been a reliable predictor of fouling propensity [108]. This was attributed to localised variations in interaction forces leading to ‘sticky’ sites accounting for areas on the PA membrane causing a high degree of fouling, despite having a lower mean adhesion force than the CA membrane [112]. When alginate was present in solution adhesion forces for the two membranes were similar, but adhesion forces were observed at a longer distance from the membrane for the PA membrane. The authors suggested that this was due to a greater adsorption of alginate onto this membrane meaning that larger complexes of polysaccharide are connecting the probe to the membrane than in the case of the CA membrane. In Fig. 13 (reproduced from [112]) representative adhesive forces are shown alongside diagrammatic representations of the proposed mechanisms leading to the long range interaction behaviour. In keeping with the proposed interaction model the CA membrane force curve shows a single pull-off event, whereas the PA force curve shows multiple pull-off events. In addition prior to each pull off event the PA retract curve shows the characteristic curve shape observed due to the entropic elasticity of macromolecules when stretched during force distance measurements [113]. An alternative approach to studying protein adhesion on membrane surface was reported by Zhan et al. [114], who used L-cysteine functionalised gold coated AFM probe tips as an adjunct for protein. Analysis of adhesive force and rupture distance from force curves carried out with different polymer membranes in parallel with membrane fouling studies led to the conclusion that electrostatic

Fig. 13. Adhesion force curves obtained using a CML colloidal probe measured with a) CA and b) PA membrane surfaces in the presence and absence of alginate, showing different mechanisms of surface fouling in the presence of Ca2+. Reproduced from [112] with permission from Elsevier.

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interactions with amine modified membranes were responsible for both greater measured adhesion forces and greater protein fouling propensity as seen for this membrane type. Johnson et al. [115] used a colloidal probe consisting of a polystyrene microsphere covalently functionalised with humic acid to probe adhesion forces between several nanofiltration membranes and foulants in both clean water and in model textile dye wastewater (MTDW). The membranes were fabricated by adding polymeric bicontinuous emulsion layers to a commercial PES ultrafiltration membrane, which formed a scaffold [116]. Measurement of adhesion forces allowed the membranes to be ranked according to lowest adhesion forces and hence greatest resistance to initial colonisation by humic acid particulates. However, the relative adhesive forces measured for the different membranes showed a completely different pattern in pure water than that observed in the MTDW. This was attributed to the components of the MTDW, which included a mixture of surfactants, salts, glucose and textile dyes [117], which would have chemically modified the surface of both the membranes and probe. The complex chemistry of both the MTDW and HA serve to make elucidation of the exact chemical changes difficult, but serves to demonstrate that when designing new membranes the chemical composition of the feed water to be treated with the membranes should be clearly considered. Nisola et al. [118] examined polysulfone (PS) ultrafiltration membranes which had been modified using polyether-block-polyamide copolymer (PEBA) and silver nanoparticles (nano-AG) to reduce bacterial fouling of membranes. The degree of fouling of membranes after surface contact tests using Escherichia coli were assessed by AFM measurements. For the unmodified PS membrane, fouling was assessed as severe, with a high degree of roughness of the membrane after tests. After modification of the membrane surface by PEBA only partial fouling was observed and almost none was visible in images for PS surfaces modified with both PEBA and nano-Ag. The authors noted that the PEBA improved resistance to fouling by both living and dead E. coli cells, prevented pore blockage and reduced flux decline, whereas the nano-AG prevented the formation of a stable biofilm. As a result the PEBA and nano-AG combined treatments were more effective than modification by PEBA alone. It was also observed that the presence of the nano-Ag in the membrane lead to a rougher membrane surface prior to fouling, which may have explained the larger than expected contact angle observed for the nano-Ag modified membrane. The effects of abiotic and biotic fouling of cellulose acetate (CA ultrafiltration) membranes on the surface morphology were examined by Zaky et al. [119], who discovered differences in the fouling of membranes when viable and inactive cells were used. As well as examining the roughness of the membranes (Sq), the skewness (Sk) of the roughness profiles was also of interest. After 4 hours of filtration with both active and inactive cells Sq and Sk were both reported to be similar to that of the unfouled membrane. For inactive cells the roughness stayed low after 11 h filtration and increased after 24 h. The Sk remained low throughout. When active cells were filtered after 11 h the roughness was comparable to that observed for the inactive cells after 11 h, but the Sk was seen to show a deviation from uniformity with a value of 1.3, suggesting an increase in the amount of asperities on the surface. After 24 h the roughness had increased, but the Sk had returned to near zero. The authors speculated that the difference in uniformity shown by the different Sk values at 11 h shows a difference in the composition of the adsorbed biofilm between the active and inactive cells [119]. A later study by the same authors found that biofilms, as monitored by increased membrane roughness, were more evenly distributed when using ‘synthetic’ water, consisting of organic molecules and bacterial cells in water, than was observed with biofouling from filtering tap water alone [120]. With tap water alone biofilms were uneven and had lower overall surface coverage, with a greater roughness observed on areas of the membrane near the permeate outlet compared with membrane surface near the flow inlet, attributed to the greater shear rates near the flow inlet. Skewness analysis also showed that the biofilm

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formed from the ‘synthetic’ water was more uniform than the tap water derived biofilm. This was attributed to the faster growth and production of EPS with the ‘synthetic’ water fouling. Some authors have reported the use of AFM as a complementary tool during membrane autopsy of fouled filtration plant membranes. Roughness measurements confirmed the efficacy of membrane cleaning in removing foulant layers from a reverse osmosis membrane [121] and the presence of bacteria adsorbed to the membrane surface [122]. This suggests a potential for AFM based techniques to be used in a membrane separation industry setting in addition to the research laboratories where it currently finds its home. 3.5. Modelling of surface properties using AFM derived data The quantitative data which may be obtained from AFM which describes the surface morphology of membranes can be used to inform computer models of fouling behaviour. Demneth et al. [123] used AFM imaging to evaluate different thermodynamic adsorption models by observing changes in surface morphology after fouling of microfiltration and ultrafiltration membranes by proteins, polysaccharides and humic acid. Chen et al. [124] used a number of parameters including the average roughness, RMS roughness, the peak count and the surface area difference (the ratio of the actual surface area to the plane scan area) which was used to construct mathematical models of CA, PVDF and PES membrane surfaces and to model their fouling by soluble microbial products (SMP). This data was then compared with actual laboratory filtration tests. The calculated interaction energies between SMPs and membrane surfaces showed deviations from that seen for ideal smooth surfaces. Hydrophobicity and zeta-potential together were found to be insufficient to predict membrane fouling behaviour. It was observed that rougher surfaces fouled more readily because shortrange effects on colloidal interactions and surface properties lead to a decreased energy barrier to attachment which resulted in rough surfaces being more suitable for colloidal adsorption. It was also concluded that the CA membrane had the highest fouling potential [124]. In contrast a study of the effects of roughness on fouling of PVC membranes, modified with pthalic acid esters, found that higher surface roughness prevented fouling by Staphylococcus aureus and E. coli. It was noted by the authors that complex interplay between roughness and wettability of the surfaces means that in this case the prevention of fouling may have been caused by heterogeneous wetting of the rougher surfaces [125]. 4. Conclusions Over the past few decades AFM and associated techniques have demonstrated an ability to characterise and quantify a number of factors pertaining to membrane surfaces and fouling of membranes of interest to anyone studying the performance and fouling resistance of such materials. Used in conjunction with other techniques it can play a powerful role in the development of new membranes with altered physical properties, particularly in the field of enhancing the ability to reject foulant molecules and particulates. As only small pieces of membrane are needed for tests, this opens up the possibility of producing relatively large numbers of different membrane samples for initial testing at relatively low cost before commencing with larger scale production of the most promising membranes for further testing by conventional means. Over the coming years there is still much room for the development of these techniques and better integration of them into the battery of characterisation methods currently used in the membrane fabrication industry. Acknowledgement We would like to acknowledge the Qatar Foundation for funding this work.

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