W. Richard Bowen and Nidal Hilal 4

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C H A P T E R 4 Investigating Membranes and Membrane Processes with Atomic Force Microscopy W. Richard Bowen and Nidal Hilal O U T L I N E 4.1 Introduction 108 4.2 The Range of Possibilities for Investigating Membranes 109 4.3 Correspondence between Surface Pore Dimensions from AFM and MWCO 113 4.4 Imaging in Liquid and the Determination of Surface Electrical Properties 116 4.5 Effects of Surface Roughness on Interactions with Particles 120 4.6 ‘Visualisation’ of the Rejection of a Colloid by a Membrane Pore and Critical Flux 123 4.7 The Use of AFM in Membrane Development 124 4.8 Characterisation of Metal Surfaces 126 4.8.1 Effect of Electropolishing of Steel Surfaces 130 4.8.2 Corrosion of Metal Surfaces 132 4.8.3 Biofilms at Metal Surfaces 135 4.9 Conclusions 135 Acknowledgements 136 List of Abbreviations 136 References 136 Atomic Force Microscopy in Process Engineering 107 © 2009, Elsevier Ltd
108 4. INvEsTIgATINg MEMbRANEs ANd MEMbRANE PROCEssEs 4.1 InTRODUCTIOn Membrane processes are one of the most significant developments in process engineering in recent times. The worldwide annual sales of membranes and membrane equipment are now worth in excess of 1 billion Euros. Membranes find widespread application in fields as diverse as water treatment, pharmaceutical processing, food processing, biotechnology, sensors and batteries. Membranes are most usually thin polymeric sheets, having pores in the range from the micrometre to sub-nanometre, that act as advanced filtration materials. This chapter is particularly concerned with pressure-driven membrane processes – microfiltration, ultrafiltration, nanofiltration and reverse osmosis. These are usually classified according to the size of materials that they separate, with ranges typically given as 10.0 � 0.1 �m for microfiltration (MF), 0.1–5 nm for ultrafiltration, �1 nm for nanofiltration (NF) and �1 nm for reverse osmosis (RO). The human imagination, including the scientific imagination, is highly visual. We are most easily convinced of the existence of phenomena and processes in the physical world if we can see them. The importance of seeing is deeply imbedded in language. Thus, when we finally understand some highly complex and abstract explanation, we may comment, ‘I see that now’. However, the size range of objects that the unaided human eye can directly observe is limited. We can use optical microscopes to extend the lower limit of this range down to objects of sizes comparable to the wavelength of light. Beyond this limit, we need to use other means to ‘see’. Atomic force microscopy (AFM) is one means of imaging objects of dimensions from about the wavelength of light to those below a nanometre. Thus, in the case of membranes, it is possible to visualise the membrane surface properties, such as pores and morphology, using AFM. Fortuitously, the size range of objects that may be visualised by AFM corresponds closely to the size range of surface features that determine the separation characteristics of membranes. However, the separation characteristics of membrane interfaces do not depend solely on the physical form of surface features. In liquids, surface electrical properties and the adhesion of solutes to membrane surfaces may also have profound effects on separation performance. It is thus exceedingly fortunate that an Atomic Force Microscope may also be used to determine both of these additional controlling factors. Finally, means may be devised to quantify all of these controlling factors in liquid environments that match those of process streams. The first intention of this chapter is to provide a concise review of the potential of AFM for the investigation of membranes and membrane processes, using examples from our own studies. The chapter will begin with illustrative examples that outline the range of possibilities of AFM
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xiv About thE Editors Professor Nid
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xvi List of Contributors Dr Daniel
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2 1. BAsIC PRINCIPLEs OF ATOMIC FOR
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20 1. BAsIC PRINCIPLEs OF ATOMIC FO
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32 2. MEASUREMENT OF PARTICLE ANd S
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Adhesion force (mN m -1 ) 20 15 10
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Adhesion force (mN m -1 ) 10 8 6 4
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5.3 MODIFICATION OF MEMBRANEs 161 F
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5.4 MODIFICATION OF MEMBRANEs wITH
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5.4 MODIFICATION OF MEMBRANEs wITH
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Å 200 100 0 5.4 MODIFICATION OF ME
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AbbrEvIAtIonS And SyMbolS NOM Natur
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REFERENCEs 171 [29] W.R. Bowen, T.A
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174 6. NANOSCALE ANALySIS Of PHARMA
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196 7. MICRO/NANOENgINEERINg ANd AF
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226 8. ATOMIC FORCE MICROSCOPy ANd
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246 9. APPLICATION OF ATOMIC FORCE
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276 10. FuTuRE PRosPECTs most AFM s
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278 10. FuTuRE PRosPECTs targeted d
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A Actin stress fiber, 197 Adhesion,
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Natural organic matter, 140 Negativ
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W. Richard Bowen and Nidal Hilal 4

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