Depletion-induced phase separation in colloid-polymer mixturesIntroductionDepletion interaction in monodisperse and neutral polymer-colloid mixtures: theoryAn exact result: the interaction between parallel plates due to ideal polymer chainsPenetrable hard sphere (PHS) approachMean-field approachesTheoretical models for the `protein limit;s'Liquid-state theory approachesNegative adsorption methodPolydispersity effects on depletion interactionThe role of charges in depletion interactionDepletion interaction in polymer-colloid mixtures: experimentsEarly experimental findings of depletion interactionDepletion-induced phase transitionsStudies on the depletion layer thicknessDetailed studies on phase behaviorModern techniques measuring the depletion-induced attractionScatteringForce methodsTotal internal reflection microscopy (TIRM)Optical tweezer measurementsInterfacial propertiesConclusions and outlookAcknowledgementsReferencesAdvances in Colloid and Interface Science103 (2003) 1–310001-8686/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0001-8686(02)00081-7Depletion-induced phase separation in colloid–polymer mixturesR. Tuinier *, J. Rieger , C.G. de Kruifa,b, ca,dNIZO Food Research, P.O. Box 20, 6710 BA Ede, The NetherlandsaForschungszentrum Julich, Institut fur Festkorperforschung, Weiche Materie, 52425 Julich, Germanyb¨¨¨ ¨BASF Aktiengesellschaft, Polymer Physics, 67056 Ludwigshafen, GermanycVan’t Hoff Laboratory, Debye Research Institute, University of Utrecht, Padualaan 8,d3584 CH Utrecht, The NetherlandsAbstractPhase separation can be induced in a colloidal dispersion by adding non-adsorbingpolymers. Depletion of polymer around the colloidal particles induces an effective attraction,leading to demixing at sufficient polymer concentration. This communication reviewstheoretical and experimental work carried out on the polymer-mediated attraction betweenspherical colloids and the resulting phase separation of the polymer–colloid mixture.Theoretical studies have mainly focused on the limits where polymers are small or large ascompared to the colloidal size. Recently, however, theories are being developed that cover awider colloid–polymer size ratio range. In practical systems, size polydispersity andpolyelectrolytes (instead of neutral polymers) andyor charges on the colloidal surfaces playa role in polymer–colloid mixtures. The limited amount of theoretical work performed onthis is also discussed. Finally, an overview is given on experimental investigations withrespect to phase behavior and results obtained with techniques enabling measurement of thedepletion-induced interaction potential, the structure factor, the depletion layer thickness andthe interfacial tension between the demixed phases of a colloid–polymer mixture.䊚 2003 Elsevier Science B.V. All rights reserved.Keywords: Depletion; Polymers; Colloids; Phase behavior; Polydispersity; Charges*Corresponding author.E-mail address: [email protected] (R. Tuinier).2 R. Tuinier et al. / Advances in Colloid and Interface Science 103 (2003) 1–31Contents1.Introduction ............................................................................................... 022. Depletion interaction in monodisperse and neutral polymer–colloid mixtures: theory ....... 042.1. An exact result: the interaction between parallel plates due to ideal polymer chains .. 042.2. Penetrable hard sphere (PHS) approach ....................................................... 042.3. Mean-field approaches ............................................................................ 082.4. Theoretical models for the ‘protein limit’ ..................................................... 122.5. Liquid-state theory approaches .................................................................. 132.6. Negative adsorption method ..................................................................... 153. Polydispersity effects on depletion interaction ...................................................... 164. The role of charges in depletion interaction ......................................................... 185. Depletion interaction in polymer–colloid mixtures: experiments ................................ 215.1. Early experimental findings of depletion interaction ........................................ 215.2. Depletion-induced phase transitions ............................................................ 215.3. Studies on the depletion layer thickness ....................................................... 225.4. Detailed studies on phase behavior ............................................................. 235.5. Modern techniques measuring the depletion-induced attraction ........................... 245.5.1. Scattering ..................................................................................... 245.5.2. Force methods ................................................................................ 245.5.3. Total internal reflection microscopy (TIRM) ........................................... 255.5.4. Optical tweezer measurements ............................................................ 265.5.5. Interfacial properties ........................................................................ 266. Conclusions and outlook ................................................................................ 27Acknowledgements .......................................................................................... 28References ..................................................................................................... 281. IntroductionDepletion interaction between colloidal spherical particles due to non-adsorbingpolymers is a subject that has gained a gradually increasing amount of attentionsince the 1980s. Understanding depletion phenomena is relevant in many ways. Itfirst helps to understand when and why phase separation occurs in mixtures ofpolymers and colloids, which are often jointly present in applications. Furthermore,depletion-induced phase separation is a way of concentrating colloidal dispersionsin a convenient way. If the colloids andyor polymers are polydisperse, depletion-induced phase separation can be used for (large-scale) size fractionation in industrialapplications. Besides these practical reasons, depletion studies provide an accessibleway of changing the range of the interaction potential between the colloidal particlesby varying the size ratio. This
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