Inference of Polymer Adsorption from Electrical Double Layer Measurements
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546.57*151:541.183.53:541.64:678.744.72 MEDEDELINGEN LANDBOUWHOGESCHOOL WAGENINGEN • NEDERLAND • 78-12 (1978) INFERENCE OF POLYMER ADSORPTION FROM ELECTRICAL DOUBLE LAYER MEASUREMENTS THE SILVER IODIDE-POLYVINYL ALCOHOL SYSTEM L. K. KOOPAL Laboratory for Physical and Colloid Chemistry, Agricultural University, Wageningen, The Netherlands (with a summary in Dutch) (received 19-XII-77) H.VEENMA N &ZONE N B.V. - WAGENINGEN - 1978 Mededelingen Landbouwhogeschool Wageningen 78-12 (1978) (Communications Agricultural University) isals o published as a thesis CONTENTS 1. INTRODUCTION 1 1.1. General introduction and aim of this study 1 1.2. Polymer adsorption 2 1.3. System used and outline of this study 4 2. MATERIALS 6 2.1. General 6 2.2. Silver iodide 6 2.2.1. Sols 6 2.2.2. Precipitates 6 2.3. Polyvinyl alcohol 7 3. SPECIFIC SURFACE AREA OF THE SILVER IODIDE DISPERSIONS ... 8 3.1. Introduction 8 3.2. Capacitance measurements 8 3.2.1. Basic principle 8 3.2.2. Experimental method and results 9 3.2.3. Discussion 10 3.3. Gas adsorption 11 3.3.1. Evaluation of the method 11 3.3.2. Experimental methods and results 12 3.3.3. Comparison with capacitance areas 12 3.4. Adsorption from solution 13 3.4.1. General features 13 3.4.2. Experimental 13 3.4.3. Results and discussion 14 3.4.4. Comparison with other methods 19 3.5. Electron microscopy 20 3.5.1. Introduction 20 3.5.2. Experimental 21 3.5.3. Results 21 3.5.4. Comparison with capacitance areas and discussion 23 3.6. Further comparison and discussion 25 3.7. Summary 27 4. POLYVINYL ALCOHOL: CHARACTERIZATION AND SOLUTION PROPERTIES 28 4.1. Synthesis and properties 28 4.2. Spectroscopic characterization 32 4.2.1. UV spectroscopy 32 4.2.2. IR spectroscopy 33 4.3. Molecular weight and molecular weight distribution 35 4.3.1. Viscometry 35 4.3.2. Gel permeation chromatography 39 4.4. Conformation and solution parameters 46 4.4.1. Theoretical introduction 46 4.4.2. Heterodispersity effects 50 4.4.3. Results and discussion 53 4.5. Summary 57 5. ADSORPTION OF POLYVINYL ALCOHOL ON SILVER IODIDE .... 59 5.1. Introduction 59 5.2. The adsorption isotherm 59 5.2.1. General features 59 5.2.2. Effect of molecular weight 60 5.2.3. The silver iodide -polyviny l alcohol system 61 5.3. Experimental procedures 62 5.3.1. Adsorption measurements 62 5.3.2. Determination of the PVA concentration 63 5.4. Results and discussion 64 5.4.1. General 64 5.4.2. Effect of surface charge 67 5.4.3. Effect of molecular weight and acetate content 68 5.4.4. Influences of the silver iodide concentration and the presence of salt ... 69 5.5. Summary 71 6. CHARACTERIZATION OF THE ADSORBED POLYMER LAYER BY DOUBLE LAYER MEASUREMENTS 73 6.1. Introduction 73 6.2. Basic principles and outline of the double layer studies 73 6.3. Experimental 76 6.3.1. Surface charge versuspAg curves 76 6.3.2. Electrophoretic mobilities 81 6.4. Results and discussion 82 6.4.1. Surface charge versuspAg curves 82 6.4.2. General features 85 6.4.3. Degree of occupancy of the first layer 90 6.4.4. Electrophoresis; degree of occupancy of the first layer and effective polymer layer thickness 95 6.4.5. Influences of molecular weight, acetate content and surface charge on 8 and A 104 6.5. Summary 106 7. GENERAL DISCUSSION 107 7.1. Fraction of segments adsorbed in the first layer and amount adsorbed in loops and tails 107 7.2. Average volume fractions and segment densities in the adsorbed layer 112 7.3. Comparison with a theoretical treatment 113 7.3.1. Polymer adsorption models 113 7.3.2. The Hoeve theory 114 7.4. Summary 121 SUMMARY 123 ACKNOWLEDGEMENTS 126 SAMENVATTING 127 REFERENCES 130 GLOSSARY OF SYMBOLS AND ABBREVIATIONS 138 1. INTRODUCTION 1.1. GENERAL INTRODUCTION AND AIM OF THIS STUDY The adsorption of macromolecules from solution onto solids isno t merely of academic interest but it is also a basic phenomenon in many applied processes. For example adsorbed macromolecules can play a decisive role as stabilizers of colloidal dispersions (LYKLEMA 1968, VINCENT 1974), but also as flocculants (KITCHENER 1972), they are of great importance for the action of adhesives (PATRICK 1967, SCHRADER and BLOCK 1971) and lubricants (FORBES et al. 1970) and can be used as soil improvers (GREENLAND 1972). To a large extent, these applications are based upon two characteristic features: 1. The adsorbed molecules are firmly attached to the surface due to their ability to form multiple adsorption bonds, and 2. an adsorbed polymer layer usually has a considerable thickness, giving rise to an interfacial zone with special properties. To gain insight into these phenomena and in the factors influencing them much work has been done, both experimental and theoretical. See for instance reviews by PATAT et al. (1964); STROMBERG (1967), SILBERBERG (1971), LIPATOV and SERGEEVA (1974,1976)an d ROE(1974) . Although not generally recognized, polymer adsorption from aqueous solution is especially significant. More often than not, particles dispersed in aqueous solution carry a charge on their surfaces. This charge, together with its counterpart in solution around the particle, forms an electrical double layer. Consequently, polymers adsorbed on such particles are as a rule situated in this electrical double layer, which may have important implications for either or both the adsorbed polymer and the electrical double layer. It is this neglected field of study that the present thesis isconcerne d with. Our aim is to investigate how the double layer properties of charged particles are influenced by the presence of adsorbed polymer molecules and to obtain information on the conformation of the polymer layer from observed alterations in the double layer properties. In particular this latter aspect may conveniently be studied if a neutral polymer is chosen, which does not strongly interact with the charges in the double layer. The basic principle, of such a study lies in the possibility of controlling the thickness of the solution part of the electrical double layer. At high electrolyte concentration (e.g. 10"1 M) this layer is very thin. Changes in its properties due to adsorbed polymer molecules are then essentially caused by alterations in this thin region, thus reflecting the presence of segments in direct contact with the surface. On the other hand, at low electrolyte concen tration (e.g. 10"3 M) the solution part of the electrical double layer is merely diffuse and comparable in thickness with an adsorbed polymer layer. Changes in the double layer properties under these conditions may then be associated Meded. Landbouwhogeschool Wageningen 78-12(1978) 1 with the presence of loops* and tails* of polymer dangling in the solution phase. The information thus obtained has a relatively wide range of appli cability: it is for instance of particular interest of the explanation of polymer induced stability in aqueous dispersions. 1.2. POLYMER ADSORPTION Unlike the isotherms observed for the adsorption of low molecular weight species,thos e for polymers arerelativel y uninformative. In addition to knowing the mass adsorbed, rp, it isals o necessary to know how the material is distribu ted. As the conformation of an adsorbed polymer is continually changing, this can only be expressed in terms of statistical parameters. The best charac teristic of a given adsorbed polymer layer is the average segment density distribution, p, as a function of the distance, x, from the surface. However, p(x) is a complicated parameter, composed of the average density distribu tions of segments in trains**, loops and tails (see e.g. HOEVE 1965, HESSELINK 1971, 1975). Apart from its compounded character, p(x) is not experimentally accessible and recourse has to be made to more available characteristics of an adsorbed polymer layer, sucha s: a. the fraction, p, of segments that is in direct contact with the surface, b. the fraction, 0, of the surface occupied with train segments and c. some measure of the average thickness, A, of the adsorbed polymer layer. Information on these parameters and the factors influencing them can be gained by both theoretical techniques and experiments on model systems. Important contributions to the theory of homopolymer adsorption have been made by SILBERBERG (1968, 1972, 1975),HOEVE (1971, 1976), ROE (1974), SCHEUTJENS (1976) and others. Although details of the results of these various approaches are different, their predictions are qualitatively similar. All theories conclude that thestructur e ofa n adsorbed polymer layer mainly depends on the polymer-surface, polymer-solvent, polymer-polymer and solvent-surface in teractions. In addition, the mean molecular weight, the polymer concentration and the chain flexibility play a role. A critical evaluation of these theories requires experimental data on the dimensions and numbers of adsorbed poly mer loops, tails and trains and estimates of the magnitudes of the different interactions. However, direct application of the models to practical data is often thwarted by the observed irreversible nature of polymer adsorption, leading to quasi equilibrium states. Confining our selves to the three important quantities rp, 6(o rp) and Ä, it is expedient to briefly review what isavailabl e in literature. 1. Theamounts adsorbedor the adsorption isotherm. Literature abunds on this, see for example review articles of PATAT et al. (1964) and LIPATOV and SER- * A loop isa sequence ofpolyme r segments adsorbed at each end, the intervening segments protruding into the solution. A taili sa serieso f segments terminally adsorbed at one end. ** A train isa n uninterrupted sequenceo f segmentsi ndirec t contact with the surface.