Physics and Chemistry of Interfaces

Transcription

1 Hans-Jürgen Butt, Karlheinz Graf, Michael Kappl Physics and Chemistry of Interfaces WILEY-VCH GmbH & Co. KGaA

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3 Hans-Jürgen Butt, Karlheinz Graf, Michael Kappl Physics and Chemistry of Interfaces

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5 Hans-Jürgen Butt, Karlheinz Graf, Michael Kappl Physics and Chemistry of Interfaces WILEY-VCH GmbH & Co. KGaA

6 Authors Hans-Jürgen Butt MPI für Polymerforschung Mainz Karlheinz Graf MPI für Polymerforschung Mainz Michael Kappl MPI für Polymerforschung Mainz Cover Pictures The left picture shows aggregates of silicon oxide particles with a diameter of 0.9 μm (see example 1.1). At the bottom an atomic force microscope image of cylindrical CTAB micelles adsorbed to gold(111) is shown (see example 12.3, width: 200 nm). The right image was also obtained by atomic force microscopy. It shows the surface of a selfassembled monolayer of long-chain alkylthiols on gold(111) (see fig. 10.2, width: 3.2 nm). This book was carefully produced. Nevertheless, authors and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate Library of Congress Card No. applied for British Library Cataloguing-in-Publication Data: A catalogue record for this book is available from the British Library. Bibliographic information published by Die Deutsche Bibliothek Die Deutsche Bibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data is available in the Internet at < WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form by photoprinting, microfilm, or any other means nor transmitted or translated into machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not be considered unprotected by law. printed in the Federal Republic of Germany printed on acid-free paper Composition Uwe Krieg, Berlin Printing betz-druck GmbH, Darmstadt Bookbinding Litges&Dopf Buchbinderei GmbH, Heppenheim ISBN

7 Preface Interface science has changed significantly during the last years. This is partially due to scientific breakthroughs. For example, the invention of scanning probe microscopy and refined diffraction methods allow us to look at interfaces under wet conditions with unprecedented accuracy. This change is also due to the greatly increased community of interfacial scientists. One reason is certainly the increased relevance of micro- and nanotechnology, including lab-on-chip technology, microfluids, and biochips. Objects in the micro- and nanoworld are dominated by surface effects rather than gravitation or inertia. Therefore, surface science is the basis for nanotechnology. The expansion of the community is correlated with an increased interdisciplinarity. Traditionally the community tended to be split into dry surface scientists (mainly physicists working under ultrahigh vacuum conditions) and wet surface scientists (mainly colloid chemists). In addition, engineers dealing with applications like coatings, adhesion, or lubrication, formed a third community. This differentiation is significantly less pronounced and interface science has become a really interdisciplinary field of research including, for example, chemical engineering and biology. This development motivated us to write this textbook. It is a general introduction to surface and interface science. It focuses on basic concepts rather than specific details, on understanding rather than learning facts. The most important techniques and methods are introduced. The book reflects that interfacial science is a diverse field of research. Several classical scientific or engineering disciplines are involved. It contains basic science and applied topics such as wetting, friction, and lubrication. Many textbooks concentrate on certain aspects of surface science such as techniques involving ultrahigh vacuum or classical wet colloid chemistry. We tried to include all aspects because we feel that for a good understanding of interfaces, a comprehensive introduction is required. Our manuscript is based on lectures given at the universities of Siegen and Mainz. It addresses (1) advanced students of engineering, chemistry, physics, biology, and related subjects and (2) scientists in academia or industry, who are not yet specialists in surface science but want to get a solid background knowledge of the subject. The level is introductory for scientists and engineers who have a basic knowledge of the natural sciences and mathematics. Certainly no advanced level of mathematics is required. When looking through the pages of this book you will see a substantial number of equations. Please do not be scared! We preferred to give all transformations explicitly rather than writing as can easily be seen and stating the result. Chapter Thermodynamics of Interfaces is the only exception. To appreciate it a basic knowledge of thermodynamics is required. You can skip and still be able to follow the rest. In this case please read and try to get an intuitive understanding of what surface excess is (Section 3.1) and what the Gibbs adsorption equation implies (Section 3.4.2).

8 VI Preface A number of problems with solutions were included to allow for private studies. If not mentioned otherwise, the temperature was assumed to be 25 C. At the end of each chapter the most important equations, facts, and phenomena are summarized to given students a chance to concentrate on important issues and help instructors preparing exams. One of the main problems when writing a textbook is to limit its content. We tried hard to keep the volume within the scope of one advanced course of roughly 15 weeks, one day per week. Unfortunately, this means that certain topics had to be cut short or even left out completely. Statistical mechanics, heterogeneous catalysis, and polymers at surfaces are issues which could have been expanded. This book certainly contains errors. Even after having it read by different people independently, this is unavoidable. If you find an error, please write us a letter (Max-Planck-Institute for Polymer Research, Ackermannweg, Mainz, Germany) or an (butt@mpipmainz.mpg.de) so that we can correct it and do not confuse more students. We are indebted to several people who helped us collecting information, preparing, and critically reading this manuscript. In particular we would like thank R. von Klitzing, C. Lorenz, C. Stubenrauch, D. Vollmer, J. Wölk, R. Wolff, K. Beneke, J. Blum, M. Böhm, E. Bonaccurso, P. Broekmann, G. Glasser, G. Gompper, M. Grunze, J. Gutmann, L. Heim, M. Hillebrand, T. Jenkins, X. Jiang, U. Jonas, R. Jordan, I. Lieberwirth, G. Liger-Belair, M. Lösche, E. Meyer, P. Müller-Buschbaum, T. Nagel, D. Quéré, J. Rabe, H. Schäfer, J. Schreiber, M. Stamm, M. Steinhart, G. Subklew, J. Tomas, K. Vasilev, K. Wandelt, B. Wenclawiak, R. Wepf, R. Wiesendanger, D.Y. Yoon, M. Zharnikov, and U. Zimmermann. Hans-Jürgen Butt, Karlheinz Graf, and Michael Kappl Mainz, August 2003

9 Contents Preface V 1 Introduction 1 2 Liquid surfaces Microscopicpictureof the liquid surface Surface tension Equation of Young and Laplace Curved liquid surfaces Derivation of the Young Laplace equation Applying the Young Laplace equation Techniques tomeasurethesurface tension The Kelvinequation Capillary condensation Nucleationtheory Summary Exercises Thermodynamics of interfaces The surfaceexcess Fundamental thermodynamic relations Internal energy and Helmholtz energy Equilibrium conditions Locationof the interface Gibbs energy and definition of the surface tension Free surface energy, interfacial enthalpy and Gibbs surface energy Thesurfacetensionofpureliquids Gibbsadsorption isotherm Derivation System of two components Experimental aspects The Marangoni effect Summary Exercises

10 VIII Contents 4 The electric double layer Introduction Poisson Boltzmann theory of the diffuse double layer The Poisson Boltzmann equation Planar surfaces Thefull one-dimensionalcase TheGrahame equation Capacity of the diffuse electric double layer Beyond Poisson Boltzmann theory Limitations of the Poisson Boltzmann theory TheStern layer The Gibbs free energy of the electric double layer Summary Exercises Effects at charged interfaces Electrocapillarity Theory Measurement of electrocapillarity Examples of charged surfaces Mercury Silveriodide Oxides Mica Semiconductors Measuring surface charge densities Potentiometric colloid titration Capacitances Electrokinetic phenomena: The zeta potential TheNavier Stokesequation Electro-osmosisand streamingpotential Electrophoresis and sedimentation potential Typesof potentials Summary Exercises Surface forces Vander Waalsforcesbetweenmolecules The van der Waals force between macroscopic solids Microscopicapproach Macroscopic calculation Lifshitz theory Surface energy and Hamaker constant Conceptsfor thedescription of surfaceforces TheDerjaguin approximation The disjoining pressure

11 Contents IX 6.4 Measurementofsurface forces The electrostatic double-layer force Generalequations Electrostatic interaction between two identical surfaces TheDLVO theory Beyond DLVO theory The solvation force and confined liquids Non DLVO forces in an aqueous medium Steric interaction Propertiesof polymers Force between polymer coated surfaces Spherical particlesin contact Summary Exercises Contact angle phenomena and wetting Young s equation Thecontactangle Derivation Theline tension Complete wetting Important wetting geometries Capillary rise Particlesin the liquid gasinterface Networkof fibres Measurementofthe contactangle Experimental methods Hysteresis in contact angle measurements Surface roughness and heterogeneity Theoretical aspects of contact angle phenomena Dynamics of wetting and dewetting Wetting Dewetting Applications Flotation Detergency Microfluidics Adjustable wetting Summary Exercises Solid surfaces Introduction Description of crystalline surfaces Thesubstratestructure

12 X Contents Surface relaxation and reconstruction Descriptionof adsorbatestructures Preparation of clean surfaces Thermodynamics of solid surfaces Surface stressandsurface tension Determination ofthe surfaceenergy Surface stepsand defects Solid solid boundaries Microscopy of solid surfaces Opticalmicroscopy Electron microscopy Scanning probe microscopy Diffraction methods Diffraction patterns of two-dimensional periodic structures Diffraction with electrons, X-rays, and atoms Spectroscopic methods Spectroscopy using mainly inner electrons Spectroscopy with outer electrons Secondary ion mass spectrometry Summary Exercises Adsorption Introduction Definitions The adsorption time Classificationof adsorption isotherms Presentationof adsorption isotherms Thermodynamics of adsorption Heatsof adsorption Differential quantities of adsorption and experimental results Adsorption models The Langmuir adsorption isotherm The Langmuir constant and the Gibbs energy of adsorption Langmuir adsorption with lateral interactions TheBET adsorptionisotherm Adsorption on heterogeneous surfaces The potential theory of Polanyi Experimental aspects of adsorption from the gas phase Measurementof adsorptionisotherms Procedures to measure the specific surface area Adsorption on porous solids hysteresis Specialaspectsof chemisorption Adsorption from solution Summary Exercises

13 Contents XI 10 Surface modification Introduction Chemical vapor deposition Soft matter deposition Self-assembled monolayers Physisorption ofpolymers Polymerization on surfaces Etching techniques Summary Exercises Friction, lubrication, and wear Friction Introduction Amontons and Coulomb s Law Static, kinetic, and stick-slip friction Rolling friction Friction andadhesion Experimental Aspects Techniquesto measurefriction Macroscopicfriction Microscopicfriction Lubrication Hydrodynamic lubrication Boundary lubrication Thin film lubrication Lubricants Wear Summary Exercises Surfactants, micelles, emulsions, and foams Surfactants Spherical micelles, cylinders, and bilayers Thecritical micelleconcentration Influenceof temperature Thermodynamics of micellization Structure of surfactant aggregates Biologicalmembranes Macroemulsions Generalproperties Formation Stabilization Evolution andaging Coalescence and demulsification

14 XII Contents 12.4 Microemulsions Sizeof droplets Elasticproperties of surfactantfilms Factors influencing the structure of microemulsions Foams Classification, application and formation Structure of foams Soapfilms Evolution of foams Summary Exercises Thin films on surfaces of liquids Introduction Phases of monomolecular films Experimental techniques to study monolayers Optical methods X-ray reflectionanddiffraction Thesurface potential Surface elasticityandviscosity Langmuir Blodgett transfer Thick films spreading of one liquid on another Summary Exercises Solutions to exercises 299 Appendix A Analysis of diffraction patterns 321 A.1 Diffraction atthreedimensional crystals A.1.1 Bragg condition A.1.2 Laue condition A.1.3 The reciprocal lattice A.1.4 Ewald construction A.2 Diffraction at Surfaces A.3 Intensity of diffraction peaks B Symbols and abbreviations 331 Bibliography 335 Index 355

15 1 Introduction An interface is the area which separates two phases from each other. If we consider the solid, liquid, and gas phase we immediately get three combinations of interfaces: the solid liquid, the solid gas, and the liquid gas interface. These interfaces are also called surfaces. Interface is, however, a more general term than surface. Interfaces can also separate two immiscible liquids such as water and oil. These are called liquid liquid interfaces. Solid solid interfaces separate two solid phases. They are important for the mechanical behavior of solid materials. Gas gas interfaces do not exist because gases mix. Often interfaces and colloids are discussed together. Colloids are disperse systems, in which one phase has dimensions in the order of 1 nm to 1 μm (seefig.1.1). Theword colloid comes from the Greek word for glue and has been used the first time in 1861 by Graham 1. He applied it to materials which seemed to dissolve but were not able to penetrate a membrane, such as albumin, starch, and dextrin. A dispersion is a two-phase system which is uniform on the macroscopic but not on the microscopic scale. It consists of grains or droplets of one phase in a matrix of the other phase. Different kinds of dispersions can be formed. Most of them have important applications and have special names (Table 1.1). While there are only five types of interface, we can distinguish ten types of disperse system because we have to discriminate between the continuous, dispersing (external) phase and the dispersed (inner) phase. In some cases this distinction is obvious. Nobody will, for instance, mix up fog with a foam although in both cases a liquid and a gas are involved. In other cases the distinction between continuous and inner phase cannot be made because both phases might form connected networks. Some emulsions for instance tend to form a bicontinuous phase, in which both phases form an interwoven network. Continuous phase 1nm-1μm Dispersed phase Figure 1.1: Schematic of a dispersion. Colloids and interfaces are intimately related. This is a direct consequence of their enormous specific surface area. More precisely: their interface-to-volume relation is so large, that their behavior is completely determined by surface properties. Gravity is negligible in the 1 Thomas Graham, British chemist, professor in Glasgow and London.