physics and chemistry of interfaces - buch.de · hans-jürgen butt, karlheinz graf, michael kappl...

Hans-Jrgen Butt, Karlheinz Graf, Michael Kappl

Physics and Chemistry of Interfaces

WILEY-VCH GmbH & Co. KGaA

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WILEY-VCH GmbH & Co. KGaA

Authors

Hans-Jrgen ButtMPI fr Polymerforschung Mainze-mail: [email protected]

Karlheinz GrafMPI fr Polymerforschung Mainze-mail: [email protected]

Michael KapplMPI fr Polymerforschung Mainze-mail: [email protected]

Cover PicturesThe left picture shows aggregates of siliconoxide particles with a diameter of 0.9 m (seeexample 1.1). At the bottom an atomic forcemicroscope image of cylindrical CTAB mi-celles adsorbed to gold(111) is shown (seeexample 12.3, width: 200 nm). The rightimage was also obtained by atomic force microscopy. It shows the surface of a self-assembled monolayer of long-chain alkylthiolson gold(111) (see fig. 10.2, width: 3.2 nm).

This book was carefully produced. Never-theless, authors and publisher do not warrantthe information contained therein to be freeof errors. Readers are advised to keep inmind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate

Library of Congress Card No. applied for

British Library Cataloguing-in-PublicationData: A catalogue record for this book is available from the British Library.

Bibliographic information published by Die Deutsche BibliothekDie Deutsche Bibliothek lists this publicationin the Deutsche Nationalbibliografie; detailed bibliographic data is available in theInternet at .

2003 WILEY-VCH Verlag GmbH & Co.KGaA, Weinheim

All rights reserved (including those of trans-lation into other languages). No part of thisbook may be reproduced in any form byphotoprinting, microfilm, or any othermeans nor transmitted or translated intomachine language without written permis-sion from the publishers. Registered names,trademarks, etc. used in this book, evenwhen not specifically marked as such, are not be considered unprotected by law.

printed in the Federal Republic of Germanyprinted on acid-free paper

Composition Uwe Krieg, BerlinPrinting betz-druck GmbH, DarmstadtBookbinding Litges&Dopf BuchbindereiGmbH, Heppenheim

ISBN 3-527-40413-9

Preface

Interface science has changed significantly during the last 1015 years. This is partially due toscientific breakthroughs. For example, the invention of scanning probe microscopy and refineddiffraction methods allow us to look at interfaces under wet conditions with unprecedentedaccuracy. This change is also due to the greatly increased community of interfacial scien-tists. One reason is certainly the increased relevance of micro- and nanotechnology, includinglab-on-chip technology, microfluids, and biochips. Objects in the micro- and nanoworld aredominated by surface effects rather than gravitation or inertia. Therefore, surface science isthe basis for nanotechnology.

The expansion of the community is correlated with an increased interdisciplinarity. Tradi-tionally the community tended to be split into dry surface scientists (mainly physicists work-ing under ultrahigh vacuum conditions) and wet surface scientists (mainly colloid chemists).In addition, engineers dealing with applications like coatings, adhesion, or lubrication, formeda third community. This differentiation is significantly less pronounced and interface sciencehas become a really interdisciplinary field of research including, for example, chemical engi-neering and biology.

This development motivated us to write this textbook. It is a general introduction to surfaceand interface science. It focuses on basic concepts rather than specific details, on understand-ing 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 scien-tific or engineering disciplines are involved. It contains basic science and applied topics suchas wetting, friction, and lubrication. Many textbooks concentrate on certain aspects of surfacescience 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, acomprehensive introduction is required.

Our manuscript is based on lectures given at the universities of Siegen and Mainz. It ad-dresses (1) advanced students of engineering, chemistry, physics, biology, and related subjectsand (2) scientists in academia or industry, who are not yet specialists in surface science butwant to get a solid background knowledge of the subject. The level is introductory for sci-entists 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 ofthis book you will see a substantial number of equations. Please do not be scared! We pre-ferred to give all transformations explicitly rather than writing as can easily be seen andstating the result. Chapter Thermodynamics of Interfaces is the only exception. To ap-preciate it a basic knowledge of thermodynamics is required. You can skip and still be ableto follow the rest. In this case please read and try to get an intuitive understanding of whatsurface excess is (Section 3.1) and what the Gibbs adsorption equation implies (Section 3.4.2).

VI Preface

A number of problems with solutions were included to allow for private studies. If notmentioned otherwise, the temperature was assumed to be 25C. At the end of each chapter themost important equations, facts, and phenomena are summarized to given students a chanceto 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 tokeep the volume within the scope of one advanced course of roughly 15 weeks, one day perweek. Unfortunately, this means that certain topics had to be cut short or even left out com-pletely. Statistical mechanics, heterogeneous catalysis, and polymers at surfaces are issueswhich could have been expanded.

This book certainly contains errors. Even after having it read by different people indepen-dently, this is unavoidable. If you find an error, please write us a letter (Max-Planck-Institutefor Polymer Research, Ackermannweg, 55128 Mainz, Germany) or an e-mail ([email protected]) so that we can correct it and do not confuse more students.

We are indebted to several people who helped us collecting information, preparing, andcritically reading this manuscript. In particular we would like thank R. von Klitzing, C. Lorenz,C. Stubenrauch, D. Vollmer, J. Wlk, R. Wolff, K. Beneke, J. Blum, M. Bhm, E. Bonac-curso, P. Broekmann, G. Glasser, G. Gompper, M. Grunze, J. Gutmann, L. Heim, M. Hille-brand, T. Jenkins, X. Jiang, U. Jonas, R. Jordan, I. Lieberwirth, G. Liger-Belair, M. Lsche,E. Meyer, P. Mller-Buschbaum, T. Nagel, D. Qur, J. Rabe, H. Schfer, 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-Jrgen Butt, Karlheinz Graf, and Michael Kappl

Mainz, August 2003

Contents

Preface V

1 Introduction 1

2 Liquid surfaces 42.1 Microscopic picture of the liquid surface . . . . . . . . . . . . . . . . . . . . 42.2 Surface tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.3 Equation of Young and Laplace . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.3.1 Curved liquid surfaces . . . . . . . . . . . . . . . . . . . . . . . . . 82.3.2 Derivation of the YoungLaplace equation . . . . . . . . . . . . . . 102.3.3 Applying the YoungLaplace equation . . . . . . . . . . . . . . . . . 11

2.4 Techniques to measure the surface tension . . . . . . . . . . . . . . . . . . . 122.5 The Kelvin equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.6 Capillary condensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.7 Nucleation theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.9 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3 Thermodynamics of interfaces 263.1 The surface excess . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.2 Fundamental thermodynamic relations . . . . . . . . . . . . . . . . . . . . . 29

3.2.1 Internal energy and Helmholtz energy . . . . . . . . . . . . . . . . . 293.2.2 Equilibrium conditions . . . . . . . . . . . . . . . . . . . . . . . . . 303.2.3 Location of the interface . . . . . . . . . . . . . . . . . . . . . . . . 313.2.4 Gibbs energy and definition of the surface tension . . . . . . . . . . . 323.2.5 Free surface energy, interfacial enthalpy and Gibbs surface energy . . 32

3.3 The surface tension of pure liquids . . . . . . . . . . . . . . . . . . . . . . . 343.4 Gibbs adsorption isotherm . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.4.1 Derivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.4.2 System of two components . . . . . . . . . . . . . . . . . . . . . . . 373.4.3 Experimental aspects . . . . . . . . . . . . . . . . . . . . . . . . . . 383.4.4 The Marangoni effect . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403.6 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

VIII Contents

4 The electric double layer 424.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424.2 PoissonBoltzmann theory of the diffuse double layer . . . . . . . . . . . . . 43

4.2.1 The PoissonBoltzmann equation . . . . . . . . . . . . . . . . . . . 434.2.2 Planar surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444.2.3 The full one-dimensional case . . . . . . . . . . . . . . . . . . . . . 464.2.4 The Grahame equation . . . . . . . . . . . . . . . . . . . . . . . . . 494.2.5 Capacity of the diffuse electric double layer . . . . . . . . . . . . . . 50

4.3 Beyond PoissonBoltzmann theory . . . . . . . . . . . . . . . . . . . . . . . 504.3.1 Limitations of the PoissonBoltzmann theory . . . . . . . . . . . . . 504.3.2 The Stern layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

4.4 The Gibbs free energy of the electric double layer . . . . . . . . . . . . . . . 544.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 554.6 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

5 Effects at charged interfaces 575.1 Electrocapillarity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

5.1.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585.1.2 Measurement of electrocapillarity . . . . . . . . . . . . . . . . . . . 60

5.2 Examples of charged surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 615.2.1 Mercury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 625.2.2 Silver iodide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 635.2.3 Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655.2.4 Mica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665.2.5 Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

5.3 Measuring surface charge densities . . . . . . . . . . . . . . . . . . . . . . . 685.3.1 Potentiometric colloid titration . . . . . . . . . . . . . . . . . . . . . 685.3.2 Capacitances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

5.4 Electrokinetic phenomena: The zeta potential . . . . . . . . . . . . . . . . . 725.4.1 The NavierStokes equation . . . . . . . . . . . . . . . . . . . . . . 725.4.2 Electro-osmosis and streaming potential . . . . . . . . . . . . . . . . 735.4.3 Electrophoresis and sedimentation potential . . . . . . . . . . . . . . 76

5.5 Types of potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 775.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795.7 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

6 Surface forces 806.1 Van der Waals forces between molecules . . . . . . . . . . . . . . . . . . . . 806.2 The van der Waals force between macroscopic solids . . . . . . . . . . . . . 84

6.2.1 Microscopic approach . . . . . . . . . . . . . . . . . . . . . . . . . 846.2.2 Macroscopic calculation Lifshitz theory . . . . . . . . . . . . . . 876.2.3 Surface energy and Hamaker constant . . . . . . . . . . . . . . . . . 92

6.3 Concepts for the description of surface forces . . . . . . . . . . . . . . . . . 936.3.1 The Derjaguin approximation . . . . . . . . . . . . . . . . . . . . . 936.3.2 The disjoining pressure . . . . . . . . . . . . . . . . . . . . . . . . . 95

Contents IX

6.4 Measurement of surface forces . . . . . . . . . . . . . . . . . . . . . . . . . 966.5 The electrostatic double-layer force . . . . . . . . . . . . . . . . . . . . . . 98

6.5.1 General equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 986.5.2 Electrostatic interaction between two identical surfaces . . . . . . . . 1016.5.3 The DLVO theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

6.6 Beyond DLVO theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1046.6.1 The solvation force and confined liquids . . . . . . . . . . . . . . . . 1046.6.2 Non DLVO forces in an aqueous medium . . . . . . . . . . . . . . . 106

6.7 Steric interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1076.7.1 Properties of polymers . . . . . . . . . . . . . . . . . . . . . . . . . 1076.7.2 Force between polymer coated surfaces . . . . . . . . . . . . . . . . 108

6.8 Spherical particles in contact . . . . . . . . . . . . . . . . . . . . . . . . . . 1116.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156.10 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

7 Contact angle phenomena and wetting 1187.1 Youngs equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

7.1.1 The contact angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1187.1.2 Derivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1197.1.3 The line tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1217.1.4 Complete wetting . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

7.2 Important wetting geometries . . . . . . . . . . . . . . . . . . . . . . . . . . 1227.2.1 Capillary rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1227.2.2 Particles in the liquidgas interface . . . . . . . . . . . . . . . . . . 1237.2.3 Network of fibres . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

7.3 Measurement of the contact angle . . . . . . . . . . . . . . . . . . . . . . . 1267.3.1 Experimental methods . . . . . . . . . . . . . . . . . . . . . . . . . 1267.3.2 Hysteresis in contact angle measurements . . . . . . . . . . . . . . . 1287.3.3 Surface roughness and heterogeneity . . . . . . . . . . . . . . . . . . 129

7.4 Theoretical aspects of contact angle phenomena . . . . . . . . . . . . . . . . 1317.5 Dynamics of wetting and dewetting . . . . . . . . . . . . . . . . . . . . . . 133

7.5.1 Wetting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1337.5.2 Dewetting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

7.6 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1387.6.1 Flotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1387.6.2 Detergency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1407.6.3 Microfluidics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1417.6.4 Adjustable wetting . . . . . . . . . . . . . . . . . . . . . . . . . . . 142


8 Solid surfaces 1458.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1458.2 Description of crystalline surfaces . . . . . . . . . . . . . . . . . . . . . . . 146

8.2.1 The substrate structure . . . . . . . . . . . . . . . . . . . . . . . . . 146

X Contents

8.2.2 Surface relaxation and reconstruction . . . . . . . . . . . . . . . . . 1478.2.3 Description of adsorbate structures . . . . . . . . . . . . . . . . . . . 149

8.3 Preparation of clean surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 1508.4 Thermodynamics of solid surfaces . . . . . . . . . . . . . . . . . . . . . . . 153

8.4.1 Surface stress and surface tension . . . . . . . . . . . . . . . . . . . 1538.4.2 Determination of the surface energy . . . . . . . . . . . . . . . . . . 1548.4.3 Surface steps and defects . . . . . . . . . . . . . . . . . . . . . . . . 157

8.5 Solidsolid boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1598.6 Microscopy of solid surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 162

8.6.1 Optical microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 1628.6.2 Electron microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 1628.6.3 Scanning probe microscopy . . . . . . . . . . . . . . . . . . . . . . 164

8.7 Diffraction methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1678.7.1 Diffraction patterns of two-dimensional periodic structures . . . . . . 1688.7.2 Diffraction with electrons, X-rays, and atoms . . . . . . . . . . . . . 168

8.8 Spectroscopic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1718.8.1 Spectroscopy using mainly inner electrons . . . . . . . . . . . . . . . 1718.8.2 Spectroscopy with outer electrons . . . . . . . . . . . . . . . . . . . 1738.8.3 Secondary ion mass spectrometry . . . . . . . . . . . . . . . . . . . 174


9 Adsorption 1779.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

9.1.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1779.1.2 The adsorption time . . . . . . . . . . . . . . . . . . . . . . . . . . 1789.1.3 Classification of adsorption isotherms . . . . . . . . . . . . . . . . . 1799.1.4 Presentation of adsorption isotherms . . . . . . . . . . . . . . . . . . 181

9.2 Thermodynamics of adsorption . . . . . . . . . . . . . . . . . . . . . . . . . 1829.2.1 Heats of adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . 1829.2.2 Differential quantities of adsorption and experimental results . . . . . 184

9.3 Adsorption models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1859.3.1 The Langmuir adsorption isotherm . . . . . . . . . . . . . . . . . . . 1859.3.2 The Langmuir constant and the Gibbs energy of adsorption . . . . . . 1889.3.3 Langmuir adsorption with lateral interactions . . . . . . . . . . . . . 1899.3.4 The BET adsorption isotherm . . . . . . . . . . . . . . . . . . . . . 1899.3.5 Adsorption on heterogeneous surfaces . . . . . . . . . . . . . . . . . 1929.3.6 The potential theory of Polanyi . . . . . . . . . . . . . . . . . . . . . 193

9.4 Experimental aspects of adsorption from the gas phase . . . . . . . . . . . . 1959.4.1 Measurement of adsorption isotherms . . . . . . . . . . . . . . . . . 1959.4.2 Procedures to measure the specific surface area . . . . . . . . . . . . 1989.4.3 Adsorption on porous solids hysteresis . . . . . . . . . . . . . . . 1999.4.4 Special aspects of chemisorption . . . . . . . . . . . . . . . . . . . . 201

9.5 Adsorption from solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2029.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2039.7 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

Contents XI

10 Surface modification 20610.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20610.2 Chemical vapor deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . 20710.3 Soft matter deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

10.3.1 Self-assembled monolayers . . . . . . . . . . . . . . . . . . . . . . 20910.3.2 Physisorption of Polymers . . . . . . . . . . . . . . . . . . . . . . . 21210.3.3 Polymerization on surfaces . . . . . . . . . . . . . . . . . . . . . . . 215

10.4 Etching techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21710.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22110.6 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

11 Friction, lubrication, and wear 22311.1 Friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223

11.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22311.1.2 Amontons and Coulombs Law . . . . . . . . . . . . . . . . . . . . 22411.1.3 Static, kinetic, and stick-slip friction . . . . . . . . . . . . . . . . . . 22611.1.4 Rolling friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22811.1.5 Friction and adhesion . . . . . . . . . . . . . . . . . . . . . . . . . . 22911.1.6 Experimental Aspects . . . . . . . . . . . . . . . . . . . . . . . . . 23011.1.7 Techniques to measure friction . . . . . . . . . . . . . . . . . . . . . 23011.1.8 Macroscopic friction . . . . . . . . . . . . . . . . . . . . . . . . . . 23211.1.9 Microscopic friction . . . . . . . . . . . . . . . . . . . . . . . . . . 232

11.2 Lubrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23611.2.1 Hydrodynamic lubrication . . . . . . . . . . . . . . . . . . . . . . . 23611.2.2 Boundary lubrication . . . . . . . . . . . . . . . . . . . . . . . . . . 23811.2.3 Thin film lubrication . . . . . . . . . . . . . . . . . . . . . . . . . . 23911.2.4 Lubricants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240

11.3 Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24111.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24411.5 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

12 Surfactants, micelles, emulsions, and foams 24612.1 Surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24612.2 Spherical micelles, cylinders, and bilayers . . . . . . . . . . . . . . . . . . . 250

12.2.1 The critical micelle concentration . . . . . . . . . . . . . . . . . . . 25012.2.2 Influence of temperature . . . . . . . . . . . . . . . . . . . . . . . . 25212.2.3 Thermodynamics of micellization . . . . . . . . . . . . . . . . . . . 25312.2.4 Structure of surfactant aggregates . . . . . . . . . . . . . . . . . . . 25512.2.5 Biological membranes . . . . . . . . . . . . . . . . . . . . . . . . . 258

12.3 Macroemulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25912.3.1 General properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 25912.3.2 Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26112.3.3 Stabilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26212.3.4 Evolution and aging . . . . . . . . . . . . . . . . . . . . . . . . . . 26512.3.5 Coalescence and demulsification . . . . . . . . . . . . . . . . . . . . 267

XII Contents

12.4 Microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26812.4.1 Size of droplets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26812.4.2 Elastic properties of surfactant films . . . . . . . . . . . . . . . . . . 26912.4.3 Factors influencing the structure of microemulsions . . . . . . . . . . 270

12.5 Foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27212.5.1 Classification, application and formation . . . . . . . . . . . . . . . 27212.5.2 Structure of foams . . . . . . . . . . . . . . . . . . . . . . . . . . . 27412.5.3 Soap films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27412.5.4 Evolution of foams . . . . . . . . . . . . . . . . . . . . . . . . . . . 277


13 Thin films on surfaces of liquids 28013.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28013.2 Phases of monomolecular films . . . . . . . . . . . . . . . . . . . . . . . . . 28313.3 Experimental techniques to study monolayers . . . . . . . . . . . . . . . . . 286

13.3.1 Optical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28613.3.2 X-ray reflection and diffraction . . . . . . . . . . . . . . . . . . . . . 28713.3.3 The surface potential . . . . . . . . . . . . . . . . . . . . . . . . . . 29013.3.4 Surface elasticity and viscosity . . . . . . . . . . . . . . . . . . . . . 292

13.4 LangmuirBlodgett transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . 29313.5 Thick films spreading of one liquid on another . . . . . . . . . . . . . . . . 29513.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29713.7 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

14 Solutions to exercises 299

Appendix

A Analysis of diffraction patterns 321A.1 Diffraction at three dimensional crystals . . . . . . . . . . . . . . . . . . . . 321

A.1.1 Bragg condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321A.1.2 Laue condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321A.1.3 The reciprocal lattice . . . . . . . . . . . . . . . . . . . . . . . . . . 323A.1.4 Ewald construction . . . . . . . . . . . . . . . . . . . . . . . . . . . 325

A.2 Diffraction at Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325A.3 Intensity of diffraction peaks . . . . . . . . . . . . . . . . . . . . . . . . . . 327

B Symbols and abbreviations 331

Bibliography 335

Index 355

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 solidliquid,the solidgas, and the liquidgas interface. These interfaces are also called surfaces. Interfaceis, however, a more general term than surface. Interfaces can also separate two immiscibleliquids such as water and oil. These are called liquidliquid interfaces. Solidsolid interfacesseparate two solid phases. They are important for the mechanical behavior of solid materials.Gasgas interfaces do not exist because gases mix.

Often interfaces and colloids are discussed together. Colloids are disperse systems, inwhich one phase has dimensions in the order of 1 nm to 1 m (see Fig. 1.1). The wordcolloid comes from the Greek word for glue and has been used the first time in 1861 byGraham1. He applied it to materials which seemed to dissolve but were not able to penetrate amembrane, such as albumin, starch, and dextrin. A dispersion is a two-phase system which isuniform on the macroscopic but not on the microscopic scale. It consists of grains or dropletsof one phase in a matrix of the other phase.

Different kinds of dispersions can be formed. Most of them have important applicationsand have special names (Table 1.1). While there are only five types of interface, we can distin-guish 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 isobvious. Nobody will, for instance, mix up fog with a foam although in both cases a liquid anda gas are involved. In other cases the distinction between continuous and inner phase cannotbe made because both phases might form connected networks. Some emulsions for instancetend to form a bicontinuous phase, in which both phases form an interwoven network.

Continuousphase

Dispersedphase

1 nm - 1 m

Figure 1.1: Schematic of a dispersion.

Colloids and interfaces are intimately related. This is a direct consequence of their enor-mous specific surface area. More precisely: their interface-to-volume relation is so large, thattheir behavior is completely determined by surface properties. Gravity is negligible in the

1 Thomas Graham, 18051869. British chemist, professor in Glasgow and London.

Physics and Chemistry of InterfacesPrefaceContents1 Introduction2 Liquid surfaces2.1 Microscopic picture of the liquid surface2.2 Surface tension2.3 Equation of Young and Laplace2.3.1 Curved liquid surfaces2.3.2 Derivation of the YoungLaplace equation2.3.3 Applying the YoungLaplace equation

2.4 Techniques to measure the surface tension2.5 The Kelvin equation2.6 Capillary condensation2.7 Nucleation theory2.8 Summary2.9 Exercises

3 Thermodynamics of interfaces3.1 The surface excess3.2 Fundamental thermodynamic relations3.2.1 Internal energy and Helmholtz energy3.2.2 Equilibrium conditions3.2.3 Location of the interface3.2.4 Gibbs energy and definition of the surface tension3.2.5 Free surface energy, interfacial enthalpy and Gibbs surface energy

3.3 The surface tension of pure liquids3.4 Gibbs adsorption isotherm3.4.1 Derivation3.4.2 System of two components3.4.3 Experimental aspects3.4.4 The Marangoni effect

3.5 Summary3.6 Exercises

4 The electric double layer4.1 Introduction4.2 PoissonBoltzmann theory of the diffuse double layer4.2.1 The PoissonBoltzmann equation4.2.2 Planar surfaces4.2.3 The full one-dimensional case4.2.4 The Grahame equation4.2.5 Capacity of the diffuse electric double layer

4.3 Beyond PoissonBoltzmann theory4.3.1 Limitations of the PoissonBoltzmann theory4.3.2 The Stern layer

4.4 The Gibbs free energy of the electric double layer4.5 Summary4.6 Exercises

5 Effects at charged interfaces5.1 Electrocapillarity5.1.1 Theory5.1.2 Measurement of electrocapillarity

5.2 Examples of charged surfaces5.2.1 Mercury5.2.2 Silver iodide5.2.3 Oxides5.2.4 Mica5.2.5 Semiconductors

5.3 Measuring surface charge densities5.3.1 Potentiometric colloid titration5.3.2 Capacitances

5.4 Electrokinetic phenomena: The zeta potential5.4.1 The NavierStokes equation5.4.2 Electro-osmosis and streaming potential5.4.3 Electrophoresis and sedimentation potential

5.5 Types of potentials5.6 Summary5.7 Exercises

6 Surface forces6.1 Van der Waals forces between molecules6.2 The van der Waals force between macroscopic solids6.2.1 Microscopic approach6.2.2 Macroscopic calculation Lifshitz theory6.2.3 Surface energy and Hamaker constant

6.3 Concepts for the description of surface forces6.3.1 The Derjaguin approximation6.3.2 The disjoining pressure

6.4 Measurement of surface forces6.5 The electrostatic double-layer force6.5.1 General equations6.5.2 Electrostatic interaction between two identical surfaces6.5.3 The DLVO theory

6.6 Beyond DLVO theory6.6.1 The solvation force and confined liquids6.6.2 Non DLVO forces in an aqueous medium

6.7 Steric interaction6.7.1 Properties of polymers6.7.2 Force between polymer coated surfaces

6.8 Spherical particles in contact6.9 Summary6.10 Exercises

7 Contact angle phenomena and wetting7.1 Youngs equation7.1.1 The contact angle7.1.2 Derivation7.1.3 The line tension7.1.4 Complete wetting

7.2 Important wetting geometries7.2.1 Capillary rise7.2.2 Particles in the liquidgas interface7.2.3 Network of fibres

7.3 Measurement of the contact angle7.3.1 Experimental methods7.3.2 Hysteresis in contact angle measurements7.3.3 Surface roughness and heterogeneity

7.4 Theoretical aspects of contact angle phenomena7.5 Dynamics of wetting and dewetting7.5.1 Wetting7.5.2 Dewetting

7.6 Applications7.6.1 Flotation7.6.2 Detergency7.6.3 Microfluidics7.6.4 Adjustable wetting


8 Solid surfaces8.1 Introduction8.2 Description of crystalline surfaces8.2.1 The substrate structure8.2.2 Surface relaxation and reconstruction8.2.3 Description of adsorbate structures

8.3 Preparation of clean surfaces8.4 Thermodynamics of solid surfaces8.4.1 Surface stress and surface tension8.4.2 Determination of the surface energy8.4.3 Surface steps and defects

8.5 Solidsolid boundaries8.6 Microscopy of solid surfaces8.6.1 Optical microscopy8.6.2 Electron microscopy8.6.3 Scanning probe microscopy

8.7 Diffraction methods8.7.1 Diffraction patterns of two-dimensional periodic structures8.7.2 Diffraction with electrons, X-rays, and atoms

8.8 Spectroscopic methods8.8.1 Spectroscopy using mainly inner electrons8.8.2 Spectroscopy with outer electrons8.8.3 Secondary ion mass spectrometry


9 Adsorption9.1 Introduction9.1.1 Definitions9.1.2 The adsorption time9.1.3 Classification of adsorption isotherms9.1.4 Presentation of adsorption isotherms

9.2 Thermodynamics of adsorption9.2.1 Heats of adsorption9.2.2 Differential quantities of adsorption and experimental results

9.3 Adsorption models9.3.1 The Langmuir adsorption isotherm9.3.2 The Langmuir constant and the Gibbs energy of adsorption9.3.3 Langmuir adsorption with lateral interactions9.3.4 The BET adsorption isotherm9.3.5 Adsorption on heterogeneous surfaces9.3.6 The potential theory of Polanyi

9.4 Experimental aspects of adsorption from the gas phase9.4.1 Measurement of adsorption isotherms9.4.2 Procedures to measure the specific surface area9.4.3 Adsorption on porous solids hysteresis9.4.4 Special aspects of chemisorption

9.5 Adsorption from solution9.6 Summary9.7 Exercises

10 Surface modification10.1 Introduction10.2 Chemical vapor deposition10.3 Soft matter deposition10.3.1 Self-assembled monolayers10.3.2 Physisorption of Polymers10.3.3 Polymerization on surfaces

10.4 Etching techniques10.5 Summary10.6 Exercises

11 Friction, lubrication, and wear11.1 Friction11.1.1 Introduction11.1.2 Amontons and Coulombs Law11.1.3 Static, kinetic, and stick-slip friction11.1.4 Rolling friction11.1.5 Friction and adhesion11.1.6 Experimental Aspects11.1.7 Techniques to measure friction11.1.8 Macroscopic friction11.1.9 Microscopic friction

11.2 Lubrication11.2.1 Hydrodynamic lubrication11.2.2 Boundary lubrication11.2.3 Thin film lubrication11.2.4 Lubricants

11.3 Wear11.4 Summary11.5 Exercises

12 Surfactants, micelles, emulsions, and foams12.1 Surfactants12.2 Spherical micelles, cylinders, and bilayers12.2.1 The critical micelle concentration12.2.2 Influence of temperature12.2.3 Thermodynamics of micellization12.2.4 Structure of surfactant aggregates12.2.5 Biological membranes

12.3 Macroemulsions12.3.1 General properties12.3.2 Formation12.3.3 Stabilization12.3.4 Evolution and aging12.3.5 Coalescence and demulsification

12.4 Microemulsions12.4.1 Size of droplets12.4.2 Elastic properties of surfactant films12.4.3 Factors influencing the structure of microemulsions

12.5 Foams12.5.1 Classification, application and formation12.5.2 Structure of foams12.5.3 Soap films12.5.4 Evolution of foams


13 Thin films on surfaces of liquids13.1 Introduction13.2 Phases of monomolecular films13.3 Experimental techniques to study monolayers13.3.1 Optical methods13.3.2 X-ray reflection and diffraction13.3.3 The surface potential13.3.4 Surface elasticity and viscosity

13.4 LangmuirBlodgett transfer13.5 Thick films spreading of one liquid on another13.6 Summary13.7 Exercises

14 Solutions to exercisesA Analysis of diffraction patternsA.1 Diffraction at three dimensional crystalsA.1.1 Bragg conditionA.1.2 Laue conditionA.1.3 The reciprocal latticeA.1.4 Ewald construction

A.2 Diffraction at SurfacesA.3 Intensity of diffraction peaks

B Symbols and abbreviationsBibliographyIndex

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