Physical Chemistry of Surfaces

Physical Chemistry of Surfaces Fifth Edition ARTHUR W. ADAMSON Department of Chemistry, University of Southern California Los Angeles, California >) A WILEY-INTERSCIENCE PUBLICATION John Wiley &. Sons, Inc. NEW YORK / CHICHESTER / BRISBANE / TORONTO / SINGAPORE

Contents I. General Introduction 1 II. Capillarity 4 1. Surface Tension and Surface Free Energy 4 2. The Equation of Young and Laplace 7 3. Some Experiments with Soap Films 9 4. The Treatment of Capillary Rise 10 A. Introductory Discussion 10 B. Exact Solutions to the Capillary Rise Problem 12 C. Experimental Aspects of the Capillary Rise Method 18 5. The Maximum Bubble Pressure Method 18 6. The Drop Weight Method 21 7. The Ring Method 23 8. Wilhelmy Slide Method 25 9. Methods Based on the Shape of Static Drops or Bubbles 28 A. Pendant Drop Method 28 B. Sessile Drop or Bubble Method 30 C. Deformed Interfaces 36 10. Dynamic Methods of Measuring Surface Tension 38 A. Flow Methods 38 B. Capillary Waves 40 11. Surface Tension Values as Obtained by Different Methods... 40 12. Problems 45 General References 48 Textual References 49 III. The Nature and Thermodynamics of Liquid Interfaces 53 1. One-Component Systems 53 A. Surface Thermodynamic Quantities for a Pure Substance... 53 B. The Total Surface Energy, E s 56 C. Change in Vapor Pressure for a Curved Surface 58 D. Effect of Curvature on Surface Tension 59 E. Effect of Pressure on Surface Tension 60 2. The Structural and Theoretical Treatment of Liquid Interfaces 61 A. Further Development of the Thermodynamic Treatment of the Surface Region 63 XI

xii CONTENTS B. Calculation of the Surface Energy and Free Energy of Liquids 66 3. Orientation at Interfaces 69 4. The Surface Tension of Solutions 71 5. Thermodynamics of Binary Systems The Gibbs Equation.. 75 A. Definition of Surface Excess 75 B. The Gibbs Equation 77 C. Alternate Methods of Locating the Dividing Surface 79 D. The Thermodynamics of Surfaces Using the Concept of a Surface Phase 80 E. Other Surface Thermodynamic Relationships 81 6. Determination of Surface Excess Quantities 82 A. Experimental Methods 82 B. Historical Footnote and Commentary 85 C. Theoretical Calculation of Surface Excess Quantities 85 7. Gibbs Monolayers 86 A. The Two-Dimensional Ideal Gas Law 88 B. Nonideal Two-Dimensional Gases 90 C. The Osmotic Pressure Point of View 93 D. Surface Elasticity 95 E. Traube's Rule 96 F. Some Further Comments on Gibbs Monolayers 97 8. Problems 97 General References 101 Textual References 101 IV. Surface Films on Liquid Substrates 106 1. Introduction 106 2. The Spreading of One Liquid on Another 109 A. Criteria for Spreading 110 B. Empirical and Theoretical Treatments 113 C. Kinetics of Spreading Processes 116 D. The Marangoni Effect 117 E. Lenses Line Tension 118 3. Experimental Techniques for the Study of Monomolecular Films 119 A. Measurement of TT 119 B. Surface Potentials 122 C. Surface Viscosities 124 D. Optical Properties of Monolayers 127 E. The Ultramicroscope 129 F. Electron Microscopy and Diffraction 129 G. Other Techniques 130 4. States of Monomolecular Films 131 5. Correspondence between TT and a Three-Dimensional Pressure 134 6. Further Discussion of the States of Monomolecular Films 135 A. Gaseous Films 135 B. The L\-G Transition 136

CONTENTS xiii C. The Liquid Expanded State 137 D. Intermediate and L 2 Films 139 E. The Solid State 139 F. Effect of Changes in the Aqueous Substrate 141 G. Rheology of Monolayers 143 H. General Correlations between Molecular Structure and the Type of Film Formed 144 7. Mixed Films 145 8. Evaporation Rates through Monomolecular Films 151 9. Rate of Dissolving of Monolayers 153 10. Reactions in Monomolecular Films 155 A. Kinetics of Reactions in Films 155 B. Kinetics of Formation and Hydrolysis of Esters 156 C. Other Chemical Reactions 158 11. Films of Biological and Polymerie Materials 160 A. Protein and Other Polymer Films 160 B. Films of Biological Substances Other than Proteins 163 C. Films at the Oil-Water Interface 166 D. Polymer Films 168 12. Films at Liquid-Liquid Interfaces and on Liquid Surfaces Other than Water 168 13. Charged Films 169 A. Equation of State of a Charged Film 169 B. Interfacial Potentials 174 14. Capillary Waves 176 A. Externally Generated Waves 178 B. Thermal Waves 179 15. Langmuir-Blodgett Films 181 A. Built-Up Films 181 B. Monolayers 183 16. Bilayers and Vesicles 184 17. Problems 185 General References 188 Textual References 189 V. Electrical Aspects of Surface Chemistry 203 1. Introduction 203 2. The Electrical Double Layer 203 3. Units the SI System 208 A. Potential 208 B. Coulomb's Law and Equations of Electrostatics 209 4. The Stern Treatment of the Electrical Double Layer 210 5. Further Treatment of the Stern and Diffuse Layers 212 6. The Free Energy of a Diffuse Double Layer 214 7. Repulsion between Two Plane Double Layers 215 8. The Zeta Potential 218 A. Electrophoresis 218 B. Electroosmosis 220

XIV CONTENTS C. Streaming Potential 221 D. Sedimentation Potential 223 E. Cross Relationships in the Theory of Electrokinetic Phenomena 223 F. Potential, Surface Charge, and the Stability of CoUoidal Suspensions 223 9. Electrocapillarity 226 A. Thermodynamics of the Electrocapillary Effect 229 B. Experimental Methods 231 C. Results for the Mercury-Aqueous Solution Interface 233 D. Effect of Uncharged Solutes and Changes of Solvent 236 E. Other Electrocapillary Systems 237 10. The Electrified Solid-Liquid Interface 238 A. Electrode-Solution Interface 238 B. Dispersed Particle-Solution Interface 239 C. Photo-Electrochemistry: Solar Energy Conversion 239 11. Types of Potentials and the Meaning of Potential Differences When Two Phases Are Involved 240 A. The Various Types of Potentials 240 B. Volta Potentials, Surface Potential Differences, and the Thermionic Work Function 243 C. Electrode Potentials 244 D. Irreversible Electrode Phenomena 246 12. Problems 249 General References 252 Textual References 252 VI. Long-Range Forces 258 1. Introduction 258 2. Forces between Atoms and Molecules 259 3. The SI System 264 4. Long-Range Forces 265 A. The Microscopic Approach 265 B. The Retarded Dispersion Attraction 267 C. The Macroscopic Approach Lifshitz Theory 268 D. Experimental Verification 269 E. Hamaker Constant Values 272 5. Long-Range Forces in Solution 272 A. Dispersion Attraction in a Condensed Medium 273 B. Electrical Double-Layer Repulsion 274 C. The Discrete Region 276 6. The Disjoining Pressure 278 7. Evidence for Deep Surface Orientation 279 8. Anomalous Water 282 9. Dipole-Induced Dipole Propagation 283 10. Problems 284 General References 286 Textual References 287

CONTENTS xv VII. Surfaces of Solids 291 1. Introduction 291 A. The Surface Mobility of Solids 291 B. Effect of Past History on the Condition of Solid Surfaces 293 2. Thermodynamics of Crystals 294 A. Surface Tension and Surface Free Energy 294 B. The Equilibrium Shape of a Crystal 296 C. The Kelvin Equation 297 3. Theoretical Estimates of Surface Energies and Free Energies 298 A. Covalently Bonded Crystals 298 B. Rare Gas Crystals 299 C. Ionic Crystals 302 D. Molecular Crystals 304 E. Metals 305 4. Factors Affecting the Surface Energies and Surface Tensions of Actual Crystals 306 A. State of Subdivision 306 B. Deviations from Ideal Considerations 306 C. Fractal Surfaces 308 D. Dislocations 310 5. Experimental Estimates of Surface Energies and Free Energies 313 A. Methods Depending on the Direct Manifestation of Surface Tensional Forces 313 B. Surface Energies and Free Energies from Heats of Solution 315 C. Relative Surface Tensions from Equilibrium Shapes of Crystals 316 D. Dependence of Other Physical Properties on Surface Energy Changes at a Solid Interface 316 6. Reactions of Solid Surfaces 318 7. Problems 321 General References 323 Textual References 323 VIII. Surfaces of Solids: Microscopy and Spectroscopy 329 1. Introduction 329 2. Microscopy of Surfaces 329 A. Optical and Electron Microscopy 329 B. Scanning Electron Microscope 338 C. Profilometry. Scanning Tunneling and Atomic Force Microscopy 338 D. Field Emission and Field Ion Microscopy 339 3. Low Energy Electron Diffraction (LEED) 344 4. Spectroscopic Methods 349 A. Auger Electron Spectroscopy (AES) 349 B. Photoelectron Spectroscopy (ESCA) 351 C. Ion Scattering (ISS, LEIS) 352 5. Other Spectroscopies 354

XVI CONTENTS 6. Problems 354 General References 356 Textual References 356 IX. The Formation of a New Phase Nucleation and Crystal Growth 364 1. Introduction 364 2. Classical Nucleation Theory 365 3. Results of Nucleation Studies 371 4. Crystal Growth 373 5. Problems 375 General References 376 Textual References 376 X. The Solid-Liquid Interface Contact Angle 379 1. Introduction 379 2. Surface Free Energies from Solubility Changes 379 3. Surface Energy and Free Energy Differences from Immersion, Adsorption, and Engulfment Studies 381 A. Heat of Immersion 381 B. Surface Energy and Free Energy Changes from Adsorption Studies 382 C. Engulfment 385 4. Contact Angle 385 A. Young's Equation 385 B. Nonuniform Surfaces 387 5. Experimental Methods and Results of Contact Angle Measurements 389 A. Measurement of Contact Angle 389 B. Hysteresis in Contact Angle Measurements 392 C. Results of Contact Angle Measurements 396 6. Some Theoretical Aspects of Contact Angle Phenomena 403 A. Thermodynamics of the Young Equation 403 B. Semiempirical Models The Girifalco-Good-Fowkes- Young Equation 406 C. Potential-Distortion Model 408 D. The Microscopic Meniscus Profile 410 7. Problems 411 General References 413 Textual References 414 XI. The Solid-Liquid Interface Adsorption from Solution 421 1. Adsorption of Nonelectrolytes from Dilute Solution 421 A. Adsorption Isotherms 422 B. Qualitative Results of Adsorption Studies Traube' s Rule 426 C. Multilayer Adsorption 430 2. Adsorption of Polymers 431 3. Irreversible Adsorption 433

CONTENTS xvii 4. Surface Area Determination 435 5. Adsorption in Binary Liquid Systems 436 A. Adsorption at the Solid-Solution Interface 436 B. Heat of Adsorption at the Solid-Solution Interface 441 6. Adsorption of Electrolytes 441 A. Stern Layer Adsorption 442 B. Surface Areas from Negative Adsorption 446 C. Counterion Adsorption Ion Exchange 447 7. Photophysics and Photochemistry of the Adsorbed State 448 A. Photophysics of Adsorbed Species 448 B. Photochemistry at the Solid-Solution Interface 449 8. Problems 450 General References 452 Textual References 453 XII. Friction and Lubrication Adhesion 460 1. Introduction 460 2. Friction Between Unlubricated Surfaces 460 A. Amontons' Law 460 B. Nature of the Contact Between Two Solid Surfaces 461 C. Role of Shearing and Plowing Explanation of Amontons' Law 463 D. Static and Stick-Slip Friction 465 E. Rolling Friction 466 3. Two Special Cases of Friction 467 A. Use of Skid Marks to Estimate Vehicle Speeds 467 B. Ice and Snow 468 4. Metallic Friction Effect of Oxide Films 469 5. Friction Between Nonmetals 470 A. Relatively Isotropie Crystals 470 B. Layer Crystals 470 C. Plastics 470 6. Some Further Aspects of Friction 471 7. Friction Between Lubricated Surfaces 473 A. Boundary Lubrication 473 B. The Mechanism of Boundary Lubrication 475 C. Forces and Friction Between Smooth Surfaces 482 8. Adhesion 482 A. Ideal Adhesion 483 B. Practical Adhesion 485 9. Problems 487 General References 488 Textual References 488 XIII. Wetting, Flotation, and Detergency 493 1. Introduction 493 2. Wetting 493 A. Wetting as a Contact Angle Phenomenon 493 B. Wetting as a Capillary Action Phenomenon 495

XV111 CONTENTS 3. Water Repellency 497 4. Flotation 498 A. The Role of Contact Angle in Flotation 499 B. Flotation of Metallic Minerals 503 C. Flotation of Nonmetallic Minerals 505 5. Detergency 506 A. General Aspects of Soil Removal 507 B. Properties of Colloidal Electrolyte Solutions 508 C. Factors in Detergent Action 513 D. Adsorption of Detergents on Fabrics 515 E. Detergents in Commercial Use 516 F. Additional Effects and Applications Involving Micellar Systems 516 6. Problems 517 General References 519 Textual References 519 XIV. Emulsions, Foams, and Aerosols 525 1. Introduction 525 2. Emulsions General Properties 526 3. Factors Determining Emulsion Stability 528 A. Macroscopic Theories of Emulsion Stabilization 528 B. Specific Chemical and Structural Effects 529 C. Long-Range Forces as a Factor in Emulsion Stability 531 D. Stabilization of Emulsions by Solid Particles 533 4. The Aging and Inversion of Emulsions 534 A. Flocculation and Coagulation Kinetics 535 B. Inversion and Breaking of Emulsions 536 5. The Hydrophile-Lipophile Balance 537 6. Microemulsions 539 7. Foams Their Structure 544 8. Foam Drainage 546 A. Drainage of Single Films 546 B. Drainage of Foams 548 9. The Stability of Foams 549 10. Aerosols 551 11. Problems 551 General References 552 Textual References 553 XV. The Solid-Gas Interface General Considerations 560 1. Introduction 560 2. The Surface Area of Solids 561 A. The Meaning of Surface Area 561 B. Surfaces as Having a Fractal Geometry 563 C. Methods Requinng Knowledge of the Surface Free Energy or Total Energy 564 D. Rate of Dissolving 566

CONTENTS xix E. The Mercury Porosimeter 567 F. Other Methods of Surface Area Estimation 570 3. The Structural and Chemical Nature of Solid Surfaces 571 4. The Nature of the Solid-Adsorbate Complex 572 A. Effect of Adsorption on Adsorbate Properties 572 B. Effect of the Adsorbate on the Adsorbent 580 C. The Adsorbate-Adsorbent Bond 582 5. Problems 583 General References 585 Textual References 585 XVI. Adsorption of Gases and Vapors on Solids 591 1. Introduction 591 2. The Adsorption Time 593 3. The Langmuir Adsorption Isotherm 595 A. Kinetic Derivation 595 B. Statistical Thermodynamic Derivation 598 C. Adsorption Entropies 601 D. Lateral Interaction 604 E. Experimental Applications of the Langmuir Equation 606 4. Experimental Procedures 606 5. The BET and Related Isotherms 609 A. Derivation of the BET Equation 610 B. Properties of the BET Equation 612 C. Modifications of the BET Equation 613 6. Isotherms Based on the Equation of State of the Adsorbed Film 614 A. Film Pressure-Area Diagrams from Adsorption Isotherms 614 B. Adsorption Isotherms from Two-Dimensional Equations of State 615 7. The Potential Theory 617 A. The Polanyi Treatment 617 B. Correspondence between the Potential Theory and That of a Two-Dimensional Film 619 C Isotherms Based on an Assumed Variation of Potential with Distance 620 D. The Polarization Model 622 8. Comparison of the Surface Areas from the Various Multilayer Models 623 9. The Characteristic Isotherm and Related Concepts 625 10. Potential Theory as Applied to Submonolayer Adsorption... 627 11. Adsorption Steps and Phase Transformations 630 12. Thermodynamics of Adsorption 633 A. Theoretical Considerations 633 B. Experimental Heats and Entropies of Adsorption 639 13. Critical Comparison of the Various Models for Adsorption... 646 A. The Langmuir-BET Model 647 B. Two-Dimensional Equation of State Treatments 648 C. The Potential Model 648

XX CONTENTS 14. Adsorption on Heterogeneous Surfaces 649 A. Site Energy Distributions 650 B. Thermodynamics of Adsorption on Heterogeneous Surfaces 657 C. Geometrie Heterogeneity 658 15. Rate of Adsorption 658 16. Adsorption on Porous Solids Hysteresis 659 A. Molecular Sieves 659 B. Capillary Condensation 661 C. Micropore Analysis 666 17. Problems 669 General References 672 Textual References 673 XVII. Chemisorption and Catalysis 682 1. Introduction 682 2. Chemisorption The Molecular View 683 A. LEED Structures 683 B. Spectroscopy of Chemisorbed Species 685 C. Work Function and Related Measurements 687 D. Programmed Desorption 690 3. Chemisorption Isotherms 692 A. Variable Heat of Adsorption 693 B. Effect of Site and Adsorbate Coordination Number 696 C. Adsorption Thermodynamics 697 4. Kinetics of Chemisorption 697 A. Activation Energies 697 B. Rates of Adsorption 699 C. Rates of Desorption 702 5. Surface Mobility 703 6. The Chemisorption Bond 706 A. Some General Aspects 706 B. Metals 707 C. Semiconductors 710 D. Acid-Base Systems 711 7. Mechanisms of Heterogeneous Catalysis 713 A. Adsorption or Desorption as the Rate-determining Step.. 714 B. Reaction Within the Adsorbed Film as the Rate-determining Step 715 8. Influence of the Adsorption Isotherm on the Kinetics of Heterogeneous Catalysis 717 A. Unimolecular Surface Reactions 718 B. Bimolecular Surface Reactions 720 9. Mechanisms of a Few Catalyzed Reactions 721 A. Ammonia Synthesis 722 B. Fischer-Tropsch Reactions 724 C. Hydrogenation of Ethylene 726 D. Catalytic Cracking of Hydrocarbons and Related Reactions 728 E. Photochemical and Photoassisted Processes at Surfaces.. 730

CONTENTS xxi 10. Problems 731 General References 733 Textual References 734 Index 747