Myers d surfaces, interfaces and colloids (2ed , wiley, 1999)(t)(519s)

Surfaces, Interfaces, and Colloids: Principles and Applications, Second Edition Drew Myers Copyright
1999 John Wiley & Sons, Inc ISBNs: 0-471-33060-4 (Hardback); 0-471-23499-0 (Electronic) SURFACES, INTERFACES, AND COLLOIDS SURFACES, INTERFACES, AND COLLOIDS Principles and Applications SECOND EDITION Drew Myers New York • Chichester • Weinheim • Brisbane • Singapore • Toronto Designations used by companies to distinguish their products are often claimed as trademarks In all inatances where John Wiley & Sons, Inc., is aware of a claim, the product names appear in initial capital or ALL CAPITAL LETTERS Readers, however, should contact the appropriate companies for more complete information regarding trademark and registration Copyright © 1999 by John Wiley & Sons, Inc All rights reserved No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic or mechanical, including uploading, downloading, printing, decompiling, recording or otherwise, except as permitted under Sections 107 or 108 of the 1976 United States Copyright Act, without the prior written permission of the Publisher Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 605 Third Avenue, New York, NY 10158-0012, (212) 850-6008, fax (212) 850-6008, E-Mail: PERMREQ@WILEY.COM This publication is designed to provide accurate and authoritative information in regard to the subject matter covered It is sold with the understanding that the publisher is not engaged in rendering professional services If professional advice or other expert assistance is required, the services of a competent professional person should be sought ISBN 0-471-22111-2 This title is also available in print as ISBN 0-471-41717-3 For more information about Wiley products, visit our web site at www.Wiley.com This work is dedicated to: Christine Betty Jimmy Linda CONTENTS Preface to the Second Edition xvii Preface to the First Edition xix Surfaces and Colloids: The Twilight Zone 1.1 1.2 1.3 Introduction: The World of Neglected Dimensions An Historical Prospective A View to the Future Problems Surfaces and Interfaces: General Concepts 2.1 2.2 3.3 3.4 21 Basic Structural Requirements for Surface Activity 21 Surfactant Structures and Sources 23 3.2.1 The Classification of Surfactants 24 3.2.2 Building Blocks for Surfactant Molecules 25 3.2.3 Surfactant Solubilizing Groups 29 3.2.4 Common Surfactant Hydrophobic Groups 30 The Economic Importance of Surfactants 34 Surfactants in the Environment 36 3.4.1 Biodegradation of Surfactants 36 3.4.2 Rules for Biodegradation 37 Problems 38 Attractive Forces 4.1 4.2 The Nature of Interfaces Surface Free Energy 10 2.2.1 The Work of Cohesion and Adhesion 13 2.2.2 Standard Reference States 18 2.2.3 The Molecular Nature of the Interfacial Region 18 Problems 20 Surface Activity and Surfactant Structures 3.1 3.2 40 Chemical and Physical Interactions 40 The Importance of Long-Range Physical Forces 41 vii viii CONTENTS 4.3 4.4 4.5 4.6 4.7 Classification of Physical Forces 42 4.3.1 Coulombic or Electrostatic Interactions 43 4.3.2 Other Interactions Involving Ions 45 van der Waals Forces 55 4.4.1 Dipole–Dipole Interactions 55 4.4.2 Angle-Averaged Dipolar Interactions 57 4.4.3 Dipole-Induced Dipole Interactions 57 4.4.4 The London–van der Waals (Dispersion) Force 58 4.4.5 Total van der Waals Interactions between Polar Molecules 62 4.4.6 Effects of a Nonvacuum Medium 64 Interactions between Surfaces and Particles 66 4.5.1 Surface Interactions in Nonvacuum Media 67 4.5.2 Dipole, Induced Dipole, and Hydrogen Bonding (Acid–Base) Interactions at Interfaces 68 Lifshitz Theory: A Continuum Approach 69 4.6.1 Some Shortcomings of the Hamaker and Lifshitz Theories 72 4.6.2 Hard Sphere Diameter Effects 72 Hydrodynamic Flow Effects in Interfacial Interactions 74 Problems 77 Electrostatic Forces and the Electrical Double Layer 5.1 5.2 5.3 Sources of Interfacial Charge 79 5.1.1 Differential Ion Solubility 81 5.1.2 Direct Ionization of Surface Groups 81 5.1.3 Substitution of Surface Ions 82 5.1.4 Specific-Ion Adsorption 82 5.1.5 Anisotropic Crystals 83 Electrostatic Theory: Coulomb’s Law 83 5.2.1 Boltzman’s Distribution and the Electrical Double Layer 84 5.2.2 Double-Layer Thickness: The Debye Length 85 5.2.3 Specific-Ion Adsorption and the Stern Layer 88 Electrokinetic Phenomena 91 5.3.1 Particle Electrophoresis 92 5.3.2 Moving-Boundary Electrophoresis 93 5.3.3 Gel (Zone) Electrophoresis 93 79 CONTENTS ix 5.3.4 Some Practical Comments on Electrokinetic Characteristics 94 Problems 96 Capillarity 6.1 6.2 6.3 Fluid Properties and Dynamics 97 A Capillary Model 100 Capillary Driving Forces in Liquid–Fluid Systems 101 6.3.1 Solid–Liquid–Fluid Systems: the Effect of Contact Angle 103 6.3.2 Capillary Flow and Spreading Processes 104 6.3.3 Geometric Considerations in Capillary Flow 107 6.3.4 Measurement of Capillary Driving Forces 109 6.3.5 Complications to Capillary Flow Analysis 112 6.3.6 Rates and Patterns of Capillary Flow 117 6.4 Some Practical Capillary Systems 118 97 6.4.1 6.4.2 6.4.3 Wetting in Woven Fibers and Papers 118 Waterproofing or Repellency Control 121 Capillary Action in Detergency Processes 122 Problems 123 Solid Surfaces 7.1 7.2 7.3 7.4 Surface Mobility in Solids 125 7.1.1 Sintering 128 ‘‘History’’ and the Characteristics of Solid Surfaces 129 Solid Surface Free Energy vs Surface Tension 130 The Formation of Solid Surfaces 132 7.4.1 Crystalline Surfaces 132 7.4.2 Nucleation Processes 133 7.4.3 Amorphous Solid Surfaces 135 Problems 138 Liquid–Fluid Interfaces 8.1 125 The Nature of a Liquid Surface: Surface Tension 140 8.1.1 Surface Mobility 142 8.1.2 Temperature Effects on Surface Tension 143 140 x CONTENTS 8.2 8.3 8.4 8.5 8.6 8.1.3 8.1.4 Surface 8.2.1 The Effect of Surface Curvature 144 Dynamic Surface Tension 145 Tensions of Solutions 147 Surfactants and the Reduction of Surface Tension 150 8.2.2 Effects of Phase Densities 151 Surfactant Adsorption and Gibbs Monolayers 151 8.3.1 Efficiency, Effectiveness, and Surfactant Structure 152 8.3.2 Adsorption Effectiveness 154 Insoluble Monomolecular Films 158 8.4.1 Surface Pressure 160 8.4.2 Surface Potential 161 8.4.3 Surface Rheology 161 The Physical States of Monolayer Films 162 8.5.1 Gaseous Films 163 8.5.2 Liquid Films 164 8.5.3 Condensed Films 165 8.5.4 Some Factors Affecting the Type of Film Formed 167 8.5.5 Mixed-Film Formation 170 8.5.6 Surface Films of Polymers and Proteins 171 8.5.7 Monolayer Films at Liquid–Liquid Interfaces and on Nonaqueous Liquids 172 8.5.8 Deposited Monolayers and Multilayer Films 173 A Final Comment 174 Problems 174 Adsorption 9.1 9.2 9.3 Introduction 179 9.1.1 The Gibbs Surface Excess 180 9.1.2 The Gibbs Adsorption Equation 183 Adsorption at the Solid–Vapor Interface 186 9.2.1 Energetic Considerations: Physical Adsorption versus Chemisorption 187 9.2.2 Chemisorption and Heterogeneous Catalysis 190 9.2.3 Catalytic Promoters and Poisons 193 Solid–Vapor Adsorption Isotherms 193 9.3.1 Classification of Adsorption Isotherms 194 9.3.2 The Langmuir Isotherm 196 9.3.3 The Freundlich Adsorption Isotherm 197 179 CONTENTS xi 9.3.4 9.4 9.5 9.6 The Brunauer–Emmett–Teller (BET) Isotherm 198 9.3.5 Surface Areas from the BET Isotherm 198 Adsorption at Solid–Liquid Interfaces 199 The Adsorption Model 200 Quantification of Surfactant Adsorption 202 9.6.1 Adsorption Isotherms in Solid–Liquid Systems 202 9.6.2 Adsorption and Modification of the Solid–Liquid Interface 204 9.6.3 Adsorption and Nature of the Adsorbent Surface 204 9.6.4 Environmental Effects on Adsorption 208 9.6.5 Effects of Adsorption on the Nature of the Solid Surface 210 Problems 211 10 Colloids and Colloidal Stability 10.1 10.2 10.3 10.4 10.5 10.6 10.7 The Importance of Colloids and Colloidal Phenomena 214 Colloids: A Working Definition 215 10.2.1 Colloid Structure 216 10.2.2 Colloid Size 218 10.2.3 Some Points of Nomenclature 218 Mechanisms of Colloid Formation 219 10.3.1 Comminution or Dispersion Methods 219 10.3.2 Condensation Methods 221 The ‘‘Roots’’ of Colloidal Behavior 222 Ground Rules for Colloidal Stability 223 10.5.1 A Problem of Semantics 225 10.5.2 Mechanisms of Stabilization 226 10.5.3 A Review of Basic Intermolecular Forces 226 10.5.4 Fundamental Interparticle Forces 228 10.5.5 Attractive Interactions in Nonvacuum Media 229 Sources of Colloidal Stability 230 10.6.1 Charged Surfaces and the Electrical Double Layer 231 10.6.2 Some Complicating Factors 231 Steric or Enthalpic Stabilization 233 10.7.1 The Mechanism of Steric Stabilization 234 10.7.2 Solvent Effects in Steric Stabilization 236 10.7.3 Effects of Polymer Molecular Weight 237 10.7.4 Depletion Flocculation 238 214 xii CONTENTS 10.8 10.9 Coagulation Kinetics 236 10.8.1 Kinetics of Particle Collisions: Fast Coagulation 239 10.8.2 Slow Coagulation 241 10.8.3 Critical Coagulation Concentration 243 10.8.4 The Deryagin–Landau–Verwey–Overbeek (DLVO) Theory 244 10.8.5 Reversible Flocculation and the Secondary Minimum 247 The Complete Interaction Curve 248 Problems 248 11 Emulsions 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 253 Fundamental Concepts in Emulsion Science and Technology 253 Emulsion Formation 254 Emulsions and the Liquid–Liquid Interface 255 11.3.1 Classification of Emulsifiers and Stabilizers 256 11.3.2 What Determines Emulsion Types? 258 Adsorption at Liquid–Liquid Interfaces 259 General Considerations of Emulsion Formation and Stability 261 Some Mechanistic Details of Stabilization 262 11.6.1 Polymeric Emulsifiers and Stabilizers 263 11.6.2 Solid Particles 264 11.6.3 Surfactants 265 11.6.4 Surfactant Structure and Emulsion Performance 265 11.6.5 Liquid Crystals and Emulsion Stability 266 11.6.6 Mixed Surfactant Systems and Interfacial Complexes 267 11.6.7 Emulsion Type 268 11.6.8 The Hydrophile–Lipophile Balance (HLB) 270 11.6.9 Cohesive Energies and the Solubility Parameter 273 Solubility Parameters, Surfactants, and Emulsions 278 The Relationship between HLB and Solubility Parameter 281 The Geometric Approach 282 11.9.1 Phase Inversion Temperature (PIT) 283 11.9.2 Application of HLB and PIT in Emulsion Formulation 284 11.9.3 Some Other Factors Affecting Stability 286 19.3 PRACTICAL ADHESION 479 joints with materials interacting through the universal dispersion energy, with no need to invoke stronger molecular interactions, mechanical interlocking, or chemical bond formation Experience, unfortunately, generally indicates otherwise A simple analysis of the situation indicates that the adhesive strength between two dissimilar materials should be greater than the cohesive strength of the weaker of the two, leading to cohesive failure rather than adhesive failure In some instances, particularly in the adhesive component of friction (see Chapter 18) that concept seems to be borne out since shearing of the contact area between surfaces normally occurs in the weaker material In adhesive joints, however, the area of contact between surfaces is several orders of magnitude greater than that in friction, and the question of the completeness of interfacial contact becomes important A liquid spreading over a rough surface (the normal situation) can easily trap air in depressions in the surfaces, leading to the formation of a composite surface (Fig 19.2) That will be the case whether the spreading liquid has a small or large contact angle, ␪, although a small advancing angle, ␪A, will obviously improve matters When a portion of the composite involves air– adhesive interfaces, the actual area of adhesive contact is greatly reduced In addition, the three phase boundaries thus formed represent excellent sites for the initiation of cracks and flaws in the system The net result—a significantly weakened joint Similar effects can be seen if the entrapped material is water, oils, or other materials with significantly lower adhesional interactions or cohesion strength In practice, it is usually found that the actual adhesive strength of a joint prepared with a ‘‘good’’ (i.e., wetting) adhesive will be at least one order of magnitude less than the ideal value Poorly wetting systems would be expected to perform correspondingly less effectively The primary reason for the discrepancy between ideal and real adhesive strength appears to lie in the almost invariable presence of bubbles, cracks, and flaws that are associated with the interfacial zone (Fig 19.3) When stress is applied to a joint, it tends to concentrate at such flaws, so that the ‘‘local’’ stress is significantly greater than the average value When the local stress exceeds the local strength (already lessened by the presence of the composite interface, e.g.), failure occurs Gas bubbles trapped by spreading adhesive FIGURE 19.2 Since all surfaces have a certain degree of roughness, it is common that an adhesive applied to such surfaces will entrap air bubbles, reducing the area of contact of adhesive with the surfaces and reducing the effectiveness of the bond or weld 480 ADHESION (a) (b) (c) (d) FIGURE 19.3 In what is generally classified as ‘‘adhesive failure,’’ breakage may occur at various locations including entrapped bubbles (a), at the interface (‘‘true’’ adhesive failure) (b), in the substrate (cohesive failure) (c), or in the adhesive (also cohesive failure) (d) The fracture of practical adhesive joints involves two primary processes— cohesive or adhesive failure at or near the joint and work (reversible and irreversible) involved in plastic, elastic, or viscoelastic deformation of one or all of the components of the joint—one of the two solid surfaces or the adhesive (Fig 19.4) As indicated in the preceding chapter on friction, cohesive failure of the weaker of two solids in contact is common The same can be said for normal adhesive joints, in that actual adhesive failure (i.e., exactly at the interface) is less common that cohesive failure, of, for example, the adhesive material, near the interface What, then, are the necessary conditions for obtaining ‘‘good’’ adhesion between two surfaces? Tension Adhesive failure Adhesive deformation zone Stress crack - cohesive failure FIGURE 19.4 When a tension or shear strain is placed on a joint, the energy may be dissipated by several mechanisms including adhesive and cohesive failure at various points, as already mentioned, but also by the plastic, elastic, or viscoelastic stretching of the adhesive and/or one or both substrates 19.4 SOME CONDITIONS FOR ‘‘GOOD’’ ADHESION 481 19.4 SOME CONDITIONS FOR ‘‘GOOD’’ ADHESION Any workable theory of adhesion must take into consideration all of the possible aspects of energy transfer across an adhesive joint, not only molecular forces or ideal adhesive strengths, but also the presence, number, and size of flaws, various energy dissipation processes, and irreversible fracture processes It has already been demonstrated that in an ideal situation, forces of molecular interaction should be sufficient to produce a very strong adhesive joint, assuming perfect contact across the interface, but that reality lies far from the ideal For reference purposes, Table 19.2 lists the various attractive molecular forces that may operate across an interface along with the approximate range of strengths they cover Since van der Waals forces, for example, fall off rapidly with distance of separation by r⫺3, for such forces to be effective the interacting surfaces must be as close as possible, typically 0.2–0.5 nm Beyond that distance, such interactions will be quite weak and the ability of the joint to transmit any applied stress will be accordingly reduced Clearly, intimate molecular contact between phases is a necessary condition for good adhesion—necessary but not sufficient! It is a fact of life that in practical adhesion problems, materials properties are often as important as interfacial forces It has been stated that a significant portion of the fracture energy of a joint is dissipated in various deformation processes Obviously, the nature of the joint interface in terms of physical ‘‘mixing’’ is an important aspect of the overall problem The intermolecular forces listed in Table 19.2 are common to all liquids and solids, depending on chemical composition, yet liquids have significantly lower mechanical strength than comparable solids In solids, the intermolecular distances are generally smaller and are reinforced by the forces stemming from the more ordered structure (crystal lattice, etc.) In polymers, which constitute the majority of adhesives, mechanical strength (e.g., the ability to TABLE 19.2 Values of Attractive Molecular Interactions at Interfaces Type of Interaction Van der Waals Dispersion Dipole–dipole Dipole-induced dipole Hydrogen bonding Chemical bonds Covalent bonds Ionic bonds Metallic bonds Approximate Energy Range (kcal mol⫺1) 0–10 0–0.5 0–40 15–170 140–250 27–83 482 ADHESION transmit stress without failure) is also a function of molecular weight Below a certain value, strength falls off rapidly with molecular weight In such a system, the intermolecular forces between chains are essentially unchanged, so that the loss in strength must be a result of some more physical phenomenon, in this case so-called molecular entanglement That is, as the polymer molecules become longer, they become more ‘‘wrapped up’’ in their neighbors (Fig 19.5) For the polymer to fail under stress, the entangled chains must slide past one another and disentangle, a process that requires a significant amount of energy Molecular entanglement, therefore, serves as an excellent process for the dissipation of the stress forces, and provides a good mechanism to back up intermolecular forces for producing good adhesion This dissipation process depends directly on the amount of chain entanglement and the nature of the forces acting between chains In a bulk polymer, the degree of entanglement is a direct result of the nature of the polymer (molecular weight, branching, etc.) and its method of preparation (e.g., cast from the melt, from solvent, spun) At an interface, entanglement is more problematical—the process (for polymeric surfaces, in any case) may be assisted, for instance, by the use of a solvent that swells the adherend surface, allowing interpenetration of adhesive and adherend; the application of heat, which increases the mobility of the polymer chains; or the use of a monomeric Applied tension Low molecular weight polymer Cohesive failure (a) Applied tension High molecular weight polymer No cohesive failure (b) FIGURE 19.5 For polymeric adhesives (and polymers in general) molecular weight can have an important effect on the cohesive strength of the material For low-molecular-weight polymers (a), there is relatively weak interaction among adjacent chains; movement of one chain past another is easy and the tensile or shear strength is low In high-molecular-weight materials (b), chain interactions are greatly increased, they may become tangled, and the material exhibits a much greater strength: stretching and ‘‘necking’’ as shear is applied, but resisting a much greater force before failing 19.4 SOME CONDITIONS FOR ‘‘GOOD’’ ADHESION 483 system that is polymerized after applications In any case, if entanglement can be increased, the strength of the bond at the interface will be improved If entanglement does occur, it should be obvious that one can no longer talk about a sharp interface, in the classic sense, but must consider an interfacial zone, the structure of which will be a primary determinant of the strength of the joint For simplicity, one may consider two types of interfaces—sharp, in which the primary component of strength derives from classical intermolecular attractive forces, and diffuse, in which entanglement plays a significant role These can be further classified by the relative strengths of the molecular interactions to give four general classes of adhesive behavior (Fig 19.6) The first, and simplest, class (Fig 19.6a) can be described as having a sharp interface and weak intermolecular interactions An example might be a joint between a nonpolar polymer (e.g., polyethylene) and a polar polymer (e.g., polyvinyl alcohol–polyvinyl acetate copolymer) In such a system, the only molecular interaction is that due to dispersion forces, with little tendency for chain entanglement due to the inherent incompatibility of the two polymers The mechanical strength of the resulting joint derives solely from the dispersion forces, which will not be able to inhibit the movement or slippage of the joint significantly A joint of very poor strength is the result The second class (Fig 19.6b) is a joint with a sharp interface, but with significant specific chemical interactions between the two phases For example, a polymer containing groups capable of significant hydrogen bonding or acid– base interactions (UOH, UNHU, or UCOOH) can interact strongly with, say, a metal or metal oxide surface (Mn⫹ or UMUOUMU), even though no significant entanglement is possible The resulting joint would have significant mechanical strength because of the larger magnitude of the interactive forces, even without the assistance of entanglement In the third class (Fig 19.6c), there is significant interpenetration of the adhesive into the surfaces to be bound The interpenetration may be at the (a) (b) (c) (d) FIGURE 19.6 It is usually possible to estimate the probable strength of an adhesive joint based on the types of interactions present between adhesive and adherend: (a) Dispersion forces alone usually produce joints of limited strength; (b) the presence of polar interactions will usually improve the situation significantly; (c) penetration of adhesive polymer chains into the adherend surface also adds greatly to the potential strength of the joint; (d) physical interlocking, coupled with any or all of the other mechanisms, usually insures the strongest practical joint 484 ADHESION molecular level, in the case that the adhesive polymer and polymeric adherend are mutually miscible, or more at the microscopic bulk level, as for porous substrates If the interface is diffuse and significant entanglement occurs, a strong joint may be expected, regardless of the nature of the intermolecular forces acting between elements In such a case, the entanglement of entire polymer molecules is not necessary for significant strength to be developed Entanglement may be considered a sufficient condition for good adhesion, even in the absence of strong intermolecular interactions The fourth class (Fig 19.6d) would be that involving some form of physical or mechanical interlocking of the two surfaces Such a situation might be encountered when a molten polymer or pre-polymer mixture is applied to a rough surface under condition where it can flow into the solid surface irregularities If one considers the action necessary to displace the two surfaces in a system with a sharp boundary, it can be seen that for the first case, the molecular forces being overcome are not only small (relative to specific interactions) but are essentially limited to a two-dimensional action across the interface (i.e., a plane approximately parallel to the two surfaces being bonded) as illustrated in Figure 19.7a If entanglement occurs, the same small forces will be acting in three dimensions (Fig.19.7b)—that is, each entangled molecule will have nearest neighbors on all sides, which will mean—roughly speaking—that the interactions will be multiplied many times, depending on the degree of interpenetration If stronger specific interactions are present (mechanical interlocking), or if chemical bonds are formed, so much the better! From the preceding discussion, then, it appears that the optimum conditions for good adhesion include a diffuse interfacial zone and/or strong specific intermolecular interactions between phases One may add to that (in the opinion of some) the existence of a direct physical interlocking between surfaces With all the best-designed practical systems, however, adhesive joints Adhesive interpenetration and/or interlocking zone (a) (b) FIGURE 19.7 For an adhesive joint with a smooth boundary (a) the tension on the joint can be dissipated only in a plane parallel to the joint, otherwise joint failure will be immediate In a joint where interpenetration or interlocking occur (b), the tension may be dissipated in the additional direction perpendicular to the joint The added ‘‘option’’ results in a stronger joint 19.5 ADHESIVE FAILURE 485 never attain the strength one would predict on the basis of theory The following section addresses the question of why life can be so difficult 19.5 ADHESIVE FAILURE When an adhesive bond fails under a small applied stress, it is commonly described as a ‘‘weak’’ bond or ‘‘poor’’ adhesion In fact, such a description may be misleading since failure may have occurred at the interface, near the interface within one of the phases comprising the system, or well away from the interface These scenarios are illustrated in Figure 19.3 It is only the first case that one can accurately describe as being a result of poor adhesion The other two are more properly called ‘‘failures’’ of the bulk materials, that is, failure of cohesion, which is not the same thing However, usage dictates that failure of any kind be termed ‘‘adhesion failure.’’ Of course, in an actual joint, failure may be due to a combination of all three processes When failure occurs exactly at the joint interface or well away from it (say, more than 100 nm), then identification of the locus of failure is generally a simple process It is when the failure occurs within 10–100 nm of the interface that identification becomes a problem 19.5.1 Importance of Failure Identification Correct identification of the locus of failure can be of great practical and theoretical importance If one can determine that failure occurred cohesively near the actual interface, it can be inferred that improvement in bond strength can be obtained by increasing the cohesive strength of the ruptured material without worrying about the nature of the interactions at the interface (i.e., molecular attraction or entanglement) If the failure is found to occur at the interface, on the other hand, it will clearly be necessary to change the chemical nature of the components to increase the intermolecular attractive forces— introduce more specific interactions, form chemical bonds, and/or increase interpenetration of the phases If the locus of failure is not correctly identified, a great deal of time, energy, and money may be wasted solving problems that not exist! All of that depends, of course, on the fact that the bond in question was actually between the phases expected (i.e., a ‘‘proper’’ joint), and that some cohesively weak contaminant (moisture, oil, air, dirt, etc.) is not the primary cause of failure On the basis of the simple calculations of ideal adhesive bond strength given earlier, it has been suggested that bond failure in a ‘‘proper’’ adhesive joint will seldom occur at the interface Instead, failure will occur in a weak boundary layer near the true interface, or within the weaker of the two bonded phases Modern experimental techniques and theoretical considerations, however, indicate that all three possibilities for failure do, in fact, occur, depending on the given situation 486 ADHESION For a system with a sharp interface and weak intermolecular interaction, as in the nonpolar–polar polymer system mentioned above, failure exactly at the interface is a distinct possibility (or even probability) Thus, interfacial separation may be expected when interfacial strength is weaker than the bulk strength of the bonded materials As we have seen, if the intermolecular interactions across the interface are more specific (including chemical bond formation) or if significant interpenetration of polymer chains occurs, rupture at the interface becomes less likely In some cases, the locus of failure may depend on the rate at which stress is applied; rapid application leads to cohesive failure and very slow application, tending more toward true adhesive failure (since slow application of stress gives more time for the entangled molecules to ‘‘slide past’’ one another) A great deal of theory has grown up around the subject of failure in adhesive joints It would be prohibitive to attempt to cover the subject here; however, the practical importance of understanding the topic cannot be overemphasized One has only to consider the large number of critical structural joints employed in modern construction (e.g., of airplanes) to see how important the subject has become 19.5.2 The Role of Joint Flaws in Adhesive Failure To this point the discussion has focused primarily on so-called ‘‘proper’’ or ideal adhesive joints, assuming intimate contact between components and the absence of flaws and contaminants In the real world, such conditions are difficult to attain, so that the question of practical joint failure may not be concerned so much with intermolecular forces and entanglement, but with the mechanics of stress propagation in the system That subject, like so many related to practical applications of surface chemistry, is very extensive and beyond the scope of this book However, the basic principles involved are such that a few words may serve a useful purpose A typical flaw, for purposes of the present discussion, would be an entrapped bubble of air or other contaminant that is itself relatively weak cohesively (Fig 19.8) In the presence of such flaws, little or no energy can be transmitted through the flaw so that the stress becomes concentrated at the junctions of the flaws with the interface or in one of the bulk materials The ‘‘local’’ stress, therefore, is greater than the average value and is more likely to exceed the adhesive or cohesive strength of the system near the flaw As a result of that situation, the applied stress induces the formation of a crack that continues to propagate along the line of least resistance (with continued application of stress) until joint failure results Cracks or other such flaws present in the adherend surface may also serve as foci for crack propagation under stress leading to apparent adhesive failure In joints or welds that undergo cyclic mechanical (bending) or thermal (hot– cold–hot cycling) stress may also develop stress cracks at or near a joint or PROBLEMS 487 Trapped air Dirt and oil contaminants (a) Contaminants and flaws in adhesive (b) Contaminants and flaws in substrate (c) FIGURE 19.8 In summary, adhesive joint failure may be caused by a number of factors some not connected with the actual adhesive bond A common source of problems is the presence of trapped air or other contaminants that serve as loci for failure at the interface (a) Perhaps less common, but of significance, is the presence of contaminants or flaws in the bulk of the adhesive (b) or in the substrate (c) that weaken the physical properties of that phase and lead to joint failure weld, again leading to apparent adhesive failure when in fact the problem lies within the adherend alone Because there are so many geometries of adhesive joints encountered, and so many types of stress applied (tension, shear, torsion, thermal, etc.), the analysis of a given system must be tailored to meet the specific application The processes of experimental design and data analysis, therefore, become quite complicated It should also be kept in mind that flaws such as those often implicated in adhesive failure can also lead to apparent cohesive failure in the bulk material PROBLEMS 19.1 Calculate the surface tension that a liquid should have so that the work of adhesion to a surface with ␴c ⫽ 20 mN cm⫺1 will be a maximum What will be the contact angle? 19.2 The statement has been made that the work of adhesion between two dissimilar substances should be larger than the work of cohesion of the weaker one Demonstrate whether this should always be correct or show circumstances in which it will not be so 19.3 A surgeon wishes to repair a hernia with a biodegradable adhesive instead of normal sutures The tension of the skin at the lesion is 290 N m⫺1 If the total surface area to be bound is cm2, what must be the minimum adhesive strength of the bond to ensure a safe closure? 19.4 For bonding two smooth glass surfaces, one might select a hot-melt polyethylene adhesive, an aqueous acrylic latex adhesive, or two- 488 ADHESION component epoxy adhesive Which material would you expect to produce the strongest bond? Why? 19.5 In the photographic industry it is necessary to coat hydrophilic gelatin emulsions onto essentially nonionic acetate or polyester surfaces Direct application of the aqueous phase to the surface often results in a system with poor adhesion—that is, the dried gelatin phase peels away from the polymer surface Suggest the reason for the observed result 19.6 In order to overcome the adhesion problem described in Problem 19.6, it has been found useful to apply a thin coating of an acrylic polymer containing small amounts of free acrylic acid prior to the application of the gelatin emulsion Suggest an explanation for any observed improvement in the resulting adhesion 19.7 In some cases, an improvement similar to that in Problem 19.7 can be obtained by treating the hydrophobic surface with electron beams, a high-intensity short-wavelength light (corona discharge), or with a corrosive etching chemical Suggest the mechanism operating to improve adhesion in those cases? 19.8 Roughing a surface can often produce an improvement in adhesion Suggest a mechanism by which such improvement might be explained 19.9 The use of surface ‘‘roughing’’ to improve adhesion may produced the opposite effect if not properly applied Give two problems that may arise from surface roughing that could result in poor adhesion Surfaces, Interfaces, and Colloids: Principles and Applications, Second Edition Drew Myers Copyright 䉷 1999 John Wiley & Sons, Inc ISBNs: 0-471-33060-4 (Hardback); 0-471-23499-0 (Electronic) Bibliography GENERAL READINGS Adamson, A W., Physical Chemistry of Surfaces, 4th ed., Wiley-Interscience: New York 1984 Aveyard, R., Haydon, D A., An Introduction to the Principles of Surface Chemistry, Cambridge University Press, Cambridge, 1973 Everett, D H., Basic Principles of Colloid Science, Royal Society of Chemistry Paperbacks, Royal Society of Chemistry, London, 1988 Evans, D F., Wennerstrom, H., The Colloidal Domain, VCH Publishers, New York, 1994 Hiemenez, P C., Principles of Colloids and Surface Chemistry, 2nd ed., Marcel Dekker, New York, 1986 Hunter, R J., Foundations of Colloid Science, Oxford University Press, New York, 1987 Israelachvili, J., Intermolecular and Surface Forces, 2nd ed., Academic Press, San Diego, 1991 Jaycock, M J., Parfitt, G D., Chemistry of Interfaces, Ellis Horwood, Chichester, U.K., 1981 Kruyt, H R., Colloid Science, Vol 1, Elsevier, New York, 1952 Shaw, D J., Introduction to Colloid and Surface Chemistry, 3rd ed., Butterworths: London, 1980 Tanford, C., The Hydrophobic Effect Formation of Micelles and Biological Membranes, 2nd ed., Wiley, New York, 1980 Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Reading, MA, 1983 CHAPTER READINGS Chapter van Olphen, H., Mysels, K J., Physical Chemistry: Enriching Topics from Colloid and Surface Science, Theorex, La Jolla, CA 1975 Chapter Adamson, A W., Physical Chemistry of Surfaces, 4th ed., Wiley-Interscience, New York, 1984, Chapters and 489 490 BIBLIOGRAPHY Aveyard, R., Haydon, D A., An Introduction to the Principles of Surface Chemistry, Cambridge University Press, Cambridge, 1973, Chapters 3, 5, and Shaw, D J., Introduction to Colloid and Surface Chemistry, 3rd ed., Butterworths: London, 1980, Chapters 4–6 Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Reading, MA, 1983, Chapters 1, 3, and Chapter Myers, D Y., Surfactant Science and Technology, 2nd ed., VCH Publishers, New York, 1992, Chapter Rosen, M J., Surfactants and Interfacial Phenomena, Wiley-Interscience, New York, 1976 Rieger, M M., Rhein, L D., eds., Surfactants in Cosmetics, Marcel Dekker, New York, 1997, Chapter Chapter Adamson, A W., Physical Chemistry of Surfaces, 4th ed Wiley-Interscience, New York, 1984, Chapter Israelachvili, J., Intermolecular and Surface Forces, 2nd ed., Academic Press, San Diego, 1991, Chapters 2–6 Mahanty, J., Ninham, B W., Dispersion Forces, Academic Press, New York, 1976 Rieger, M M., Rhein, L D., eds., Surfactants in Cosmetics, 2nd ed., Marcel Dekker, New York, 1997, Chapter Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Reading, MA, 1983, Chapter Chapter Adamson, A W., Physical Chemistry of Surfaces, 4th ed., Wiley-Interscience, New York, 1984, Chapter Aveyard, R., Haydon, D A., An Introduction to the Principles of Surface Chemistry, Cambridge University Press, Cambridge, 1973, Chapter Israelachvili, J., Intermolecular and Surface Forces, 2nd ed., Academic Press, San Diego, 1991, Chapter Rieger, M M., Rhein, L D., eds., Surfactants in Cosmetics, 2nd ed Marcel Dekker, New York, 1997, Chapter Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Inc.: Reading, MA, 1983, Chapter Chapter Adamson, A W., Physical Chemistry of Surfaces, 4th ed., Wiley-Interscience, New York, 1984, Chapter Aveyard, R., Haydon, D A., An Introduction to the Principles of Surface Chemistry, Cambridge University Press, Cambridge, 1973, Chapter Evans, D F., Wennerstrom, H., The Colloidal Domain, VCH Publishers, New York, 1994, Chapter CHAPTER READINGS 491 Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Reading, MA, 1983, Chapter Chapter Adamson, A W., Physical Chemistry of Surfaces, 4th ed., Wiley-Interscience, New York, 1984, Chapter Aveyard, R., Haydon, D A., An Introduction to the Principles of Surface Chemistry, Cambridge University Press, Cambridge, 1973, Chapter Shaw, D J., Introduction to Colloid and Surface Chemistry, 3rd ed., Butterworths: London, 1980, Chapter Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Reading, MA, 1983, Chapter Zettlemoyer, A C., ed., Nucleation, Marcel Dekker, New York, 1969 Chapter Adamson, A W., Physical Chemistry of Surfaces, 4th ed., Wiley-Interscience, New York, 1984, Chapter 12 Aveyard, R., Haydon, D A., An Introduction to the Principles of Surface Chemistry, Cambridge University Press, Cambridge, 1973, Chapter Shaw, D J., Introduction to Colloid and Surface Chemistry, 3rd ed., Butterworths, London, 1980, Chapter Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Reading, MA, 1983, Chapter Chapter Adamson, A W., Physical Chemistry of Surfaces, 4th ed., Wiley-Interscience, New York 1984, Chapters and 12–14 Aveyard, R., Haydon, D A., An Introduction to the Principles of Surface Chemistry, Cambridge University Press, Cambridge, 1973, Chapters and Shaw, D J., Introduction to Colloid and Surface Chemistry, 3rd ed., Butterworths: London, 1980, Chapters and Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Reading, MA, 1983, Chapters 2–4 Chapter 10 Adamson, A W., Physical Chemistry of Surfaces, 4th ed., Wiley-Interscience, New York, 1984, Chapter Evans, D F., Wennerstrom, H., The Colloidal Domain, VCH Publishers, New York, 1994, Chapter Shaw D J., Introduction to Colloid and Surface Chemistry, 3rd ed., Butterworths, London, 1980, Chapters and Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Reading, MA, 1983, Chapters 6–10 Chapter 11 Adamson, A W., Physical Chemistry of Surfaces, 4th ed., Wiley-Interscience, New York, 1984, Chapter 11 492 BIBLIOGRAPHY Becher, P., Emulsions: Theory and Practice, 2nd ed., Reinhold, New York, 1965 Becher, P., ed., Encyclopedia of Emulsion Technology, Vols 1–4, Marcel Dekker, New York, 1985 Davies, J T., Rideal, E K., Interfacial Phenomena, Academic Press, New York, 1961 Evans, D F., Wennerstrom, H., The Colloidal Domain, VCH Publishers, New York, 1994, Chapter 11 Griffith, W C., J Soc Cosmetic Chemists 5, 249 (1954) Rieger, M M., Rhein, L D., eds., Surfactants in Cosmetics, 2nd ed., Marcel Dekker, New York, 1997 Shaw, D J., Introduction to Colloid and Surface Chemistry, 3rd ed., Butterworths: London, 1980, Chapter 10 Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Reading, MA, 1983, Chapter 12 Chapter 12 Adamson, A W., Physical Chemistry of Surfaces, 4th ed., Wiley-Interscience, New York, 1984, Chapter 11 Berkman S., Egloff, G., Emulsions and Foams, Reinhold, New York, 1961 Bikerman, J J., Foam, Springer-Verlag, New York, 1973 Little, R C., J Colloid Interface Sci 65, 587 (1978) Shaw, D J., Introduction to Colloid and Surface Chemistry, 3rd ed., Butterworths, London, 1980, Chapter 10 Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Reading, MA, 1983, Chapter 12 Chapter 13 Evans, D F., Wennerstrom, H., The Colloidal Domain, VCH Publishers, New York, 1994, Chapter Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Reading, MA, 1983, Chapter 11 Chapter 14 Evans, D F., Wennerstrom, H., The Colloidal Domain, VCH Publishers, New York, 1994, Chapter Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Reading, MA, 1983, Chapters 13–15 Chapter 15 Evans, D F., Wennerstrom, H., The Colloidal Domain, VCH Publishers, New York 1994, Chapter Israelachvili, J., Intermolecular and Surface Forces, 2nd ed., Academic Press, San Diego, 1991, Chapters 16 and 17 Rosen, M J., Surfactants and Interfacial Phenomena, Wiley-Interscience, New York, 1976 CHAPTER READINGS 493 Shinoda, K., Nakawaga, T., Tamamushi, B., Isemura, T., Colloidal Surfactants, Some Physico-Chemical Properties, Academic Press, New York, 1963 Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Reading MA, 1983, Chapter 18 Chapter 16 Elworthy, P H., Florence, A T., Macfarlane, C B., Solubilization by Surface-Active Agents, Chapman and Hall, London, 1968 Evans, D F., Wennerstrom, H., The Colloidal Domain, VCH Publishers, New York, 1994, Chapters and 11 Fendler, J H., Fendler, E J., Catalysis in Micellar and Macromolecular Systems, Academic Press, New York, 1975 Prince, L M., ed., Microemulsions Theory and Practice, Academic Press, New York 1977 Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Reading, MA, 1983, Chapter 19 Chapter 17 Adamson, A W., Physical Chemistry of Surfaces, 4th ed., Wiley-Interscience, New York 1984, Chapter 10 Aveyard, R., Haydon, D A., An Introduction to the Principles of Surface Chemistry, Cambridge University Press, Cambridge, 1973, Chapter Shaw, D J., Introduction to Colloid and Surface Chemistry, 3rd ed., Butterworths: London, 1980, Chapter Vold, M J., Vold, R D., Colloid and Interface Chemistry, Addison-Wesley, Reading, MA, 1983, Chapter 14 Chapter 18 Adamson, A W., Physical Chemistry of Surfaces, 4th ed., Wiley-Interscience, New York 1984, Chapter Girifalco, L A., Good, R J., J Phys Chem 61, 904 (1957) Fox, H W., Zisman, W A., J Colloid Sci 5, 514 (1950) Chapter 19 Adamson, A W., Physical Chemistry of Surfaces, 4th ed., Wiley-Interscience, New York, 1984, Chapter Bickerman, J J., The Science of Adhesive Joints, Academic Press, New York, 1961 Houwink, R., Salomon, D., eds., Adhesion and Adhesives, Elsevier, New York, 1965 Kaelble, D H., Physical Chemistry of Adhesion, Wiley-Interscience, New York, 1971