Surfactants and Polymers in Aqueous Solution.
`Krister Holmberg, Bo J¨onsson, Bengt Kronberg and Bj¨orn Lindman
`Copyright  2002 John Wiley & Sons, Ltd.
`ISBN: 0-471-49883-1
`
`SURFACTANTS
`AND POLYMERS
`IN AQUEOUS
`SOLUTION
`
`Page 1
`
`KASHIV EXHIBIT 1050
`IPR2019-00791
`
`

`

`SURFACTANTS
`AND POLYMERS
`IN AQUEOUS
`SOLUTION
`
`SECOND EDITION
`
`Krister Holmberg
`
`Chalmers University of Technology, S-412 96, GoÈteborg, Sweden,
`
`Bo JoÈnsson
`
`Chemical Centre, Lund University, POB 124, S-221 00, Lund, Sweden
`
`Bengt Kronberg
`
`Institute for Surface Chemistry, POB 5607, S-114 87, Stockholm, Sweden
`
`and
`
`BjoÈrn Lindman
`
`Chemical Centre, Lund University, POB 124, S-221 00, Lund, Sweden
`
`Page 2
`
`

`

`Copyright # 2003 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester,
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`
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`Library of Congress Cataloging-in-Publication Data
`
`Surfactants and polymers in aqueous soltion.±2nd ed./ Krister Homberg . . . [et al.].
`p. cm.
`Includes bibliographical references and index.
`ISBN 0±471±49883±1 (acid-free paper)
`1. Surface active agents. 2. Polymers. 3. Solution (Chemistry) I. Holmberg, Krister, 1946-
`
`TP994 .S863 2002
`6680.1±dc21
`
`2002072621
`
`British Library Cataloguing in Publication Data
`
`A catalogue record for this book is available from the British Library
`
`ISBN 0 471 49883 1 Cloth: 2nd edition
`(ISBN 0 471 974422 6 Cloth: 1st edition)
`(ISBN 0 471 98698 5 Paper: 1st edition)
`
`Typeset in 10/12pt Times by Kolam Information Services Pvt Ltd, Pondicherry, India.
`Printed and bound in Great Britain by Biddles Ltd, Guildford, Surrey.
`This book is printed on acid-free paper responsibly manufactured from sustainable forestry
`in which at least two trees are planted for each one used for paper production.
`
`Page 3
`
`

`

`CONTENTS
`
`Preface to the second edition
`Preface to the Wrst edition
`
`1.
`
`2.
`
`INTRODUCTION TO SURFACTANTS
`Surfactants Adsorb at Interfaces
`Surfactants Aggregate in Solution
`Surfactants are Amphiphilic
`Surface Active Compounds are Plentiful in Nature
`Surfactant Raw Materials May be Based on Petrochemicals or
`Oleochemicals
`Surfactants are ClassiWed by the Polar Head Group
`Dermatological Aspects of Surfactants are Vital Issues
`The Ecological Impact of Surfactants is of Growing Importance
`The Rate of Biodegradation Depends on Surfactant Structure
`Environmental Concern is a Strong Driving Force for
`Surfactant Development
`Bibliography
`
`SURFACTANT MICELLIZATION
`DiVerent Amphiphile Systems
`Surfactants Start to Form Micelles at the CMC
`CMC Depends on Chemical Structure
`Temperature and Cosolutes AVect the CMC
`The Solubility of Surfactants may be Strongly Temperature
`Dependent
`Driving Forces of Micelle Formation and Thermodynamic
`Models
`The Association Process and Counterion Binding can be
`Monitored by NMR Spectroscopy
`Hydrophobic Compounds can be Solubilized in Micelles
`Micelle Size and Structure may Vary
`A Geometric Consideration of Chain Packing is Useful
`Kinetics of Micelle Formation
`
`xiii
`xv
`
`1
`1
`3
`3
`5
`
`7
`8
`24
`27
`30
`
`32
`36
`
`39
`39
`39
`43
`46
`
`49
`
`52
`
`55
`57
`58
`60
`61
`
`Page 4
`
`

`

`vi
`
`3.
`
`4.
`
`Surfactants and Polymers in Aqueous Solution
`
`Surfactants may Form Aggregates in Solvents other than Water
`General Comments on Amphiphile Self-Assembly
`Bibliography
`
`PHASE BEHAVIOUR OF CONCENTRATED
`SURFACTANT SYSTEMS
`Micelle Type and Size Vary with Concentration
`Micellar Growth is DiVerent for DiVerent Systems
`Surfactant Phases are Built Up by Discrete or InWnite
`Self-Assemblies
`Micellar Solutions can Reach Saturation
`Structures of Liquid Crystalline Phases
`How to Determine Phase Diagrams
`Binary and Ternary Phase Diagrams are Useful Tools: Two
`Components
`Binary and Ternary Phase Diagrams are Useful Tools: Three
`Components
`Surfactant Geometry and Packing Determine Aggregate
`Structure: Packing Parameter and Spontaneous Curvature of the
`Surfactant Film are Useful Concepts
`Polar Lipids Show the same Phase Behaviour as other
`Amphiphiles
`Liquid Crystalline Phases may Form in Solvents other than
`Water
`Bibliography
`
`PHYSICOCHEMICAL PROPERTIES OF
`SURFACTANTS AND POLYMERS CONTAINING
`OXYETHYLENE GROUPS
`Polyoxyethylene Chains make up the Hydrophilic
`Part of many Surfactants and Polymers
`CMC and Micellar Size of Polyoxyethylene-Based
`Surfactants are Strongly Temperature Dependent
`Temperature Dependence can be Studied using Phase Diagrams
`The L3 or `Sponge' Phase
`Sequence of Self-Assembly Structures as a Function of
`Temperature
`The Critical Packing Parameter and the Spontaneous Curvature
`Concepts are Useful Tools
`Clouding is a Characteristic Feature of
`Polyoxyethylene-Based Surfactants and Polymers
`
`62
`64
`66
`
`67
`67
`70
`
`74
`76
`77
`80
`
`82
`
`85
`
`89
`
`93
`
`94
`95
`
`97
`
`97
`
`98
`100
`103
`
`103
`
`103
`
`109
`
`Page 5
`
`

`

`Contents
`
`Physicochemical Properties of Block Copolymers
`Containing Polyoxethylene Segments Resemble those of
`Polyoxyethylene-Based Surfactants
`Temperature Anomalies of Oxyethylene-Based Surfactants
`and Polymers are Ubiquitous
`Temperature Anomalies are Present in Solvents other than Water
`Bibliography
`
`5. MIXED MICELLES
`Systems of Surfactants with Similar Head Groups
`Require no Net Interaction
`General Treatment of Surfactants Mixtures Requires a Net
`Interaction
`The Concept of Mixed Micelles can also be Applied to
`Amphiphiles not Forming Micelles
`Mixed Surfactant Systems at Higher Concentrations Show
`Interesting Features
`Mixed Surfactant Systems are used Technically
`Appendix
`Bibliography
`
`6. MICROEMULSIONS
`The Term Microemulsion is Misleading
`Phase Behaviour of Oil±Water±Surfactant Systems can be
`Illustrated by Phase Diagrams
`The Choice of Surfactant is Decisive
`Ternary Phase Diagrams can be Complex
`How to Approach Microstructure?
`Molecular Self-DiVusion can be Measured
`ConWnement, Obstruction and Solvation Determine
`Solvent Self-DiVusion in Microemulsions
`Self-DiVusion Gives Evidence for a Bicontinuous Structure at
`Balanced Conditions
`The Microstructure is Governed by Surfactant Properties
`Bibliography
`
`vii
`
`111
`
`113
`117
`118
`
`119
`
`119
`
`124
`
`130
`
`131
`134
`136
`138
`
`139
`139
`
`140
`143
`146
`146
`147
`
`148
`
`151
`152
`154
`
`7.
`
`157
`INTERMOLECULAR INTERACTIONS
`157
`Pair Potentials Act between Two Molecules in a Vacuum
`159
`The Intermolecular Interaction can be Partitioned
`EVective Pair Potentials Act between Two Molecules in a Medium 167
`Bibliography
`174
`
`Page 6
`
`

`

`viii
`
`Surfactants and Polymers in Aqueous Solution
`
`8. COLLOIDAL FORCES
`Electric Double-Layer Forces are Important for Colloidal Stability
`Other Types of Forces Exist
`Colloidal Forces can be Measured Directly
`Bibliography
`
`9. POLYMERS IN SOLUTION
`Polymer Properties are Governed by the Choice of Monomers
`The Molecular Weight is an Important Parameter
`Dissolving a Polymer can be a Problem
`Polymers in Solution can be Characterized by Viscosity
`Measurements
`Polymer Solutions may Undergo Phase Separation
`Polymers Containing Oxyethylene Groups Phase-Separate
`Upon Heating in Aqueous Systems
`Solvents and Surfactants have Large EVects on Polymer
`Solutions
`The Solubility Parameter Concept is a Useful Tool for Finding
`the Right Solvent for a Polymer
`The Theta Temperature is of Fundamental Importance
`There are Various Classes of Water-Soluble Polymers
`Polyelectrolytes are Charged Polymers
`Polymer ConWgurations Depend on Solvent
`Conditions
`Bibliography
`
`10. REGULAR SOLUTION THEORY
`Bragg±Williams Theory Describes Non-ideal Mixtures
`Flory±Huggins Theory Describes the Phase Behaviour
`of Polymer Solutions
`Bibliography
`
`11. NOVEL SURFACTANTS
`Gemini Surfactants have an Unusual Structure
`Cleavable Surfactants are Environmentally Attractive but
`are of Interest for other Reasons as well
`Polymerizable Surfactants are of Particular Interest
`for Coatings Applications
`Polymeric Surfactants Constitute a Chapter of their Own
`Special Surfactants Give Extreme Surface Tension Reduction
`Bibliography
`
`175
`175
`181
`189
`191
`
`193
`193
`195
`196
`
`196
`197
`
`199
`
`199
`
`201
`203
`205
`207
`
`207
`214
`
`215
`215
`
`223
`226
`
`227
`227
`
`235
`
`246
`258
`258
`259
`
`Page 7
`
`

`

`Contents
`
`12. SURFACE ACTIVE POLYMERS
`Surface Active Polymers can be Designed in DiVerent Ways
`Polymers may have a Hydrophilic Backbone and
`Hydrophobic Side Chains
`Polymers may have a Hydrophobic Backbone and
`Hydrophilic Side Chains
`Polymers may Consist of Alternating Hydrophilic and
`Hydrophobic Blocks
`Polymeric Surfactants have Attractive Properties
`Bibliography
`
`13. SURFACTANT±POLYMER SYSTEMS
`Polymers can Induce Surfactant Aggregation
`Attractive Polymer±Surfactant Interactions Depend on both
`Polymer and Surfactant
`Surfactant Association to Surface Active Polymers can be
`Strong
`The Interaction between a Surfactant and a Surface Active
`Polymer is Analogous to Mixed Micelle Formation
`Phase Behaviour of Polymer-Surfactant Mixtures Resembles
`that of Mixed Polymer Solutions
`Phase Behaviour of Polymer±Surfactant Mixtures in Relation to
`Polymer±Polymer and Surfactant±Surfactant Mixtures
`Polymers may Change the Phase Behaviour of InWnite
`Surfactant Self-Assemblies
`There Are Many Technical Applications of Polymer±Surfactant
`Mixtures
`DNA is Compacted by Cationic Surfactants, which gives
`Applications in Gene Therapy
`Bibliography
`
`14. SURFACTANT±PROTEIN MIXTURES
`Proteins are Amphiphilic
`Surfactant±Protein Interactions have a Broad Relevance
`Surface Tension and Solubilization give Evidence for Surfactant
`Binding to Proteins
`The Binding Isotherms are Complex
`Protein±Surfactant Solutions may have High Viscosities
`Protein±Surfactant Solutions may give rise to Phase Separation
`Surfactants may Induce Denaturation of Proteins
`Bibliography
`
`ix
`
`261
`261
`
`262
`
`267
`
`272
`276
`276
`
`277
`277
`
`281
`
`283
`
`285
`
`288
`
`295
`
`298
`
`299
`
`301
`303
`
`305
`305
`306
`
`306
`308
`310
`311
`314
`315
`
`Page 8
`
`

`

`x
`
`Surfactants and Polymers in Aqueous Solution
`
`15. AN INTRODUCTION TO THE RHEOLOGY OF
`POLYMER AND SURFACTANT SOLUTIONS
`Rheology Deals with how Materials Respond to Deformation
`The Viscosity Measures how a Simple Fluid Responds to Shear
`The Presence of Particles Changes the Flow Pattern and the
`Viscosity
`The Relationship between Intrinsic Viscosity and Molecular
`Mass can be Useful
`The Rheology is often Complex
`Viscoelasticity
`The Rheological Behaviour of Surfactant and Polymer
`Solutions Shows an Enormous Variation: Some
`Further Examples
`Bibliography
`
`16. SURFACE TENSION AND ADSORPTION AT THE
`AIR±WATER INTERFACE
`Surface Tension is due to Asymmetric Cohesive Forces
`at a Surface
`Solutes AVect Surface Tension
`Dynamic Surface Tension is Important
`The Surface Tension is Related to Adsorption
`Surfactant Adsorption at the Liquid±Air Surface is Related to
`the Critical Packing Parameter
`Polymer Adsorption can be Misinterpreted
`Measurement of Surface Tension
`The Surface and Interfacial Tensions can be Understood in
`Terms of Molecular Interactions
`Surface Tension and Adsorption can be Understood in
`Terms of the Regular Solution Theory
`Bibliography
`
`17. ADSORPTION OF SURFACTANTS AT SOLID
`SURFACES
`Surfactant Adsorption is Governed both by the Nature of
`the Surfactant and the Surface
`Model Surfaces and Methods to Determine Adsorption
`Analysis of Surfactant Adsorption is Frequently Carried out
`in Terms of the Langmuir Equation
`Surfactants Adsorb on Hydrophobic Surfaces
`Surfactants Adsorb on Hydrophilic Surfaces
`Competitive Adsorption is a Common Phenomenon
`Bibliography
`
`317
`317
`317
`
`322
`
`324
`324
`327
`
`329
`335
`
`337
`
`337
`339
`340
`342
`
`343
`346
`347
`
`349
`
`351
`355
`
`357
`
`358
`359
`
`362
`365
`372
`380
`387
`
`Page 9
`
`

`

`Contents
`
`18. WETTING AND WETTING AGENTS,
`HYDROPHOBIZATION AND
`HYDROPHOBIZING AGENTS
`Liquids Spread at Interfaces
`The Critical Surface Tension of a Solid is a Useful Concept
`The Critical Surface Tension can be Applied to Coatings
`Surface Active Agents can Promote or Prevent Wetting and
`Spreading
`Measuring Contact Angles
`Bibliography
`
`19.
`
`INTERACTION OF POLYMERS WITH SURFACES
`The Adsorbed Amount Depends on Polymer Molecular Weight
`The Solvent has a Profound InXuence on the Adsorption
`Electrostatic Interactions AVect the Adsorption
`Polyelectrolyte Adsorption can be Modelled Theoretically
`Polyelectrolytes Change the Double-Layer Repulsion
`Polymer Adsorption is Practically Irreversible
`The Acid±Base Concept can be Applied to Polymer Adsorption
`Measurement of Polymer Adsorption
`Bibliography
`
`20. FOAMING OF SURFACTANT SOLUTIONS
`There are Transient Foams and Stable Foams
`Two Conditions must be FulWlled for a Foam to be Formed
`There are Four Forces Acting on a Foam
`The Critical Packing Parameter Concept is a Useful Tool
`Polymers might Increase or Decrease Foam Stability
`Particles and Proteins can Stabilize Foams
`Various Additives are Used to Break Foams
`Bibliography
`
`21. EMULSIONS AND EMULSIFIERS
`Emulsions are Dispersions of One Liquid in Another
`Emulsions can be Very Concentrated
`Emulsions can Break Down According to DiVerent Mechanisms
`The Emulsion Droplets Need a Potential Energy Barrier
`The DVLO Theory is a Cornerstone in the Understanding of
`Emulsion Stability
`EmulsiWers are Surfactants that Assist in Creating an Emulsion
`The HLB Concept
`The HLB Method of Selecting an EmulsiWer is Crude but Simple
`The PIT Concept
`
`xi
`
`389
`389
`391
`394
`
`395
`399
`402
`
`403
`404
`407
`408
`416
`419
`427
`428
`431
`435
`
`437
`437
`438
`440
`442
`446
`447
`448
`450
`
`451
`451
`452
`452
`453
`
`456
`458
`459
`461
`462
`
`Page 10
`
`

`

`xii
`
`Surfactants and Polymers in Aqueous Solution
`
`The PIT Method of Selecting an EmulsiWer is often Useful
`DiVerent Types of Non-Ionic Surfactants can be Used as
`EmulsiWers
`Bancroft's Rule may be Explained by Adsorption
`Dynamics of the Surfactant
`Bancroft's Rule may be Related to the Surfactant Geometry
`Hydrodynamics may Control what Type of Emulsion will Form
`Bibliography
`
`22. MICROEMULSIONS FOR SOIL AND OIL REMOVAL
`Surfactant-Based Cleaning Formulations may act by in situ
`Formation of a Microemulsion (Detergency)
`Microemulsion-Based Cleaning Formulations are EYcient
`Microemulsions were once Believed to be the Solution to
`Enhanced Oil Recovery
`Bibliography
`
`23. CHEMICAL REACTIONS IN
`MICROHETEROGENEOUS SYSTEMS
`Microemulsions can be used as Minireactors for Chemical
`Reactions
`Surface Active Reagents may be Subject to Micellar Catalysis
`Microemulsions are Good Solvents for Organic Synthesis
`Microemulsions are Useful as Media for Enzymatic Reactions
`Microemulsions can be Used to Prepare Nanosized Lattices
`Nanosized Inorganic Particles can be Prepared in
`Microemulsions
`Mesoporous Materials can be Prepared from Surfactant Liquid
`Crystals
`Bibliography
`
`Appendices
`Index
`
`466
`
`466
`
`468
`469
`471
`471
`
`473
`
`473
`484
`
`486
`492
`
`493
`
`493
`494
`496
`502
`507
`
`511
`
`516
`517
`
`519
`527
`
`Page 11
`
`

`

`PREFACE TO SECOND
`EDITION
`
`The basic concept behind `Surfactants and Polymers in Aqueous Solution', i.e.
`to combine in one book the physicochemical behaviours of both surfactants
`and water-soluble polymers, has evidently been attractive. The Wrst edition of
`this book has sold well and has found a place as a course book at universities
`and as a reference book for researchers in the area. We, ourselves, use it
`extensively in our own teaching and research and receive constant feedback
`from course participants and from research colleagues. The additions and
`revisions made in this new edition of `Surfactants and Polymers in Aqueous
`Solution' are based on suggestions that we have obtained through these years
`and also from our own ambition to keep the content up-to-date with respect to
`recent developments in the Weld.
`The interaction between surfactants and polymers is a core topic of the book
`and constituted one chapter in the previous edition. Surfactant±protein inter-
`action is a related theme of major importance in the life sciences area and one
`new chapter now deals with this issue. Rheology related to the behaviour of
`amphiphiles in solution is a subject of practical interest in many areas. This
`issue was only marginally covered in the Wrst edition but is now the topic of a
`complete chapter.
`Surfactants are widely used as wetting agents and we have received many
`comments on the fact that the Wrst edition did not cover this aspect. A chapter
`treating both the wetting of a liquid on another liquid and on a solid, and also
`discussing the role of the wetting agent, has now been included.
`In order to keep up with recent developments in the surfactant area, a
`contribution on novel surfactants has now been added. This chapter includes
`polymerizable surfactants, which were also covered in the Wrst edition, but now
`contains, in addition, new sections on gemini surfactants and cleavable surfac-
`tants.
`All of the chapters from the Wrst edition that reappear in this second volume
`have been fully up-dated and revised. In most of these, new material has been
`added, usually describing the results obtained from recent research. A section
`on the dermatological aspects of surfactants has been included in the general
`chapter on surfactants. The chapter dealing with polymers in solution has been
`
`Page 12
`
`

`

`xiv
`
`Surfactants and Polymers in Aqueous Solution
`
`extended to include a section which describes diVerent types of water-soluble
`polymers. In the chapter on interaction of polymers with surfaces the polyelec-
`trolyte adsorption has been restructured. Within the chapter that deals with
`emulsiWers a general treatment of emulsions has been included, while in the
`chapter discussing chemical reactions in microheterogeneous media a section
`has been added on mesoporous materials made via surfactant self-assembly.
`Finally, mistakes and indistinct descriptions in the Wrst edition that have
`been brought to our attention have been taken care of. We believe that this
`second edition is a more complete and a more coherent book than the Wrst
`edition. However, we also realize that there is still a long way to go until the
`book is `perfect' and therefore encourage comments and suggestions for further
`improvements.
`
`GoÈ teborg, Lund and Stockholm
`April, 2002
`
`Krister Holmberg
`Bo JoÈ nsson
`Bengt Kronberg
`BjoÈ rn Lindman
`
`Page 13
`
`

`

`PREFACE TO THE FIRST
`EDITION
`
`Surfactants are used together with polymers in a wide range of applications. In
`areas as diverse as detergents, paints, paper coatings, food and pharmacy,
`formulations usually contain a combination of a low molecular weight surfac-
`tant and a polymer which may or may not be highly surface active. Together,
`the surfactant and the polymer provide the stability, rheology, etc., needed for
`speciWc application. The solution behaviour of each component is important,
`but the performance of the formulated product depends to a large extent on the
`interplay between the surfactant and the polymer. Hence, knowledge about
`physicochemical properties of both surfactants and polymers and not least
`about polymer±surfactant interactions, is essential in order to make formula-
`tion work more of a science than an art.
`There are books on surfactants and books dealing with water-soluble poly-
`mers, but to our knowledge no single work treats both in a comprehensive way.
`Researchers in the areas involved need to go to diVerent sources to obtain basic
`information about surfactants and polymers. More serious than the inconveni-
`ence of having to consult several books is the considerable variation in the
`description of physiochemical phenomena from one book to another. Such
`diVerences in the treatments can make it diYcult to get a good understanding of
`the solution behaviour of surfactant±polymer combinations. In our opinion
`there has been a long-standing need for a book covering both surfactants and
`water-soluble polymers and bringing the two topics together. This book is
`intended to Wll that gap.
`This book is practical rather than theoretical in scope. It is written as a
`reference book for scientists and engineers both in industry and academia. It is
`also intended as a textbook for courses for employees in industry and for
`undergraduate courses at universities. It has already been used as such, at the
`manuscript stage, at the University of Lund.
`The book originates from a course on `Surfactants and Polymers in Aqueous
`Solution' that we have been giving annually at diVerent places in southern
`Europe since 1992. The course material started with copies of overhead pic-
`tures, grew into extended summaries of the lectures and developed further into
`a compendium which after several rounds of polishing has become this volume.
`
`Page 14
`
`

`

`xvi
`
`Surfactants and Polymers in Aqueous Solution
`
`We thank the course participants throughout these years for many valuable
`comments and suggestions.
`We would also like to thank Akzo Nobel Surface Chemistry AB, and in
`particular Dr Lennart Dahlgren, for economic support towards the production
`of the book. We are grateful to Mr Malek Khan, for his skilful drawing of the
`Wgures. We thank many colleagues in Lund and in Stockholm for providing
`material and for helpful discussions.
`
`Stockholm and Lund
`November, 1997
`
`Krister Holmberg
`Bo JoÈ nsson
`Bengt Kronberg
`BjoÈ rn Lindman
`
`Page 15
`
`

`

`Surfactants and Polymers in Aqueous Solution.
`Krister Holmberg, Bo J¨onsson, Bengt Kronberg and Bj¨orn Lindman
`Copyright  2002 John Wiley & Sons, Ltd.
`ISBN: 0-471-49883-1
`
`1 INTRODUCTION TO
`SURFACTANTS
`
`Surfactants Adsorb at Interfaces
`
`Surfactant is an abbreviation for surface active agent, which literally means
`active at a surface. In other words, a surfactant is characterized by its tendency
`to absorb at surfaces and interfaces. The term interface denotes a boundary
`between any two immiscible phases; the term surface indicates that one of the
`phases is a gas, usually air. Altogether Wve diVerent interfaces exist:
`
`Solid±vapour
`Solid±liquid
`Solid±solid
`Liquid±vapour
`Liquid±liquid
`
`surface
`
`surface
`
`The driving force for a surfactant to adsorb at an interface is to lower the free
`energy of that phase boundary. The interfacial free energy per unit area
`represents the amount of work required to expand the interface. The term
`interfacial tension is often used instead of interfacial free energy per unit
`area. Thus, the surface tension of water is equivalent to the interfacial free
`energy per unit area of the boundary between water and the air above it. When
`that boundary is covered by surfactant molecules, the surface tension (or the
`amount of work required to expand the interface) is reduced. The denser the
`surfactant packing at the interface, then the larger the reduction in surface
`tension.
`Surfactants may adsorb at all of the Wve types of interfaces listed above.
`Here, the discussion will be restricted to interfaces involving a liquid phase. The
`liquid is usually, but not always water. Examples of the diVerent interfaces and
`products in which these interfaces are important are given in Table 1.1.
`In many formulated products several types of interfaces are present at the
`same time. Water-based paints and paper coating colours are examples of
`familiar but, from a colloidal point of view, very complicated systems contain-
`ing both solid-liquid (dispersed pigment particles) and liquid±liquid (latex or
`other binder droplets) interfaces. In addition, foam formation is a common
`
`Page 16
`
`

`

`2
`
`Surfactants and Polymers in Aqueous Solution
`
`Table 1.1 Examples of interfaces involving a liquid phase
`
`Interface
`
`Type of system Product
`
`Suspension
`Solid±liquid
`Emulsion
`Liquid±liquid
`Liquid±vapour Foam
`
`Solvent-borne paint
`Milk, cream
`Shaving cream
`
`(but unwanted) phenomenon at the application stage. All of the interfaces are
`stabilized by surfactants. The total interfacial area of such a system is immense:
`the oil±water and solid±water interfaces of one litre of paint may cover several
`football Welds.
`As mentioned above, the tendency to accumulate at interfaces is a funda-
`mental property of a surfactant. In principle, the stronger the tendency, then
`the better the surfactant. The degree of surfactant concentration at a boundary
`depends on the surfactant structure and also on the nature of the two phases
`that meet at the interface. Therefore, there is no universally good surfactant,
`suitable for all uses. The choice will depend on the application. A good surfac-
`tant should have low solubility in the bulk phases. Some surfactants (and
`several surface active macromolecules) are only soluble at the oil±water inter-
`face. Such compounds are diYcult to handle but are very eYcient in reducing
`the interfacial tension.
`There is, of course, a limit to the surface and interfacial tension lowering
`eVect by the surfactant. In the normal case that limit is reached when micelles
`start to form in bulk solution. Table 1.2 illustrates what eVective surfactants
`can do in terms of lowering of surface and interfacial tensions. The values
`given are typical of what is attained by normal light-duty liquid detergents.
`With special formulations, so-called ultra-low interfacial tensions, i.e. values
`in the range of 103 mN/m or below, can be obtained. An example of a
`system giving ultra-low interfacial tensions is a three-phase system compris-
`ing a microemulsion in equilibrium with excess water and oil phases. Such
`systems are of interest for enhanced oil recovery and are discussed in
`Chapter 22.
`
`Table 1.2 Typical values of surface and interfacial
`tensions (mN/m)
`
`Air±water
`Air±10% aqueous NaOH
`Air±aqueous surfactant solution
`Aliphatic hydrocarbon±water
`Aromatic hydrocarbon±water
`Hydrocarbon±aqueous surfactant solution
`
`72±73
`78
`40±50
`28±30
`20±30
`1±10
`
`Page 17
`
`

`

`Introduction to Surfactants
`
`Surfactants Aggregate in Solution
`
`3
`
`As discussed above, one characteristic feature of surfactants is their tendency to
`adsorb at interfaces. Another fundamental property of surface active agents is
`that unimers in solution tend to form aggregates, so-called micelles. (The free or
`unassociated surfactant is referred to in the literature either as `monomer' or
``unimer'. In this text we will use `unimer' and the term `monomer' will be
`restricted to the polymer building block.) Micelle formation, or micellization,
`can be viewed as an alternative mechanism to adsorption at the interfaces for
`removing hydrophobic groups from contact with water, thereby reducing the
`free energy of the system. It is an important phenomenon since surfactant
`molecules behave very diVerently when present in micelles than as free unimers
`in solution. Only surfactant unimers contribute to surface and interfacial
`tension lowering and dynamic phenomena, such as wetting and foaming, are
`governed by the concentration of free unimers in solution. The micelles may be
`seen as a reservoir for surfactant unimers. The exchange rate of a surfactant
`molecule between micelle and bulk solution may vary by many orders of
`magnitude depending on the size and structure of the surfactant.
`Micelles are already generated at very low surfactant concentrations in
`water. The concentration at which micelles start to form is called the critical
`micelle concentration, or CMC, and is an important characteristic of a surfac-
`tant. A CMC of 1 mM, a reasonable value for an ionic surfactant, means that
`the unimer concentration will never exceed this value, regardless of the amount
`of surfactant added to the solution. Surfactant micellization is discussed in
`detail in Chapter 2.
`
`Surfactants are Amphiphilic
`
`The name amphiphile is sometimes used synonymously with surfactant. The
`word is derived from the Greek word amphi, meaning both, and the term relates
`to the fact that all surfactant molecules consist of at least two parts, one which
`is soluble in a speciWc Xuid (the lyophilic part) and one which is insoluble (the
`lyophobic part). When the Xuid is water one usually talks about the hydrophilic
`and hydrophobic parts, respectively. The hydrophilic part is referred to as the
`head group and the hydrophobic part as the tail (see Figure 1.1).
`
`Hydrophilic
`head group
`
`Hydrophobic tail
`
`Figure 1.1 Schematic illustration of a surfactant
`
`Page 18
`
`

`

`4
`
`Surfactants and Polymers in Aqueous Solution
`
`In a micelle the surfactant hydrophobic group is directed towards the interior
`of the cluster and the polar head group is directed towards the solvent. The
`micelle, therefore, is a polar aggregate of high water solubility and without
`much surface activity. When a surfactant adsorbs from aqueous solution at a
`hydrophobic surface, it normally orients its hydrophobic group towards the
`surface and exposes its polar group to the water. The surface has become
`hydrophilic and, as a result, the interfacial tension between the surface and
`water has been reduced. Adsorption at hydrophilic surfaces often results in
`more complicated surfactant assemblies. Surfactant adsorption at hydrophilic
`and hydrophobic surfaces is discussed in Chapter 17.
`The hydrophobic part of a surfactant may be branched or linear. The polar
`head group is usually, but not always, attached at one end of the alkyl chain.
`The length of the chain is in the range of 8±18 carbon atoms. The degree of
`chain branching, the position of the polar group and the length of the chain
`are parameters of importance for the physicochemical properties of the surfac-
`tant.
`The polar part of the surfactant may be ionic or non-ionic and the choice of
`polar group determines the properties to a large extent. For non-ionic surfac-
`tants the size of the head group can be varied at will; for the ionics, the size is
`more or less a Wxed parameter. As will be discussed many times throughout this
`book, the relative size of the hydrophobic and polar groups, not the absolute
`size of either of the two, is decisive in determining the physicochemical behav-
`iour of a surfactant in water.
`A surfactant usually contains only one polar group. Recently, there has been
`considerable research interest in certain dimeric surfactants, containing two
`hydrophobic tails and two head groups linked together with a short spacer.
`These species, generally known under the name gemini surfactants, are not yet
`of commercial
`importance. They show several
`interesting physicochemical
`properties, such as very high eYciency in lowering surface tension and very
`low CMC. The low CMC values of gemini surfactants can be illustrated by a
`comparison of the value for the conventional cationic surfactant dodecyltri-
`methylammonium bromide (16 mM) and that of the corresponding gemini
`surfactant, having a 2 carbon linkage between the monomers (0.9 mM). The
`diVerence in CMC between monomeric and dimeric surfactants could be of
`considerable practical importance. A typical gemini surfactant is shown in
`Figure 1.2. Gemini surfactants are discussed further in Chapter 11.
`Weakly surface active compounds which accumulate at interfaces but which
`do not readily form micelles are of interest as additives in many surfactant
`formulations. They are referred to as hydrotropes and serve the purpose of
`destroying the ordered packing of ordinary surfactants. Thus, addition of a
`hydrotrope is a way to prevent the formation of highly viscous liquid crystalline
`phases which constitutes a well-known problem in surfactant formulations.
`Xylene sulfonate and cumene sulfonate are typical examples of hydrotropes
`
`Page 19
`
`

`

`Introduction to Surfactants
`
`5
`
`−
`
`Br
`
`H3C
`
`N +
`
`CH2CH2
`
`N +
`
`CH3
`
`−
`
`Br
`
`Figure 1.2 A gemini surfactant
`
`used, for instance, in detergent formulations. Short-chain alkyl phosphates
`have found speciWc use as hydrotropes for longer-chain alcohol ethoxylates.
`
`Surface Active Compounds are Plentiful in