Surfactants and Interfacial Phenomena

Milton J. Rosen, PhD, is Professor Emeritus of Chemistry at Brooklyn College of the City University of New York. He is also the Director (ret.) of the university's Surfactant Research Institute, a pioneering organization that he founded in 1987.

Joy T. Kunjappu, PhD, DSc, is a chemistry educator, consultant, and former Adjunct Professor at Columbia University and Brooklyn College. His areas of research interest include surfactant and surface science, organic chemistry, and photochemistry.

1 Characteristic Features of Surfactants 1

I. Conditions under which Interfacial Phenomena and Surfactants Become Significant 2

II. General Structural Features and Behavior of Surfactants 2

A. General Use of Charge Types 4

B. General Effects of the Nature of the Hydrophobic Group 5

1. Length of the Hydrophobic Group 5

2. Branching, Unsaturation 5

3. Aromatic Nucleus 5

4. Polyoxypropylene or Polyoxyethylene (POE) Units 5

5. Perfluoroalkyl or Polysiloxane Group 6

III. Environmental Effects of Surfactants 6

A. Surfactant Biodegradability 6

B. Surfactant Toxicity; Skin Irritation 7

IV. Characteristic Features and Uses of Commercially Available Surfactants 8

A. Anionics 9

1. Carboxylic Acid Salts 9

2. Sulfonic Acid Salts 11

3. Sulfuric Acid Ester Salts 15

4. Phosphoric and Polyphosphoric Acid Esters 17

5. Fluorinated Anionics 18

B. Cationics 19

1. Long-Chain Amines and Their Salts 20

2. Acylated Diamines and Polyamines and Their Salts 20

3. Quaternary Ammonium Salts 20

4. Polyoxyethylenated Long-Chain Amines 22

5. Quaternized POE Long-Chain Amines 22

6. Amine Oxides 22

C. Nonionics 23

1. Polyoxyethylenated Alkylphenols, Alkylphenol "Ethoxylates" 23

2. Polyoxyethylenated Straight-Chain Alcohols 24

3. Polyoxyethylenated Polyoxypropylene Glycols 25

4. Polyoxyethylenated Mercaptans 25

5. Long-Chain Carboxylic Acid Esters 26

6. Alkanolamine "Condensates," Alkanolamides 27

7. Tertiary Acetylenic Glycols and Their "Ethoxylates" 28

8. Polyoxyethylenated Silicones 28

9. N-Alkylpyrrolid(in)ones 29

10. Alkylpolyglycosides 29

D. Zwitterionics 30

1. pH-Sensitive Zwitterionics 30

2. pH-Insensitive Zwitterionics 32

E. Newer Surfactants Based Upon Renewable Raw Materials 32

1. α-Sulfofatty Acid Methyl Esters (SME) 32

2. Acylated Aminoacids 33

3. Nopol Alkoxylates 34

V. Some Useful Generalizations 34

VI. Electronic Searching of the Surfactant Literature 35

References 36

Problems 37

2 Adsorption of Surface-Active Agents at Interfaces: The E lectrical Double Layer 39

I. The Electrical Double Layer 40

II. Adsorption at the Solid–Liquid Interface 44

A. Mechanisms of Adsorption and Aggregation 44

B. Adsorption Isotherms 48

1. The Langmuir Adsorption Isotherm 50

C. Adsorption from Aqueous Solution onto Adsorbents with Strongly Charged Sites 53

1. Ionic Surfactants 53

2. Nonionic Surfactants 59

3. pH Change 59

4. Ionic Strength 60

5. Temperature 60

D. Adsorption from Aqueous Solution onto Nonpolar, Hydrophobic Adsorbents 60

E. Adsorption from Aqueous Solution onto Polar Adsorbents without Strongly Charged Sites 63

F. Effects of Adsorption from Aqueous Solution on the Surface Properties of the Solid Adsorbent 63

1. Substrates with Strongly Charged Sites 63

2. Nonpolar Adsorbents 65

G. Adsorption from Nonaqueous Solution 65

H. Determination of the Specific Surface Areas of Solids 66

III. Adsorption at the Liquid–Gas (L/G) and Liquid–Liquid (L/L) Interfaces 66

A. The Gibbs Adsorption Equation 67

B. Calculation of Surface Concentrations and Area Per Molecule at the Interface by Use of the Gibbs Equation 69

C. Effectiveness of Adsorption at the L/G and L/L Interfaces 71

D. The Szyszkowski, Langmuir, and Frumkin Equations 99

E. Efficiency of Adsorption at the L/G and L/L Interfaces 100

F. Calculation of Thermodynamic Parameters of Adsorption at the L/G and L/L Interfaces 104

G. Adsorption from Mixtures of Two Surfactants 113

References 115

Problems 121

3 Micelle Formation by Surfactants 123

I. The Critical Micelle Concentration (CMC) 123

II. Micellar Structure and Shape 126

A. The Packing Parameter 126

B. Surfactant Structure and Micellar Shape 127

C. Liquid Crystals 128

D. Rheology of Surfactant Solutions 131

III. Micellar Aggregation Numbers 132

IV. Factors Affecting the Value of the CMC in Aqueous Media 140

A. Structure of the Surfactant 140

1. The Hydrophobic Group 140

2. The Hydrophilic Group 158

3. The Counterion in Ionic Surfactants; Degree of Binding to the Micelle 160

4. Empirical Equations 164

B. Electrolyte 166

C. Organic Additives 167

1. Class I Materials 167

2. Class II Materials 168

D. The Presence of a Second Liquid Phase 169

E. Temperature 170

V. Micellization in Aqueous Solution and Adsorption at the Aqueous Solution–Air or Aqueous Solution–Hydrocarbon Interface 170

A. The CMC/C20 Ratio 171

VI. CMCs in Nonaqueous Media 179

VII. Equations for the CMC Based on Theoretical Considerations 180

VIII. Thermodynamic Parameters of Micellization 184

IX. Mixed Micelle Formation in Mixtures of Two Surfactants 191

References 192

Problems 200

4 Solubilization by Solutions of Surfactants: Micellar Catalysis 202

I. Solubilization in Aqueous Media 203

A. Locus of Solubilization 203

B. Factors Determining the Extent of Solubilization 206

1. Structure of the Surfactant 207

2. Structure of the Solubilizate 209

3. Effect of Electrolyte 209

4. Effect of Monomeric Organic Additives 210

5. Effect of Polymeric Organic Additives 211

6. Mixed Anionic–Nonionic Micelles 212

7. Effect of Temperature 212

8. Hydrotropy 214

C. Rate of Solubilization 214

II. Solubilization in Nonaqueous Solvents 215

A. Secondary Solubilization 218

III. Some Effects of Solubilization 218

A. Effect of Solubilization on Micellar Structure 218

B. Change in the CPs of Aqueous Solutions of Nonionic Surfactants 219

C. Reduction of the CMC 223

D. Miscellaneous Effects of Solubilization 223

IV. Micellar Catalysis 224

References 229

Problems 233

5 Reduction of Surface and Interfacial Tension by Surfactants 235

I. Efficiency in Surface Tension Reduction 239

II. Effectiveness in Surface Tension Reduction 241

A. The Krafft Point 241

B. Interfacial Parameter and Chemical Structural Effects 242

III. Liquid–Liquid Interfacial Tension Reduction 256

A. Ultralow Interfacial Tension 257

IV. Dynamic Surface Tension Reduction 262

A. Dynamic Regions 262

B. Apparent Diffusion Coefficients of Surfactants 265

References 266

Problems 270

6 Wetting and Its Modification by Surfactants 272

I. Wetting Equilibria 272

A. Spreading Wetting 273

1. The Contact Angle 275

2. Measurement of the Contact Angle 277

B. Adhesional Wetting 278

C. Immersional Wetting 281

D. Adsorption and Wetting 282

II. Modification of Wetting by Surfactants 285

A. General Considerations 285

B. Hard Surface (Equilibrium) Wetting 286

C. Textile (Nonequilibrium) Wetting 288

D. Effect of Additives 299

III. Synergy in Wetting by Mixtures of Surfactants 300

IV. Superspreading (Superwetting) 300

References 303

Problems 306

7 Foaming and Antifoaming by Aqueous Solutions of Surfactants 308

I. Theories of Film Elasticity 309

II. Factors Determining Foam Persistence 313

A. Drainage of Liquid in the Lamellae 313

B. Diffusion of Gas through the Lamellae 314

C. Surface Viscosity 315

D. The Existence and Thickness of the Electrical Double Layer 315

III. The Relation of Surfactant Chemical Structure to Foaming in Aqueous Solution 316

A. Efficiency as a Foaming Agent 317

B. Effectiveness as a Foaming Agent 317

C. Low-Foaming Surfactants 325

IV. Foam-Stabilizing Organic Additives 326

V. Antifoaming 329

VI. Foaming of Aqueous Dispersions of Finely Divided Solids 330

VII. Foaming and Antifoaming in Organic Media 331

References 332

Problems 334

8 E mulsification by Surfactants 336

I. Macroemulsions 337

A. Formation 338

B. Factors Determining Stability 338

1. Physical Nature of the Interfacial Film 339

2. Existence of an Electrical or Steric Barrier to Coalescence on the Dispersed Droplets 341

3. Viscosity of the Continuous Phase 342

4. Size Distribution of Droplets 342

5. Phase Volume Ratio 343

6. Temperature 343

C. Inversion 345

D. Multiple Emulsions 345

E. Theories of Emulsion Type 347

1. Qualitative Theories 347

2. Kinetic Theory of Macroemulsion Type 349

II. Microemulsions 350

III. Nanoemulsions 354

IV. Selection of Surfactants as Emulsifying Agents 355

A. The Hydrophile–Lipophile Balance (HLB) Method 356

B. The PIT Method 358

C. The Hydrophilic Lipophilic Deviation (HLD) Method 361

V. Demulsification 361

References 363

Problems 366

9 Dispersion and Aggregation of Solids in Liquid Media by Surfactants 368

I. Interparticle Forces 368

A. Soft (Electrostatic) and van der Waals Forces: Derjaguin and Landau and Verwey and Overbeek (DLVO) Theory 369

1. Limitations of the DLVO Theory 374

B. Steric Forces 376

II. Role of the Surfactant in the Dispersion Process 378

A. Wetting of the Powder 378

B. Deaggregation of Fragmentation of Particle Clusters 379

C. Prevention of Reaggregation 379

III. Coagulation or Flocculation of Dispersed Solids by Surfactants 379

A. Neutralization or Reduction of the Potential at the Stern Layer of the Dispersed Particles 380

B. Bridging 381

C. Reversible Flocculation 381

IV. The Relation of Surfactant Chemical Structure to Dispersing Properties 382

A. Aqueous Dispersions 382

B. Nonaqueous Dispersions 387

C. Design of New Dispersants 387

References 388

Problems 390

10 Detergency and Its Modification by Surfactants 392

I. Mechanisms of the Cleaning Process 392

A. Removal of Soil from Substrate 393

1. Removal of Liquid Soil 394

2. Removal of Solid Soil 395

B. Suspension of the Soil in the Bath and Prevention of Redeposition 398

1. Solid Particulate Soil: Formation of Electrical and Steric Barriers; Soil Release Agents 398

2. Liquid Oily Soil 399

C. Skin Irritation (see Chapter 1, Section IIIB) 400

D. Dry Cleaning 401

II. Effect of Water Hardness 402

A. Builders 402

B. LSDAs 404

III. Fabric S ofteners 405

IV. The Relation of the Chemical Structure of the Surfactant to its Detergency 407

A. Effect of Soil and Substrate 407

1. Oily Soil 407

2. Particulate S oil 409

3. Mixed Soil 410

B. Effect of the Hydrophobic Group of the Surfactant 411

C. Effect of the Hydrophilic Group of the Surfactant 412

D. Dry Cleaning 414

V. Biosurfactants and Enzymes in Detergent Formulations 415

VI. Nanodetergents (see Chapter 14, S ection IIIF) 416

References 416

Problems 419

11 Molecular Interactions and Synergism in Mixtures of Two Surfactants 421

I. Evaluation of Molecular Interaction Parameters 422

A. Notes on the Use of Equations 11.1–11.4 423

II. Effect of Chemical Structure and Molecular Environment on Molecular Interaction Parameters 427

III. Conditions for the Existence of Synergism 440

A. Synergism or Antagonism (Negative Synergism) in Surface or Interfacial Tension Reduction Efficiency 441

B. Synergism or Antagonism (Negative S ynergism) in Mixed Micelle Formation in an Aqueous Medium 442

C. Synergism or Antagonism (Negative Synergism) in Surface or Interfacial Tension Reduction Effectiveness 445

D. Selection of Surfactant Pairs for Optimal Interfacial Properties 447

IV. The Relation between Synergism in Fundamental Surface Properties and Synergism in Surfactant Applications 448

References 453

Problems 456

12 Gemini Surfactants 458

I. Fundamental Properties 459

II. Interaction with Other Surfactants 463

III. Performance Properties 466

References 467

Problems 470

13 Surfactants in Biology 471

I. Biosurfactants and Their Application Areas 471

II. Cell Membranes 480

III. Surfactants in Cell Lysis 486

IV. Protein Denaturing and Electrophoresis with Surfactants 491

V. Pulmonary Surfactants 491

VI. Surfactants in Biotechnology 493

A. Mineral Engineering 494

B. Fermentation 495

C. Enzymatic Deinking 495

D. EOR and Oil Bioremediation 495

E. Enzyme Activity in Surfactant Media 496

F. Carbon Dioxide “Fixing” in Bioreactors 496

G. Soil Remediation 496

H. Effluent Purification 497

I. Surfactants in Horticulture 497

J. Vesicle Manipulation 497

K. Genetic Engineering and Gene Therapy 497

References 498

Problems 501

14 Surfactants in Nanotechnology 502

I. Special Effects of the Nanostate 503

II. Role of Surfactants in the Preparation of Nanostructures 503

A. Bottom-Up Methods 504

1. Surfactant Self-Assembly 504

2. Synthetic Processes 508

B. Top-Down Methods 517

III. Surfactants in Nanotechnology Applications 517

A. Nanomotors 517

B. Other Nanodevices 520

C. Drug Delivery 522

D. Nanostructural Architectural Control of Materials 522

E. Nanotubes 525

F. Nanodetergents 525

G. Surfactant Nanoassemblies in the Origin of Life 526

References 528

Problems 529

15 Surfactants and Molecular Modeling 531

I. Molecular Mechanics Methods 533

A. Parametrization from Experiments 534

B. Classes of FF Methods 534

II. Quantum Mechanical Methods 534

A. Application to the Electronic Problem 536

B. The Hartree Product (HP) Description 537

C. Minimal and Larger Basis Sets 538

D. Electron Correlation Method 539

E. Density Functional Theory (DFT) 540

III. Energy Minimization Procedure 540

IV. Computer Simulation Methods 541

V. Surfactant Systems 542

VI. Five Selected Systems 542

A. Aggregation in a Liquid (i) 542

B. Aggregation in a Liquid (ii) 543

C. Liquid–liquid and Liquid–Gas Interface 545

D. Solid–Liquid Interface 547

E. Solid–Liquid Interface and Aggregation in a Liquid 549

VII. Summary of Representative Modeling Studies 550

General References 568

Problems 568

Answers to Selected Problems 569

Index 576

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