The Science of Defoaming

xiii

© 2010 Taylor & Francis Group, LLC

Contents

Preface.................................................................................................................. xxiii

Author ....................................................................................................................xxv

Chapter 1 Some General Properties of Foams ......................................................1

1.1 Introduction ...............................................................................1

1.2 Structure of Foams ....................................................................1

1.3 Foam Films ................................................................................5

1.3.1 Surface Tension Gradients and Foam Film Stability ...5

1.3.2 Drainage Processes in Foam Films ..............................9

1.3.3 Disjoining Forces and Foam Film Stability ............... 14

1.4 Processes Accompanying Aging of Foam ............................... 18

1.4.1 Capillary Pressure Gradients ..................................... 18

1.4.2 Foam Drainage ...........................................................21

1.4.3 Bubble Coarsening (Diffusional Disproportionation) ....24

1.5 Summarizing Remarks ............................................................28

Acknowledgment ................................................................................29

References ..........................................................................................29

Chapter 2 Experimental Methods for Study of Foam and Antifoam Action ......33

2.1 Introduction .............................................................................33

2.2 Measurement of Foam ............................................................. 33

2.2.1 Bartsch Method: Hand Shaking

MeasuringCylinders .................................................. 33

2.2.2 Automated Shake Tests ..............................................35

2.2.3 Ross–Miles Method ...................................................35

2.2.4 Tumbling Cylinders ....................................................36

2.2.5 Gas Bubbling .............................................................. 37

2.2.6 Direct Air Injection ....................................................38

2.2.7 Measurement of Bubble Size Distributions ................39

2.3 Observations with Single Foam Films .................................... 41

2.3.1 Scheludko Cells .......................................................... 41

2.3.1.1 Films Containing Antifoam Drops ............. 41

2.3.1.2 Measurement of Disjoining

PressureIsotherms of Air–Liquid–Air

Foam Films ................................................. 43

2.3.2 Dippenaar Cell ...........................................................44

2.3.3 Large Vertical Films .................................................. 45

xiv Contents

© 2010 Taylor & Francis Group, LLC

2.4 Air–Water–Oil Pseudoemulsion Films ....................................46

2.4.1 Direct Observation of Pseudoemulsion Films ............46

2.4.2 Measurement of Disjoining Pressure Isotherms

for Air–Water–Oil Pseudoemulsion Films .................48

2.4.3 Direct Measurements of Pseudoemulsion Film

Rupture Pressures .......................................................49

2.5 Spreading Behavior of Oils ..................................................... 52

2.6 Summarizing Remarks ............................................................ 53

References ..........................................................................................54

Chapter 3 Oils at Interfaces: Entry Coefcients, Spreading Coefcients,

and Thin Film Forces ......................................................................... 57

3.1 Introduction .............................................................................57

3.2 Classic Entry and Spreading Coefcients ............................... 58

3.3 Generalized Entry Coefcients, Pseudoemulsion Films,

and Thin Film Forces .............................................................. 61

3.3.1 Denitions .................................................................. 61

3.3.2 Case Where Generalized Entry Coefcient, E

g

> 0 ... 65

3.3.3 Case Where Generalized Entry Coefcient, E

g

< 0 ...68

3.3.4 Magnitude of Generalized Entry Coefcients ...........70

3.4 Mode of Rupture of Pseudoemulsion Films ............................ 72

3.4.1 Disjoining Pressures and Stability of

Pseudoemulsion Films ................................................ 72

3.4.2 Surface Tension Gradients and Stability of

Pseudoemulsion Films ................................................ 78

3.5 Generalized Spreading Coefcients and Thin Film Forces ....79

3.6 Spreading Behavior of Typical Antifoam Oils on

Aqueous Surfaces ....................................................................85

3.6.1 Hydrocarbon Oils ....................................................... 85

3.6.1.1 Spreading and Wetting Behavior of

Hydrocarbons on Surface of Pure Water ....85

3.6.1.2 Complete and Pseudo-Partial Wetting

Behavior of Hydrocarbons on the

Surfaces of Aqueous Surfactant

Solutions ................................................ 87

3.6.1.3 Non-Spreading (Partial Wetting) by

Hydrocarbons on the Surfaces of

Aqueous Surfactant Solutions .....................94

3.6.2 Polydimethylsiloxane Oils ..........................................96

3.6.2.1 Complete and Pseudo-Partial Wetting

Behavior of Polydimethylsiloxanes

on the Surfaces of Pure Water and

Aqueous Surfactant Solutions .....................96

xvContents

© 2010 Taylor & Francis Group, LLC

3.6.2.2 Rates of Spreading of

Polydimethylsiloxanes on the Surfaces

of Pure Water and Aqueous Surfactant

Solutions ...................................................104

3.6.3 Effect of Spread Hydrocarbon and

Polydimethylsiloxane Oils on the Stability of

Pseudoemulsion Films .............................................. 106

3.7 Non-Equilibrium Effects due to Surfactant Transport ..........108

3.8 Summarizing Remarks .......................................................... 108

References ........................................................................................ 110

Chapter 4 Mode of Action of Antifoams .......................................................... 115

4.1 Introduction ........................................................................... 115

4.2 Antifoam Effects due to Solubilized Oils ............................. 116

4.3 Effect on Foamability of Mesophase Precipitation in

Aqueous Surfactant Solutions ............................................... 121

4.4 Surface Tension Gradients and Theories of Antifoam

Mechanism ............................................................................128

4.4.1 General Considerations ............................................128

4.4.2 Surface Tension Gradients Induced by Spreading

Antifoam .................................................................. 129

4.4.3 Elimination of Surface Tension Gradients ............... 138

4.5 Oil Bridges and Antifoam Mechanism ................................. 141

4.5.1 Oil Bridges in Foam Films ....................................... 141

4.5.2 Line Tensions and Antifoam Behavior of Oil Lenses ...153

4.5.3 Oil Bridges in Plateau Borders and Stability of

Pseudoemulsion Films .............................................. 157

4.6 Antifoam Behavior of Emulsied Liquids ............................ 165

4.6.1 Antifoam Effects of Neat Oils in Aqueous

Foaming Systems ..................................................... 165

4.6.1.1 Early Work ................................................ 165

4.6.1.2 Short-Chain Alcohols ............................... 167

4.6.1.3 n-Alkanes .................................................. 175

4.6.1.4 Neat Polydimethylsiloxane Oils ................ 180

4.6.2 Antifoam Effects of Neat Oils in Non-Aqueous

Foaming Systems ..................................................... 183

4.6.3 Partial Miscibility and Antifoam Effects ................. 185

4.6.3.1 Origins of Partial Miscibility in Binary

Liquid Mixtures ........................................ 185

4.6.3.2 Systems with Lower Critical

Temperatures ............................................ 188

4.6.3.3 Systems with Higher Critical

Temperatures ............................................ 198