ABSTRACT
Gravitational Coagulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 1.6 Gravitational Coagulation in the Primary Minimum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 1.7 Classification of Regimes of Gravitational Coagulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 1.8 Delineation of Different Particle Loss Mechanisms. Rapid Coagulation . . . . . . . . . . . . . . 61
1.8.1 Gravity-Induced Flocculation versus Brownian Flocculation . . . . . . . . . . . . . . . . . . 61 1.8.2 Gravity-Induced Flocculation versus Creaming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
1.8.3 Domains of Dominant Particle Loss Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 1.8.4 Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
1.9 Coupling of Flocculation and Coalescence in Dilute Oil-In-Water Emulsions . . . . . . . . . 70 1.9.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
1.9.1.1 Kinetic and Thermodynamic Stability in Macroemulsions and Mini-Emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
1.9.1.2 Current State of Emulsion Stability Science . . . . . . . . . . . . . . . . . . . . . . . . 71 1.9.1.3 The Specificity of Emulsion Characterization . . . . . . . . . . . . . . . . . . . . . . . 72 1.9.1.4 Scope of Section 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
1.9.2 Coupling of Coalescence and Flocculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 1.9.2.1 Singlet-Doublet Quasi-Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 1.9.2.2 Kinetic Equation for Coupling of Flocculation and Intradoublet
Coalescence in Monodisperse Emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 1.9.2.3 Coalescence in a Singlet-Doublet System at Quasi-Equilibrium . . . . . . 75 1.9.2.4 Reduced Role of Fragmentation with Decreasing τc . . . . . . . . . . . . . . . . . 76 1.9.2.5 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
1.9.2.5.1 Application of Video Enhanced Microscopy Combined with the Microslide Technique for Investigation of Singlet-Doublet Equilibrium and Intradoublet Coalescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
1.9.2.5.2 Improving the Experimental Technique with the Use of Low Density Contrast Emulsions . . . . . . . . . . . . . . . . . . . . . 78
1.9.2.5.3 The Measurement of Coalescence Time and Doublet Fragmentation Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
1.9.2.6 Perspective for Generalization of the Theory for Coupling of Coalescence and Flocculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
1.9.3 Coupling of Coalescence and Coagulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 1.9.3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 1.9.3.2 Average Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
1.9.3.2.1 The Model by Borwankar et al. . . . . . . . . . . . . . . . . . . . . . . . . . 81 1.9.3.2.2 The Limiting Cases of Fast and Slow Coalescence . . . . . . . 82
1.9.3.3 DIGB Model for the Simultaneous Processes of Coagulation and Coalescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
1.9.4 Doublet Fragmentation Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 1.9.4.1 Theory of Doublet Fragmentation Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 1.9.4.2 Doublet Fragmentation Time of Uncharged Droplets . . . . . . . . . . . . . . . . 87 1.9.4.3 Lifetime of a Doublet of Charged Droplets and
Coagulation/Flocculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 1.9.5 Coalescence Coupled with Either Coagulation or Flocculation in Dilute
Emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 1.9.5.1 Fragmentation of Primary Flocs in Emulsions and the Subsequent
Reduction of Coalescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 1.9.5.2 Domains of Coalescence Coupled Either with Coagulation or with
Flocculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 1.9.5.3 Hydration Forces Initiate Flocculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
1.9.6 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 1.9.6.1 Long-Term Prediction of Emulsion Stability . . . . . . . . . . . . . . . . . . . . . . . . 95 1.9.6.2 Perfection of Methods for Emulsion Stabilization (Destabilization)
by Means of the Effect on Both Coalescence and Flocculation . . . . . . . 96
1.9.6.2.1 Combining Surfactants and Polymers in Emulsion Stabilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
1.9.6.2.2 Strong Influence of Low Concentrations of Ionic Surfactant on Doublet Fragmentation Time and Coalescence Time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
1.9.6.3 Standardization of the Measurement of τc and τd . . . . . . . . . . . . . . . . . . . 97 1.9.6.4 Experimental-Theoretical Emulsion Dynamics Modeling . . . . . . . . . . . . 97
1.9.6.4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 1.9.6.4.2 Combined Approach in Investigations of Dilute and
Concentrated Emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 1.9.6.4.3 Kernel Determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 1.9.6.4.4 Singlet-Doublet Quasi-Equilibrium . . . . . . . . . . . . . . . . . . . . . . 99 1.9.6.4.5 Substitution of the Coalescence Kernels . . . . . . . . . . . . . . . . . 99
1.10 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
For emulsion stability the surface chemistry, the dynamic of adsorption, the surface rheology, and the physicochemical kinetics are important. In contrast to the large success in industrial application of the emulsion surface chemistry the physicochemical kinetics is almost not used in emulsion technology. Meanwhile, the population balance equation (PBE) enables prediction of the evolution in time for droplet size distribution if the family of subprocesses including droplet aggregation, aggregate fragmentation, droplet coalescence, and droplet (floccula) creaming are quantified. These subprocesses are characterized in PBE by means of kinetic coefficients (kernels). The coupling of these four subprocesses, droplet poly-dispersity, and immense variety in droplet aggregate configurations cause the extreme difficulty in emulsion dynamics modeling (EDM). Three subprocesses, namely aggregation, fragmentation, and creaming, can be quantified. The systematic consideration of these three subprocesses with account for both Brownian and gravitational aggregation is accomplished in this chapter as it is necessary for EDM. In contrast to those three subprocesses, the experimental approach only is effective now concerning the emulsion film stability and coalescence kernel quantification for EDM. Accordingly, an experimental theoretical approach for the coalescence time determination, based on a dilute emulsion characterization in its simplest state, namely at the singlet doublet quasi-equilibrium, is elaborated. Information about elementary acts of coalescence and fragmentation obtained in experiments with dilute emulsion preserves its significance for concentrated emulsion as well. For modeling of nondiluted emulsions the combining of experimental investigation of the simplest emulsion model system with computer simulation accounting for the characteristics of a concentrated emulsion is proposed. Emulsion dynamics modeling combined with experiments using dilute emulsions at singlet doublet equilibrium may result in: (1) the quantification of emulsion film stability, namely the establishment of the coalescence time dependence on the physicochemical specificity of the adsorption layer of a surfactant (polymer), its structure and the droplet dimensions. This quantification can
form a base for the optimization of emulsifier and demulsifier selection and their synthesis for emulsion technology applications, instead of the current empirical level applied in the area; (2) the elaboration of a commercial device for coalescence time measurement, which in combination with EDM will represent a useful approach to the optimization of emulsion technology with respect to stabilization and destabilization.
