Mechanism of emulsification

Combination of Gmix, Gei with Ga gives the total energy of interaction Gt (theory of steric stabilisation) (Eq. 6.9). 

The schematic representation of the variation of G ix, Gei and Ga with h given in Eigure 6.10 shows that there is only one minimum (Gmm), whose depth depends on R,S and A. When ho 23, strong repulsion occurs and it increases very sharply with further decrease in ho- At a given particle size and Hamaker constant, the larger the adsorbed layer thickness, the smaller the depth of the minimum. If Gmin is made sufficiently small (large 3 and small R), one may approach thermodynamic stability. This explains the case with nanoemulsions, which will be discussed in a separate chapter. 

To prepare an emulsion, oil, water, surfactant and energy are needed [9, 10). This can be considered from an examination of the energy required to expand the interface, AAy (where AA is the increase in interfacial area when the bulk oil with area 

Ai produces numerous droplets with area A2 Ai Ai, y is the interfacial tension). Since y is positive, the energy required to expand the interface is large and positive. This energy term cannot be compensated by the small entropy of dispersion TAS (which is also positive), and, as already discussed, the total free energy of formation of an emulsion, AG is positive. 

Near 1 there is only one radius of curvature R, whereas near 2 there are two radii of curvature Rb,i and Rb,2- Consequently, the stress needed to deform the 

In order to break up a drop into smaller units it must be strongly deformed, and this deformation increases Ap. Surfactants play major roles in the formation of emulsions [8] by lowering the interfacial tension Ap is reduced, and hence the stress needed to break up a drop is reduced. Surfactants also prevent the coalescence of newly formed drops. 

To describe emulsion formation two main factors must be considered, namely hydrodynamics and interfacial science. In hydrodynamics, consideration must be given to the type of flow, whether laminar or turbulent, and this depends on the Reynolds number (as wiU be discussed later). 

To assess emulsion formation, the normal approach is to measures the droplet size distribution using, for example laser diffraction techniques. A useful average diameter d is, 

In most cases d 2 (the volume/surface average or Sauter mean) is used, while the width of the size distribution can be given as the variation coefficient. The latter is the standard deviation of the distribution weighted with d divided by the corresponding average d. Generally C2 will be used which corresponds to d 2 An alternative way to describe the emulsion quahty is to use the specific surface area A (the surface area of all emulsion droplets per unit volume of emulsion). 

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It will be seen that the scientific understanding of the mechanism of emulsification has not yet proceeded very far. The reader can gain from Clayton s Emulsions some idea of the great complexity of the phenomena. 

The conductometric titration of aqueous hexadecyltrimethyl-ammonium bromide-cetyl alcohol mixtures with benzene allows determination of the mechanism of emulsification. However, the shapes of the conductometric titration curves are complex the most sta-... 

Rulinson CJ, Lochhead RY, Bui HS, Pierce TD. Investigation of the mechanism of emulsification by hydrophobically modified hydrogels. VI. Dilute systems. ACS Polym Prepr Div Polym Chem 1993 34 863-864. 

The mechanisms of emulsification in oil reservoirs when chemical slugs are injected into the reservoir are largely unknown. This study was conducted to understand and characterize these mechanisms by microscopic observations and high speed cine-pho tomicrography. 

Mechanistic interpretations The results of the dynamic and equilibrium displacement experiments are used to evaluate and further define mechanisms by which alkaline floods increase the displacement and recovery of acidic oil in secondary mode and the tertiary mode floods. The data sets used in the mechanistic interpretations of alkaline floods are (a) overall and incremental recovery efficiencies from dynamic and equilibrium displacement experiments, (b) production and effluent concentration profiles from dynamic displacement experiments, (c) capillary pressure as a function of saturation curves and conditions of wettability from equilibrium displacement experiments, (d) interfacial tension reduction and contact angle alteration after contact of aqueous alkali with acidic oil and, (e) emulsion type, stability, size and mode of formation. These data sets are used to interpret the results of the partially scaled dynamic experiments in terms of two-stage phase alteration mechanisms of emulsification followed by entrapment, entrainment, degrees and states of wettability alteration or coalescence. 

The cleaning process proceeds by one of three primary mechanisms solubilization, emulsification, and roll-up [229]. In solubilization the oily phase partitions into surfactant micelles that desorb from the solid surface and diffuse into the bulk. As mentioned above, there is a body of theoretical work on solubilization [146, 147] and numerous experimental studies by a variety of spectroscopic techniques [143-145,230]. Emulsification involves the formation and removal of an emulsion at the oil-water interface the removal step may involve hydrodynamic as well as surface chemical forces. Emulsion formation is covered in Chapter XIV. In roll-up the surfactant reduces the contact angle of the liquid soil or the surface free energy of a solid particle aiding its detachment and subsequent removal by hydrodynamic forces. Adam and Stevenson s beautiful photographs illustrate roll-up of lanoline on wood fibers [231]. In order to achieve roll-up, one requires the surface free energies for soil detachment illustrated in Fig. XIII-14 to obey... 

The effect of ultrasound on liquid-liquid interfaces between immiscible fluids is emulsification. This is one of the major industrial uses of ultrasound (74-76) and a variety of apparatus have been devised which will generate micrometer-sized emulsions (9). The mechanism of ultrasonic emulsification lies in the shearing stresses and deformations created by the sound field of larger droplets. When these stresses become greater than the interfacial surface tension, the droplet will burst (77,78). The chemical effects of emulsification lie principally in the greatly increased surface area of contact between the two immiscible liquids. Results not unlike phase transfer catalysis may be expected. 

N. Shahidzadeh, D. Boim, and J. Meunier A New Mechanism of Spontaneous Emulsification Relation to Surfactant Properties. Europhys. Lett 40, 459 (1997). R.W. Greiner and D.F. Evans Spontaneous Formation of a Water-Continuous Emulsion from a W/O Microemulsion. Langmuir 6, 1793 (1990). 

M.G. Wakerly, C.W. Pouton, B.J. Meakin, and E.S. Morton Self emulsification of vegetable oil non-ionic mixtures a proposed mechanism of action. In Phenomena in Mixed Surf actant Systems. American Chemical Society, Washington, DC (1986). 

Wakerly, M.G., Pouton, C.W., Meakin, . J., and Morton, F.S. (1986). Self-emulsification of vegetable oil-nonionic surfactant mixtures A proposed mechanism of action. A.C.S. Symposium, 311, 242-255. 

D. Q. Craig, S. A. Barker, D. Banning, and S. W. Booth, Investigation into the mechanisms of self-emulsification using particle size analysis and low frequency dielectric spectroscopy, Int. J. Pharmaceut. 114 103-110 (1995). 

The adsorption mechanisms of surfactant at interfaces have been extensively studied in order to understand their performance in many processes such as dispersion, coating, emulsification, foaming and detergency. These interfaces are liquid-gas (foaming), liquid-liquid (emulsification) and liquid-solid (dispersion, coating and detergency). 

The mechanism of this emulsification has been explained in detail by Finkle, Draper, and Hildebrand,6 and by Ramsden.6 It was seen in 17 that if the contact angle is acute in liquid A, the greater part of the bulk of the particles, including the widest cross-section, is in that fluid. Fig. 41 shows a number of particles in contact in an interface, their greatest cross-section being in A. Any contraction of the interface must bend the surface with its load of solid particles so as to become concave to fluid B. Thus the contraction of the interface, after bringing the particles into contact, curves the interface towards that fluid which wets the powder least, in which the contact angle is obtuse. 

In this work the potential of biphasic synthesis mixtures for the synthesis of microporous silicoalumino-phosphates is investigated. The influence of emulsification of the synthesis mixture, agitation and temperature on the crystallization is studied. The structural identity of MCM-1 and A1P0.-H3, which is an aluminum phosphate hydrate that crystallizes in absence of template, is demonstrated. Special attention is paid to the mechanism of Si substitution in MCM-1, to its catalytic activity and its intracrystalline void structure. 

Prior to this discovery, in 1954 Silberberg and Kuhn (62) were first to study the polymer-in-polymer emulsion containing ethylcellulose and polystyrene in a nonaqueous solvent, benzene. The mechanisms of polymer emulsification, demixing, and phase reversal were studied. Wetzel and Hocks discovery would then equate the pressure-sensitive adhesive to a polymer-polymer emulsion instead of a polymer-polymer suspension. Since the interface is liquid-liquid, the adhesion then becomes one type of R-R adhesion (35, 36). According to our previous discussion, diffusion is not operative unless both resin and rubber have an identical solubility parameter. The major interfacial interaction is physical adsorption, which, in turn, determines adhesion. Our previous work on the wettability of elastomers (37, 38) can help predict adhesion results. Detailed studies on the function of tackifiers have been made by Wetzel and Alexander (69), and by Hock (20, 21), and therefore the subject requires no further elaboration. 

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