Free energy emulsion formation

As mentioned in Chapter 10, the preparation of an emulsion requires oil, water, a surfactant, and energy. This can be considered on the basis of the energy required to expand the interface, AAy (where AA is the increase in interfacial area when the bulk oil with area Aj produces a large number of droplets with area Aj Aj Aj, where y is the interfacial tension). Since y is positive, the energy 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 the total free energy of formation of an emulsion, AG is positive. 

The free energy of formation of the emulsion from the bulk oil (state 1) is given by the following ... 

Y is positive, the energy 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 the total free energy of formation of an emulsion, AG is positive. 

There is a very large increase in interfacial surface area when an emulsion is formed from layers of two immiscible liquids. This can be a 7-8 order of magnitude increase. To accomplish this, work must be done, some of which remains in the system as potential energy and some of which is dissipated as heat. The free energy of formation, Gform-, is a function of the work done and the increase in configurational entropy ... 

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... 

Emulsion formation enhances the area of the interface between the two immiscible solvents and as a result also enhances the free energy of the system, which may be designated by the following expression ... 

Any surfactant adsorption will lower the oil-water interfacial tension, but these calculations show that effective oil recovery depends on virtually eliminating y. That microemulsion formulations are pertinent to this may be seen by reexamining Figure 8.11. Whether we look at microemulsions from the emulsion or the micellar perspective, we conclude that the oil-water interfacial free energy must be very low in these systems. From the emulsion perspective, we are led to this conclusion from the spontaneous formation and stability of microemulsions. From a micellar point of view, a pseudophase is close to an embryo phase and, as such, has no meaningful y value. 

Block or graft copolymers in a selective solvent can form structures due to their amphiphilic nature. Above the critical micelle concentration (CMC), the free energy of the system is lower if the block copolymers associate into micelles rather than remain dispersed as single chains. Often the micelles are spherical, with a compact core of insoluble polymer chains surrounded by a corona of soluble chains (blocks) [56]. Addition of a solvent compatible with the insoluble blocks (chains) and immiscible with the continuous phase leads to the formation of swollen micelles or polymeric micro emulsion. The presence of insoluble polymer can be responsible for anomalous micelles. 

Concerning thermodynamically unstable emulsions, the creation of new interfaces from the disruption of the disperse phase increases the free energy of the system, which tends to return to the original two separate systems. Therefore, the use of emulsifier is necessary not only to reduce the interracial tension, but also to avoid the coalescence and the formation of macroaggregates thanks to electrostatic repulsion between adsorbed emulsifier. 

In the formation of an emulsion, one of the two immiscible liquids is broken up into droplets which are dispersed in the other liquid. The dispersion of one liquid in another immiscible liquid leads to a large increase in interfacial free energy because of the increase in the area of the interface. The emulsifying agent stabilises the emulsion by adsorbing at the liquid-liquid interface as an oriented interfacial film. This film reduces the interfacial tension between the liquids and also decreases the rate of coalescence of the dispersed droplets by forming mechanical, steric and/or electrical barriers arormd them. 

Let us assume that the total free energy of the emulsion can be separated into several independent contributions. Considering hypothetically the formation or coalescence of emulsion of two immiscible liquids (e.g. oil and water), such that external field forces are absent. The total free energy (Gg) of the system just before emulsification process can be expressed in the form (10)... 

GjE is larger than that before emulsification. The free energy of emulsion formation can be written in the form (10)... 

These are transparent or translucent systems covering the size range from 5 to 50nm. Unlike emulsions and nanoemulsions (which are only kinetically stable), microemulsions are thermodynamically stable as the free energy of their formation is either zero or negative. Microemulsions are better considered as swollen micelles normal micelles can be swollen by some oil in the core of the micelle to form O/W microemulsions. Reverse micelles can be swollen by water in the core to form W/O microemulsions. 

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