Spontaneity of emulsification

Spontaneous emulsification was first demonstrated by Gad [13], who observed that when a solution of lauric acid in oil is carefully placed onto an aqueous alkaline solution, an emulsion is spontaneously formed at the interface. As explained in Ghapter 10, such spontaneous emulsification could be due to the very low (or transient negative) interfacial tension produced by the surfactant. Using an aqueous alkaline solution causes partial neutralisation of lauric acid. A mixture of lauric acid 

The second mechanism that may account for spontaneous emulsification is based on diffusion and stranding. This is best illustrated by carefully placing an ethanol-toluene mixture (containing say 10% alcohol) onto water. The aqueous layer eventually becomes turbid due to the presence of toluene droplets [20]. In 

Such a phase transition might be expected to occur when the third component increases the mutual solubility of the two previously immiscible phases. 

The third mechanism of spontaneous emulsification may be due to the production of an ultralow (or transiently negative) interfacial tension. This mechanism is thought to be the cause of formation of microemulsions when two surfactants, one essentially water soluble and one essentially oil soluble, are used [22, 23]. This mechanism is described in detail in Chapter 10 on microemulsions. 

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J.C. Lopez-Montilla, P.E. Herrera-Morales, and D.O. Shah New Method to Quantitatively Determine Spontaneity of Emulsification Process. Langmuir 18, 4258 (2002). 

In all of the above formulations, the role of surface chemistry is crucial, both in the formulation of the product and its subsequent application. Even for simple formulations of water-soluble actives, surface-active agents (sometimes referred to as wetters ) are needed to enable the spray solution to adhere to the target surface and spread over a large area. The surface-active agents also play a more subtle role in optimization of biological efficacy. With self-emulsifiable oils (referred to as emulsifiable concentrates (ECs)), surfactants are added in high concentrations to ensure the spontaneity of emulsification on dilution. The adsorption and conformation of the surfactant molecules at the oil/water (OAV) interface is crucial for spontaneous emulsification of the oil... 

Formation of Hposomal vesicles under controlled conditions of emulsification of Hpids with phosphoHpids has achieved prominence in the development of dmgs and cosmetics (42). Such vesicles are formed not only by phosphoHpids but also by certain nonionic emulsifying agents. Formation is further enhanced by use of specialized agitation equipment known as microfluidizers. The almost spontaneous formation of Hposomal vesicles arises from the self-assembly concepts of surfactant molecules (43). Vesicles of this type are unusual sustained-release disperse systems that have been widely promoted in the dmg and cosmetic industries. 

At approximately optimum salinity, spontaneous emulsification of brine drops in the oil phase began in both systems. This phenomenon resulted from local supersaturation of the oil phase, as explained in the discussion section below. The amount of emulsification tended to increase with increasing salinity. As a result, the cloud of emulsion drops began to obscure the interface between the microemulsion and oil, making interface position measurements difficult. These observations of spontaneous emulsification confirm the results of the earlier contacting experiments performed in the horizontal configuration ( 4). 

There are three similarities between macroemulsions and foams (1) They both consist of a dispersion of an immiscible state of matter in a liquid phase. Foams are dispersions of a gas in a liquid emulsions are dispersions of a liquid in a second immiscible liquid. (2) The tension y7 at the relevant interface is always greater than zero, and since there is a marked increase in interfacial area AA during the process (of emulsification or foaming), the minimum work involved is the product of the interfacial tension and the increase in interfacial area (Vkmm = AA x y7. (3) The system will spontaneously revert to two bulk phases unless there is an interfacial... 

In some cases under the conditions similar to those corresponding to the formation of lyophilic colloidal systems, a spontaneous formation of emulsions, the so-called self-emulsification, may take place. This is possible e.g. when two substances, each of which is soluble in one of the contacting phases, react at the interface to form a highly surface active compound. The adsorption of the formed substance under such highly non-equilibrium conditions may lead to a sharp decrease in the surface tension and spontaneous dispersion (see, Chapter III, 3), as was shown by A.A. Zhukhovitsky [42,43], After the surface active substance has formed, its adsorption decreases as the system reaches equilibrium conditions. The surface tension may then again rise above the critical value, acr. Similar process of emulsification, which is an effective method for preparation of stable emulsions, may take place if a surfactant soluble in both dispersion medium and dispersed liquid is present. If solution of such a surfactant in the dispersion medium is intensively mixed with pure dispersion medium, the transfer of surfactant across the low surface tension interface occurs (Fig. VIII-10). This causes turbulization of interface... 

In this technique, a transition in the affinity is obtained by changing the water volume fraction, instead of changing the temperature. By successively adding water into oil, initially water droplets are formed in a continuous oil phase. Increasing the water volume fraction changes the spontaneous curvature of the surfactant from initially stabilizing a w/o microemulsion to an o/w microemulsion at the inversion locus. This transition is referred to as PIC. PIC method of emulsification involves... 

Sacanna S, PhiUpse AP (2007) A generic single-step synthesis of monodisperse core/sheU colloids based on spontaneous Pickering emulsification. Adv Mater 19(22) 3824—3826... 

Figure 11.13 The process of emulsification of a W/0 gel emulsion by the spontaneous emulsification method. An oil-in-water (0/W) microemulsion at 7°C (a) is rapidly heated to 40 °C and a gel emulsion is spontaneously formed after 38 s (b). At this stage, the emulsion does not flow if the container is turned upside down (c)...

It is quite clear, first of all, that since emulsions present a large interfacial area, any reduction in interfacial tension must reduce the driving force toward coalescence and should promote stability. We have here, then, a simple thermodynamic basis for the role of emulsifying agents. Harkins [17] mentions, as an example, the case of the system paraffin oil-water. With pure liquids, the inter-facial tension was 41 dyn/cm, and this was reduced to 31 dyn/cm on making the aqueous phase 0.00 IM in oleic acid, under which conditions a reasonably stable emulsion could be formed. On neutralization by 0.001 M sodium hydroxide, the interfacial tension fell to 7.2 dyn/cm, and if also made O.OOIM in sodium chloride, it became less than 0.01 dyn/cm. With olive oil in place of the paraffin oil, the final interfacial tension was 0.002 dyn/cm. These last systems emulsified spontaneously—that is, on combining the oil and water phases, no agitation was needed for emulsification to occur. 

A (macro)emulsion is formed when two immiscible Hquids, usually water and a hydrophobic organic solvent, an oil, are mechanically agitated (5) so that one Hquid forms droplets in the other one. A microemulsion, on the other hand, forms spontaneously because of the self-association of added amphiphilic molecules. During the emulsification agitation both Hquids form droplets, and with no stabilization, two emulsion layers are formed, one with oil droplets in water (o /w) and one of water in oil (w/o). However, if not stabilized the droplets separate into two phases when the agitation ceases. If an emulsifier (a stabilizing compound) is added to the two immiscible Hquids, one of them becomes continuous and the other one remains in droplet form. 

Different methods are used in microemulsion formation a low-energy emulsification method by dilution of an oil surfactant mixture with water and dilution of a water-surfactant mixture with oil and mixing all the components together in the final composition. These methods involve the spontaneous formation of microemulsions and the order of ingredient addition may determine the formation of the microemulsion. Such applications have been performed with lutein and lutein esters. ... 

In buffered surfactant-enhanced alkaline flooding, it was found that the minimum in interfacial tension and the region of spontaneous emulsification correspond to a particular pH range, so by buffering the aqueous pH against changes in alkali concentration, a low interfacial tension can be maintained when the amount of alkali decreases because of acids, rock consumption, and dispersion [1826]. 

A similar technique, the so-called spontaneous emulsification solvent diffusion method, is derived from the solvent injection method to prepare liposomes [161]. Kawashima et al. [162] used a mixed-solvent system of methylene chloride and acetone to prepare PLGA nanoparticles. The addition of the water-miscible solvent acetone results in nanoparticles in the submicrometer range this is not possible with only the water-immiscible organic solvent. The addition of acetone decreases the interfacial tension between the organic and the aqueous phase and, in addition, results in the perturbation of the droplet interface because of the rapid diffusion of acetone into the aqueous phase. 

T Niwa, H Takeuchi, T Hino, N Kunou, Y Kawa-shima. Preparations of biodegradable nanospheres of water-soluble and insoluble drugs with d,l-lacti-de/glycolide copolymer by a novel spontaneous emulsification solvent diffusion method, and the drug release behavior. J Control Rel 25 89-98, 1993. 

Bouchemal, K., Briangon, S., Perrier, E. and Fessi, H. (2004) Nano-emulsion formulation using spontaneous emulsification solvent, oil and surfactant optimisation. International Journal of Pharmaceutics, 280, 241-251. 

S. Sugiura, M. Nakajima, N. Kumazawa, S. Iwamoto, and M. Seki Characterization of Spontaneous Transformation-Based Droplet Formation During MicroChannel Emulsification. J. Phys. Chem. B 106, 9405 (2002). 

M.J. Rang and C.A. Miller Emulsions and Microemulsions—Spontaneous Emulsification of Oil Drops Containing Surfactants and Medium-Chain Alcohols. Prog. Colloid Polym. Sci. 109, 101 (1998). 

M. J. Rang and C.A. Miller Spontaneous Emulsification of Oils Containing Hydrocarbon, Nonionic Surfactant, and Oleyl Alcohol. J. Colloid Interface Sci. 209, 179 (1999). 

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). 

These equations, referring to completely unstirred systems, are not usually valid in practice complications such as spontaneous interfacial turbulence and spontaneous emulsification often arise during transfer, while, if external stirring or agitation is applied to decrease Ri and R2, the hydrodynamics become complicated and each system must be considered separately. The testing of the above equations will be discussed below, after a consideration of overall coefficients and of interfacial turbulence. 

Study of the eflSciency of packed columns in liquid-liquid extraction has shown that spontaneous interfacial turbulence or emulsification can increase mass-transfer rates by as much as three times when, for example, acetone is extracted from water to an organic solvent (84, 85). Another factor which may be important for flow over packing has been studied by Ratcliff and Reid (86). In the transfer of benzene into water, studied with a laminar spherical film of water flowing over a single sphere immersed in benzene, they found that in experiments where the interface was clean... 

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