Preparation of microemulsions

It may be clear now that preparation of microemulsions L, or L2 would involve controlled addition of the intended dispersed phase and its solubilization into a 

To prepare a reverse microemulsion by the titration method, a known amount of oil is taken, along with the surfactant, in a stoppered test tube at a constant temperature. This mixture is then titrated with water (using a microburette) to reach and pass through a compositional array with optical clarity. The appearance of a permanent turbidity indicates the maximum amount of water (the value of which has just been crossed) that can be solubilized. A similar procedure can be adopted to prepare a O/W microemulsion also, the difference being that in this case, the oil phase is titrated into the aqueous phase [122, 123]. 

In the contact method, in order to prepare a W/O microemulsion, the oil phase containing the surfactant is mixed with an excess aqueous phase and allowed to settle. The oil phase (containing dispersed aqueous droplets) is then in equilibrium with an excess aqueous phase forming a Winsor II system. The water content in the oil phase is estimated, usually by Karl Fischer titration. 

There are distinct differences in the water uptake and composition of the water pool in the W/O microemulsions obtained by the two methods described above. In case of the titration method, it is the composition of the titrant that determines the composition of the water pool. If some electrolyte is present in the titrant, its concentration affects the amount of water solubilized. In case of the contact method, however, the composition of the water pool depends on the exchange equilibrium of ions between the water pool and the excess aqueous phase, and this exchange 

The above observations tell us that the water content in reverse micelles as determined by the two methods can indeed be totally different. Whatever the case may be, there are various other reasons (many of them relating to the composition) 

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Hou A, Chen S (2010) Preparation of microemulsions of the polysiloxanes modified with different amines and their effect on the color shade of dyed cellulose. J Dispersion Sci Technol 31 102-107... 

Phosphate-borate buffer is the most frequently used solution for the preparation of microemulsion systems. An increase in the buffer concentration... 

Emulsions are two-phase systems formed from oil and water by the dispersion of one liquid (the internal phase) into the other (the external phase) and stabilized by at least one surfactant. Microemulsion, contrary to submicron emulsion (SME) or nanoemulsion, is a term used for a thermodynamically stable system characterized by a droplet size in the low nanorange (generally less than 30 nm). Microemulsions are also two-phase systems prepared from water, oil, and surfactant, but a cosurfactant is usually needed. These systems are prepared by a spontaneous process of self-emulsification with no input of external energy. Microemulsions are better described by the bicontinuous model consisting of a system in which water and oil are separated by an interfacial layer with significantly increased interface area. Consequently, more surfactant is needed for the preparation of microemulsion (around 10% compared with 0.1% for emulsions). Therefore, the nonionic-surfactants are preferred over the more toxic ionic surfactants. Cosurfactants in microemulsions are required to achieve very low interfacial tensions that allow self-emulsification and thermodynamic stability. Moreover, cosurfactants are essential for lowering the rigidity and the viscosity of the interfacial film and are responsible for the optical transparency of microemulsions [136]. 

Tsang JW, Moffatt JR. (1998) Preparation of microemulsion and Micellar color inks from modified water-soluble color chromaphores for thermal ink-jet printing, US5749952. 

Preparation of Microemulsions. Oil, lipopeptide and water are mixed in the same way as in emulsion preparation the mixture is then titrated, at room temperature, with the cosurfactant until transparency is obtained (9). 

Microemulsions are fluid, transparent, thermodynamically stable oil and water systems, stabilized by a surfactant usually in conjunction with a cosurfactant that may be a short-chain alcohol, amine, or other weakly amphiphilic molecule. An interesting characteristic of microemulsions is that the diameter of the droplets is in the range of 100-1000 A, whereas the diameter of droplets in a kinetically stable macroemulsion is 5000 A. The small droplet size allows the microemulsion to act as carriers for drugs that are poorly soluble in water. The suggested method of preparation of microemulsions is as follows the surfactant, oil, and water are mixed to form a milky emulsion and titrated with a fourth component, the cosurfactant,... 

Ethyl oleate is primarily used as a vehicle in certain parenteral preparations intended for intramuscular administration. It has also been used as a solvent for drugs formulated as biodegradable capsules for subdermal implantation and in the preparation of microemulsions containing cyclosporin. ... 

In the preparation of microemulsions and self-emulsifying systems, emulsions, or aqueous suspensions of medium-chain triglycerides, care should be taken to avoid microbiological contamination of the preparation, since lipase-producing microorganisms, which become active in the presence of moisture, can cause hydrolysis of the triglycerides. Hydrolysis of the triglycerides is revealed by the characteristic unpleasant odor of free medium-chain fatty acids. 

Preparation of microemulsions. Microemulsions were prepared of oil (cyclohexanol, cyclohexane or heptane), of surfactant Tween 80 - polyoxiethylen(20)sorbitan monoleate with M=1309.68 and, for comparison, of AOT (sodium bis(2-ethylhexyl)sulfosuccinate) or CTAB (cetyltrimethyl ammonium bromide) and water. The molar ratio R of water and surfactant moved in the range 270 - 1110. 

Considerable research is being done to increase the utility of CO2 as a solvent. In the area of pharmaceutical applications, supercritical fluids are useful as solvents in the production of particulate drugs, the extraction and separation of active ingredients, and the preparation of microemulsions and sustained drug delivery systems (2). A major disadvantage, insofar as solvent... 

Ho, H.-O. Hsiao, C.-C. Sheu, M.-T. Preparation of microemulsions using polyglycerol fatty acid esters as surfactant for the delivery of protein drugs. J.Pharm.Sci., 1996, 85, 138-143... 

In addition to the aforementioned differences, their methods of preparation also differ distinctly. Dining preparation of macroemulsions, a large input of energy is required, whereas preparation of microemulsions does not require energy. In contrast to ordinary emulsions, microemulsions form upon simple mixing of the components and do not require the high shear conditions generally used in the formation of ordinary emulsions. ... 

Preparation of microemulsion requires less amount of energy and is spontaneously formed. 

PIT method, which was introduced by Shinoda, is an anulsiiication technique for preparation of microemulsions with low energy. Advantages like being low cost have made the PIT method more attractive in recent years. The PIT concept is based on one type of phase inversion in anulsions (transitional inversion) induced by changing tanperature, which affects the HLB of the system. 

The PIC emulsification method for preparation of microemulsions enjoys many advantages, such as low preparation cost, absence of organic solvents, good production feasibility, long stability, and thermodynamic stability." ... 

Generally, water/oil/surfactant phase diagrams are not worked out in so much detail [47, 105] before particle synthesis and often, only the area important for synthesis is delineated for obvious practical reasons (partial phase diagrams, as in Fig. 3.3). We shall describe at the end of this Section, for the uninitiated, the basics of the phase diagrams and the procedures of setting them up. Preparation of microemulsions will be taken up in Section 3.3. 

Figure 3.7. A pictorial representation showing methods of preparation of microemulsions (a) contact method and (b) titration method. Reprinted from Rabie et al. [ 122], Copyright (1997), with permission from Elsevier Science.

A microemulsion is water/hydrocarbon dispersion stabilized by an ionic surfactant such as a soap, alkyl sulphate or sul-phonate and most often also contains a cosurfactant in the form of a medium chain length alcohol (pentanol). Of these four components water, surfactant and cosurfactant are called the structure forming elements since they form colloidal association structures similar to the microemulsions with no hydrocarbon present. The formulation and preparation of microemulsions is greatly enhanced by a knowledge of these composition dependent structures, hence an introductory description of them will be given. 

Microemulsions are thermodynamically (indefinitely) stable dispersions of droplets less than 100 nm in diameter. Microemulsions are spontaneously formed from mixtures of water and monomer containing large amounts of surfactant (at about 10 percent). The high surfactant concentration required for the preparation of microemulsions can be a disadvantage from an applications standpoint. An intermediate chain length alcohol (e.g., hexanol) is often used as a cosurfactant in preparing microemulsions. Because interfacial and... 

There are a wide range of other surfactants that form microemulsions, including nonionic, anionic, and cationic molecules. For instance, nonionic alkyl oligo(ethylene oxide) (CEp surfactants can be also used in the preparation of microemulsions with the traditional ILs ethylammonium nitrate ([C2NH3][N03]) or propylammonium nitrate ([C3NH3][NOj]) and alkanes (octane, decane, dodecane, tetradecane, and hexadecane) [46,47],... 

This section will summarise the basic principles involved in the preparation of microemulsions and the origin of their thermodynamic stability (see Chapter 10 for more details). A sub-section is devoted to emulsifier selection for both O/W and W/0 microemulsions. Physical methods that may be applied for characterization of microemulsions will be briefly described. Finally a sub-section is devoted to the possible enhancement of biological efficacy using microemulsions. The role of microemulsions in enhancing wetting, spreading and penetration will be discussed. Solubilization is also another factor that may enhance the penetration and uptake of an insoluble agrochemical. 

Chadonnet, S., H. Korstredt, A. Siciliano, Preparation of microemulsions by micro-fluidization, Soap/Cosmet./Chem. Specialties, 61, 2, p. 37, 1985. 

For the preparation of microemulsions 264 p,l water was emulsified in 6 ml Isoctane containing 140 mmol/1 AOT. 

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