Polar oils, surfactants

All the results we obtained are qualitative in nature. The materials used in the experiments are mostly not 100% pure but should be usable in comparing performance result of industrial applications. And the conclusions we obtained do not have the limitation of a pure oil (or surfactant) or a particular oil (or surfactant). One important factor we have not discussed is the electrical effect. This may contribute significantly in anionic surfactant solubilization with polar oil. We have neither discussed the mechanism of solubilization nor the specific effect of a certain oil to a certain surfactant. To understand clearly such specific effects and to derive more quantitative relations governing the result of solubilization, we are planning to conduct future experiments with pure sample. 

Recently, nonionic ortho ester surfactants have been used as emulsifiers for squalene, a polar oil [65]. In this case a polymer is used together with the surfactant. The emulsification is made under acidic conditions, and the surfactant breaks down rapidly after the emulsion is formed, leaving a surfactant-free, polymer stabilized emulsion with reasonable stability. 

It is worth noting here that this difference between the interface and in the bulk is not specific to surfactant mixtures. While oil mixtures of very similar substances, such as n-alkanes, exhibit a linear mixing rule written in terms of equivalent alkane carbon number or EACN [62-64], mixtures of oils containing substances with very different polarities behave in a non-ideal way and exhibit a segregation near the interface, which results in an accumulation most polar oil components close to the interface [65]. 

Kabalnov described water-based inkjet ink compositions that are miniemulsions, i.e., an aqueous vehicle having emulsified oil particles with dissolved dye molecules, where the oil droplets have a diameter of less than 1 m. In his patent, Kabalnov mentioned the advantages of miniemulsions in comparison to microemulsions, namely the surfactant nature and concentration which allow better penetration control to the printed papers, and the dye load in miniemulsions can also be increased compared to microemulsions at acceptable viscosity limits. According to this invention, the aqueous inkjet ink composition is comprised of an oil-soluble dye, a solvent, and an aqueous vehicle wherein particles of the oil-soluble dye are dissolved in low-polarity oil particles having a particle size of less than 1 m, the particles forming miniemulsions in water. 

Minana-Perez, M., Graciaa, A., Lachaise, J. and Salager, J.L. (1995) Solubilization of polar oils with extended surfactants. Colloids Surf A, 100, 217-224. 

The phenomenon of microemulsification is mainly governed by factors such as (1) nature and concentration of the oil, surfactant, co-surfactant and aqueous phase, (2) oil/surfactant and surfactant/co-surfactant ratio, (3) temperature, (4) pH of the environment and (5) physicochemical properties of the API such as hydrophilicity/lipophilicity, plformulating microemulsions. From a pharmaceutical perspective, one of the most important factors to be considered is acceptability of the oil, surfactant and co-surfactant for the desired route of administration. This factor is very important while developing micro emulsions for parenteral and ocular delivery as there is only limited number of excipients which are approved for the parenteral and ocular route. In Chapter 3 of this book a more general overview of formulating microemulsions is given and formulation considerations with respect to the components of microemulsions used in pharmaceutical applications are discussed below. 

Fig. III-7. The orientation of surfactant molecules adsorbed at different interfaces a - nonpolar solid/surfactant aqueous solution b - polar solid/surfactant solution in non-polar liquid (oil phase) [4]...

Oils are solubilized into the interior of micelles where they allow the micelle to swell to a larger radius, hence giving rise to cubic (12) and hexagonal (H2) phases at smaller a values than for the surfactant alone. Polar oils can also reside at the micelle surface to some extent, reducing micelle curvature and inducing the occurrence of lamellar and inverse phases. This behavior is typified by the behavior of the commercial nonionic surfactant nonylphenol-(probably branched)-decaethylene oxide with hexadecane and p-xylene [40]. 

Selecting a suitable surfactant is a crucial step for the preparation of microemnlsions, because formulation of a microemulsion requires the use of moderate to high concentration of surfactant. Surfactants are those compounds, which lower the interfacial tension between two liquids or between a liquid and a solid. Surfactants are usually amphiphilic compounds, that is, the molecule of a surfactant has two parts one part is soluble in polar solvents, which is hydrophilic, and the other part is insoluble in the polar solvent, which is hydrophobic. - The polar part of the surfactant molecule is referred as head, and the nonpolar part of the molecule is referred as tail. This amphiphilic nature confers unique capabilities to the surfactant molecules tendency to adsorb at the surfaces and interfaces, which causes the decrease of the surface tension, and also formation of the aggregates inside the solutions, resulting in the formation of the microemulsions. - - Surfactants will diffuse in water and adsorb at interfaces between air and water or at the interface between oil and water, in the case where water is mixed with oil. Surfactant in solution below its... 

Even minerals with naturally hydrophilic surfaces, such as coal, graphite or talc, may benefit from the use of a collector such as an emulsified non-polar, oil-like kerosene [18]. A review of collector surfactant types is given by Leja [15]. 

Bicontinuous microemulsions have been formulated to solubilize polar oils, even of the triglyceride type, because of a new generation of the so-called extended surfactants, which contain a poly(xl0)propylene oxide intermediate polarity chain in between the (ionic) hydrophilic and lipophilic groups [51,52,126]. 

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