Soybean lecithin-water system

The phase diagreun of a pure soybean lecithin-water system is shown in Fig. 5.5. The excess water region, relevant to emulsions based on this surfactant, consists of a dispersion of the lamellar hquid crystalline phase in the form of liposomes. 

Fig. 5.5 Binary phase diagram of soybean lecithin-water system.

Soybean lecithins can be chemically altered to modify their emulsifying properties and improve their dispersibihty in aqueous systems. Phospholipids may be hydrolyzed by acid, base, or enzyme to achieve better hydrophilic and emulsification properties. Hydroxylation of lecithin improves its oil-in-water emulsification property and water dispersibihty. Acetylation creates improved fluidity and emulsification, water dispersion properties, and heat stability (200). 

From Stockmann and Schwarz (1999). Oil-in-water emulsions were prepared with 20% corn oil and 1% emulsifier in 0.2 M acetate buffer (pH 5). SDS = sodium dodecyl sulfate, CTAB = cetyltrimethyl ammonium bromide, Brij 58 = polyoxyethylene 20 cetyl ether, PHLC = partially hydrolysed soybean lecithin. Amount of solute solubilized in oil lipid phase of emulsions was calculated from the difference between the total amount in the system and the amount of oil solubilized in the aqueous phase determined by ultrafiltration and semiequiUbrium dialysis. 

A vesicle system prepared with purified soybean lecithin in 1 mM KCl has an inner radius of 15 nm. The system is then diluted with a large excess of distilled water. Estimate the time required for the encapsulated ions to transfer into the outer continuous phase given that the permeability coefficient for both ions is 10 cm/s and that the dimensions of the vesicles do not change despite the change in osmotic pressure. 

Nature utilizes surfactants for a variety of additional roles. A natural surfactant, using a strict definition, is a surfactant taken directly from a natural source (isolated by a separation procedure from either a plant or an animal origin).42 Lecithin, obtained either from soybean or from egg yolk, is probably the best example of a truly natural surfactant. Other natural originated surfactants are the various soap-like surfactants for the removal of fatty/oily substances. These compounds produce a rich lather when dispersed into water and are found in various natural systems (such as chestnuts, in leaves and seeds of Saponaria Officinalis (soapwort), in the bark of the South American soaptree Quillaja saponaria Molina and in the fruits of Acacia Auriculiformis (Figure 7.2) 43,44... 

Lecithin-based o/w MEs for parenteral use were formulated using polysorbate 80, IPM (Isopropyl myristate), lecithin, and water at different lecithin-polysorbate 80 weight ratios [115]. The formulated systems were shown to be highly stable and of minimal toxicity when evaluated in vitro. Phospholipid-based ME formulations of all-trans retinoic acid (ATRA) for parenteral administration were prepared and tested in vitro [116]. ATRA is effective against acute promyelocytic leukemia with highly variable oral bioavailability. Parenteral ME of ATRA was prepared using pharmaceutically acceptable ingredients, namely phospholipids and soybean oil. The inhibitory effect of ATRA on two human cancer cell lines (HL-60 and MCF-7) was not affected by incorporation into a ME formulation. 

Another important class of surfactants in cosmetics is the phospholipids (e.g., lecithin obtained from egg yolk or soybean), which are used as emulsifiers as well as for the formation of liposomes and vesicles. Liposomes are multi-lamellar bUayers of phospholipids that on sonication produce singular bilayers or vesicles. They are ideal systems for cosmetic applications. They offer a convenient method for solubilizing water insoluble active substances in the hydrocarbon core of the bilayer. They will always form a lamellar liquid crystalline structure on the skin and, therefore, they do not disrupt the structure of the stratum comeum. Phospholipid liposomes may be used as an indicator for studying skin irritation by surfactants. 

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