Polyoxyethylene sterols

Synthesis of polyoxyethylene sterol was first reported by Scotney and Truter [6] in 1971. They simply mixed cholesterol and the appropriate amount of ethylene oxide in a sealed vial during 4 months. Later workers have applied more sophisticated methods to synthesize better defined products. Khachadurian et al. [7] synthesized both polyoxyethylene and methoxypolyoxyethylene derivatives of cholesterol. The former reaction was carried out in dichloromethane using boron trifluoride as catalyst. Derivatives with a methoxy terminal were prepared by condensing methoxypoly(oxyethylene)methanesulfonate with cholesterol in xylene. Commercially available products are produced by the reaction of sterol with ethylene oxide, resulting in a product with a distribution in the number of ethylene oxide (EO) units in the hydrophilic group. Meissner et al. also described the synthesis of sterol sulfate and sulfonate with oxy-ethylene spacers [8], The synthesis of sterol surfactants has been covered in a review by Folmer [9]. There are only a few producers of polyoxyethylene... 

There has been relatively few reports regarding polyoxyethylene sterols, as compared with other polyoxyethylene surfactants. However, in recent years there has been an increase in the number of published reports concerning the surface activity and solubility of polyoxyethylene derivatives of different sterols. 

The aggregation behavior of these surfactants appears to be unusual and requires a deeper analysis. A comparison of the CMC values reported for different polyoxyethylene sterols and polyoxyethylene octylphenols, which have lipophilicities similar to that of cholesterol, is shown in Fig. 2. As the hydrophilicity of the surfactant increases, the trend in the CMC values is quite opposite the CMC values for sterol surfactants are reduced when the number of oxyethylene units is increased, which is contrary to that found for other polyoxyethylene surfactants. The observation of lanolin alcohols can be viewed as an intermediate behavior. At lower numbers of oxyethylene units the CMC trend follows the behavior of sterols, whereas at a higher degree of ethoxylation the behavior becomes dominated by the ethoxylated alcohols. These comprise only about 20% of the weight but approximately 40% of the molar composition. 

Because of the beneficial effects on biological tissues that have been reported for sterol derivatives and also because of the rather exclusive natural raw materials, the use of this type of products has mainly been reported in personal care applications. Manufacturers recommend the products to be used in day creams, body lotions, and hair conditioners. In addition, the use of biological materials like sterols as a hydrophobic group has attracted studies of these surfactants in drug delivery applications. This section of the chapter discusses the use of polyoxyethylene sterols in these two very different applications. In addition, it can also be noted that unexpected and surprising uses of these types of surfactants in ink-jet inks was recently patented [32]. 

Stadler and coworkers [45] synthesized a bifunctional, or bola form, polyoxyethylene sterol derivative (Fig. 5). This compound was designed as a prototype for a new class of compounds intended to serve as functional equivalents to the antibiotic drug amphotericin B. Analysis of the surface pressure-area isotherm, at the air-water surface, indicated a limiting area of about. 0.60 nm per molecule. This value was consistent with a model in which... 

Name Polyoxyethylene soy sterol Sample preparation Capillary film, sodium chloride cell... 

FIG. 2 Critical micelle concentration (CMQ of polyoxyethylene derivatives of (-0-) cholesterols (from [11,13]), ( ) lanolin alcohols [10], (x) phytosterol [16], ( ) cholesteryl carbonate [24], and (A) octylphenol [21]. Increasing the length of the polyoxyethylene chain causes a decrease in the CMC for the sterol derivatives, initially causes a decrease in the CMC (minimum at 15 oxyethylene imits) for the lanolin alcohols and then an increase, while for the octylphenol derivatives the CMC increases with increasing hydrophilicity. 

The increase in spontaneous curvature at a given concentration with increasing length of the hydrophihc chain is clear as the number of units increases from 3 to 50 [13,16,19]. The cubic phases for 20 35 oxyethylene units are located between the miceUar and the hexagonal phases, which implies micellar cubic phases. It can be noted that the temperature stability of the liquid crystalline phases for sterol surfactants with 20 and 30 polyoxyethylene units is remarkably high compared with those formed by polyoxyethylene alkyl surfactants (see Fig. 4c, d) [16]. 

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