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its anhydrides. The space-filling model of a representative component of polysorbate 20 is shown in Table 6.8. The polysorbates are miscible with water, as reflected in their higher HLB values (see Table 6.8), and are used as emulsifying agents for oil-in-water emulsions.

Cetomacrogol 1000 BP and other macrogol ethers

Cetomacrogol is a water-soluble substance with the general structure CH3(CH2) m (OCH2CH2)„OH, where m may be 15 or im and the number of oxyethylene groups, „, is between 20 and 24. It is used in the form of Cetomacrogol Emulsifying Wax BP in the preparation of oil-in-water emulsions and also as a solubilising agent for volatile oils. The cmc and micellar molecular weight in aqueous solution are 6 x 10 2 g dm 3 and 1.01 x 105, respectively. Other macrogol ethers are com mercially available as the Brij series -for example, Brij 30 [polyoxyethylene (4) lauryl ether, C12H35(OCH2CH2)4OH], Brij 72 [polyoxyethylene (2) stearyl ether, C18H37 (0CH2CH2)20H] and Brij 97 [polyoxyethylene (10) oleyl ether, C18H35(OCH2CH2)10OH].

Cremophor EL is a polyoxyethylated castor oil containing approximately 40 oxyethylene groups to each triglyceride unit. It is used as a solubilising agent in the preparation of intravenous anaesthetics and other products.

Poloxamers

Poloxamers are synthetic block copolymers of hydrophilic poly(oxyethylene) and hydro-phobic poly(oxypropylene) with the general formula EmP„Em, where E = oxyethylene (OCH2 CH2) and P = oxypropylene (OCH2CHCH3) and the subscripts m and n denote chain lengths. Properties such as viscosity, HLB and physical state (liquid, paste or solid) are dependent on the relative chain lengths of the hydrophilic and hydrophobic blocks. The convention for naming these compounds is to use a number of which the first two digits, when multiplied by 100, correspond to the approximate average molecular weight of the poly(oxypropylene) block and the third digit, when multiplied by 10, corresponds to the percentage by weight of the poly(oxyethylene) block. For example, the poly(oxypropylene) block of poloxamer 188 has a molecular weight of approximately 1800 and about 80% by weight of the molecule is poly(oxypropylene). These copolymers were introduced to the market in 1951 by the Wyandotte Chemical Corp. (now BASF-Wyandotte) under the trade name Pluronic. The nomenclature adopted by this company indicates the physical state by a letter (F, P or L, denoting solid, paste or liquid, respectively) followed by a two- or three-digit number. The last digit of this number is the same as that for the equivalent poloxamer and is approximately one-tenth of the weight percentage of poly(oxyethylene), the first digit (or two digits in a three-digit number) multiplied by 300 gives a rough estimate of the molecular weight of the hydrophobe. So, for example, Pluronic F68 is a solid, the molecular weight of the hydrophobe is approximately 1800 and the poly(oxyethylene)

Table 6.9

Nomenclature of EmPnEm and PnEmPn(*) block cc

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