Info

a Reproduced from B. W. Barry and D. I. D. El Eini, J. Pharm. Pharmacol., 28, 210 (1976).

a Reproduced from B. W. Barry and D. I. D. El Eini, J. Pharm. Pharmacol., 28, 210 (1976).

Figure 6.40 Moles of tropicamide solubilised per mole of Pluronic plotted against the oxyethylene content of the poloxamer (see Table 6.9 for details of the Pluronics).

Reproduced from M. F. Saettone, B. Giannaccini, G. Delmonte, et al., Int. J. Pharm., 43, 67 (1988) with permission.

Figure 6.40 Moles of tropicamide solubilised per mole of Pluronic plotted against the oxyethylene content of the poloxamer (see Table 6.9 for details of the Pluronics).

Reproduced from M. F. Saettone, B. Giannaccini, G. Delmonte, et al., Int. J. Pharm., 43, 67 (1988) with permission.

polarisability and chain length) have been explored, it has not been possible to establish simple correlations between them. In general, a decrease in solubility in a surfactant solution occurs when the alkyl chain length of a homologous series is increased. Unsaturated compounds are generally more soluble than their saturated counterparts. Branching of the hydrocarbon chain of the solubilisate has little effect, but increased solubilisation is often noted following cyclisation. Unfortunately, these generalisations apply only to very simple solubilisates.

More specific rules can be formulated for particular series of solubilisates. For example, it is clear from studies of the effect of steroid structure on solubilisation by a series of surfactants that the more hydrophilic is the substituent in position 17 of the ring structure the lower is the quantity of surfactant required to effect solubilisation of the hormone. Thus the extent of solubilisation of hormones in sodium lauryl sulfate follows the series proges terone < testosterone < deoxycorticosterone, the C17 substituents being —COCH3, —OH and —COCH2OH, respectively (Box 6.5).

A relationship between the lipophilicity of the solubilisate, expressed by the partition coefficient between octanol and water, Poctanol (see Chapter 5), and its extent of solubilisation has been noted for the solubilisation of substituted barbituric acids by polyoxyethylene stearates, of substituted benzoic acids by polysorbate 20, and of several steroids by polyoxyethylene nonionic surfactants. An exhaustive survey of data for the solubilisation of some 64 drugs by bile salt micelles revealed linear relationships between log Pm (partition coefficient between micelles and water) and log Po ctanol for each of seven bile salts examined.21

Effect of temperature

In most systems the amount solubilised increases as temperature increases. The effect

Box 6.5 Structures of some steroid solubilisâtes

0 0

Post a comment