Solubilizate molar volume

The solubilizing capacity for a given surfactant system is a complex function of the physicochemical properties of the two components which, in turn, influence the location or sites where the drug is bound to the micelle. The molar volume of the solubilizate together with its lipophilicity are important factors, the former reducing and the latter increasing solubilization. ... 

Partial molar volumes and compressibilities of solutes at infinite dilution have proved valuable as a tool for studying water-solute interactions in aqueous solution, and a number of systems have been investigated. It has therefore been of interest to determine these quantities for solubilizates in the micellar state. By using Equation 6.23 these quantities can be obtained. 

The effect of the curvation of the micelle on solubilization capacity has been pointed out by Mukerjee (1979, 1980). The convex surface produces a considerable Laplace pressure (equation 7.1) inside the micelle. This may explain the lower solubilizing power of aqueous micellar solutions of hydrocarbon-chain surfactants for hydrocarbons, compared to that of bulk phase hydrocarbons, and the decrease in solubilization capacity with increase in molar volume of the solubilizate. On the other hand, reduction of the tension or the curvature at the micellar-aqueous solution interface should increase solubilization capacity through reduction in Laplace pressure. This may in part account for the increased solubilization of hydrocarbons by aqueous solutions of ionic surfactants upon the addition of polar solubilizates or upon the addition of electrolyte. The increase in the solubilization of hydrocarbons with decrease in interfacial tension has been pointed out by Bourrel (1983). 

Molar volume has been studied widely [2, 128, 148, 149] but no simple relationship has been shown between molar volume and the amount of solubilizate dissolved. Stearns et al. [148], studying hexane, heptane, and octane, and benzene, toluene, ethylbenzene, propylbenzene, and butylbcnzene concluded that there was inverse proportionality between the volume of hydrocarbon solubilized and molar volume. The slope of the plots of ml hydrocarbon dissolved per 100 g solution against molar volume of hydrocarbon are different for the aliphatic and aromatic series. Klevens [16], with polycyclic compounds in sodium laurate, found linear relationships between the log volume solubilized and molar volume, the slope of plots for linear polycyclics varying from that for the nonlinear polycyclics. Schwuger [149] reported that the amount of naphthalene, anthracene, pyrene, perylene and dibenzanthracene solubilized by micelles of dodecylpentaglycol ether was inversely related to the molecular size of these solubilizates. 

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