PREDICTION OF SOLUBILITY OF NONPOLAR GASES IN MICELLAR SOLUTIONS OF IONIC SURFACTANTS

The solubilities of methane, carbon dioxide, ethane, and propane have been determined experimentally at 1 atm partial pressure and 25 degree s C in aqueous solutions of sodium dodecyl sulfate and cetyltrimethyla mmonium bromide. For this purpose a specially developed apparatus is u sed, which is relatively simple compared to those reported in the lite rature hitherto. Above the CMC, the solubility of each gas increases l inearly with surfactant concentration indicating micellar solubilizati on. The degree of solubilization is greatest for propane and decreases in the order propane > ethane > carbon dioxide > methane. Both the le ngth of the alkyl chain and the properties of the head-group region (i .e., the micelle-water interfacial tension) are observed to determine the intramicellar solubility. A method for estimating the solubilities is also proposed based on the rationale that the micellar phase may b e represented by a droplet of a ''model solvent'' into which the solub ilizate molecule dissolves as it would in a bulk solvent. The mixture of the solute species and the model solvent is assumed to behave as a regular solution. Accordingly, a characteristic solubility parameter i s estimated for the model solvent from the interaction potential betwe en two adjacent surfactant molecules in a micelle. The solubility comp uted using the regular solution approximation is further corrected for the Laplace pressure that acts across the curved micelle-water interf ace and generally reduces the intramicellar solubility. It is conclude d that for small nonpolar solutes, micellar solubilities may be reason ably well predicted by assuming that the model solvent has a solubiliz ing capacity (i.e., solubility parameter) close to that of a bulk equi valent alkane. (C) 1997 Academic Press.