Sodium Lauryl Ether Sulfate Micellization and Water Solubility Enhancement Towards Naphthalene and Pyrene: Effect of the Degree of Ethoxylation

摘要

The effect of ethoxylation on self-aggregation behavior in aqueous media of sodium lauryl ether sulfate (SLES), C_(12)H_(25) (OCH2CH2)_xOSO3Na, where x = 1 (SLE1S), x = 2 (SLE2S), and x = (SLE3S) was investigated. CMCs were determined from the effect of surfactant concentration on (2,2'-bipyridine dichloro-ruthenium(II) hexahydrate (Ru(bipy)_3~(2+),2Cl~-) fluorescence emission. Aggregation numbers and micelle concentrations were estimated from the fluorescent quenching of (Ru(bipy)_3~(2+),2Cl~-) by 9-methylanthracene. Interestingly, critical micelle concentrations (0.80 mM), average aggregation numbers (N = 43), and micelle concentrations ([M] = 40 μM) were found to be independent of the degree of ethoxylation. Water solubilization enhancement of naphthalene and pyrene by SLES was also investigated. Water solubility enhancement for naphthalene and pyrene was observed with increasing degrees of ethoxylation and follow the order SLE3S > SLE2S > SLE1S. The higher impact of oxyethylene (OE) groups on solubilization enhancement of naphthalene over pyrene was taken as an indication for a preferential distribution of the relatively polar organic naphthalene molecule at the outer layer of the micelle in close contact with the hydrated oxyethylene units, thereby allowing for a strong polycy-clic aromatic hydrocarbon (PAH)-micelle interaction. On the other hand, pyrene molecule is preferentially distributed in the core of the micelle, relatively far away from the OE units located at the micelle-water interface for PAH-micelle interaction to occur. Water solubility enhancement for both PAHs was also was associated with an increase in SLES micelle size with the degree of ethoxylation as well as to the accessibility of the locus solubilization for the solute.