Effect of 1-Pentanol on Size and Shape of Sodium Dodecyl Sulfate Micelles as Studied by Positron Annihilation Lifetime Spectroscopy

Abstract

The changes in size and shape of micelles formed in a 0.2 mol dm-3 sodium dodecyl sulfate (SDS) aqueous solution are studied as a function of added 1-pentanol concentration at 294 K, by using positron annihilation lifetime spectroscopy (LS). As in a previous work, the trapping of ortho-positronium (o-Ps) from the aqueous to the organic micellar subphase results in the appearance of two o-Ps states with distinct lifetimes. The LS data are analyzed on the basis of a time-dependent trapping rate coefficient, which involves two parameters characteristic of the micellar system: the mean core radius, Rcore, and the mean aggregation number, Nag. Setting the Nag values from a previous experimental work, the LS data give thus access to Rcore. In parallel to the variation of Nag, Rcore is found to decrease slightly, then to increase steadily as the alcohol concentration is increased. A quantitative agreement is found between the experimental values of Rcore and those expected on the basis of the volume of the hydrocarbon tails of both SDS and 1-pentanol with different geometries, strongly suggesting that the micelles evolve from spherical to rod shaped. In addition, the lifetime (τ4) of those o-Ps atoms trapped in the micelles shows an initial increase, at low alcohol concentration, which is attributed to a decrease in the local surface tension inside the dodecane pools due to the presence of the shorter 1-pentanol hydrocarbon tail. On adding more alcohol, τ4 shows a somewhat linear decrease, indicating a deeper penetration of the pentanol molecules inside the micelles with a resulting increase in their surface tension.