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

摘要

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 l-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 appeara11ce of two o-Ps states with distinct lifetimes. The LS data are analyze4 on. the basis of a time-dependent tr!iPping rat~ cgefficient; which involves two parameters characteristic of the micellar system: the mean core radius, Rcore, and the mean aggregation number, N_(ag). Setting the N_(ag) values from a previous experimental work, the LS data give thus access to R_(core). In parallel to the variation of Nag.. Rcoreis found to decrease slightly, then to increase steadily as the alcohol concentration is increased. A quantitative agreementis.found between the experimeQtal values of R_(core) and those expected on the basis of the volume of the hydrocarbon tails of both SDS and l-pentanol with different geometries, strongly suggesting that the micelles evolve from spherical to rod shaped. In addition, the lifetime (.4) of those o-Ps atoms trapped in the micelles shows an initial increase, at low alcoh9l concentration, which is attributed to a decrease in the local surface tension inside the 4odecane pools due to the presence of the shorter l-pentanol hydrocarbon tail. On adding more alcohol, .4 shows a somewhat linear decrease, indicating a deeper penetration of the pentanol molecules inside the micelles with a resulting increase in theiLsurface tension.