Bulk and interfacial physical properties of aqueous solutions of sodium lauryl sulphate: Part IV: Dilute aqueous solution behavior by electron spin resonance studies and by pH and surface tension measurements

The electron spin resonance spectrum of the nitroxide label 2,2,6,6 – TetraMethyl – 4 – Piperidone – Oxide (TEMPO) has been investigated in aqueous solutions of a purified and commercially available impure samples of sodium lauryl sulphate (NaLS) for a wide range of concentrations. The spectra were recorded at 24°C as a function of surfactant concentration. The reorientational correlation times Tθ of the TEMPO label in aqueous solutions were calculated using the Kivelson's theory. Using the Tθ data qualitative analysis has been carried out in an attempt to understand the microscopic effects produced by the hydrophobic part of the surfactant on the water structure and the hydrophobic-hydrophobic interactions in water. Four major discontinuities in the τθ results have been identified with the purified NaLS system. They are found to occur at NaLS concentrations of 0.008 wt.%, 0.015 wt.%, 0.23 and 1.73 wt.%. The latter two concentrations represent the first and the second critical micelle concentration (CMC) of the surfactant while the first two concentrations reveal the existence of premicellar association and dissociation processes, respectively. These discontinuities have been also confirmed by the pH data and by surface tension results from the du Nouy ring method. The effect of addition of lauryl alcohol and sodium chloride on τθ values was also studied. The additive action has been found to produce an additional discontinuity, corresponding to the mixture CMC, in the τθ versus concentration plot. Analysis of other effects such as solution aging and hydrolysis of NaLS are also included. The applicability of the spin labelling technique for the study of aqueous solution behavior of NaLS is discussed by comparing data reported in the literature. On a etudie les spectres de resonance paramagnetique electronique d'un compose du type d'oxyde d'azote, a savoir le 2,2,6,6, – tetramethyl – 4-piperidone – oxyde (TEMPO); cette etude a ete faite dans des solutions aqueuses d'echantillons purifies et commerciaux impurs de l'agent tensioactif sulfate de lauryle et de sodium (SLNa), et ce, sur une large gamme de concentrations. Les spectres ont ete enregistres a 24°C en fonction de la concentration du surfactant. On a calcule, en employant la theorie de Kivelson, les temps de correlation Tθ de la reorientation, du compose TEMPO dans des solutions aqueuses. En utilisant les donnees de Tθ, on a fait une analyse qualitative dans le but de comprendre les effets microscopiques que produit la partie hydrophobique de l'agent tensioactif sur la structure de l'eau, et les interactions entre deux parties hydrophobiques dans l'eau. On a identifie quatre discontinuites importantes dans les resultats de τθ. dans le cas de l'echantillon purifie de SLNa; on les rencontre lorsque les concentrations de cet agent tensio-actif sont 0.008%, 0.015%, 0.23% et 1.73% en poids. Les denx dernieres concentrations representent respectivement la premiere et la seconde concentrations critiques en micelles (CCM) de l'agent tensioactif, tandis que les deux premieres concentrations revelent respectivement l'existence d'une association premicellaire et de processus de dissociation. Les don-nees de pH et les resultats obtenus par la methode de l'anneau de Du Nouy pour la tension superficielle, ont apporte une confirmation de l'existence des discontinuites precitees. On a aussi etudie l'effet que produisait l'addition d'alcool de lauryle et de chlorure de sodium sur les valeurs de τθ; cette addition creait une autre discontinuite (correspondant aux concentrations CCM) dans la representation graphique de τθ en fonction de la concentration. On presente egalement l'analyse d'autres effets, tels que le vieillissement des solutions et l'hydrolyse de SLNa. On discute l'applicabilite de la technique d'identification des spins a l'etude du comportement d'une solution aqueuse de SLNa, en comparant les donnees publiees.