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

Abstract

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 Noüy 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.

Adsorption and Premicellar Aggregation of CTAB Molecules and Fabrication of Nanosized Platinum Lattice on the Glass Surface

“…They evidenced some premicellar changes in the correlation times of a spin probe, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). These results are in agreement with ref , revealing premicellar association in sodium dodecyl sulfate (SDS) solutions.…”

“…Using four different types of spin probes in their experiments and after extensive analysis of the data, they have suggested that the observed aggregation processes cannot be attributed to any probe-induced effect. A rather detailed study of the probe correlation time in a wide premicellar concentration range was reported in a paper . The authors have suggested the formation of micellar type of aggregates.…”

“…A rather detailed study of the probe correlation time in a wide premicellar concentration range was reported in a paper. 36 The authors have suggested the formation of micellar type of aggregates. However, no one could propose an idea that would clarify the nature of such aggregates.…”

“…For aggregation processes with correlation times in range between 0.01 and 10 ns the rate of nitroxide molecule rotation can be characterized by rotation correlation time τ c , which can be calculated as follows: τ c = 6.5 · 10 − 10 · W 0 ( h 0 h − 1 + h 0 h + 1 − 2 ) goodbreak0em1em⁣ [ s ] where W is the line width of central spectral line, h 0 , h +1 , h –1 are peak to peak intensity of central, low-field and high-field lines, respectively . Correlation time τ c is defined as a time it takes the molecule of probe to rotate to an angle of 1 rad.…”

“…where W is the line width of central spectral line, h 0 ,h +1 ,h respectively. 36 Correlation time τ c is defined as a time it takes the molecule of probe to rotate to an angle of 1 rad.…”