Size, Hydration, and Shape of SDS/Heptane Micelles Investigated by Time-Resolved Fluorescence Quenching and Electron Spin Resonance

Abstract

Time-resolved fluorescence quenching (TRFQ) and electron spin resonance (ESR) are used to investigate sodium dodecyl sulfate/heptane micelles in water in the presence of varying amounts of sodium chloride (NaCl). Aggregation numbers are obtained from TRFQ and the micelle hydration from ESR. At any zero or any fixed sodium chloride concentration ≤375 mM, micelles grow with heptane while maintaining the volume fraction of water in the polar shell of the micelle, referred to as hydration index, constant. Core−shell model calculations, using simple geometry, show that some hydrocarbon density extends into the polar shell. The shell comprises about 15−17% of the core hydrocarbon in addition to the headgroups and hydration water. The rate of micelle growth with sodium chloride is smaller in the presence of heptane, and at higher salt concentrations (>375 mM), the micelles decrease in size with heptane. The rate of variation of hydration index with [NaCl] or aggregation number is indicative of micelle shape and shows that the sphere-to-rod transformation occurs at higher aggregation numbers and higher salt concentrations when heptane is present.

DiPyMe in SDS Micelles: Artifacts and Their Implications in the Interpretation of Micellar Properties

“…Above 0.3 M NaCl, the microviscosity stopped to increase. The microviscosity profile obtained with DiPyMe in Figure 6B is quite similar to that obtained by ESR using 5-doxylstearic acid methyl ester, 44 although the absolute µη values are different probably due to the fact that the different probes might interact with different parts of the SDS micelles. Nevertheless the overall agreement between the two techniques that predict an increase in µη with increasing salt concentration is quite remarkable.…”

“…Analysis of the line width of the ESR signal yielded the rotational time of the ESR probe which in turn provided the µηSDS values which were plotted in Figure 6B. 44 As usual, an absolute match is difficult to obtain between different probes such as 5-DSE and DiPyMe, but the overall trend as a function of salt concentration between the two probes shows a remarkably good agreement, with µηSDS increasing with increasing salt concentration up to a salt concentration of 0.3 M and remaining constant at salt concentration larger than 0.3 M.…”

“…39,44 While the presence of a free pyrenyl label affects the absolute value of the IE/IM ratio of a bipyrenyl molecule, the IE/IM ratio still represents a relative measure of the microviscosity of the local environment as long as the fluorescence signal of the pyrene monomer is not overwhelmed by free pyrene labels. The problem with DiPyMe is that the IE/IM ratio decreases continuously with increasing irradiation time (see Figure 3A) as DiPyMe is being hydrolyzed in aqueous solution.…”

Clogging of microchannels by nano-particles due to hetero-coagulation in elongational flow

“…The anionic sodium dodecyl sulfate (Carl Roth Ltd, Germany) has a molar mass of 288 g/mol and provides a thin stabilizing layer owing to the small size of the SDS molecule. The SDS molecules contain 12 carbons and the length of a molecule in the stretched configuration is only about 1.7 nm [92]. The non-ionic polymeric surfactant, known as Thanemul FD (TANATEX Chemicals, Germany) is based on stearyl alcohol polyglycol ether and contains polymer chains of different lengths.…”

Additive‐induced association in unconventional systems: A case of the hydrotrope

“…Bile salts are peculiar amphiphiles that, in aqueous environments, aggregate step by step: in molecular dimers, in small micelles, in elongated structures, and in liquid crystals (21,22). Additives influence both the thermodynamic process and the kinetic mechanism of aggregation (23,24).…”