Sodium lauryl ether sulfates, pivotal surfactants for formulations: Rationalization of their assembly properties

Hypothesis: Sodium lauryl ether sulfates (SLES) are a class of surfactants, among the most used in the formulations of healthcare products. Although largely used in industry, literature studies are hereto completely missing of the characterization of some species, as sodium lauryl triether sulfates (E3S), and of the effect of several assembly parameters, as temperature. Moreover, drastic disparity of assembly performance subsists among laboratory synthesized and industrial raw SLES, generating problems in the real-world applications of formulations design. Experiments: We report on the self-assembly analysis of industrial grade sodium lauryl monoether sulfate (E1S), sodium lauryl diether sulfate (E2S) and laboratory synthesized E3S. The surfactants were studied in a large range of NaCl concentrations (0.1-1 M NaCl) and as function of the temperature by crossing Surface Tension, SmallAngle X-ray Scattering and Dynamic Light Scattering measurements. Findings: Such analysis enabled to establish systematic relationship between the molecular structure and assembly properties of the surfactants and to define size and morphologies variations of the aggregates. We observed that i) core-shell spherical aggregates evolve towards elongated core-shell structures upon addition of NaCl, ii) the elongation is higher the lower the number of the ether groups iii) E3S is greatly stable in the analyzed temperature range (20 degrees C - 60 degrees C), iv) the industrial grade SLES exhibit a minimum of the surface tension and an anticipated self-assembly as a function of concentration, probably due to the presence of hydrophobic impurities. Impurities are also detected by DLS in unfiltered samples of the industrial grade SLES as large particles. Moreover, a case of industrial interest was investigated: the instability of formulations comprising of E3S and 3,4,5-trimethoxybenzoic acid. Such information enriches the quite small literature on pure SLES self-assembly, rationalizing new and literature data within a unified framework.