Physicochemical characterization of sodium laureth sulfate with different ethylene oxide groups in shampoo technologies

Abstract

Shampoos play a pivotal role in the daily personal care routine and hygiene of scalp and hair. Their main function is to remove sebum, with the additional benefit of depositing hairconditioning compounds on the fiber’s surface. This simultaneous cleaning-and-conditioning relies on the synergy of an anionic surfactant with polymers to form very specific, multiphase systems known as Polyelectrolyte-Surfactant Complexes (PESCs). Sodium laureth sulfate (SLES) is a commonly used anionic surfactant in cosmetics, being present in several shampoo formulations. SLES is an alkyl sulfate composed of a 12-carbon hydrophobic tail and a hydrophilic ethoxysulfate group, which is heterogeneous in number. The number of ethylene oxide groups used in the cosmetic industry varies mostly between 1 and 3. The number of groups can impact PESC adsorption onto the hair fiber and also several physicochemical properties. Therefore, a small change in the molecular structure of the surfactant can result in several changes in the cleaning and conditioning performance of the whole formulation. The objective of the present work is to assess the impact of removing an ethylene oxide group from SLES in two market shampoos: a clear silicone-free (CSF) and clear shampoo with silicone (CWS). The impact of this change on product perception is evaluated by exploring the main differences between sodium laureth-2 sulfate (SLES 2EO) and sodium laureth-1 sulfate (SLES 1EO) through physicochemical characterization of surfactant solutions in water, as well as the full shampoo formulations, where the interaction with polymers and cosurfactants is assessed. The formulations were characterized by the means of rheology, surface tension, bulk contact angle, foamability and detergency tests.Results for the CSF shampoo show that the substitution of SLES2EO by SLES1EO causes a significant increase in bulk viscosity, regardless of surfactant concentration. Additional zero-shear rate viscosity analysis show that a precipitate is formed between the substitution ratios of 0.22 and 1.0, indicating an instability triggered by the loss of the EO group. Surface tension and contact angle tests showed that use of SLES1EO decreases surface tension between the bulk and hair, while detergency results show statistically similar results between both surfactants.For the CWS shampoo, a 1:1 substitution of SLES2EO by SLES1EO was able to maintain the same rheology profile and overall effective viscosity. However, organoleptic analysis also shows the formation of an unstable precipitate, with the loss of the EO group resulting in a formula more turbid and darker than the reference with SLES2EO. Surface tension and contact angle analysis show that SLES1EO decreases surface tension when substituting SLES2EO at the same concentration, which might correlate with increased PESC deposition and conditioning benefits. Additionally, the loss of an EO did not cause statistically significant changes in detergency or foam volume.