Objective The purpose of this study is to understand the impact of the cationic polymer merquat on the rheological behavior of the mixed surfactant system of sodium lauryl ether sulfate (SLES) and cocamidopropyl betaine (CapB) as well as the impact of varying formulation conditions on the wet lubrication performance of the SLES‐CapB‐Merquat system. Methods Rotation mechanical Rheometry was used to study the rheological response of the SLES‐CapB‐Merquat systems. Frequency sweeps were conducted to analyze the rheological properties of the system at low frequency ranges and bulk viscosity of the system was studied at high shear rates at varying salt and polymer concentrations. Wet combing tests and hair friction tests were run on the Dia‐stron MTT175 flexible miniature tensile tester in order to evaluate the conditioning effects of the different formulations. Results The SLES‐CapB system on its own exhibits very little viscosity. The bulk rheology results show that the addition of Merquat enhances the viscosity and viscoelastic properties of the SLES‐CapB‐Merquat system indicating the presence of electrostatic interactions between the surfactant and polyelectrolyte. Addition of salt had a significant impact on the system’s rheological response due to the charge screening effect of salt. Wet combing force data indicate that the charged polyelectrolyte binds to the hair substrate resulting in reduced combing force values after the product was applied. The addition of silicone oil to the formulation seemed to greatly enhance the conditioning effect of the formulation. Conclusion The charge interactions between SLES, CapB and Merquat results in the formation of an integrated gel like network, thus building the viscosity of the system. Variation of parameters like polymer and salt concentration has the potential to modify the bulk rheological properties and consequently the wet lubrication properties of the system.

The purpose of this study was to understand the impact of the biopolymer chitosan on the rheological behaviour of the biosurfactant sophorolipid as well as the effects of ionization and electrolyte addition on the chitosan-sophorolipid system. METHODS: Rotation mechanical rheometry was used to study the rheological response of the chitosan-SL systems. Frequency sweeps were conducted to analyse the rheological properties of the system at low-frequency ranges, and bulk viscosity of the system was studied at high shear rates for each sample. RESULTS: The biosurfactant sophorolipid on its own has very low viscosity. The bulk rheology results show that the addition of chitosan enhances the viscosity and viscoelastic properties of the chitosan-sophorolipid system indicating the presence of synergistic interactions between the two systems. Electrolyte addition had a significant impact on the system's rheological response. Addition of salt built the viscosity of pure chitosan. However due to charge screening effects, it resulted in a decrease in viscosity for the chitosan-sophorolipid system. On further increasing the salt concentration, an increase in viscosity was observed but not beyond the value obtained for the chitosan-SL system without any salt. An increase in pH results in increased ionization of the carboxylic acid groups in acidic SL, which in turn enhances the synergistic interactions between chitosan and SL. CONCLUSION: The strong charge interactions between chitosan and sophorolipid lead to formation of an integrated gel-like network, thus building the viscosity of the system. A variation in parameters like biopolymer concentration, electrolyte and ionic strength has the potential to modify the bulk rheological properties of the chitosan-SL system. CONCLUSION: Les fortes interactions de charge entre le chitosan et le sophorolipide conduisent a la formation d'un r eseau de type gel int egr e, renforc ßant ainsi la viscosit e du syst eme. Une variation de param etres tels que la concentration en biopolym ere, l' electrolyte et la force ionique a le potentiel de modifier les propri et es rh eologiques globales du syst eme chitosan-SL.