Effects of formulation conditions on micellar interactions and solution rheology in multi-component micellar systems

Abstract

Surfactants are crucial to the personal care industry due to their unique surface activity, cleansing, and self assembly properties. Typically, multi-component systems are used in order to maximize mildness, hard water tolerance, and foaming. System morphology and viscosity are controlled through chemistry and solution conditions. An experimental study was conducted to determine how variations in solution chemistry (surfactant headgroup and blend stoichiometry) and solution conditions (pH and [NaCl]: [anionic + zwitterionic surfactant] ratio) affect the structure and rheology of surfactant solutions. This study examined binary systems of Sodium Laureth Sulfate (SLES) and Lauramidopropyl Betaine (LAPB) or SLES and Lauramidopropyl Hydroxysultaine (LAPHS) as well as ternary systems of SLES/LAPB/PEG-80 Sorbitan Laurate (PEG-80 SL) and SLES/LAPB/Polysorbite-20 (Tween-20). Using dynamic light scattering and rheometic measurements, system morphology was determined. In the SLES/LAPB system, it was found that there was a break in system viscosity at a critical [NaCl]: [anionic + zwitterionic surfactant] ratio, 0.16:1 (R*). Micelles only had the ability to entangle, thus increasing viscosity, above this ratio. When the system pH decreased such that pH ~ pKa of LAPB, all [NaCl]:[anionic + zwitterionic surfactant] ratios had the ability to entangle, and entanglement began at lower surfactant concentrations. At these pH values, LAPB protonated and created a pseudo-ternary system with SLES, LAPB0 , and LAPB*. There was no measured variation in system morphology in the SLES/LAPHS system with [NaCl]: [anionic + zwitterionic surfactant] ratio, most likely because the minimum ratio achievable was above R* due to a high salt content in the raw materials. In addition, there was no measured variation in system morphology in the SLES/LAPHS system with variation in pH, most likely because the system was not tested at pH ~ pKa of LAPHS. The addition of a third surfactant drastically decreased the system viscosity and drove the system towards the formation of spherical micelles because the nonionic surfactant of choice decreased the packing parameter due to its relatively large size as compared to that of SLES and LAPB.