Non-equilibrium micelles formed by sugar-based surfactants under their Krafft temperature

Abstract

Sugar-based surfactants are renewable substitutes to petroleum-based ones as wetting, dispersing and emulsifying agents in the formulation industry. In a substitution approach, knowledge of physico-chemical properties (e. g. critical micelle concentration – CMC – or minimum surface tension) of the sugar-based surfactant candidates is crucial. However, conflicting physico-chemical properties, such as widely different CMC and surface tensions at CMC (γCMC) can be published about of a given surfactant. Such conflict is illustrated and investigated for two surfactants as case studies, octyl-α-D-glucoside and dodecyl-β-d-glucoside surfactants, which have typical behavior and a Krafft temperature (Tk, which characterizes the solubility of solid surfactant) above room temperature. Tensiometry curves have been measured and discussed for a range of surfactant solutions prepared using different temperature conditions. Complementary methods such as colorimetry, spectrophotometry and dynamic light scattering were used in combination with tensiometry curves. Overall, this study evidences the persistence of out of equilibrium micelles below Tk for a noticeable time on realistic experimental conditions, for both octyl-α-d-glucoside and dodecyl-β-d-glucoside surfactants, as a potential explanation for the conflicting CMC and surface tensions at CMC published in literature. This study evidences that investigating the solubility of the surfactant in water as represented by Tk to complement tensiometric curves analysis is important to confirm the derived CMC and γCMC values.