Phase behavior of laundry surfactants in polar solvents

Laundry surfactants are usually mixtures of ionic and nonionic detergents that exhibit a complex phase behavior. Here the ternary phase behavior of an isotropic and a liquid crystalline (LC) surfactant mixture has been examined in water/solvent systems. The size of the LC area in the ternary phase diagram was correlated to solvent parameters including the dielectric constant and the Gordon cohesiveness parameter. The Gordon parameter was found to have a linear relationship with the amount of solvent needed to go from an LC to an isotropic state over a wide range of solvents from polar to apolar. For solvents in which no surfactant aggregation (micellar or inverted micellar) is expected, the size of the LC area is linear with the reciprocal of the dielectric constant of the solvent. On diluting practical detergent liquids with water, a large LC area can be avoided by using solvents with a relatively low dielectric constant and with a relatively low molecular weight. The aggregated state of the surfactant mixtures in the isotropic regions of the phase diagram was studied using the solvatochromic fluorescent probe Nile Red. In the water corner of the phase diagram, the surfactants are aggregated into micelles. In strongly polar solvents, such as glycerol, ethylene glycol, formamide, and ethanolamine, the surfactants are also aggregated into micelles. In somewhat less polar solvents, such as methanol, ethanol, and t-butanol, the surfactant molecules are randomly distributed. In the surfactant-rich corner of the phase diagram of the isotropic mixture, the surfactant forms inverted micelles. An inverted micelle-to-micelle transition could be observed on dilution in ethylene glycol as a discontinuity in the trend of the Nile Red fluorescence maxima.