"Whiter than white": interactions between optical brighteners and surfactants in detergents

Abstract

The interactions between an optical brightener and surfactants commonly found in laundry detergents were studied. Three techniques were used; fluorescence spectroscopy was used to determine the effect of the addition of surfactants on the rate of brightener deposition, while small-angle X-ray scattering (SAXS) and molecular dynamics (MD) simulations were used to determine the effect of the brightener on the micellar structure pre-deposition. In addition, the effect of calcium ions on micellar structure was studied using these techniques. The effect of surfactant addition on CaCO3 crystallisation was also studied.
It was found that the addition of nonionic surfactants inhibited the deposition of brightener onto fabric surfaces, whereas the addition of anionic surfactants aided this deposition process. This was correlated to the micellar structures found using SAXS and MD simulations; the anionic surfactant system formed ellipsoidal structures, in which the brightener was incorporated onto the surface of the hydrocarbon core, whereas the nonionic surfactant formed spherical structures where the brightener was held within the outer shell, which appeared to hamper its ability to be deposited. In addition, the brightener was shown to affect micellar radius and aggregation number of the nonionic surfactant, in a manner similar to the addition of anionic surfactant to nonionic surfactant.
The addition of calcium ions at concentrations up to 0.36 g L-1 Ca2+ (for SAXS) and 0.77 g L-1 Ca2+ (for MD simulations) to nonionic and anionic surfactant systems was not seen to significantly affect their micellar structure, although a greater affinity was seen between the calcium and the anionic surfactant than between the calcium and the nonionic surfactant. The effect of the addition of surfactants on the rate of CaCO3 crystallisation and the polymorphs produced was determined.