Complex fluids for active delivery in laundry application
Dye – Surfactant interactions

Abstract

Everyone has noticed the yellowing of our white fabrics over time. Consequently, industrial companies have introduced hueing dyes to bring a brightening effect in their soluble unit doses (SUD). However, unwanted staining by the dye can occur when the SUD is badly dissolved and directly in contact with the fabric. Internal P&G works have shown that the risk of staining could be reduced using nonionic surfactants. Thus, this thesis concerns the fundamental understanding of the underlying mechanisms and interactions in complex fluids, focussing on dye- surfactant interactions, to minimise the staining risk and maximise the consumer’s benefits.
An increase of the hydrophilicity of the surfactant increased its solubility and cloud point, as expected. Two hueing dyes, E4210 and V200, were tested with two nonionic surfactant series (containing 10 or 13 carbons, with a number of EO groups from 3 to 20). In water, V200 aggregates at a concentration above 2 x 10-5 mol L-1. Using the spectrophotometric method, we observed an interaction between dye and surfactant molecules below their CMC, with an intensity increase of the absorption band. When the CMC is reached, a bathochromic shift occurs due to the dye incorporation into surfactant micelles. E4210 molecules were located at the hydrophilic/hydrophobic interface within surfactant micelles, co-micellising with surfactant. Surfactant interactions decrease the dye deposition onto fabric while the viscosity of the solution increases the risk of fabric staining. For V200, monomers adsorb onto the fabric, depleting monomers from solution. However, the monomer- aggregate equilibrium in solution is quickly re-established with the aggregates dispersing into monomers.
Through observations and data results, it has been found that the dye-surfactant interactions have a role in controlling the dye adsorption kinetics by slowing or limiting the dye-fibre interactions, reducing dye adsorption onto fabric and thus the risk of staining. Surfactants also participate in the dye removal by reducing dye redeposition by keeping dye molecules in suspension within surfactant micelles.