Colloidal stability of di-chain cationic and ethoxylated nonionic surfactant mixtures used in commercial fabric softeners

Abstract

Mixed surfactant systems find extensive use in laundry for fabric cleaning and fabric conditioning and in personal products as shampoos and hair conditioners. This paper studies the colloidal stability of a system containing co-exiting liposome and bilayer fragment phases used in fabric conditioners.Di-chain cationic surfactants form Lβ liposomal structures in water, which on addition of ethoxylated nonionic surfactants transform into intermediate Lβ bilayer fragments or disks and eventually to mixed micelles. Co-exiting liposome-fragment structures show a smaller phase volume compared to liposomal dispersions and therefore have lower viscosities and better flow properties.In contrast to the colloidal forces that control the stability of liposomes the stability in fragment containing dispersions is mainly controlled by the steric repulsion provided by the ethoxylate chains that are partitioned at the edges of the fragments to reduce their hydrophobic edge tension. We show here that the gel formation phenomenon observed in fragment containing dispersions at elevated temperatures is a manifestation of the well-known ‘cloud point’ depression effect associated with the de-hydration of ethoxylate chains. This shortens the range of the steric repulsion between the chains leading to the edge-to-edge coalescence of the fragments and hence the gelation and viscosity increase on ageing.