Surfactant-polymer interactions: molecular architecture does matter.

Polymer-surfactant mixtures are found in many industrial formulations, and hence there is a significant interest in understanding, at a molecular level, how the self-assembly of surfactant is affected by oppositely-charged polyelectrolytes (PEs). We use self-consistent field modeling and show that the modes of interaction of PEs strongly depend on the architecture of the PE on the segmental level. Hydrophilic cationic PEs with their charge proximal to the linear backbone are expected to bind electrostatically to the outsides of the coronas of the spherical micelles of anionic surfactants, such as sodium laureth sulphate (SLES). As a result, the surfactant aggregation number increases, but at the same time the colloidal stability deteriorates, due to bridging of the PEs between micelles. PEs with their charge somewhat displaced from the backbone by way of short hydrophobic spacers, are expected to be present inside a micelle at the core-corona boundary. In this case the aggregation number decreases, yet the colloidal stability is retained. Hence, SLES tends to remove hydrophilic PEs from an aqueous solution, whereas it solubilizes more hydrophobic ones. The binding isotherm shows that the uptake of PEs remains typically below charge compensation and in this case the spherical micelle topology remains the preferred state.

Chronological ageing of human hair keratin fibres.

Examination of very long hair (length > 2.4 m) using a large range of evaluation methods including physical, chemical, biochemical and microscopic techniques has enabled to attain a detailed understanding of natural ageing of human hair keratin fibres. Scrutinizing hair that has undergone little or no oxidative aggression--because of the absence of action of chemical agents such as bleaching or dyeing--from the root to the tip shows the deterioration process, which gradually takes place from the outside to the inside of the hair shaft: first, a progressive abrasion of the cuticle, whilst the cortex structure remains unaltered, is evidenced along a length of roughly 1 m onwards together with constant shine, hydrophobicity and friction characteristics. Further along the fibre, a significant damage to cuticle scales occurs, which correlates well with ceramides and 18-Methyl Eicosanoic Acid (18-MEA) decline, and progressive decrease in keratin-associated protein content. Most physical descriptors of mechanical and optical properties decay significantly. This detailed description of natural ageing of human hair fibres by a fine analysis of hair components and physical parameters in relationship with cosmetic characteristics provides a time-dependent 'damage scale' of human hair, which may help in designing new targeted hair care formulations.