| Summary: | Di-alkyl chain anionic surfactants are a major constituent in hair and clothes care products. Commercially they are frequently formulated in conjunction with perfumes. Despite considerable progress over the last two decades in understanding the behaviour of surfactant mixtures, the solution and surface behaviour of surfactant - perfume mix- tures have been neglected. Small angle neutron scattering and neutron reflectivity have been used to characterise the surface and solution behaviour of the di-alkyl chain an- ionic surfactant sodium para-dodecyl benzene sulphonate, Na-6-DBS, in the presence of two model perfumes, the hydrophilic 2-phenyl ethanol and the more hydrophobic linalool. Additional work focused on the addition of a divalent salt, CaCh or a polyelec- trolyte, polyethyleneimine, PEI. The solution phase behaviour for the different anionic surfactant - perfume mixtures has been determined in some detail. For the hydrophilic phenyl ethanol the Na-6-DBS preferred highly curved micellar structures while with linalool more planar structures predominated. At the highest perfume concentrations there was a switch from globular micelles to swollen micelles. At higher solution con- centrations the phenyl ethanol observes a shift towards lamellar structures while the linalool promotes a mixture of vesicles and lamellar structures. The surface behaviour is characterised by a marked departure from ideal mixing for both phenyl ethanol and linalool with Na-6-DBS. The linalool can compete a lot more effectively at the interface than the phenyl ethanol. Replacing the surfactant with sodium dodecyl sulphate, SDS, enhances the adsorption of both perfumes while the addition of an electrolyte, NaCl to the SDS causes a significant reduction in perfume adsorption. In the presence of PEI significant linalool adsorption is observed which is not the case for phenyl ethanol, Clearly while the PEI - linalool interaction is strong enough to compete with the SDS PEI for the interface, the PEI - phenyl ethanol interaction is not, with the result that no adsorption occurs.
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