Influence of structure of polar head on the micellization of lactose-based surfactants. Small-angle X-ray and neutron scattering study

We investigated here the micellization in water of three surfactants: (N-dodecylamino)lactitol, (N-dodecyl)lactobionamide, and (N-acetyl N-dodecyl)lactosylamine, having n-alkyl chain lengths of 12 carbon atoms and different lactose-based polar heads. The micellization is studied by small-angle neutron and X-ray scattering. The characteristic dimensions of the micelles are derived from the I(q) curve fittings. We demonstrated the existence of spherical or oblate ellipsoidal micellar structures. The differences were attributed to differences in steric hindrance of the polar head enabling or preventing formation of spherical micelles. We pointed out the particular role of steric hindrance at the junction between the hydrophobic chain and the polar head. The formation of oblate ellipsoidal micelles attests to a relatively low steric hindrance at this junction ((N-dodecylamino)lactitol and (N-dodecyl)lactobionamide) whereas the formation of spherical micelle displays an important steric hindrance as observed for (N-acetyl N-dodecyl)lactosylamine which includes a bulky linkage. The surface areas per polar head in the micelles are confronted to the minimum areas per molecules deduced from surface pressure measurements. The results are in good agreement excepted in the case of (N-dodecyl)lactobionamide. The experimental results obtained with micellar solutions of N-dodecyllactobionamide from both neutron scattering and surface pressure measurements exhibit a particular comportment of this surfactant compared with two other surfactants studied in regard to the intermicellar interactions and the minimum area per molecule at the air/aqueous solution interface. We suggest that this particular behavior could be the consequence of a very important solvation of the (N-dodecyl)lactobionamide polar heads due to the presence of an amide group in this molecule. This important solvation could modify the interactions between the micelles and the apparent size of isolated molecules at the air/aqueous solution interface. Over parallel explanations may be suggested as formation of dimers or trimers below the critical micelle concentration (cmc) and above the cmc the existence of relatively strong intermicellar interactions stemming from the distribution of dipoles over the surface in these micelles due to the presence of the amide group.