Structure of Hybrid Materials Based on Halloysite Nanotubes Filled with Anionic Surfactants

The structures of pristine halloysite nanotubes (HNTs) and ones functionalized by anionic surfactants (sodium dodecanoate and sodium dodecyl sulfate) were investigated by small angle neutron scattering (SANS). These experiments evidenced the structural organization of the surfactants adsorbed onto the HNT cavity and the importance of the surfactant headgroup. Contrast matching experiments were employed in order to mask the dominant scattering effect of the clay hollow nanotubes and to focus on the surfactant organization within the lumen. Further investigation on the mesoscopic structure of the investigated materials was carried out by electric birefringence (EBR), which allowed study of the rotational mobility of both pristine and functionalized HNTs. The gained structural insights were used to deduce some relevant properties of the hybrids, such as their surfactant loading, charge, and solubilization ability toward hydrophobic compounds. For the latter, the HNT lumen hydrophobization was straightforwardly demonstrated by both fluorescence spectroscopy and fluorescence correlation spectroscopy (FCS) using Nile Red as fluorescent probe. This paper correlates the structural properties of the hybrid material with micellar properties based on HNT and anionic surfactants; showing that for the dodecanoate a much more pronounced aggregation tendency within the HNT cavity prevails compared to the dodecyl sulfate. The attained knowledge is crucial for designing innovative sustainable nanostructures that are based on ecofriendly halloysite and anionic surfactants that can be used for the solubilization and delivery of hydrophobic compounds from such hybrid materials.