Adsorption characteristics of spiropyran-modified cationic surfactants at the silica/aqueous solution interface

We have characterized the adsorption of a spiropyran-modified cationic surfactant at the silica/aqueous solution interface and the subsequent photoresponsive nature of the adsorbed molecular assemblies. The surfactant used in this study was 1'(6-trimethylammoniohexyl)-3'3'-dimethyl-6-nitrospiro-(2H-1-benzopyran-2,2'-indoline) bromide (SP-Me-6). Visible light incident to the aqueous solution of SP-Me-6 results in the isomerization from the merocyanine (MC) form to the spiro (SP) form, whereas the isomerized SP form reverts to the MC form when the surfactant solution is stored in the dark. The adsorption isotherms on colloidal silica particles reveal that the combination of electrostatic and intermolecular hydrophobic interactions drives the adsorption for both of the isomers, and the more hydrophobic SP form gives a greater adsorbed amount when compared with the zwitterionic MC form. In situ atomic force microscopy (AFM) has demonstrated that disklike or flattened spherical surface aggregates are formed in the adsorption plateau region on a flat silica plate, where the SP form gives larger surface aggregates as a result of its greater hydrophobic/associative character. Such a difference in the hydrophobic/associative nature between the two isomers induces reversible changes in the dispersion stability of silica suspensions as well as in the surface force, in response to the photoisomerization. By taking the in situ and ex situ color changes into consideration, it seems likely that the photoisomerization occurs reversibly even after surfactant adsorption on silica.