Adsolubilization by a photo-responsive surfactant

We have characterized the adsolubilization of 2-naphthol into the adsorbed layer of a photo-responsive surfactant. 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. In our previous paper, we reported that the photo-isomerization occurs even after the surfactant adsorption on silica. In our current study, the amounts of adsorbed SP-Me-6 and adsolubilized 2-naphthol were measured through a depletion method in a colloidal silica system. The adsolubilized amount of 2-naphthol is observed to be greater for the MC form than for the SP form. It seems likely that the planar and zwitterionic MC structure facilitates the adsolubilization of 2-naphthol. We also performed quartz crystal microbalance with dissipation monitoring (QCM-D) measurements to assess conformational information about the SP-Me-6 layer formed at the silica/aqueous solution interface. One of the key findings in these QCM-D studies is the fact that the adsolubilization induces swelling of the adsorbed aggregates, and this swelling occurs more significantly for the MC form than for the SP form.