Photorheological response of aqueous wormlike micelles with photocleavable surfactant

Recently, we have reported a new cinnamic acid-type photocleavable surfactant, C4-C-N-PEG9 that experiences a photocleavage through UV-induced cyclization in aqueous solution, yielding a coumarin derivative (7-butoxy-2H-chromen-2-one) and an aminated polyoxyethylene compound. Here, we have studied the effects of C4-C-N-PEG9 on the photorheological behavior of viscoelastic wormlike micelles formed by aqueous mixture of nonionic surfactants, polyoxyethylene phytosterol ether (PhyEO20) and tetraoxyethylene dodecyl ether (C12EO4). The 4.9 wt % PhyEO20/H2O + 2.4 wt % C12EO4 solution forms wormlike micelles, and its viscosity is ~10 Pa·s. We have found that the addition of C4-C-N-PEG9 into this viscous, non-Newtonian fluid system decreases the viscosity. Viscosity decreased in parallel to the C4-C-N-PEG9 concentration reaching ~0.003 Pa·s at 2.5 wt % of C4-C-N-PEG9. However, viscosity of the C4-C-N-PEG9 incorporated system increased significantly (~200 times at 1.5 wt % of C4-C-N-PEG9 system) upon UV irradiation. Small-Angle X-ray scattering studies have shown that addition of C4-C-N-PEG9 favors wormlike-to-sphere type transition in the micellar structure. However, UV irradiation in the C4-C-N-PEG9 incorporated system causes one-dimensional micellar growth. Since C4-C-N-PEG9 has relatively bigger headgroup size compared to the C12EO4, addition of C4-C-N-PEG9 into wormlike micelles reduces the critical packing parameter resulting in the formation of spherical aggregates. UV irradiation induced one-dimensional micellar growth is caused due to photocleavage of the C4-C-N-PEG9 into a less surface-active coumarin derivative and an aminated polyoxyethylene compound, as confirmed by UV-vis spectrometry and HPLC measurements. The hydrophobic coumarin derivative formed after cleavage of C4-C-N-PEG9 goes to the micellar core and is responsible for decreasing the viscosity. However, the hydrophilic aminated polyoxyethylene prefers to reside at the vicinity of headgroup of PhyEO20 reducing the interhead repulsion, increasing the critical packing parameter and the viscosity as well.