Rheological properties of photo-responsive system composed of cationic surfactant and azobenzene

Abstract

A “smart” fluid has been considered as an important part of softer matters, which undergoes a change in some macroscopic property in response to an external stimulus, such as light, pH, temperature and so on. Among these stimulus, light could serve as a trigger which possesses a series of advantages compared with other kinds of stimulus. Firstly, light is more temperate and non-invasive, since it need not any additives. Secondly, light signal is more available. Thirdly, light could aim at a special functional group, which is valuable on medical applications. This smart fluid is usually named as photorheological (PR) fluids, which show changes in their rheological and flow properties under irradiation of light at a given wavelength. Therefore, PR fluids has been widely employed in research and development for photo-responsive micelles. But it is difficult to synthesize some new complex photosensitive molecules, so it is worth to develop simple classes of PR fluid that require no special synthesis and make full use of inexpensive chemicals to combination. In this research, we investigated a class of such photo-responsive systems obtained by mixing traditional cationic surfactants, cetyltrimethylammonium salicylate (C 16 TASal) or cetyltrimethylammonium bromide (CTAB) and a common photosensitive compound azobenzene (AZB) and their photosensitive properties by employing the rheological measurements, freeze-etching electron microscopy (FF-TEM), Fourier transform infrared spectrometer (IR) and nuclear magnetic resonance ( 1 H NMR). The rheological properties of the C 16 TASal/AZB mixed system indicate that the rheological behavior is corresponding with the Maxwell model. In the solution of C 16 TASal, salicylate ions didn’t penetrate into the palisade layer but adsorbed on the interface of the micelles due to the bridges between carboxyl groups of salicylate anions and water molecules by hydrogen bonding. The rod-micelles connected to each other through these hydrogen bonds and then formed a pseudo-network. The addition of AZB to the pure C 16 TASal solution partially broke the pseudo-network connections between micelles leading to the decrease of viscosity of C 16 TASal solution. The dimerization of hydrogen bond among the micelles was confirmed by IR and 1 H NMR. Through UV irradiation, AZB molecules undergone a photoisomerization from trans - to cis -form, which strengthened the steric hindrance and altered the molecular packing at the micellar interface, so the network junctions between C 16 TASal micelles was completely destroyed, and the micelles become much shorter entities or more diffuse, resulting that the zero-shear viscosity of C 16 TASal/AZB decreased by about one order of magnitude compared with that without UV irradiation. Under visible light irradiation, cis -AZB transformed to trans -AZB again, and the connections between micelles formed again, leading to the increased viscosity of the system. That is to say, due to the cis - trans photoisomerization of AZB, the viscoelastic of C 16 TASal/AZB could be reversibly regulated under UV-Vis irradiation. However, because of the different conversion ratio between trans - and cis -AZB structure, the viscosity of system could not return to the previous state by cycle illumination. Subsequently we demonstrated another mixed solution of CTAB/NaSal/AZB with the same method. The results show that the viscosity of this system also reduced after the sample had been exposed to UV irradiation in accord with that of C 16 TASal/AZB mixed solution, but under visible light irradiation, the viscosity of the system did not change and the system cannot be reversible regulation, which might be related to the existence of NaBr in the CTAB/NaSal/AZB mixed solution. These results are important for the development of light-responsive mixed system formed by traditional cationic surfactants and a common photosensitive compound. Thus, the C 16 TASal/AZB system could be regarded regard as a new photorheological fluids of the smart fluids, which is conducive to understand the nature of the photosensitive aromatic compounds and exploit their application fields. rheological property, cetyltrimethylammonium salicylate, cetyltrimethylammonium bromide, azobenzene,