Light-responsive hydrophobic association of azobenzene-modified poly(acrylic acid) with neutral surfactants

Photoresponsive association between azobenzene-modified poly(acrylic acid)s (AMPs) and the nonionic surfactants tetraethylene glycol monododecyl ether and octadecyl ether (C12E4 and C18E4) has been achieved in dilute aqueous solution. The binding was investigated by (i) spectrophotometry that probes the polarity close to the azobenzene chromophore, (ii) capillary electrophoresis to obtain the amount of C12E4 bound per polymer chain, and (iii) pressure-area curves of Langmuir films to obtain information on the adsorption of AMP at the water-C18E4 interface. Increasing hydrophobicity of AMP (with increasing degree of modification with azobenzene side-groups) tightened the association with C12E4 in the dark. Exposure to UV light rapidly converted the azobenzene to their more polar cis isomer, which in turn weakened the association with surfactant. Almost complete photorelease of bound C12E4 was obtained with the optimal structure of AMP. Adsorption on large interfaces is much less sensitive to light. The possible origin of the photoresponse is analyzed in terms of AMP affinity for surfactant assemblies and azobenzene penetration in the hydrophobic core of micelles. We propose that the photoswing of polarity is amplified by the binding to small micelles because of the small number of anchors involved. A few azobenzene anchors afford tight binding in the dark, but also detach more easily than the whole AMP chain upon photoisomerization.