Role of Density and Conformational Composition in the Surface-to-Bulk Molecular Dosing of Photosensitive Surfactant Monolayers

Poorly water-soluble photosensitive monolayers might enable very precise control of the rate and number of desorbing molecules by controlling both the monolayer density and conformational composition. In this perspective, we systematically characterized the interfacial behavior of Langmuir monolayers consisting of a poorly water-soluble azobenzene-containing surfactant as a function of its trans/cis ratio. Precise control of the conformational ratio was achieved by controlling the UV irradiation time, allowing researchers to investigate compositions spanning from 100% trans to 90% cis. Our results demonstrate that in 100% trans monolayers, molecules do not desorb with compression until a threshold area is reached. Instead, the number of molecules desorbing in mixed trans-cis monolayers can be modulated by controlling both the composition and the compression rate. Additionally, the desorption rate at constant density is also strongly composition-dependent, and it accounts for two different regimes with two different characteristic times. We will show that trans molecules mostly desorb according to the slow regime while cis molecules conform to the fast one, but the two conformers mutually influence each other.