Compression Isotherms of Polydopamine Films

Polydopamine is formed via dopamine hydrochloride (DAH) polymerization in a slightly alkaline medium (pH 8.5) both in a solution bulk and at a liquid–gas interface. Atmospheric oxygen and oxygen dissolved in the bulk solution serves as an oxidizing agent. Compression isotherms of polydopamine films formed at the interface for different times at different initial DAH concentrations have been determined in this work. Initial concentration of DAH affects the rate of polydopamine film formation; however, it weakly affects the film structure. At a low DAH concentration (1 g/L) and a long surface lifetime (720 min), the compression isotherms and the dependences of the static surface elasticity on the surface pressure turn out to be close to corresponding dependences for polydopamine films formed at a relatively high DAH concentration (5 g/L) and a shorter surface lifetime (240 min). The dependences of the static surface elasticity on the surface pressure appear to be nonmonotonic in contrast to the corresponding dependences of dynamic surface elasticity. In the first case, significantly higher values of the surface pressure are achieved; however, in the region of low surface pressures (<7 mN/m), the results are close to each other. The maximum static surface elasticity for continuous uniform polydopamine films is 150 mN/m. Upon a further compression, such films begin to collapse with the formation of a three-dimensional multilayer structure.