Compression Isotherms of Polydopamine Films
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ression Isotherms of Polydopamine Films O. Yu. Milyaevaa, *, A. V. Akent’eva, A. G. Bykova, A. V. Zerova, N. A. Isakova, and B. A. Noskova, ** a
Chair of Colloid Chemistry, St. Petersburg State University, St. Petersburg, 198504 Russia *e-mail: [email protected] **e-mail: [email protected] Received May 6, 2020; revised May 10, 2020; accepted May 12, 2020
Abstract—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 (120 min), continuous films are formed, which are visible with the naked eye. Figure 3a shows the surface pressure isotherms obtained for 1 g/L DAH solutions 10, 30, 60, 240, and 720 min after surface formation (first compression cycle). When the surface lifetime is increased from 10 to 240 min, the surface pressure rises from 2 to 60 mN/m under almost ninefold compression. At a surface lifetime of 720 min, a noticeable increase in the surface pressure is already observed upon comCOLLOID JOURNAL
Vol. 82
No. 5
2020
pression by 15–20%. Therewith, two regions of almost constant surface pressure values equal to nearly 45 and 55 mN/m (plateau regions) are observed in the compression isotherm. At higher DAH concentrations, the plateau regions arise at shorter surface lifetimes (Figs. 3b, 3c). For a concentration of 5 g/L and a surface lifetime of 240 min, the plateau regions were also observed at surface pressures of about 45 and 55 mN/m (Fig. 3c). Some similarity of the compression isotherms obtained at low DAH concentrations and long lifetimes of the surface and the isotherms measured at high DAH concentrations and moderate surface lifetimes may be due to an increase in the polymerization rate with the initial DAH concentration. At the same time, the film structure seems to remain almost unchanged with an increase in DAH concentration. Note that the compression isotherm of a continuous polydopamine film formed over 720 min on the surface of a solution with an initial DAH concentra
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