Department of Chemical and Biological Engineering, University of Colorado , Boulder, Colorado 80309, United States.
Department of Chemistry and Geochemistry, Colorado School of Mines , Golden, Colorado 80401, United States.
J Am Chem Soc. 2015 Sep 30;137(38):12312-20. doi: 10.1021/jacs.5b07108. Epub 2015 Sep 18.
Polymers are found near surfaces and interfaces in a wide range of chemical and biological systems, and the structure and dynamics of adsorbed polymer chains have been the subject of intense interest for decades. While polymer structure is often inferred from dynamic measurements in bulk solution, this approach has proven difficult to implement at interfaces, and the understanding of interfacial polymer conformation remains elusive. Here we used single-molecule tracking to study the interfacial diffusion of isolated poly(ethylene glycol) molecules at oil-water interfaces. Compared to diffusion in dilute aqueous solution, which exhibited the expected dependence of the diffusion coefficient (D) upon molecular weight (M) of D ∼ M(-1/2) for a Gaussian chain, the behavior at the interface was approximately D ∼ M(-2/3), suggesting a significantly more expanded polymer conformation, despite the fact that the oil was a poor solvent for the polymer. Interestingly, this scaling remained virtually unchanged over a wide range of oil viscosity, despite the fact that at low viscosities the magnitude of the diffusion coefficient was consistent with expectations based on viscous drag (i.e., Stokes-Einstein diffusion), and for high viscosity oil, the interfacial mobility was much faster than expected and consistent with the type of intermittent hopping transport observed at the solid-liquid interface. The dependence on molecular weight, in both regimes, was consistent with results from both self-consistent field theory and previous Monte Carlo simulations, suggesting that an adsorbed polymer chain adopted a partially swollen (loop-train-tail) interfacial conformation.
聚合物存在于广泛的化学和生物体系的表面和界面附近，吸附聚合物链的结构和动力学一直是数十年来人们关注的焦点。虽然聚合物结构通常可以从体相溶液中的动态测量中推断出来，但这种方法在界面上很难实施，并且对界面聚合物构象的理解仍然难以捉摸。在这里，我们使用单分子跟踪技术研究了孤立聚乙二醇分子在油水界面上的扩散。与在稀水溶液中的扩散相比，在油水界面上的扩散行为约为 D∼M-2/3，这表明聚合物构象明显更加扩展，尽管油是聚合物的不良溶剂。有趣的是，尽管在低粘度下，扩散系数的大小与基于粘性阻力（即 Stokes-Einstein 扩散）的预期一致，而在高粘度油中，界面迁移率比预期的要快得多，并且与在固-液界面上观察到的间歇跳跃输运类型一致，但这种标度在很宽的油粘度范围内几乎保持不变。在这两种情况下，对分子量的依赖性与自洽场理论和先前的蒙特卡罗模拟的结果一致，表明吸附聚合物链采用了部分肿胀的（环-线-尾）界面构象。
