Gliemann Hartmut, Mei Yu, Ballauff Matthias, Schimmel Thomas
Institute of Nanotechnology, Forschungszentrum Karlsruhe GmbH, D-76021 Karlsruhe, Germany.
Langmuir. 2006 Aug 15;22(17):7254-9. doi: 10.1021/la060933u.
We demonstrate that the adsorption of cationic spherical polyelectrolyte brushes (SPB) on negatively charged mica substrates can be controlled in situ by the ionic strength of the suspension. The SPB used in our experiments consist of colloidal core particles made of polystyrene. Long cationic polyelectrolyte chains are grafted onto these cores that have diameters in the range of 100 nm. These particles are suspended in aqueous solution with a fixed ionic strength. Atomic force microscopy (AFM) in suspension as well as in air was used for surface characterization. In pure water the polymer particles exhibit a strong adhesion to the mica surface. AFM investigations of the dry samples show that the particles occupy the identical positions as they did in liquid. They were not removed by the capillary forces within the receding water front during the drying process. The strong interaction between the particles and the mica surface is corroborated by testing the adhesion of individual particles on the dried surface by means of the AFM tip: after a stepwise increase of the force applied to the surface by the AFM tip, the polymer particles still were not removed from the surface, but they were cut through and remained on the substrate. Moreover, in situ AFM measurements showed that particles which adsorb under liquid in a stable manner are easily desorbed from the surface after electrolyte is added to the suspension. This finding is explained by a decreasing attractive particle-substrate interaction, and the removal of the particles from the surface is due to the significant reduction of the activation barrier of the particle desorption. All findings can be explained in terms of the counterion release force.
我们证明，阳离子球形聚电解质刷（SPB）在带负电荷的云母基底上的吸附可以通过悬浮液的离子强度进行原位控制。我们实验中使用的SPB由聚苯乙烯制成的胶体核心颗粒组成。长的阳离子聚电解质链接枝到这些直径在100nm范围内的核心上。这些颗粒悬浮在具有固定离子强度的水溶液中。使用悬浮液中和空气中的原子力显微镜（AFM）进行表面表征。在纯水中，聚合物颗粒对云母表面表现出强烈的附着力。对干燥样品的AFM研究表明，颗粒占据了它们在液体中时相同的位置。在干燥过程中，它们没有被后退水前沿内的毛细力去除。通过用AFM针尖测试单个颗粒在干燥表面上的附着力，证实了颗粒与云母表面之间的强相互作用：在逐步增加AFM针尖施加到表面的力之后，聚合物颗粒仍然没有从表面去除，但它们被切断并留在基底上。此外，原位AFM测量表明，在液体中稳定吸附的颗粒在向悬浮液中添加电解质后很容易从表面解吸。这一发现可以用颗粒-基底吸引力的降低来解释，颗粒从表面的去除是由于颗粒解吸活化能垒的显著降低。所有这些发现都可以用抗衡离子释放力来解释。
