A Colloid Model System for Interfacial Sorption Kinetics

Particle adsorption to an interface may be a complicated affair, motivating detailed measurements of various processes involved, to discover better understanding of the role of particle characteristics and solution conditions on adsorption coverage and rate. Here we use micron size colloids with a weak interfacial interaction potential as a model system to track particle motion and measure the rates of desorption and adsorption. The colloid-interface interaction strength is tuned to be less than 10 kBT so that it is comparable to many nanoscale systems of interest such as proteins at interfaces. The tuning is accomplished using a combination of depletion, electrostatic, and gravitational forces. The colloids transition between an entropically trapped adsorbed state and a desorbed state through Brownian motion. Observations are made using an light-emitting diode (LED)-based total internal reflection microscopy (TIRM) setup. The observed adsorption and desorption rates are compared to theoretical predictions based on the measured interaction potential and near-wall particle diffusivity. The results demonstrate that diffusion dynamics play a significant role when the barrier energy is small. This experimental system will allow for the future study of more complex dynamics such as nonspherical colloids and collective effects at higher concentrations.粒子在界面的吸附过程往往较为复杂，因此亟需对其所涉及的各类环节开展详尽测量，以更深入地阐明粒子特性与溶液条件对吸附覆盖率及吸附速率的调控作用。本研究采用具有弱界面相互作用势的微米级胶体（micron size colloids）作为模型体系，用于追踪粒子运动并测量解吸与吸附速率。胶体-界面相互作用强度被调控至10 kBT以下，使其与诸多受关注的纳米尺度体系（如界面处的蛋白质）具有可比性。该相互作用强度的调控通过耗尽力、静电力与引力的协同作用实现。胶体粒子可通过布朗运动在熵阱束缚的吸附态与解吸态之间发生转变。实验观测基于搭载发光二极管（light-emitting diode, LED）的全内反射显微镜（total internal reflection microscopy, TIRM）平台开展。将观测得到的吸附与解吸速率，与基于实测相互作用势及近壁粒子扩散系数得到的理论预测值进行对比。研究结果表明，当能垒较小时，扩散动力学对吸附/解吸过程具有显著影响。该实验体系可为后续开展更复杂的动力学研究提供支撑，例如非球形胶体以及高浓度条件下的集体效应相关研究。