Department of Chemical and Biomolecular Engineering, University of Melbourne, Victoria 3010, Australia; Department of Mechanical Engineering, University of Tokyo, Tokyo 113-8656, Japan.
Department of Mechanical Engineering, University of Tokyo, Tokyo 113-8656, Japan.
Adv Colloid Interface Sci. 2016 Aug;234:108-131. doi: 10.1016/j.cis.2016.05.001. Epub 2016 May 6.
The manipulation of biomolecules, fluid and ionic current in a new breed of integrated nanofluidic devices requires a quantitative understanding of electrokinetics at the silica/water interface. The conventional capacitor-based electrokinetic Electric Double Layer (EDL) models for this interface have some known shortcomings, as evidenced by a lack of consistency within the literature for the (i) equilibrium constants of surface silanol groups, (ii) Stern layer capacitance, (iii) zeta (ζ) potential measured by various electrokinetic methods, and (iv) surface conductivity. In this study, we consider how the experimentally observable viscoelectric effect - that is, the increase of the local viscosity due to the polarisation of polar solvents - affects electrokinetcs at the silica/water interface. Specifically we consider how a model that considers viscoelectric effects (the VE model) performs against two conventional electrokinetic models, namely the Gouy-Chapman (GC) and Basic Stern capacitance (BS) models, in predicting four fundamental electrokinetic phenomena: electrophoresis, electroosmosis, streaming current and streaming potential. It is found that at moderate to high salt concentrations (>5×10(-3)M) predictions from the VE model are in quantitative agreement with experimental electrokinetic measurements when the sole additional adjustable parameter, the viscoelectric coefficient, is set equal to a value given by a previous independent measurement. In contrast neither the GS nor BS models is able to reproduce all experimental data over the same concentration range using a single, robust set of parameters. Significantly, we also show that the streaming current and potential in the moderate to high surface charge range are insensitive to surface charge behaviour (including capacitances) when viscoelectric effects are considered, in difference to models that do not consider these effects. This strongly questions the validity of using pressure based electrokinetic experiments to measure surface charge characteristics within this experimentally relevant high pH and moderate to high salt concentration range. At low salt concentrations (<5×10(-3)M) we find that there is a lack of consistency in previously measured channel conductivities conducted under similar solution conditions (pH, salt concentration), preventing a conclusive assessment of any model suitability in this regime.
新型集成纳米流控装置中生物分子、流体和离子电流的操控需要对二氧化硅/水界面处的电动动力学进行定量理解。传统基于电容器的电动动力学电双层 (EDL) 模型对此界面具有一些已知的缺点，例如文献中缺乏表面硅醇基的平衡常数、斯特恩层电容、各种电动动力学方法测量的 ζ 电位以及表面电导率的一致性。在本研究中，我们考虑实验可观察到的粘弹性效应（即由于极性溶剂的极化导致局部粘度增加）如何影响二氧化硅/水界面处的电动动力学。具体而言，我们考虑了一种考虑粘弹性效应的模型（VE 模型）如何与两种传统电动动力学模型（即古依-查普曼 (GC) 和基本斯特恩电容 (BS) 模型）相比，在预测四种基本电动动力学现象时的性能：电泳、电渗流、流动电流和流动电势。研究发现，在中高盐浓度 (>5×10(-3)M) 下，当唯一的附加可调参数，即粘弹性系数，设置为先前独立测量给出的值时，VE 模型的预测与实验电动动力学测量值在定量上是一致的。相比之下，GS 或 BS 模型都无法在相同的浓度范围内使用单个稳健的参数集重现所有实验数据。值得注意的是，我们还表明，在考虑粘弹性效应时，在中高表面电荷范围内，流动电流和电势对表面电荷行为（包括电容）不敏感，这与不考虑这些效应的模型不同。这强烈质疑了在这种实验相关的高 pH 值和中高盐浓度范围内使用基于压力的电动动力学实验来测量表面电荷特性的有效性。在低盐浓度 (<5×10(-3)M) 下，我们发现先前在相似溶液条件（pH 值、盐浓度）下进行的通道电导率测量存在不一致性，这使得在该范围内任何模型的适用性都无法得出明确的结论。
