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arXiv physics/0611030physics.chem-ph
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大电压下电解质动力学中的空间效应:I。双层充电

Steric effects in the dynamics of electrolytes at large applied voltages: I. Double-layer charging

Mustafa Sabri Kilic, Martin Z. Bazant, Armand Ajdari

摘要:

电解质的经典泊松-玻尔兹曼(PB)理论假设点电荷的稀溶液具有平均场静电力。然而,即使对于非常稀的溶液,它预测了荒谬的大离子浓度(超过紧密堆积),表面电位仅为零点几伏,这经常被超过,例如在微流体泵和电化学传感器中。自20世纪50年代以来,PB方程的几个修改已被提出来考虑平衡离子的有限尺寸,但在这两部分系列中,我们考虑空间效应对扩散电荷动力学(在没有电渗流的情况下)。在这第一部分中,我们回顾了文献,并分析了两个简单的模型充电的薄双层,这必须形成一个密集的离子在高电压的表面附近的凝聚层。令人惊讶的预测是,差分电容通常随所施加的电压非单调地变化,因此电解系统的响应时间也是如此。在PB理论中,电容随电压呈指数增长,但空间效应实际上使其降低到阈值电压以上,此时离子在表面附近变得拥挤。PB理论中的其他非线性效应也受到空间效应的强烈抑制:双层对外加电压的净盐吸附大大减少,扩散层中的切向“表面传导”也大大减少,以至于与体传导相比,它通常可以被忽略(小杜欣数)。

英文摘要:

The classical Poisson-Boltzmann (PB) theory of electrolytes assumes a dilute solution of point charges with mean-field electrostatic forces. Even for very dilute solutions, however, it predicts absurdly large ion concentrations (exceeding close packing) for surface potentials of only a few tenths of a volt, which are often exceeded, e.g. in microfluidic pumps and electrochemical sensors. Since the 1950s, several modifications of the PB equation have been proposed to account for the finite size of ions in equilibrium, but in this two-part series, we consider steric effects on diffuse charge dynamics (in the absence of electro-osmotic flow). In this first part, we review the literature and analyze two simple models for the charging of a thin double layer, which must form a condensed layer of close-packed ions near the surface at high voltage. A surprising prediction is that the differential capacitance typically varies non-monotonically with the applied voltage, and thus so does the response time of an electrolytic system. In PB theory, the capacitance blows up exponentially with voltage, but steric effects actually cause it to decrease above a threshold voltage where ions become crowded near the surface. Other nonlinear effects in PB theory are also strongly suppressed by steric effects: The net salt adsorption by the double layers in response to the applied voltage is greatly reduced, and so is the tangential "surface conduction" in the diffuse layer, to the point that it can often be neglected compared to bulk conduction (small Dukhin number).

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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