Lu Hongduo, Stenberg Samuel, Woodward Clifford E, Forsman Jan
Theoretical Chemistry, Chemical Centre, P.O. Box 124, S-221 00 Lund, Sweden.
Soft Matter. 2021 Apr 14;17(14):3876-3885. doi: 10.1039/d0sm02167a. Epub 2021 Mar 4.
We used a recently developed classical Density Functional Theory (DFT) method to study the structures, phase transitions, and electrochemical behaviours of two coarse-grained ionic fluid models, in the presence of a perfectly conducting model electrode. Common to both is that the charge of the cationic component is able to approach the electrode interface more closely than the anion charge. This means that the cations are specifically attracted to the electrode, due to surface polarization effects. Hence, for a positively charged electrode, there is competition at the surface between cations and anions, where the latter are attracted by the positive electrode charge. This generates demixing, for a range of positive voltages, where the two phases are structurally quite different. The surface charge density is also different between the two phases, even at the same potential. The DFT formulation contains an approximate treatment of ion correlations, and surface polarization, where the latter is modelled via screened image interactions. Using a mean-field DFT, where ion correlations are neglected, causes the phase transition to vanish for both models, but there is still a dramatic drop in the differential capacitance as proximal cations are replaced by anions, for increasing surface potentials. While these findings were obtained for relatively crude coarse-grained models, we argue that the findings can also be relevant in "real" systems, where we note that many ionic liquids are composed of a spherically symmetric anion, and a cation that is asymmetric both from a steric and a charge distribution point of view.
我们使用一种最近开发的经典密度泛函理论（DFT）方法，在存在理想导电模型电极的情况下，研究了两种粗粒度离子流体模型的结构、相变和电化学行为。两者的共同之处在于，阳离子组分的电荷比阴离子电荷更能接近电极界面。这意味着由于表面极化效应，阳离子被特异性地吸引到电极上。因此，对于带正电的电极，阳离子和阴离子在表面存在竞争，其中阴离子被正电极电荷吸引。在一定范围的正电压下，这会产生相分离，其中两相在结构上有很大差异。即使在相同电位下，两相之间的表面电荷密度也不同。DFT公式包含对离子相关性和表面极化的近似处理，其中后者通过屏蔽镜像相互作用进行建模。使用忽略离子相关性的平均场DFT，会导致两个模型的相变消失，但随着表面电位增加，当近端阳离子被阴离子取代时，微分电容仍会急剧下降。虽然这些发现是针对相对粗糙的粗粒度模型获得的，但我们认为这些发现也可能与“真实”系统相关，我们注意到许多离子液体由球形对称的阴离子和从空间位阻和电荷分布角度来看都不对称的阳离子组成。
