Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.
J Chem Phys. 2009 Nov 14;131(18):185102. doi: 10.1063/1.3257735.
We introduce a minimal free energy describing the interaction of charged groups and counterions including both classical electrostatic and specific interactions. The predictions of the model are compared against the standard model for describing ions next to charged interfaces, consisting of Poisson-Boltzmann theory with additional constants describing ion binding, which are specific to the counterion and the interfacial charge ("chemical binding"). It is shown that the "chemical" model can be appropriately described by an underlying "physical" model over several decades in concentration, but the extracted binding constants are not uniquely defined, as they differ depending on the particular observable quantity being studied. It is also shown that electrostatic correlations for divalent (or higher valence) ions enhance the surface charge by increasing deprotonation, an effect not properly accounted within chemical models. The charged phospholipid phosphatidylserine is analyzed as a concrete example with good agreement with experimental results. We conclude with a detailed discussion on the limitations of chemical or physical models for describing the rich phenomenology of charged interfaces in aqueous media and its relevance to different systems with a particular emphasis on phospholipids.
我们提出了一个最小自由能描述，用于描述包括经典静电相互作用和特定相互作用在内的带电基团和抗衡离子的相互作用。该模型的预测与描述带电界面附近离子的标准模型进行了比较，该模型由泊松-玻尔兹曼理论组成，其中包含描述离子结合的常数，这些常数是特定于抗衡离子和界面电荷的（“化学结合”）。结果表明，在几个数量级的浓度范围内，“化学”模型可以被适当描述为基础的“物理”模型，但提取的结合常数不是唯一定义的，因为它们取决于正在研究的特定可观察量。还表明，对于二价（或更高价）离子，静电相关会通过增加去质子化来增强表面电荷，这是化学模型中没有正确考虑到的效应。带电荷的磷脂酰丝氨酸被分析为一个具体的例子，与实验结果吻合良好。我们最后详细讨论了化学或物理模型在描述水相中带电界面丰富现象学方面的局限性及其与具有特定重点的不同系统（特别是磷脂）的相关性。
