Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States.
J Phys Chem B. 2023 May 18;127(19):4328-4337. doi: 10.1021/acs.jpcb.3c00588. Epub 2023 May 9.
Implicit solvent models that reduce solvent degrees of freedom into effective interaction potentials are widely used in the study of soft materials and biophysical systems. For electrolyte and polyelectrolyte solutions, coarse-graining the solvent degrees of freedom into an effective dielectric constant embeds entropic contributions into the temperature dependence of the dielectric constant. Properly accounting for this electrostatic entropy is essential to discern whether a free energy change is enthalpically or entropically driven. We address the entropic origin of electrostatic interactions in a dipolar solvent and provide a clarified physical picture of the solvent dielectric response. We calculate the potential of mean force (PMF) between two oppositely charged ions in a dipolar solvent using molecular dynamics and dipolar self-consistent field theory. We find with both techniques that the PMF is dominated by the entropy gain from the dipole release, owing to the diminished orientational polarization of the solvent. We also find that the relative contribution of the entropy to the free energy change is nonmonotonic with temperature. We expect that our conclusions are applicable to a broad range of problems involving ionic interactions in polar solvents.
隐溶剂模型将溶剂自由度简化为有效相互作用势，广泛应用于软物质和生物物理系统的研究。对于电解质和聚电解质溶液，将溶剂自由度粗粒化为有效介电常数，将熵贡献嵌入到介电常数对温度的依赖性中。正确考虑这种静电熵对于辨别自由能变化是焓驱动还是熵驱动至关重要。我们研究了偶极溶剂中静电相互作用的熵起源，并提供了溶剂介电响应的更清晰的物理图像。我们使用分子动力学和偶极自洽场理论计算了偶极溶剂中两个带相反电荷离子之间的平均力势（PMF）。我们发现，这两种技术都表明 PMF 主要由偶极释放带来的熵增益主导，这归因于溶剂取向极化的减小。我们还发现，熵对自由能变化的相对贡献随温度是非单调的。我们预计我们的结论适用于涉及极性溶剂中离子相互作用的广泛问题。
