Department of Chemical Engineering, Kyushu University, Nishi-ku, Fukuoka, 819-0395, Japan.
J Colloid Interface Sci. 2023 Sep 15;646:354-360. doi: 10.1016/j.jcis.2023.05.046. Epub 2023 May 12.
The dielectric constant of an electrolyte solution, which determines electrostatic interactions between colloids and interfaces, depends nonlinearly on the salinity and also on the type of salt. The linear decrement at dilute solutions is due to the reduced polarizability in the hydration shell around an ion. However, the full hydration volume cannot explain the experimental solubility, which indicates the hydration volume should decrease at high salinity. Volume reduction of the hydration shell is supposed to weaken dielectric decrement and thus should be relevant to the nonlinear decrement.
According to the effective medium theory for the permittivity of heterogeneous media, we derive an equation which relates the dielectric constant with the dielectric cavities created by the hydrated cations and anions, and the effect of partial dehydration at high salinity is taken into account.
Analysis of experiments on monovalent electrolytes suggests that weakened dielectric decrement at high salinity originates primarily from the partial dehydration. Furthermore, the onset volume fraction of the partial dehydration is found to be salt-specific, and is correlated with the solvation free energy. Our results suggest that while the reduced polarizability of the hydration shell determines the linear dielectric decrement at low salinity, ion-specific tendency of dehydration is responsible for nonlinear dielectric decrement at high salinity.
电解质溶液的介电常数决定了胶体和界面之间的静电相互作用，它不仅与盐度有关，而且与盐的类型有关。在稀溶液中的线性递减是由于离子水化壳的极化率降低。然而，完整的水化体积并不能解释实验溶解度，这表明水化体积应该在高盐度下减小。水化壳的体积减小被认为会削弱介电常数的衰减，因此应该与非线性衰减有关。
根据非均匀介质介电常数的有效介质理论，我们推导出一个方程，将介电常数与由水合阳离子和阴离子形成的介电空腔联系起来，并考虑了高盐度下部分脱水的影响。
对单价电解质的实验分析表明，高盐度下介电常数的减弱主要源于部分脱水。此外，还发现部分脱水的起始体积分数与盐的种类有关，与溶剂化自由能有关。我们的结果表明，虽然水化壳的极化率降低决定了低盐度下的线性介电衰减，但在高盐度下，脱水的离子特异性趋势是导致非线性介电衰减的原因。
