The strong, long-range electrostatic forces described by Coulomb's law disappear for ions in water, and the behavior of these ions is instead controlled by their water affinity – a weak, short-range force which arises from their charge density. This was established experimentally in the mid-1980s by size-exclusion chromatography on carefully calibrated Sephadex®G-10 (which measures the effective volume and thus the water affinity of an ion) and by neutron diffraction with isotopic substitution (which measures the density and orientation of water molecules near the diffracting ion and thus its water affinity). These conclusions have been confirmed more recently by molecular dynamics simulations, which explicitly model each individual water molecule. This surprising change in force regime occurs because the oppositely charged ions in aqueous salt solutions exist functionally as ion pairs (separated by 0, 1 or 2 water molecules) as has now been shown by dielectric relaxation spectroscopy; this cancels out the strong long-range electrostatic forces and allows the weak, short-range water affinity effects to come to the fore. This microscopic structure of aqueous salt solutions is not captured by models utilizing a macroscopic dielectric constant. Additionally, the Law of Matching Water Affinity, first described in 1997 and 2004, establishes that contact ion pair formation is controlled by water affinity and is a major determinant of the solubility of charged species since only a net neutral species can change phases.

中文翻译：

---

水中离子的行为受其亲水性控制

对于水中的离子，库仑定律所描述的强、长程静电力消失了，而这些离子的行为由它们的水亲和力控制——一种由它们的电荷密度产生的微弱的短程力。这是在 1980 年代中期通过尺寸排阻色谱法在经过仔细校准的 Sephadex 上实验建立的®G-10（测量有效体积，从而测量离子的水亲和力）和通过同位素取代的中子衍射（测量衍射离子附近水分子的密度和方向，从而测量其水亲和力）。这些结论最近已通过分子动力学模拟得到证实，该模拟明确地模拟了每个单独的水分子。发生这种令人惊讶的力状态变化是因为盐水溶液中带相反电荷的离子在功能上作为离子对存在（由 0、1 或 2 个水分子隔开），如介电弛豫光谱所示；这抵消了强大的远程静电力，并使弱的短程亲水效应脱颖而出。使用宏观介电常数的模型无法捕捉到盐水溶液的这种微观结构。此外，1997 年和 2004 年首次描述的匹配水亲和力定律确定接触离子对的形成受水亲和力控制，并且是带电物质溶解度的主要决定因素，因为只有净中性物质才能改变相。