The analysis of water density fluctuations in the hydration shell of nonpolar solutes provides insights into water-mediated interactions, especially hydrophobic interactions. These fluctuations are sensitive to small perturbations due to changes in thermodynamic conditions, such as temperature and pressure, but also to the presence of cosolutes, such as salts or small organic molecules. Herein, we investigate the effect of two classes of adsorbing cosolutes, using urea and methanol as representatives, on the fluctuations in energy and solvent density within the solvation shell of a model extended hydrophobic solute. We focus on the interactions of the cosolutes with the hydrophobic hydration shell, rather than with the solute itself, which though important remain largely unexplored. We calculate and analyze the interfacial partial molar energy of the cosolute, using a methodology based on the small system method. This approach provides correlated solvent density and energy fluctuations and allows us to decompose them into contributions due to interactions between the different components present in the solvation shell of the solute. The results show that adsorbed urea molecules interact more favorably with water than nonadsorbed urea molecules, which leads to the attenuation of interfacial density fluctuations and thus to the stabilization of the solvation shell. By contrast, the adsorbed methanol molecules interact preferably with other methanol molecules in the solvation shell, leading to a nano-phase segregated structure, which enhances interfacial fluctuations.

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吸附共质与疏水水合壳的相互作用

对非极性溶质水合壳中水密度波动的分析提供了对水介导的相互作用，特别是疏水相互作用的见解。这些波动对由于热力学条件（例如温度和压力）的变化而引起的小扰动很敏感，而且对共溶质（例如盐或小有机分子）的存在也很敏感。在此，我们研究了以尿素和甲醇为代表的两类吸附共溶质对模型扩展疏水性溶质的溶剂化壳内能量和溶剂密度波动的影响。我们关注的是共溶质与疏水水合壳的相互作用，而不是与溶质本身的相互作用，尽管这一点很重要，但在很大程度上仍未得到探索。我们使用基于小系统方法的方法计算和分析了共质的界面部分摩尔能。这种方法提供了相关的溶剂密度和能量波动，并允许我们将它们分解为由于溶质溶剂化壳中存在的不同组分之间的相互作用而产生的贡献。结果表明，吸附的尿素分子比未吸附的尿素分子与水的相互作用更有利，这导致界面密度波动减弱，从而稳定溶剂化壳。相比之下，吸附的甲醇分子优选与溶剂化壳中的其他甲醇分子相互作用，形成纳米相分离结构，从而增强界面波动。