We employ amplitude- and phase-resolved second harmonic generation experiments to probe interactions of fused silica:aqueous interfaces with Al³⁺, Mg²⁺, and Na⁺ cations at pH 4 and as a function of metal cation concentration. We quantify the second-order nonlinear susceptibility and the total interfacial potential in the presence and absence of a 10 mM screening electrolyte to understand the influence of charge screening on cation adsorption. Strong cation:surface interactions are observed in the absence of the screening electrolyte. The total potential is then employed to estimate the total number of absorbed cations cm^–2. The contributions to the total potential from the bound and mobile charges were separated using Gouy–Chapman–Stern model estimates. All three cations bind fully reversibly, indicating physisorption as the mode of interaction. Of the isotherm models tested, the K_d adsorption model fits the data with binding constants of 3–30 and ∼300 mol^–1 for the low (<0.1 mM) and high (0.1–3 mM) concentration regimes, corresponding to adsorption free energies of −13 to −18 and −24 kJ mol^–1 at room temperature, respectively. The maximum surface coverages are around 10¹³ cations cm^–2, matching the number of deprotonated silanol groups on silica at pH 4. Clear signs of decoupled Stern and diffuse layer nonlinear optical responses are observed and found to be cation-specific.

中文翻译：

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熔融石英上的 NaCl、MgCl2 和 AlCl3 表面覆盖率以及 pH 4 时的非线性光学吸附自由能

^{我们采用振幅和相位分辨的二次谐波发生实验来探测熔融石英：在 pH 4 下与 Al 3+}、Mg ²⁺和 Na ⁺阳离子的水界面的相互作用，以及作为金属阳离子浓度的函数。我们量化了存在和不存在 10 mM 屏蔽电解质时的二阶非线性磁化率和总界面电势，以了解电荷屏蔽对阳离子吸附的影响。在没有屏蔽电解质的情况下观察到强阳离子：表面相互作用。然后使用总电势来估计吸收的阳离子总数 cm ^–2。使用古伊-查普曼-斯特恩模型估计将束缚电荷和移动电荷对总电势的贡献分开。所有三种阳离子的结合完全可逆，表明物理吸附是相互作用的模式。在测试的等温线模型中，K _d吸附模型适合低 (<0.1 mM) 和高 (0.1–3 mM) 浓度范围的结合常数为3–30 和 ∼300 mol ^–1的数据，对应于无吸附室温下的能量分别为-13至-18和-24 kJ mol ^–1 。最大表面覆盖度约为 10 ^{13 个}阳离子 cm ^–2，与 pH 4 时二氧化硅上去质子化硅烷醇基团的数量相匹配。观察到解耦 Stern 和扩散层非线性光学响应的​​明显迹象，并发现其具有阳离子特异性。