利用基于Gouy-Chapman模型的离子有效电荷定量表征离子特异性效应

离子特异性效应在固-液界面反应中是普遍存在的. 近期研究指出, 在较低电解质浓度的某些体系中, 离子特异性效应可能并非来源于色散力、经典诱导力、离子半径或水合半径的大小等, 而是界面附近强电场中的离子极化作用. 这种作用可使界面附近的吸附态反号离子被强烈极化(高达经典极化的10⁴倍). 强烈极化的结果将导致离子在界面附近受到的库仑力远远超过离子电荷所能产生的库仑力, 这体现在离子的有效电荷将远大于离子的实际电荷. 因此胶体体系中基于这种强极化的离子有效电荷可以用来定量表征离子特异性效应的强度. 本研究在蒙脱石-胡敏酸混合悬液凝聚过程中发现了Na⁺、K⁺、Ca²⁺、Cu²⁺四种离子的离子特异性效应, 提出了基于激光散射技术测定离子有效电荷的方法, 并成功获得了被强烈极化后的离子有效电荷数值. 实验测得的Na⁺、K⁺、Ca²⁺、Cu²⁺四种离子的有效电荷值分别为: Z_{Na(effective)}=1.46, Z_K(effective)=1.86, Z_{Ca(effective)}=3.92, Z_{Cu(effective)}=6.48.该结果表明: (1) 离子在强电场中的极化将大大提高离子的有效电荷, 从而极大地增强离子所受的库仑作用力;(2) 离子的电子层数越多, 离子极化越强烈, 离子的有效电荷增加越多.

Quantitative Characterization of Specific Ion Effects Using an Effective Charge Number Based on the Gouy-Chapman Model

Specific ion effects have been observed in a wide range of phenomena at solid-liquid interfaces. Recent studies have indicated that the origin of these effects in some relatively low-electrolyte-concentration systems is the ion polarization in the strong electric field near the interface, rather than dispersion forces, classical induction forces, ionic size, or hydration effects. These effects cause the counterions near the interface to become strongly polarized (with a polarization that is nearly ten thousands times stronger than classical polarization). This strong polarization causes that the Coulomb force exerted by the polarized ions near the interface is far greater than the force generated by the ionic charge, which is reflected in the fact that the effective charge number of polarized ions is much larger than their original charge number. We therefore used the effective charge number of strongly polarized cations to quantitatively characterize the strength of specific ion effects in colloid systems. In this study, we observed the strong, specific ion effects of Na⁺, K⁺, Ca²⁺, and Cu²⁺ in the montmorillonite-humic acid composite aggregation process. Furthermore, we established a method to calculate the effective charge number of polarized cations based on the critical coagulation concentration (CCC) measured using dynamic light scattering. We successfully obtained the effective charge number of polarized ions. The experimental effective charge numbers for Na⁺, K⁺, Ca²⁺, and Cu²⁺ were Z_{Na(effective)}=1.46, Z_K(effective)=1.86, Z_{Ca(effective)}=3.92, Z_{Cu(effective)}=6.48, respectively. These results showed that the non-classical polarization greatly enhanced the effective charge number of ions, greatly enhancing the Coulomb force exerted by the ions; and that the more electronic layers the ions had and the stronger the ionic polarization, the more the effective charge of ions increased.