Why Are Hydrophobic/Water Interfaces Negatively Charged?
Why Are Hydrophobic/Water Interfaces Negatively Charged?
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DOI：
10.1002/anie.201108228
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发表时间：
2012-01-01
影响因子：
16.6
通讯作者：
Cabane, Bernard
中科院分区：
文献类型：
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作者：
Roger, Kevin;Cabane, Bernard
水和非极性介质（气体、液体或固体）之间的界面的自由能成本很高。因此，它们往往会重新组合以减少总界面面积：在水中，油滴会合并，气泡会在碰撞后重新组合。乳液、泡沫和聚合物分散体的亚稳定性是通过吸附两亲分子（离子或非离子）、大分子或颗粒来实现的，从而阻止重组。这种稳定的机制是众所周知的。[1]然而，在纯水中由纯油滴制成的非常细的乳液也被发现在不添加任何稳定剂的情况下具有亚稳态。[2-4]根据和频发生（SFG）光谱[5]和电泳迁移率测量，[2, 3]这些不含表面活性剂的乳液的液滴被电离并带有负电荷。水/空气界面也有类似的结果。[6-8]此外，这种负电荷随着 pH 值的增加而迅速增加，因此随着氢氧根离子的体积浓度的增加而迅速增加。[2,4,9]对这些现象最常见的解释是氢氧根离子吸附在疏水/水界面。虽然与这些现象的 pH 特征一致，但这种解释需要高吸附能，超过 20 倍的热能 kBT（约 50 kJ molÀ）[2, 6]，以及氢氧根离子相对于不在此类界面吸附的其他简单阴离子 [2, 6, 10, 11] 具有出色的选择性。在理论方面，一些模型试图解释这种意外的吸附，[12-14]，而另一些模型则发现疏水界面处没有氢氧根离子的积累；[15]不过，其他模型则寻找表面电荷的另一个来源。 [16]目前，对于这一有趣的现象还没有清晰、直接的理解。之前所有实验和理论研究的基本假设是这些系统具有“原始”油/水界面，即油分子与水分子接触，尽管已经提到了阴离子、表面活性杂质污染的可能性。 [17]这一假设得到了使用纯组分 (99%) 的额外纯化、彻底清洁的玻璃器皿和设备、惰性气氛以及良好的重现性的支持。
Interfaces between water and apolar media (gases, liquids, or solids) have a high cost in free energy. Therefore they tend to recombine to reduce the total interfacial area: in water, oil drops coalesce and air bubbles recombine following collision. The metastability of emulsions, foams, and polymer dispersions is achieved through adsorption of amphiphilic molecules (ionic or non-ionic), macromolecules, or particles, which block the recombination. The mechanisms of this stabilization are well understood.[1] Yet very fine emulsions made of pure oil droplets in pure water have also been found to be metastable in the absence of any added stabilizers.[2–4] According to sum frequency generation (SFG) spectroscopy [5] and electrophoretic mobility measurements,[2, 3] the droplets of these surfactant-free emulsions are ionized and carry a negative electrical charge. Similar results have been reported for the water/air interface.[6–8] Moreover, this negative charge increases rapidly with pH value and therefore with the bulk concentration of hydroxide ions.[2, 4, 9] The most frequent explanation given for these phenomena is that hydroxide ions adsorb at hydrophobic/water interfaces. While consistent with the pH signature of these phenomena, this explanation requires high adsorption energies, more than 20times the thermal energy kBT (about 50 kJ molÀ),[2, 6] and an outstanding selectivity of hydroxide ions over other simple anions [2, 6, 10, 11] that do not adsorb at such interfaces. On the theoretical side, some models attempt to account for this unexpected adsorption,[12–14] while others find no accumulation of hydroxide ions at hydrophobic interfaces;[15] still, other models look for another origin of the surface charge.[16] At present, there is no clear and straightforward understanding of this intriguing phenomenon.The basic assumption of all previous experimental and theoretical studies has been that these systems have “pristine” oil/water interfaces, that is, oil molecules in contact with water molecules, although the possibility of contamination by anionic, surface-active impurities has been mentioned.[17] This assumption is supported by the use of pure components (99%) with additional purification, thoroughly cleaned glassware and equipment, inert atmospheres, and good reprodu-