Our understanding of the dielectric response of interfacial water, which underlies the solvation properties and reaction rates at aqueous interfaces, relies on the linear response approximation: an external electric field induces a linearly proportional polarization. This implies antisymmetry with respect to the sign of the field. Atomistic simulations have suggested, however, that the polarization of interfacial water may deviate considerably from the linear response. Here we present an experimental study addressing this issue. We measured vibrational sum-frequency generation spectra of heavy water (D₂O) near a monolayer graphene electrode, to study its response to an external electric field under controlled electrochemical conditions. The spectra of the OD stretch show a pronounced asymmetry for positive versus negative electrode charge. At negative charge below 5 × 10¹² electrons per square centimetre, a peak of the non-hydrogen-bonded OD groups pointing towards the graphene surface is observed at a frequency of 2,700 per centimetre. At neutral or positive electrode potentials, this ‘free-OD’ peak disappears abruptly, and the spectra display broad peaks of hydrogen-bonded OD species (at 2,300–2,650 per centimetre). Miller’s rule¹ connects the vibrational sum-frequency generation response to the dielectric constant. The observed deviation from the linear response for electric fields of about ±3 × 10⁸ volts per metre calls into question the validity of treating interfacial water as a simple dielectric medium.

我们对界面水的介电响应的理解依赖于线性响应近似：外部电场引起线性比例极化，它是水界面溶剂化特性和反应速率的基础。这意味着关于场的符号的反对称性。然而，原子模拟表明，界面水的极化可能会大大偏离线性响应。在这里，我们提出了一项解决这个问题的实验研究。我们测量了重水的振动和频产生谱（D ₂O) 在单层石墨烯电极附近，研究其在受控电化学条件下对外部电场的响应。OD 拉伸的光谱显示正负电极电荷明显不对称。在负电荷低于每平方厘米 5 × 10 ^{12 个}电子时，以每厘米 2,700 的频率观察到指向石墨烯表面的非氢键 OD 基团的峰。在中性或正电极电位下，这个“游离 OD”峰突然消失，光谱显示氢键 OD 物质的宽峰（每厘米 2,300–2,650）。米勒法则¹将振动和频生成响应与介电常数联系起来。观察到的电场线性响应偏差约为 ±3 × 10 ⁸ 伏特/米，这对将界面水作为简单介电介质处理的有效性提出了质疑。