Abstract:
Phenolic compounds are a class of organic compounds that are prevalent in agricultural, industrial, and petroleum products. Due to their toxicity, when introduced to the aquatic environment, these compounds pose a health risk to the ecosystem. In addition, upon exposure to solar radiation these compounds photodegrade. This impacts their transport and fate throughout the environment – which includes the air-water interface.
An interface is defined as the boundary between two different phases. Molecules bound to an interface possess unique characteristics as they align to the interface with a specific orientation and possess different electronic properties than those in the bulk solution phase. Thus, they can exhibit unique surface chemical reactivity. It is therefore important to understand the changes that these compounds undergo at the air-water interface.
Surface second harmonic generation (SHG) spectroscopy has been shown to be a powerful tool in the exploration of chemical surfaces since this process is only permitted in regions that lack center of inversion symmetry, such as at the air-water interface. Using SHG, it has been demonstrated that p-nitrophenolate (PNP–, pH 13) adsorbs at the air-water interface with an
affinity of −16.0±0.3 kJ mol−1. In comparison, the neutral molecule, p-nitrophenol (PNP, pH 2) is shown to adsorb to this interface with an affinity of −24.7±0.6 kJ mol−1, which is greater than that of its conjugate anion. This work also demonstrates that the average molecular orientation of PNP– at the surface, relative to the laboratory coordinate, is dependent upon the surface population.
Furthermore, a comparison of the UV photolysis of PNP– in aqueous solution to the same process occurring at the surface is presented. In the bulk solution, the photolysis quantum efficiencies of PNP and PNP– are found to be (3.31±0.08)×10−5 mol einstein−1 and (1.33±0.04)×10−5 mol einstein−1, respectively, indicating that PNP– is more photostable. At the surface however, the kinetic process of PNP– is found to be slower than in the bulk. SHG based equilibrium and kinetic data is presented and the applicability of SHG in probing chemical reactions at this interface is discussed.
