助溶剂甲醇对化学物质紫外光解的影响

在研究微量有机化学污染物的光化学行为时,为了增加难溶于水的药品溶解度,往往采用甲醇作助溶剂.但对反应体系中助溶剂甲醇对光化学反应的影响缺乏系统研究.以高压汞灯为光源,以萘普生(NP)及其光解产物NP1和NP2为研究对象,通过对比纯水、1%甲醇和1%异丙醇反应体系中的实验结果发现,少量助溶剂甲醇在紫外光照下对羟基自由基的淬灭效果接近于异丙醇.结合检测到的中间产物,推导了NP的光解路径主要为光致脱羧、羟基自由基夺氢、自由基耦合等反应过程.在助溶剂甲醇存在的体系中,甲醇能与萘普生发生酯化反应,并通过淬灭羟基自由基,影响萘普生的光解反应,从而导致实验结果误读.因此,在光化学实验研究中,应该排除助溶剂甲醇对实验结果的影响.

Effect of CH3OH as cosolvent on ultraviolet photolysis of chemical species

Methanol is often used to improve the solubility of some insoluble organic micropollutants in photochemical research. As a result, a little methanol is introduced into the reaction system. Naproxen and its major photoproducts NP1 and NP2 were studied in aqueous solutions under monochromatic irradiation(λ=254 nm) at pH=7.0 and T=25℃. The results of UV254-photolysis in pure water, pure water-1% methanol and pure water-1% isopropanol were compared. The obtained results indicated a remarkable suppression effect of the cosolvent methanol on the photodegradation rates of NP and its major reaction intermediates. We may reasonably conclude that methanol is also a quencher of hydroxyl radical just like isopropanol,and should be excluded in photolysis experiments. The photodegradation mechanism of NP with 1% methanol is derived from the major reaction intermediates, including light induced decarboxylation, hydrogen abstraction by hydroxyl radical and free radical coupling. Methanol can react with NP to produce ester, and affect the photodegradation by quenching hydroxyl radical. As a result, methanol should be excluded in photolysis experiments.