Cavitation enhancement by organic additives: a systematic study probing the universal effect of additive vapor-pressure.

Carbon tetrachloride (CCl4) is a common additive for enhancing ultrasonic cavitation, and the governing mechanism is generally attributed to its chemical reactivity. Here, we systematically investigate chlorinated solvents, insoluble alkanes, and soluble alcohols as additives in ultrasonication, using complementary co-sonication and stepwise sonication protocols, alongside [Fe(SCN)6]4- and DPD as chromogenic probes. Unexpectedly, all additive categories exhibit a common volcano-shaped dependence of cavitation intensity on additive vapor pressure, irrespective of chlorine content or water solubility, indicating that vapor pressure rather than intrinsic chemical reactivity dominates cavitation enhancement. Stepwise sonication decouples cavitation from chromogenic reactions and reveals that chlorinated additives predominantly generate free chlorine whereas chlorine-free additives mainly produce peroxides. Based on these observations, we propose that vapor pressure facilitates bubble nucleation and at excessive values, weakens bubble collapse, leading to a peak with optimum cavitation efficiency. This work establishes a unified physical framework for understanding cavitation promotion by organic additives and provides guidance for the rational design of efficient sonochemical systems.