Ultrasonic Emulsification of Liquid, Near-Critical Carbon Dioxide−Water Biphasic Mixtures for Acceleration of a Hydrolysis Reaction

A surfactant-free method based on power ultrasound was used to emulsify biphasic mixtures of liquid, near-critical carbon dioxide and water (T = 30 °C, P = 80 bar). When ultrasound above a threshold power density (roughly 0.15 W/cm³ at 20 kHz) was applied to a biphasic system of carbon dioxide and water, emulsions were formed which remained turbid for approximately 30 min (after 2 min of continuous ultrasound). Pulsed ultrasound (20% duty of a 2.5 s cycle at 20 kHz) could be used to maintain the emulsions indefinitely. Benzoyl chloride hydrolysis was selected as a model chemical reaction because of its rapid rate in pure water and the relative insolubility of the aroyl halide in the aqueous phase as compared to the carbon dioxide phase. The kinetic rate was measured under a range of ultrasonic pulsed power densities and zero ultrasound (i.e., silently) as a control. The measured, apparent, first-order rate constant was accelerated 100-fold in the presence of ultrasound at 0.50 W/cm³ relative to silent conditions. The accelerative effect of ultrasound saturates with increasing power beginning at 0.50 W/cm³. These results demonstrate the use of power ultrasound to create liquid carbon dioxide−water emulsions that, without surfactants or other additives, accelerate hydrolysis of substrates with low water solubility.