Investigation of the Kinetics of Cotton Soapstock Saponification under Ultrasonic Illumination

Abstract

This study compares the saponification of cotton soapstock using conventional alkaline and ultrasonic (20 kHz) methods. Experiments were conducted over a temperature range of 333–368 K, with 5 K increments, and agitation times between 15 and 450 min. Under ultrasonic treatment, soapstocks with a free alkalinity of 0.2–0.3% reached the target degree of saponification in approximately 225 min at 358 K and 90 min at 368 K. Under identical conditions, the conventional process required about 270 min and 120 min, respectively. Thus, the application of ultrasound shortened the saponification time by approximately 1.2–1.3 h compared with the classical method. Microscopic analysis showed that ultrasonic irradiation reduced the dispersed phase size from ~2 µm in the conventional process to below the optical resolution limit, indicating a significant improvement in dispersion. Saponification extent was determined for both methods at 15–120 min and 338–368 K. For the conventional method, it increased from 78.78–80.86% to 93.05–98.06%, whereas ultrasound yielded 74.11–74.43% to 92.84–96.88% under the same conditions. Kinetic analysis showed that the apparent activation energy decreased from 21.40 kJ mol⁻¹ (conventional) to 18.38 kJ mol⁻¹ (ultrasound), demonstrating that ultrasonic treatment lowers the energy barrier and accelerates the reaction. Overall, ultrasound enhances both the rate and efficiency of cotton soapstock saponification, offering a faster and more effective alternative to the classical alkaline process.