Effect of ethoxylation and molecular weight of cationic surfactants on nucleate boiling in aqueous solutions

Saturated, nucleate pool boiling on a horizontal, cylindrical heater and the associated bubble dynamics in aqueous solutions of cationic surfactants of different molecular weight and ethoxylation or ethylene oxide (EO) content, are experimentally investigated. Boiling curves (q(w)" proportional to DeltaT(sat)) for different concentrations and photographic records of the salient features of the ebullient behavior are presented, along with a characterization of interfacial properties (surface tension and contact angle). The surfactant additive significantly alters the nucleate boiling in water and enhances the heat transfer. The enhancement increases with concentration, with an optimum obtained in solutions at or near the critical micelle concentration (c.m.c.) of the surfactant. The photographic and visual observations indicate a markedly different boiling behavior than that of water as well as between pre- and post-c.m.c. solutions. A lower molecular weight surfactant tends to reduce surface tension faster and show better enhancement performance than its higher molecular weight counterpart. With EO groups in its molecular chain the surfactant solution becomes more hydrophilic, and the higher wettability tends to suppress nucleation, thereby weakening the boiling process. Also, enhancement in pre-micellar solutions is shown to depend on the dynamic surface tension, and the number of EO groups in and molecular weight of the surfactant.