Thermal effects in nanoemulsification by ultrasound

A significant part of energy in sonication is dissipated as heat, which, if not controlled, can adversely affect the sonication efficiency as well as the emulsion properties. In this research, thermal effects in nanoemulsification via ultrasound are studied. Alkanes with a wide range of volatility and water solubility were used as model oils and sodium dodecylsulfate (SDS) as surfactant. Nonisothermal sonication of oil/water/surfactant mixtures resulted in the formation of emulsions whose drop size could not easily be related to formulation and process variables. Isothermal sonication produced more reproducible and finer nanoemulsions than the nonisothermal one. The difference in the drop size between two processes became wider with increasing water solubility/volatility of oils. In order to elucidate the thermal effects during sonication of the oil/water/surfactant system, the sonicationd of pure water, an aqueous solution of SDS, and an aqueous solution of SDS saturated with an oil (hexane) were thoroughly monitored. Nanobubbles of about 180 nm in diameter were detected in the sonified pure water; however, the average size of bubbles increased to about 2 μm in the presence of SDS. Bubbles formed under extensive cavitation, by using either high power sonication or volatile oil, can increase the average oil drop size by damping the shock waves (i.e., decreasing energy transfer efficiency) and, therefore, reducing the rate of drop breakup. Bubbles can also contribute to the size of emulsion droplets, if they are not removed from the emulsion prior to measurement