Interfacial Tension and Drop Phenomena of Triglycerides in Contact with pressurized CO2 and CO2-expanded Ethanol

Abstract

This study presents the interfacial tension (IFT) of fish oil triglycerides (TG) at 40, 55 and 70°C in contact with pressurized pure carbon dioxide (CO2) as well as in contact with CO2 - expanded ethanol (CX EtOH) determined according to the pendant drop method. Furthermore, drop phenomena caused by interfacial tension gradients and diffusion of CX Ethanol into the pendant TG droplet are presented.

The IFT of TG in contact with CO2 at a pressure ranging from 10 to 20 MPa decreased sharply from initially about 25- 30 mN/m to values of about 3.6 to 9.8 mN/m followed by a less pronounced decrease until IFT vanished at higher pressures depending on temperature ranging from 15 to 25 MPa. In comparison, the IFT of TG in contact with CX EtOH decreased linearly with CO2 pressure (and CO2 mole fraction in CX EtOH) from initial values of less than 1 mN/m and eventually vanished at pressures in the range of 4 to 6 MPa.

The freshly formed pendant drops of TG in CX EtOH exhibited various drop phenomena such as sideways and downward kicking motion or auto-oscillation caused by IFT gradients, likely leading to a Marangoni effect, where interfacial turbulence and internal and external fluid convection patterns including pulsating internal circulation could be observed. Images (videos) taken for TG pendant drops in contact with CX EtOH during formation and aging up to 140 s were analyzed showing the effect of CX EtOH diffusing into the pendant TG drop affecting drop density leading to an apparent increase in the values of IFT. As well, drop phenomena such as a sideways and a downwards kicking pendant drop are presented, which are caused by local variations in IFT over the drop surface leading to interfacial turbulence and rapid kicking movements of the pendant drop.

This study aims to further advance the understanding of IFT and interfacial phenomena, while emphasizing the important role of interfacial properties and phenomena for the potential applications of CX EtOH related to lipid processing and particle formation process developments.