Destabilization of Particle-stabilized Emulsions with Non-ionic Surfactants

"Particle-stabilized water-in-oil (W/O) emulsions are commonly sought for applications that demand long-term resistance against droplet coalescence. However, their remarkable stability may pose problems for uses that require controlled breakdown, such as for controlled release applications. Here, we investigated the demulsification of model W/O emulsions stabilized by glycerol monostearate (GMS) crystals solidified prior to emulsification. We studied the ability of the surfactants sorbitan monooleate (SMO), sorbitan monolaurate (SML), polyglycerol polyricinoleate (PGPR), citric acid esters of mono and diglycerides (CITREM), sorbitan trioleate (STO), and propylene glycol monolaurate (PgML) to act as demulsifiers based on their capacity to alter the wettability of interfacially-bound GMS crystals. Demulsification was promoted by the addition of SMO, SML, and CITREM, which promoted the transition of the GMS crystals from oil-wet to water-wet, thereby reducing their ability to stabilize the starting oil-continuous emulsions. Conversely, surfactants PGPR, STO, and PgML, did not sufficiently alter GMS crystal wettability to illicit demulsification. We found that two factors were necessary for a surfactant to act as a demulsifier, namely a strong affinity to the surface of GMS crystals as well as to the oil-water interface. From a compositional perspective, SMO, SML, and CITREM were effective demulsifiers because of their availability of sterically unhindered polar functional groups that can anchor to the surface of GMS crystals and polar dispersed phase droplets. Conversely, polar functional groups in PGPR and STO were sterically hindered, preventing adsorption to polar surfaces, while the propylene glycol head-group of PgML lacked polar character. Furthermore, it was shown that emulsion breakdown was concentration dependent, with surfactant concentration dominating release kinetics. Overall, this work showed that tuning the wettability of interfacially-bound GMS crystals could be used to destabilize particle-stabilized W/O emulsions, which may allow for the controllable breakdown of highly stable emulsions.