Phospholipids as emulsifiers for micro/nano droplets suitable for biotechnological systems integration

We studied the emulsifying properties of palmitoyl oleoyl phosphatidylcholine (POPC) using the biocompatible compounds water and squalene as immiscible fluid phases. We tested the solubility limit of POPC in squalene and its equilibrium distribution between bulk phases. POPC is dissolvable in squalene up to 0.3% (w/v) with an ultrasonication procedure. Above this limit, aggregates of >1 μm are formed which are resistant to prolonged ultrasonication in size and quantity. Emulsifying properties of POPC were elaborated by measuring the droplet size ranges of emulsions. Nanofluidics was studied by pressure driven transport of nanometer sized emulsion droplets through defined nanochannels whereby droplet sizes 500 nm can be produced. The mechanical properties of the emulsifying phospholipid monolayer at the water/squalene interface were studied by profile analysis tensiometry (PAT). The dynamic interfacial tension was measured and the adsorption isotherms were established from long-time approximations of the diffusion-controlled adsorption. With PAT a critical aggregation concentration was determined in the same range as the solubility limit, which was measured by dynamic light scattering. The minimum interfacial tension for POPC as emulsifier was found to be below 1 mN/m. Thus, it can be concluded that phospholipids are suitable emulsifiers for microfluidics and produce adsorbed layers of remarkably small interfacial tension