Lipids behaviour in aqueous solution of disrupted microalgae cultivated under nitrogen starving conditions: molecular simulation compared to experimental study of representative synthetic mixtures
Résumé
This work deals with the microalgae culture and biorefinery for biofuel production. Microalgae transform CO 2 into biomass and valuable molecules, including lipids. The downstream processing to valorise intracellular compounds necessitates innovative, efficient and clean processes for biomass harvesting, concentration, cell disruption and fractionation. Membrane filtration is a promising process for the concentration and the purification of lipids in a wet pathway to produce biofuel. The behaviour of the biomolecules and particles during the filtration processes is unknown. Yet it drives their performances. For example, strong interactions between lipids and polar molecules released during the cell disruption may prevent an efficient separation and valorisation of both compounds because of the water-oil interface stabilisation. Consequently, to achieve oil droplets coalescence, the interface must be disturbed during filtration. The understanding of the local organisation on the target molecules is a necessary step to develop innovative fractionation strategies and optimize the coupling of different processes. To bring understanding, a multiscale approach is proposed. The use of synthetic mixtures drives the link between the different scales. These synthetic mixtures are simplified emulsions that mimic a supernatant of a real grinded biomass, centrifugated to get rid of cell fragments. The composition is based on the characterisation of disrupted cells samples, from a P. kessleri culture, under nitrogen starving conditions. The synthetic mixtures contain water, triglycerides and polar lipids. On the one hand, coarse-grained molecular simulations were carried out, to study the behaviour of lipids in the solution. Interfacial tension of the water-lipids interface was calculated for several compositions and validated with experiments. On the other hand, the interfacial properties of complex mixtures were related to molecular organisation deduced from molecular simulation. The perspective of this project is at first the characterization of water-lipid interfaces with real products, to compare to the results with synthetic solutions. This necessitates a large-scale culture in starving conditions to recover enough lipids in the supernatant. The acquired knowledge on the behaviour and organisation of lipids at the interfaces will help the optimization of membrane processes for the concentration and coalescence of lipids before valorisation into biofuels.
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