Investigation Of Responsive Emulsions Based On Dynamic Covalent Bond/Non-Covalent Interaction

Emulsions are widely used in numerous industrial applications,including food,cosmetics,and oil-recovery.For some applications,such as food storage and bitumen emulsification,long-term emulsion stability is critical.However,in other cases,such as oil recovery,heavy oil transportation,and emulsion polymerization,only temporary emulsion stability is desired.In general,breaking stable emulsions can be achieved through chemical methods(e.g.,adding demulsifiers and salt)or physical methods(e.g.,applying high electric field or centrifugation).However,the current methods in general lead to large energy consumption and/or secondary waste production.To avoid large energy consumption and subsequent pollutions,emulsions with on and off property(responsive emulsions)were desired.Such emulsions can be prepared with responsive emulsifiers.Responsive emulsifiers undergo reversible interconversions between active and inactive forms in response to specific external stimuli,such as CO2,pH,temperature,light irradiation and redox.The most commonly used responsive emulsifiers are covalent compound.Synthesis of these covalent compound,such as tertiary-amine and amidine,is complicated and time-consuming.Easily controlled dynamic covalent bond is an effective strategy to mediate responsive micelles,vesicles and capsules.However,few reports on responsive emulsions mediated by dynamic covalent bond have been reported.This dissertation is focused on the design of novel responsive emulsions by using easily controlled dynamic covalent bond/non-covalent interactions.It not only broadens the idea of construction responsive emulsions and improves its basic theory,but also has guiding significance for practical applications.In this dissertation,responsive emulsifiers were designed and synthesized,and the responsive emulsions based on these responsive emulsifiers were studied systematically The outline and contents of this dissertation are as follows:(1)In this chapter,dynamic covalent surfactant PEI-B was designed and synthesized.The formation of dynamic imine bond was proved by Fourier Transform infrared spectroscopy(FTIR)measurement.The responsive behavior of PEI-B was proved by proton nuclear magnetic resonance spectroscopy(1H NMR)measurement.At pH 7.8,PEI-B was interfacial active.However,after lowering pH to 3.5,PEI-B lost its interfacial activity.The interfacial active state change of PEI-B was attributed to the reversible formation and decomposition of dynamic imine bond.By taking advantage of the pH-responsiveness of PEI-B,pH-responsive emulsions were prepared.At pH 7.8,interfacial active PEI-B can be used to prepare stable emulsions,because interfacial active PEI-B can be effectively adsorbed on the oil-water interface and form stable interfacial film to stabilize the droplets.However,after lowering the pH from 7.8 to 3.5,complete phase separation was achieved.As pH decreased to 3.5,PEI-B becomes dissociated into interfacial inactive PEI and benzaldehyde due to the break of dynamic imine bond.Both PEI and benzaldehyde desorbed from the oil-water interface,which leading to the phase separation.Indeed,the emulsification and demulsification was controlled by the formation and decomposition of dynamic imine bond.The responsive behavior based on dynamic covalent bond will open up a novel route for preparing responsive emulsions.(2)Compared with surfactant stabilized conventional emulsions,Pickering emulsions stabilized by particles show less surfactant consumption,lower toxicity and lower cost.Therefore,responsive Pickering emulsions are more desirable.To test the versatility of dynamic covalent bond for responsive emulsions,this strategy was used to mediate responsive Pickering emulsions.Dynamic covalent silica(SiO2-B)nanoparticles were designed and synthesized based on dynamic imine bond.The formation and responsiveness of dynamic imine bond was confirmed by FTIR and 1H NMR measurement,respectively.The partially hydrophobic character of SiO2-B was proved by contact angle measurement.Based on the responsiveness of SiO2-B,pH-responsive Pickering emulsions were successfully constructed.At pH 7.8,stable Pickering emulsions were prepared.The high stability of Pickering emulsions was attributed to that the SiO2-B was adsorbed on the oil-water interface and formed firm interfacial film,which has been proved by confocal laser scanning microscopy observation.After lowering the pH to 3.5,complete phase separation was achieved.Under such acidic environment,the dynamic imine bond in the SiO2-B decomposes,resulting in decomposition of the SiO2-B into highly hydrophilic SiO2-NH2 and surface inactive benzaldehyde.Both of them desorpted from oil-water interface,which leading to a dramatic coalescence and finally resulted in a complete phase separation of the Pickering emulsions.The essence of the pH-responsiveness of Pickering emulsions was the dynamic character of dynamic imine bond.This work provides a new strategy to prepare responsive Pickering emulsions.(3)Acid-responsive non-aqueous Pickering emulsions were constructed by using hydrophobic dynamic covalent silica(SiO2-pDB)nanoparticles.The acid-responsive mechanism was the formation and decomposition of dynamic imine bond in the SiO2-pDB nanoparticles.The acid-responsive non-aqueous Pickering emulsions have potential applications in the field of oil-based drilling fluid.(4)Simple and controllable non-covalent interactions is another effective strategy to construct responsive emulsions.In this chapter,the cellulose nanocrystals(CNCs-M2005)were prepared via simple and controllable electrostatic interaction with thermosensitive M2005.The obtained CNCs-M2005 exhibited thermo and CO2 dual-responsive properties.By taking advantage of the thermo and CO2 dual-responsive character of CNCs-M2005 Pickering emulsifier,dual-responsive Pickering emulsion was developed.The dual-responsive mechanism of the Pickering emulsions was inveatigated by DLS and TEM measurement.The development of thermo and CO2 dual-responsive bio-safe Pickering emulsions represents a significant advance for controlled drug delivery,food and cosmetic applications.(5)In this chapter,temperature-sensitive polyoxypropylene(PPO)surfactants were synthesized through non-covalent interactions-electrostatic interactions.The synthesized PPO surfactants were used as emulsifiers to prepare nanoemulsions by the phase inversion temperature(PIT)method.The droplet size and morphology of the nanoemulsions were characterized by dynamic light scattering and cryogenic transmission electron microscopy,respectively.This work extended the scope of surfactants that can be used for the preparation of nanoemulsions by the PIT method.