无机纳米颗粒在流体界面上的可逆自组装及其多元化触发机制研究

The self-assembly of nanoparticles at fluid interfaces leads to formation of dense films at fluid interface, which can be applied to prepare ultrastable multiple dispersion systems such as Pickering emulsions and foams, but also makes it more difficult to destabilize the systems. On the other hand, in some fields temporarily stabilized multiple-phase disersion systems which have to be destabilized finally are needed. It is therefore much attractive and challenging to realize reversible self-assembly of nanoparticles at fluid interfaces. This project is aimed to investigate the chemical principles and multiple triggers for realizing reversible self-assembly of non-surface active inorganic nanoparticles at fluid interfaces. The designed access is to convert the inorganic nanoparticles such as silica nanoparticles and calcium carbonate nanoparticles to surface-active in bulk aqueous media by hydrophobization in situ with suitable amphiphiles via non-covalent interaction, so that the self-assembly of the surface-active nanoparticles occurs at fluid interfaces, whereas by certain triggers the hydrophobization can be rescinded, which renders particles to original surface-inactive state and thus leads to disassembly of the particles. The reversible hydrophobization and induced reversible self-assembly of the nanoparticles can then be recycled to a certain extent and the triggers may include CO2/N2, pH, temperature and ion pair formation etc. It is expected that by the study of this project the chemical principles and possible multiple triggers involved in the double reversible phenomena will be revealed and it is allowed to construct smart self-assembly systems using ordinary inorganic nanoparticles and conventional amphiphiles.

纳米颗粒在流体界面上的自组装导致形成致密的界面膜，可用于制备超稳定多相分散体系如Pickering乳状液和泡沫，但同时也给分散体系的失稳增加了难度。另一方面，一些领域需要暂时稳定而最终失稳的多相分散体系，因此实现纳米颗粒在流体界面上的可逆自组装是非常有吸引力的和挑战性的。本项目旨在探索使无机纳米颗粒在流体界面实现可逆自组装的化学原理和多元化触发机制。具体途径是使二氧化硅和碳酸钙等纳米颗粒在水介质中通过与合适的双亲分子之间的非共价键作用而被原位疏水化，进而吸附到流体界面实现自组装，再通过某种触发机制解除原位疏水化作用，使颗粒恢复到原始状态并导致自组装解体。这种因果性双可逆现象可在一定程度上反复循环。可能的触发机制包括CO2/N2，pH，温度，离子对形成等。预期通过本项目研究，能够揭示相关双可逆现象发生的化学原理和多元化触发机制，并实现用普通无机纳米颗粒和常规双亲分子构建智能自组装体系。

智能分散体系例如Pickering乳状液和Pickering泡沫等的构建在乳液聚合、多相催化、油品乳化输送以及新材料制备等领域具有重要的应用价值，能够实现乳化剂或相应乳化体系的循环利用，从而使相关过程绿色化。本课题试图用普通商品纳米颗粒与特定表面活性物质相结合，实现颗粒在流体界面的可逆自组装，从而制备具有开关性的或刺激-响应性的Pickering乳状液和Pickering泡沫体系。具体研究内容包括两个方面：(1)借助于特定表面活性物质对纳米颗粒的原位疏水化作用，赋予颗粒表面活性，从而能够吸附到流体(油/水和气/液)界面实现自组装，稳定流体分散体系；(2)通过多元化的触发机制解除原位疏水化作用，进而解除颗粒的表面活性，导致分散体系不稳定。. 通过4年来的研究，本项目利用无机纳米颗粒与表面活性物质分子之间的非共价键作用，包括静电作用和氢键作用等，实现了对无机纳米颗粒的原位疏水化，赋予其表面活性，能够吸附到流体界面稳定Pickering乳状液和Pickering泡沫，并可以通过CO2/N2触发，离子对触发，pH触发，温度触发，CO2/N2-离子对复合触发以及CO2/N2-光双重触发等智能触发机制解除原位疏水化作用，从而解除颗粒在流体界面的自组装，导致破乳和消泡。相关理论和技术分别可应用于表面带负电荷的颗粒和带正电荷的颗粒。尤其是能够使用普通商品无机纳米颗粒和常规表面活性剂，包括阴离子、阳离子、非离子、两性离子等构建智能体系。在这些智能体系中，颗粒浓度在0.1~0.5 wt.%，表面活性物质的浓度一般仅需0.1cmc左右。.作为扩展研究，本项目取得另一项重大发现，即用无机纳米颗粒与带相同电荷的离子型表面活性剂相结合能够协同稳定一种新型乳状液，其中颗粒并不吸附于油/水界面，而是分散于水相，稳定乳状液所需要的颗粒和表面活性剂浓度最低分别仅需0.0001~0.001 wt.%和0.001cmc，并且这种新型乳状液同样具有开关性或刺激-响应性。这种新型乳状液由本团队首次报道，目前被命名为“oil-in-dispersion emulsions”。由于使用了极低的乳化剂浓度，降低了向环境的化学排放，这种新型乳状液具有十分重要的经济和环境效益。