Controllable And Switchable/stimuli-responsive Surface Active Inorganic Nanoparticles

In recent past there has been a growing interest in switchable surfactants, switchable polymers, switchable or stimuli-responsive surface-active particles, and smart systems they formed. These functional chemicals can be transformed between surface-active and surface-inactive forms via different triggers, convenient in recovering and re-use after using, and are therefore environmentally friendly green chemicals. Currently a variety of triggers have been reported, typically including electrochemistry, light irradiation, pH, temperature, magnetic-field intensity, and dual stimuli, such as pH-temperature, magnetic-field, intensity-temperature and pH-ionic strength etc.A series of smart systems have been prepared using these switchable chemicals, such as emulsions and foams, which can be transformed between stable and unstable. When switchable surfactants are used, high concentration（> cmc） is usually necessary and the dispersed systems are thermodynamically unstable. On the other hand by using switchable or stimuli-responsive surface-active particles super stable systems such as Pickering emulsions and Pickering foams can be prepared, but the synthesis or preparation of the particles is in general complicated because they are mostly functional polymers. It is well known that some commercial inorganic nanoparticles can be made surface-active by in situ hydrophobization in water via interaction with amphiphiles. If the in situ hydrophobization can be made reversible, switchable or stimuli-responsive surface-active inorganic nanoparticles can then be obtained, by which smart systems can be constructed, avoiding complicated synthesis of functional polymer particles.Based on this idea, the feasibility of making switchable or stimuli-responsive inorganic silica nanoparticles via reversible in situ hydrophobization and possible triggers are studied in this paper. Firstly, a switchable surfactant with CO₂/N2 trigger is used as an amphiphile for preparing switchable surface-active SiO₂ nanoparticles via reversible in situ hydrophobization. The switchable surfactant can be transformed between cationic（long chain alkyl amidines bicarbonate） and neutral（long chain alkyl amidines） forms by bubbling CO2/N2. When in cationic form, they adsorb to the negatively charged surfaces of the SiO₂ nanoparticles via electrostatic interaction and thus in situ hydrophobize the surfaces. While in neutral form, they desorb from particle surfaces due to lack of electrostatic interaction. The in situ hydrophobization is then removed and particles lose their surface activity. Secondly, stimuli-responsive surface-active SiO₂ nanoparticles are obtained by using conventional cationic surfactants, where the silica nanoparticles are in situ hydrophobized by the free cationic surfactants in water via electrostatic interaction, with the in situ hydrophobization removed by adding an equimolar amount of an anionic surfactant, which prefer to form ion pairs with the cationic surfactants and thus making them desorb from SiO₂ nanoparticles. The trigger is the stronger electrostatic interaction between the oppositely charged ionic surfactants than that between the cationic surfactant and the particle surfaces. Thirdly, the switchable surface-active SiO₂ nanoparticles employing temperature trigger are obtained via hydrogen-bonding between the EO groups in nonionic amphiphiles and the SiOH on the particle surfaces. The nonionic amphiphiles have significant adsorption at SiO₂ nanoparticle/water interface, with the EO groups towards particle surface and the hydrocarbon chains towards water, giving an in situ hydrophobisation. With increasing temperature the hydrogen bonding is broken and the hydrophobization is removed. Besides, a preliminary study is carried out on preparing switchable or stimuli-responsive CaCO3 nanoparticles which are positively charged in pure water. Although surface-active CaCO3 nanoparticles with controllable surface activity are obtained, the route to obtain switchable or stimuli-responsive surface-active Ca CO₃ nanoparticles relies on more studies.