A Study On Self-assembly Of Silica Nanoparticles At Fluid Interface And Its PH-responsiveness

Switchable or stimuli-responsive surface-active nanoparticles can be transformed reversibly between surface-active and surface-inactive via different triggers,and can therefore realize reversible self-assembly at fluid interface.The relative smart systems such as switchable or stimuli-responsive Pickering emulsions/foams are of great interests.Currently the triggers reported include typically light irradiation,pH,temperature,CO₂/N₂,and magnetic-field intensity etc.However,the switchable or stimuli-responsive surface-active particles are mostly functional polymeric materials or inorganic/organic hybrid materials where inorganic particle surfaces are coated with functional compounds,which are difficult or complicated to prepare in large amount.In this thesis we try to obtain stimuli-responsive particles via a simple and practical route,or by in situ hydrophobization of the ionic amphiphiles to oppositly charged inorganic nanoparticles in aqueous media to endow particles surface activity,followed by removing the hydrophobization via certain trigger to make particle turn back to surface-inactive,making their self-aseembly at fluit interface reversible.The study shows that the surface-active silica nanoparticles can be obtained by interaction of the particles in aqueous media with trace amount of zwetterionic surfactants such as dodecyl dimethyl carboxyl betaine(C₁₂B)and N-dodecyl-?-aminopropionate?DAP?.In acidic media,these two surfactants are turned to cationic types and can then adsorb at particle/water interface via electrostatic interaction forming a hydrophobic monolayer to produce hydrophobization,whereas in neutral or alkaline media,these two surfactants are turned to zwitteionic or anionic types?DAP?,and the electrastatic interaction is dramatically weaken with the hydrophobization significatly reduced or removed.With decane as oil and water/oil volume ratio = 1:1,O/W type Pickering emulsions can be prepared using 0.5 wt% silica nanoparticles in combination with 0.06 mmol/L C₁₂B?refer to aqueous phase?as emulsifiers,which are stable at pH ? 5.2 and unstable at pH ? 8.8.Thus by alternatively addition of NaOH and HCl followed by homogenization the emulsions can be cycled between stable and demulsification for many times.Correspondingly p H-responsive Pickering foams can be prepared using 0.5 wt% silica nanoparticles in combination with 0.6 mmol/L C₁₂B,which are stable at pH ? 4.3 and unstable at pH ? 10.The pH-responsive Pickering emulsions can also be obtained by using 0.5 wt% silica nanoparticles in combination with 0.06 mmol/L DAP as emulsifiers,which are stable at pH ? 4.0 and unstable at pH ? 6.0.The difference between the two systems is that for SiO₂/C₁₂B systems the p H range required for transforming the emulsions between stable and unstable is larger,which needs therefore addition of more amount of NaOH and HCl resulting in accumulation of NaCl produced in the systems,which may inhibit the in situ hydrophobization and thus result in larger droplet sizes and low foaming efficiency in multiple cycling.Whereas for SiO₂/DAP systems the pH range required for transforming the emulsions between stable and unstable is relatively small,the accumulation of the NaCl in the systems in not significant and their effects are negligible for limited multiple cycling?? 6 times?.The relative mechanism has been well explained by measuring the adsorption isotherms of the surfactants at particle/water interface,the zeta potentials of the particles dispersed in surfactant solutions,the contact angles of the aqueous solution of surfactants on quartz surface and their sensitivity to pH.This study has made it possible to construct smart stimuli-responsive systems using commercial inorganic particles and conventional surfactants.