Study On The Emulsion Stability Through Colloid Particles Adsorption And Desorption Mechanism

Pickering emulsions are emulsion systems stabilized by solid particles to stabilize the oilwater interface,have received increasing attention in both academia and industry since replacement of small molecular surfactants can offer advantageous properties like strong interfacial stabilization,low environmental impact,and easy modification of the particle surface.The stability of Pickering emulsions is a necessary guarantee to enable their wide application.There are many factors affecting the stability of emulsion,among which the adsorption and desorption of colloidal particles is the direct factor affecting the stability of emulsions.Therefore,a systematic study of the influencing factors of colloidal particle adsorption-desorption and revealing the mechanism of colloidal particle adsorption-desorption on emulsion stability can promote the in-depth understanding of Pickering emulsion stability and lay the theoretical foundation for the preparation of long-term stable Pickering emulsions.In this study,emulsions were stabilized by fluorescent silica particles to systematically investigate the factors influencing the adsorption-desorption of colloidal particles,clarify the causes of adsorption-desorption of colloidal particles at the interface,and elucidate the mechanism of particles adsorption-desorption on the stability of oil-water interface by regulating the particle size and surface properties.The main research contents are as follows.1.Firstly,we investigated the influencing factors of particles adsorption at the interface from the factors of particle charge,interfacial charge and Van der Waals force.The regulation of particle surface charge was achieved by changing the p H of the aqueous phase and adding particles of opposite charge,and the oil-water interfacial charge was changed by adding tetrabutylammonium bromide(TBAB)to make it positively charged,and different Van der Waals forces were exhibited between the particles and the oil phase according to the different refractive indices of the oil and water phases.The results showed that for hydrophilic silica particles,the average droplet diameter achieved to 50 μm,stability of emulsion was best when the particles reached near the isoelectric point.The hydrophilic particles of negatively charged could electrostatic attraction with the interfacial positive charge,thus achieving the adsorption of the particles at the interface.For superhydrophobic silica particles,the particles could be adsorbed to the interface based on three factors: the image force attraction of particles,the Van der Waals repulsive force between the oil phase and the electrostatic attraction with the interfacial charge.2.Secondly,the effect of interfacial electrostatic adsorption-desorption on emulsion stability was further investigated by using two types of silica particles with different surface charge.The silica particles with positively charged(silica-FITC particles)dispersed in toluene alone could form W/O emulsions,while addition of silica particles with negatively charged(silica-RB particles)in the aqueous phase achieved phase inversion of the emulsion through interfacial electrostatic attraction.The results showed that the particles dispersed in the two phases could form a bilayer particles structure at the interface by interfacial electrostatic attraction,and the arrangement of two particles at the interface was found to determine the type of emulsion by high-resolution CLSM and SEM images.As the size of silica-RB particles decreased,the concentration of silica-RB particles required for the phase inversion of emulsion decreased.The desorption of silica particles at the interface could be realized by adjusting the p H and salt concentration of aqueous phase,thus the emulsion types could be inversed from O/W to W/O.It was shown that the emulsion stability could be improved through this approach and the interfacial particle bilayer could enhance the maximum osmotic pressure of the emulsion droplets,up to 37 k Pa.3.Finally,emulsions were stabilized by a series of silica particles with different sizes but the same surface chemistry to further investigate the effect of the particle size factor on the competitive adsorption and displacement of particles at the interface.The silica particles of different sizes had different diffusion coefficients in the aqueous phase,resulting in a competitive adsorption relationship.And the displacement relationship between the particles occured at the interface due to the difference in kinetic energy and diffusion coefficient.The results showed that particles of smaller size had larger diffusion coefficients and could preferentially adsorb to the oil-water interface in competitive adsorption with particles of larger size.Fluorescent silica particles in the size range of 100 nm to 1 μm were used as Pickering emulsion stabilizers,and subsequent addition to different concentrations of silica particles with different sizes could displace the silica particles initially adsorbed to the interface.The displacement more effectively with the increase of particle concentration and external energy at the time of displacement.A possible mechanism,based on the balance between the diffusion coefficient and kinetic energy of silica particles in the emulsion was proposed to explain the above results.