Study On The Fluid-fluid Dispersion Systems Stabilized By Al₂O₃ Nanoparticles In Combination With Surfactants And Their Stimuli-responsive Properties

In recentyears,there have been rapid advances in research on switchable/stimuli-responsive surfactants/surface active particles and corresponding fluid dispersions,such as emulsions and foams.Many switchable or stimuli-responsive emulsion and foam systems have been constructed with the help of smart surfactants/surface active nanoparticles which can be reversibly converted between surface-active and surface-inactive by a single trigger like pH,light,magnetic field,temperature,redox,ion pair formation,CO₂/N₂,or multiple triggers composed of them.It is worth of noticing that switchable/stimuli-responsive Pickering emulsions and Pickering foams can be constructed using charged inorganic nanoparticles in aqueous media in combination with oppositely charged switchable/stimulus-responsive surfactants or even conventional surfactants,since it needs only to use common commercial inorganic nanoparticles in combined with low concentration?0.1 cmc? surfactant to obtain switchable/stimuli-responsive surface active particles by reversible in situ hydrophobization,and complex artificial synthesis of particles can be avoided.Since both the surfactants and nanoparticles are surface-active,their behavior at the fluid interface when they coexisted in a system has been a great concern.Although the interactions between oppositely charged nanoparticles and ionic surfactants and their behavior at the fluid dispersions have been well studied in recent years,those between like charged nanoparticles and ionic surfactants have not been paid attention,because there is no expected strong interaction between them.Under this academic background,this thesis tries to study the behavior of similarly charged inorganic nanoparticles and ionic surfactants in fluid dispersion systems,specifically the potential new behavior of the positively charged alumina?Al₂O₃? nanoparticles in combination with a CO₂/N₂ switchable surfactant?cationic surfactant?or an ordinary cationic surfactant in emulsion systems,aimed to explore unknown phenomena and enrich the basic theory of emulsions.In this paper,innovative research advances have been achieved mainly in the following four aspects:?1?CO₂/N₂ stimuli-responsive Pickering emulsions are constructed by using alumina nanoparticles and CO₂/N₂ switchable surfactant both positively charged with the help of trace amount of anionic surfactants.In general inorganic nanoparticles can only be hydrophobized in situ by adsorbing oppositely charged surfactant to become surface active particles to stabilize Pickering emulsions.However,the existing CO₂/N₂ switchable surfactants,such as alkylamidine bicarbonate?DDMAC?,are cationic surfactants,which have no in situ hydrophobization to the positively charged alumina nanoparticles.Therefore,CO₂/N₂ switchable or stimuli-responsive Pickering emulsions can not be constructed by using a CO₂/N₂ switchable surfactant in combination with alumina nanoparticles.However,in this study,such CO₂/N₂stimuli-responsive Pickering emulsions with n-decane as oil phase have been constructed with addition of trace amount of an anionic surfactant,such as sodium dodecyl sulfate?SDS?,into the system.The positively charged alumina nanoparticles can be hydrophobized in situ by adsorption of anionic SDS via electrostatic attraction,so as to be transformed to surface active particles,which tend to adsorb at oil/water interface to stabilize oil-in-water?O/W? Pickering emulsion.Then,if CO₂ is injected into the emulsion,the alkylamidine will be converted to cationic bicarbonate,which tends to form ion pairs with SDS and promotes desorption of SDS from particle surface.The particles are turned back to strong hydrophilic and desorb from oil/water interface leading to demulsification of the emulsions.After that,once N₂ was bubbled into the system,DDMAC is transformed into neutral amidine.The ion pairs formed by DDMAC and SDS are disintegrated,and the SDS can re-adsorb on the surface of alumina particles to establish the hydrophobization.After homogenization Pickering emulsion are formed again.This emulsification/demulsification cycle following bubbling CO₂/N₂ alternatively can be repeated multi-times depending on the concentration of the DDMAC used.?2?Novel type of O/W emulsions are constructed by using like charge nanoparticles and ionic surfactants both at ultra-low concentration,and their stabilization mechanisms are preliminarily revealed.It was found for the first time that alumina particles and cationic surfactant CTAB can synergistically stabilize a novel n-decane-in-water?O/W? emulsion,in which the concentration of surfactants and particles needed can be as low as 0.001 cmc and 0.001 wt.%,respectively.The microstructure of this novel emulsion differs from conventional emulsions and Pickering emulsions,where very low concentration of surfactant adsorbs at oil/water interface to endow droplets charges and decides droplet sizes,whereas particles distribute in the continuous?water? phase,forming a thicker aqueous lamella.The distance between the oil droplets is then increased the van der Waals attraction between the droplets greatly reduced.Moreover,both the particles and droplets have thick electric double layers and the double layer repulsion between particles,droplets and between particles and droplets prevents droplets from flocculation and coalescence.It has been shown that the synergistic stabilization between like charge nanoparticles and surfactants at ultralow concentration is universal,or it applies not only to the combination of positively charged nanoparticles and cationic surfactants,but also to the negatively charged nanoparticles and anionic surfactants.In addition,the novel emulsions apply to a variety of oils,including alkanes,aromatics and triglycerides.However,the stabilization mechanism of the novel O/W emulsions is difficult to interpret using the existing DLVO stabilization,steric stabilization,and Pickering stabilization mechanisms.The study has revealed that to stabilize the novel O/W emulsions,the particles should have a Zeta potential higher than a critical value of about±18mV.This discovery not only adds new stabilization mechanism to the emulsion stabilization theories,but also make it possible to greatly reduce the amount of surfactants and particles required for stabilizing an emulsion and their emissions into environment after use in practical applications,being both economical and environmental benign and in line with "green chemistry".?3?Stimuli-responsive conversion between a novel O/W emulsion and a Pickering emulsion is achieved.The alumina nanoparticles has an isoelectric point of pH=10.6.Therefore in acidic or neutral aqueous solutions,the surface of alumina nanoparticles are positively charged,which together with a cationic surfactants such as CTAB can stabilize a novel O/W emulsion?with n-decane as oil phase? at concentration as low as 0.1 wt.% and 0.1 cmc,respectively.However,when the pH of aqueous phase is adjusted to alkaline?>10.6?,the surface of alumina nanoparticles is turned to be negatively charged,which can be hydrophobized in situ by adsorbing cationic surfactants via electrostatic attraction to become surface active,and can then adsorb at oil/water interface to stabilize a Pickering emulsion.This pH stimuli-responsive conversion between a novel emulsion and a Pickering emulsion can only be cycled a limited number of times,because of the accumulation of inorganic salts formed during repeated acid-base neutralization,which may compress the electric double layer and thus inhibit the Zeta potentials of both particles and oil droplets.?4?The novel emulsions are made wise or switchable/stimuli-responsive.In the novel emulsion with n-decane as oil phase,using a CO₂/N₂ switchable surfactant such as DDMAC to replace the conventional cationic surfactant and working together with alumina nanoparticles,a CO₂/N₂ switchable novel O/W emulsion can be obtained.When CO₂is bubbled into the system,DDMAC is in cationic form and can co-stabilize the novel emulsions with positively charged alumina nanoparticles.Then by bubbling with N₂ into the system,DDMAC is turned to neutral form which can not adsorb at oil/water interface.The charges at the surface of droplets are eliminated resulting in demulsification.By bubbling CO₂ into the system again,DDMAC is turned back to cationic form and stable novel emulsions are formed again by homogenization.