Construction And Application Of CO₂/redox-responsive Microemulsions

Stimuli-responsive microemulsions（MEs）have attracted extensive attention because of their intelligent regulation on the stability of system.The emergence of surfactant-free microemulsion（SFME）provides great convenience for breaking the limitations of high-content surfactants in drug delivery and nanomaterial synthesis.Among various stimuli,environmentally friendly and biocompatible stimuli including redox,CO₂/N₂,and temperature,etc.,are more fascinating,but the current research on stimuli-responsive SFME is limited to CO₂/N₂ or temperature.Furthermore,as far as we know,these stimuli-responsive MEs can only respond to one stimulus,but not two or more stimuli simultaneously.However,it is undeniable that MEs with multiple stimuli responses can provide a wider range of controllability and higher precision,so it has greater advantages in application.Herein,we designed and synthesized a selenium-containing short-chain alcohol（monoselenide dipropanol,PSeP）,and successfully fabricated redox-responsive surfactant-based microemulsion（PSeP-SBME）using PSeP as a co-surfactant and surfactant-free microemulsion（PSeP-SFME）using PSeP as an amphi-solvent.Moreover,dual-responsive SFME was constructed utilizing commercial hydroxyethylamine or its combination with PSeP.These responsive MEs were used as a carrier to encapsuled bioactive molecules,or template to prepare nanomaterials.The main research contents and conclusions were summarized as follows:（1）Redox-responsive ME based on PSePPSeP-SBME was developed using octyldodecanol（ODD）as oil phase,polyoxyethylene hydrogenated castor oil-40（HCO-40）as surfactant,PSeP and diethylene glycol monohexyl ether（C₆（EO）₂）as co-surfactants（the mass ratio of HCO-40:C₆（EO）₂:PSeP is 1:1:1）.The percentage of single-phase area was as high as 39.74%.Based on the unique redox-resonsiveness of PSeP,PSeP-SBME exhibited excellent redox response performance.No matter what types of ME,demulsification could occur after oxidation,and single-phase ME was restored after adding N₂H₄·H₂O.Additionally,due to the presence of full water angle,the number of redox cycles of the O/W PSeP-SBME was infinite theoretically.After several cycles,the size of droplet only showed a small change.PSeP-SBME showed good solubility of curcumin and can increase the solubility of curcumin in oil by about 20 times.Moreover,ME can significantly improved the light stability,thermal stability and storage stability of curcumin.The existence of PSeP further raised the antioxidant properties of ME.Furthermore,the curcumin-encapsulated ME showed enhanced transdermal properties,which was approximately 3.67 times higher than the skin accumulation of curcumin-encapsulated emulsifier oil solutions.In addition,PSeP can also be used as an amphi-solvent to stabilize SFME,and the single-phase area of SFME constructed with n-octanol and water accounted for 30.30%.Substituting part of PSeP with other common amphi-solvents does not affect the redox-responsiveness of SFME,and the combination of PSeP and CO₂-responsive amphiphilic solvent could endow the SFME with redox-CO₂/N₂ dual responses.（2）CO₂/N₂-temperature dual-responsive SFME based on hydroxyethylamineCO₂/N₂-temperature dual-responsive SFMEs were successfully constructed using N,N-dimethylethanolamine（DMEA）,N-methyldiethanolamine（MDEA）,and triethanolamine（TEA）as amphi-solvents,respectively.Among them,DMEA-based SFME had the largest single-phase area47.76%.With alternatively bubbling of CO₂/N₂ or cooling/heating,both the single-phase and pre-ouzo zone of DMEA-based SFME could shrink and expand reversibly.Note that CO₂/N₂ led to a relatively larger change.The bubbling of CO₂ can lead to complete phase separation of DMEA-based SFME from Winsor IV to the oil and water phase,and the removal of CO₂ brought an oppsite transformation.After three cycles,the stability and particle size of DMEA-based SFME almost remained unchanged.Complete phase separation couldn’t be achieved by lowering the temperature.（3）Nanoparticle synthesis using SFME as templateZIF-8@SFME was synthesized using octanol/DMEA/water SFME as template.Due to the presence of excessive DMEA,the nucleation and reaction rates in DMEA-based SFME was extremely fast,as short as 5 min.ZIF-8@SFME has good thermal stability,small crystal size and narrow particle size distribution,indicating that SFME is a good template for synthesis of ZIF-8.ZIF-8@SFME featuring positive charge showed better absorption performance to anionic SDBS than cationic 1227 and non-ionic TX-100.With the increase of amount of ZIF-8,the adsorption rate of SDBS increased,but the adsorption capacity decreased.The adsorption of SDBS on ZIF-8@SFME conformed to the pseudo-second-order kinetic model and Langmuir adsorption model,and the highest adsorption capacity was 548.04 mg/g at 318 K.Thermodynamic studies indicate that adsorption is spontaneous and endothermic and thus driven by entropy.Compared with ZIF-8 synthesized using SBME,ZIF-8@SFME exhibited higher adsorption capacity for SDBS.