The Study On PH-responsive Self-assembly Behavior Of ?-carboxyl Amine Oxide Surfactant

Environmentally stimuli-responsive surfactants have the ability to regulate the transitions of self-assembled aggregates by changing the external conditions(such as light,heat,magnetism,electricity,pH,humidity,pressure,electrolyte concentration,etc.).With attractive application prospects,they are widely employed in the controlled drug release,controlled microfluidic devices and nanometer microreactors,and are closely related to the development of new materials and new technologies in the fields of energy,information and smart sensors.Therefore environmentally stimuli-responsive surfactants have become an important research direction of intelligent fluids.The smart pH-responsive surfactant fluids have the characteristics of convenient regulation,rapid response,low cost and reversible response.In the literature,the constructions of the smart pH-responsive surfactant fluids mainly consist of surfactant combination,surfactants by the addition of functional additives in solution and the single component of the surfactant.In this paper,?-carboxytetradecyldimethylamine oxide(CTAO)and ?-carboxy pentadecyldimethylamine oxide(CPAO)in the molecular structures containing both amine oxide group and carboxylic acid group were synthesized from natural fatty acids.The smart pH-responsive fluids with single component surfactant were successfully obtained without any additives to achieve the multi-molecular aggregates transition including vesicle,wormlike micelles and spherical micelles.The results showed that the rheological properties of the system changed twice greatly in the range of pH variation of about 1 unit.The effects of temperature and concentration on vesicles and wormlike micelles formed by CTAO and CPAO were investigated.The mechanism of pH-responsive self-assembly was discussed.In addition,based on the sensitive response to pH,the smart light-responsive fluids transformed by the smart pH-responsive fluids were studied.The main work is summarized as follows:The CTAO and CPAO were synthesized by three steps of chlorination,hydrolysis and oxidation using natural fatty acid as raw materials,and their structures were identified by FT-IR and 1H NMR.The pKa1,pKa2 and isoelectric points of CTAO and CPAO were determined by acid-base titration,conductivity meter and zeta potential analyzer.The rheological behaviors of CTAO and CPAO at different pH were investigated by rheometer.The self-assembled microstructures of CTAO and CPAO were characterized by cyro-transmission electron microscopy(cyro-TEM)and scanning electron microscopy(SEM).The results showed that the surfactant solution showed sensitive pH response without any additives.Vesicles are found for CTAO(pH 2.0 ~ 5.6)and CPAO(pH 2.3 ~ 5.8)when the pH is lower pH value.When the pH continues to increase,CTAO(pH 6.0 ~ 6.7)and CPAO(pH 6.2 ~ 7.4)can self-assembly into wormlike micelles possessing the viscoelastic fluids behavior.The zero-shear viscosity changes can reach three orders of magnitude in a very narrow pH range.When the pH was higher than neutral pH,the spherical micelles are formed in CTAO(pH > 7)and CPAO(pH > 8)solution.In addition,the effects of temperature and concentration on the vesicles and wormlike micelles in CTAO and CPAO solution were respectively investigated.By measuring the surface tention of CTAO solution and CPAO solution under different pH conditions,the variations of the occupied area of the two amine oxide surfactants with the pH in solution were investigated.The results showed that with the increase in pH,surface tention and critical micelle concentration of two kinds of surfactants were increased,indicating the molecular forms and intermolecular interactions both changed,which had an effect on the critical packing parameters of surfactant molecules at different pH and thus leading to self-assemblied aggregates transiton in surfactant solution.In this paper,based on the sensitivity of CTAO to pH response,we introduced photo-acid generator(PAG)to pH-responsive surfactant fluids to construct the light-responsive surfactant fluids.The UV light irradiation induced the changes coupled with of pH,which would result in the aggregates transition.