Formation And Properties Of Vesicles Of Alkyltrimethylammonium Bromides

It is generally believed that simple single-tailed amphiphiles(STAs)can form micelles but not vesicles in their single-component solutions without any additives.However,our group recently found that,under the in situ mediation of rough glass surfaces(RGSs),the micelle-tovesicle transition can occur in the single-component solutions of simple STAs,forming stable vesicles.The population of STA vesicles thus obtained is small,which coexist with a large number of micelles(the vesicle-containing systems are transparent,different from the opalescent appearance of traditional vesicle systems).This limits the research on their performances and applications.More recently,we found that the dehydration-rehydration(DHRH)cycles can enhance the transition from micelles to vesicles for 4-dodecylbenzene sulfonic acid(DBSA),a simple weakly acidic STA,in water,especially resulting in a large population of vesicles(an opalescent vesicle system).It is interesting to examine the feasibility of the DHRH method and to study its mechanism.In addition,there is still a lack of basic understanding for the properties of STAs vesicles.In this work,the DHRH method on smooth glass surfaces(SGSs)was used to induce the formation of vesicles in the single-component solutions of the simple cationic STA alkyltrimethylammonium bromides(CnTABs,n=12,14,and 16).The stability of the soobtained vesicles and the micropolarity,microviscosity,and permeability of the vesicle membranes were determined.The mechanism of DHRH was discussed.It is expected to deepen the understanding of the aggregation behavior of STAs and the properties of their vesicles.Main contents and conclusions:(1)Micelle-to-vesicle transition mediated by dehydration-rehydrationDodecyl,tetradecyl,and cetyl trimethylammonium bromides(DTAB,TTAB,and CTAB)were used as STAs models,and the DHRH-mediated micelle-to-vesicle transition was investigated in their single-component solutions.That is,a CnTAB micelle solution dropped on SGSs was first dried,and the dried CnTAB aggregates were then rehydrated in a monomer solution of CnTAB,to examine the formation,structure,and number of vesicles.The effects of hydrocarbon chain length,solution concentration,and dehydration(drying)temperature were determined.The relevant mechanisms were discussed.The results show that DHRH can induce micelle-to-vesicle transition in CnTABs solutions,mainly forming unilamellar vesicles.It is necessary that the concentration of CnTABs in the rehydration systems is higher than their critical micelle concentration,and the dehydration temperature has no obvious effect on the formation of vesicles.With an increase in alkyl chain length,all of the number,size,and film thickness of CnTABs vesicles increase.A possible mechanism of DHRH-mediated micelle-to-vesicle transition is proposed:bilayer sheets of CnTABs are formed on SGSs during dehydration,and then detached from the SGSs to spontaneously form vesicles during rehydration.A highly interdigitated structure of alkyl chains between two leaflets was identified in the bi layers,which probably is the origin of the formation and stability of STA vesicles.Compared with the RGSs-in situ-mediated method previously proposed,the DHRH process can more effectively motivate the micelle-to-vesicle transition for STAs and even produce pure vesicle systems(or vesicle-dominated systems,exhibiting milky white in appearance).This work deepens the understanding of the aggregation behavior of STAs,and the so-obtained pure STAs vesicle systems can be used for the investigation on their properties.(2)Stability,micropolarity,microviscosity,and permeability of CnTAB vesicle membranesThe stability,micropolarity,microviscosity,and permeability of CnTABs vesicle membranes were measured by the steady-state and time-resolved fluorescence techniques using calcein,pyrene,1,6-diphenyl-1,3,5-hexatriene(DPH),and riboflavin as fluorescent probes,and compared with those of their micelle systems.The results show that CnTAB vesicles have good stability,and no obvious changes in the morphology,structure,and number of CnTAB vesicles were observed after long-term storage at room temperature(25℃,at least 9 months),exposure to high temperature(80℃ for 2 h),and freeze-thaw(-20℃/25℃)cycle.The micropolarity of CnTAB vesicle membranes is lower than that of micelles,and decreases with the increase of hydrocarbon chain length.The fluorescence anisotropy(r)and microviscosity(ηm)of DPH in CnTAB vesicle membranes are higher than those in micelles,and increase with the increase of hydrocarbon chain length.The change of pH in 2.6-9.4 has little influence on r.In addition,the CnTAB vesicle membranes are permeable to small ions,such as OH-.The permeability of OH-across CnTAB membranes can be well described by the first-order kinetic model,and the transmembrane permeation rate(or first-order kinetic rate constant)decreases with the increase of hydrocarbon chain length.The results of the high stability of CnTAB vesicles and the change of membrane properties with the hydrocarbon chain length can be explained by the highly interdigitated structure of hydrocarbon chains existing in membranes.This work provides a better understanding of the properties of CnTAB or STAs vesicles.